Effect of hypertrophy induced by pressure overload or volume overload on reperfusion induced arrhythmias in anaesthetised rats.

OBJECTIVE: The aim was to determine the effect of myocardial hypertrophy on the incidence and severity of arrhythmias following reperfusion after experimental coronary artery occlusion, and to establish if the effect differed between pressure overload hypertrophy and volume overload hypertrophy. METHODS: The experiments were performed in rats. Pressure overload hypertrophy was induced by aortic banding, and volume overload hypertrophy by perforation of the aortic valve. Four weeks after surgery, coronary artery occlusion and reperfusion experiments were performed. RESULTS: Both pressure and volume overloaded hearts exhibited a similar degree of left ventricular hypertrophy of about 45% weight gain compared to control animals. The total number of ventricular premature beats during reperfusion was not significantly increased in either model of hypertrophy. In contrast to volume overload, the incidence of reperfusion induced ventricular fibrillation was significantly increased in pressure overloaded hearts. Furthermore, there was a significant correlation between number of ventricular premature beats and degree of left ventricular hypertrophy, as well as between duration of ventricular tachycardia and degree of hypertrophy in the pressure overloaded group. Such correlations were not observed in hypertrophy due to volume overload. Neither volume nor pressure overload hypertrophy significantly increased mortality from ventricular fibrillation. CONCLUSIONS: An increase in cardiac mass per se does not necessarily aggravate reperfusion arrhythmias. The susceptibility to arrhythmias in this model seems to be critically dependent on the nature of the stimulus triggering left ventricular hypertrophy.

Decreased lumen size after balloon injury despite inhibition of neointimal thickening and antivasospastic treatment.

OBJECTIVE: Both calcium antagonists and angiotensin converting enzyme (ACE) inhibitors are known to diminish the development of intimal thickening after a balloon catheter lesion. It was previously shown that narrowing of carotid artery lumen induced by balloon injury was not influenced by treatment, even though the two ACE inhibitors used inhibited neointimal thickening. The aim of the present study was to include a calcium antagonist as well, in order to investigate whether vasospasm contributes to the persistence of lumen narrowing in ACE inhibitor treated rats after a balloon lesion. METHODS: Six groups of 10 rats were subject to balloon lesion of the left carotid artery. They received spirapril (10 mg.kg-1 x d-1) or isradipine (30 or 100 mg.kg-1 x d-1) or both, given throughout the study in the food. Controls received no drug. Neointimal thickening was measured histologically two weeks after injury. The cross sectional carotid lumen area was measured in vivo by nuclear magnetic resonance imaging, both before and two weeks after balloon injury, and also postmortem by histological techniques. RESULTS: Two weeks after injury, the lumen area of the left carotid artery was significantly reduced following balloon injury, as measured by both techniques. Treatment did not modify the stenosis process as assessed by either method for measuring lumen size. Neointimal thickening, however, was inhibited by between 4% (low dose isradipine) and 59% (combined spirapril + high dose isradipine) in the various treatment groups. CONCLUSIONS: Since calcium antagonist treatment was not able to influence the reduction of lumen size, it is unlikely that the narrowing is due to reversible spasm of the carotid artery in the first two weeks after inducing a balloon lesion. Alternatively, chronic vasospasm of neointimal smooth muscle might not be very sensitive to calcium antagonists.

Effects of isradipine and other calcium antagonists on arteriovenous-shunt flow in anesthetized rabbits and cats.

The effects of vasodilators on arteriovenous (AV)-shunt flow was investigated in anesthetized cats and rabbits, using the tracer microsphere method. In cats, the calcium antagonist isradipine reduced AV-shunt flow; verapamil showed a similar tendency and nicardipine was without effect. Dihydralazine strongly increased, but nitroglycerin and dipyridamole decreased AV-shunt flow. In rabbits, the effects of isradipine and verapamil were similar to those seen in cats. Sodium nitroprusside had no effect, whereas prazosin, minoxidil, and the potassium-channel activator cromakalim increased AV-shunt flow. The contrasting effects of drugs sharing the same mechanism of action suggest that target-tissue selectivity is more important than the mechanism of action. An increase of AV-shunt flow is unlikely to be beneficial but could be associated with a number of undesirable side effects. It might negatively affect migraine sufferers and, if AV-shunt dilatation shows no tolerance development, it represents an unnecessary hemodynamic burden for the heart.

Antivasoconstrictor effects of isradipine. A quantitative approach in anesthetized rats and conscious rabbits.

Calcium antagonists blunt the constrictor effects of vasoconstrictor agents. Differences in the sensitivity of several vasoconstrictors to the constrictor effect of calcium antagonists were quantified in experiments on rats and rabbits. In rats, the dose-response curves for pressor effects to angiotensin II were shifted in parallel to the right after treatment with isradipine, but not with prazosin and dihydralazine, suggesting that the antivasoconstrictor effect of isradipine was of a specific type. Blood pressure increases elicited by angiotension II or norepinephrine (alone, or with propranolol or propranolol plus prazosin) were also measured in rabbits and, again, isradipine caused parallel shifts of the dose-response curves. Both agonists thus appear to stimulate calcium entry, at least in resistance vessels, and via the same pathway, the L-type calcium channel. However, the angiotension II pressor effects were more sensitive to isradipine than those of norepinephrine in the presence of propranolol, and even more so in the presence of propranolol plus prazosin. These results suggest subtle and as yet unexplained differences, possible related to the activity of the adenylate cyclase system, in the chain of events leading to activation of the L-channels in vivo.

Cerebrovascular, biochemical, and cytoprotective effects of isradipine in laboratory animals.

Dose-response curves of isradipine for blood pressure, total peripheral conductance, and regional cerebral conductances and blood flows were obtained in anesthetized cats and rabbits using the microsphere method. Cerebrovascular effects occurred at lower doses than systemic effects, and the effective duration was longer in the brain than in the periphery. In a rat model of embolic stroke (unilateral occlusion of the middle cerebral artery), isradipine has been shown to have cytoprotective efficacy (Sauter A, Rudin M: Stroke 1986; 17: 1228-1234; Rudin M, Sauter A, Wiederhold K-H: Therapie 1987; 42: 477-481; Sauter A, Rudin M, Wiederhold K-H: Neurochem Pathol 1989 [in press]). Using this model, dose-response curves for infarct size, measured by magnetic resonance imaging, and biochemical markers of infarction were obtained for various calcium antagonists. Isradipine showed the biggest improvements (50 to 60 percent at 2.5 mg/kg subcutaneously), followed by nimodipine (30 to 40 percent at 5 mg/kg subcutaneously), nitrendipine (30 to 40 percent at 10 mg/kg subcutaneously), darodipine (20 to 30 percent at 10 mg/kg subcutaneously), and nicardipine (10 percent at 10 mg/kg subcutaneously). It is concluded that isradipine differs in both efficacy and potency from the other calcium antagonists tested. The effects of isradipine, nimodipine, and darodipine on cerebral blood flow were further investigated in this model using the [14C]iodoantipyrine method. Despite systemic hypotension, cerebral blood flow was dose dependently increased in the normal and ischemic hemispheres. Isradipine elicited maximal improvements over a wider dose range than the other drugs tested (0.1 to 2.5 mg/kg), preferentially affecting cerebral blood flow in the ischemic areas, as further demonstrated using autoradiographic techniques. These effects are in good, quantitative agreement with the reductions in infarct size, observed by magnetic resonance imaging and histology, emphasizing the importance of cerebrovascular mechanisms for cytoprotection in stroke. The concurrence between cat, rabbit, and rat experiments suggests that the findings may also apply to humans.

Combined FTY720/cyclosporine treatment promotes graft survival and lowers the peripheral lymphocyte count in a murine cardiac allotransplantation model.

BACKGROUND: FTY720 lowers the peripheral lymphocyte count (PLC) by accelerating the migration of circulating lymphocytes to secondary lymphoid organs. We investigated the efficacy of combined FTY720+cyclosporine (CsA) treatment versus monotherapy on prolonging graft survival and on lowering the PLC. METHODS: BALB/c hearts were heterotopically grafted in C3H mice. FTY720 was administered alone or in combination with CsA. PLC and body weight were determined on day 7, day 28, or the day of rejection. RESULTS: Combining FTY720 with CsA prolonged, dose-dependently and significantly, the allograft survival. FTY720, but not CsA, lowered the PLC dose-dependently. The granulocyte count was not reduced in any group. FTY720 concentrations were not influenced by the CsA co-administration. CONCLUSIONS: Combined FTY720 and CsA treatment was well tolerated, promoted graft survival, and suppressed the inflammatory allo-response. The PLC lowering correlated well with the antirejection effects in the two-drug regimens, suggesting that the PLC might guide FTY720 therapy at low doses.

Allo- and autotransplantation of carotid artery--a new model of chronic graft vessel disease: evaluation by magnetic resonance imaging and histology.

BACKGROUND: Graft vessel disease is a special form of accelerated arteriosclerosis. Because immunological and nonimmunological factors can contribute to graft vessel disease, we developed a model that enables the study of both factors simultaneously. METHODS: A carotid artery was allografted from DA to Lewis rats, with the excised native artery autografted on the contralateral side. Five groups of six to seven rats were treated for 8 weeks with vehicle (placebo) or cyclosporine (CsA) (0.3, 1, 3, and 10 mg x kg(-1) x day(-1)), which was administered using subcutaneous osmotic minipumps. The carotid lumen area was estimated in vivo at 2, 4, and 8 weeks by magnetic resonance imaging (MRI); CsA blood levels were determined twice. Carotid neointimal thickening and medial and luminal area were measured with histological techniques. RESULTS: MRI showed bulging of the allografts but not autografts. Bulging disappeared over time with narrowing of the allograft lumina estimated by both MRI and histology. Histologically, vehicle-treated animals developed a massive neointima, which was inhibited in a dose-dependent manner by CsA. Autografts remained normal except for minimal subintimal thickening of two of four arteries in the group given the highest dose of CsA. Cellular rejection was detected in the allografts of all but the highest-dose group. The CsA blood levels were similar to those used in man at the two lower doses and about 10-fold higher at the highest dose. CONCLUSIONS: Subintimal thickening did not correlate with in vivo lumen size, a phenomenon that we have previously described for balloon catheter-induced lesions. CsA blood concentrations similar to those used in patients suppressed neointima formation in part, and 10-fold higher concentrations almost completely suppressed neointima formation.

Contribution of donor-specific antibodies to acute allograft rejection: evidence from B cell-deficient mice.

BACKGROUND: The role of T lymphocytes in acute allograft rejection is well established. The involvement of B lymphocytes in this process, however, is more controversial. A series of reports showed that mice without a functional B-cell compartment rejected allografts with the same kinetics as control animals. In rats, however, alloantibodies were found to play a decisive role in allograft rejection. To provide an explanation for the discrepant results, we readdressed the role of B cells and antibodies in mice with disrupted immunoglobulin mu chain genes. The use of cyclosporine (CsA), which strongly suppresses T cells, allowed us to focus specifically on the function of B cells. METHODS: C57BL/6 mice rendered B cell deficient by targeted disruption of the immunoglobulin mu chain gene (referred to as microMT/microMT mice) and microMT/+ control mice with one functional mu chain were heterotopically transplanted with fully MHC-disparate BALB/c hearts. CsA was administered subcutaneously by Alzet osmotic pumps. Normal and immune serum specific for donor hearts was given to assess the role of antibodies in the rejection process. RESULTS: Both B cell-deficient microMT/microMT and heterozygous microMT/+ mice were found to reject transplanted hearts within a similar period of time. In contrast, when T cells were partially suppressed with CsA, graft survival was significantly prolonged in microMT/microMT mice as compared with heterozygous controls. Passive transfer of donor-specific immune serum, obtained from microMT/+ animals rejecting allogeneic hearts, to CsA-treated microMT/microMT mice significantly accelerated allograft rejection as opposed to recipients treated with normal serum. CONCLUSIONS: B lymphocytes and antibodies play an important role in acute allograft rejection particularly when the dominant T-cell compartment is partially suppressed.

Prevention of graft vessel disease by combined FTY720/cyclosporine. A treatment in a rat carotid artery transplantation model.

BACKGROUND: Graft vessel disease (GVD) is an important problem often responsible for late graft loss. The effects of FTY720, an immunomodulator with a novel mechanism of action were investigated in combination with cyclosporine A (CsA) in a carotid artery allograft model. METHODS: A segment of the carotid artery of Lewis rats was replaced by a DA allograft. Seven groups of eight rats were treated for 8 weeks with vehicle (P), CsA 0.3 (C0.3), 1 (C1) or 3 (C3) mg x kg(-1).day(-1) or a combination of CsA 1 with FTY 0.01 (C1F0.01), 0.03 (C1F0.03), and 0.1 (C1F0.1) mg x kg(-1).day(-1). Lumen area was estimated by magnetic resonance imaging, peripheral lymphocyte count and drug concentrations were determined at 1 and 8 weeks. Neointima, media, and lumen area were measured morphometrically. Intimal and adventitial infiltration of mononuclear cells, and medial smooth muscle cells number was assessed using a score. RESULTS: FTY720 did not influence CsA blood concentrations. FTY720 but not CsA decreased the PLC dose dependently. Magnetic resonance imaging revealed that treatment groups have larger lumen size than group P. Histological and morphometric evaluation showed that all aspects of GVD were dose dependently suppressed by treatment and lumen narrowing was prevented. CONCLUSIONS: CsA, at clinically relevant blood levels, suppressed GVD only partly. The addition of FTY720 was well tolerated and completely suppressed GVD development. In vivo lumen size did not correlate with the histologically estimated neointimal thickness.

Interaction between two calcium antagonists and two beta blockers in conscious rabbits: hemodynamic consequences of differing cardiodepressant properties.

The interaction between beta-adrenoreceptor blockers and calcium antagonists may occasionally be dangerous. The effects of the new calcium antagonist PN 200-110 (isradipine) were compared with those of verapamil in 3 groups of conscious rabbits pretreated with either pindolol 0.3 mg/kg, propranolol 1 mg/kg intravenously or placebo. Each animal received PN 200-110 (0.01, 0.03 and 0.1 mg/kg) and 2 or more days later verapamil (0.1, 0.3 and 1 mg/kg). The calcium antagonists were given to lower mean blood pressure to the same extent as in the placebo group. This blood pressure effect remained unchanged after pretreatment with pindolol or propranolol. Both PIN 200-110 and verapamil increased heart rate to the same extent as in the placebo group. Both beta blockers blunted the effect of PN 200-110 on heart rate but converted the verapamil-induced tachycardia to bradycardia. Propranolol blunted the PN 200-110-induced increase in cardiac output and total peripheral conductance, whereas the high verapamil dose decreased cardiac output and caused peripheral vasoconstriction in propranolol-pretreated animals. Thus, both agents lowered blood pressure by peripheral vasodilatation in the placebo group, after beta blockade; however, the mechanism of the verapamil-induced blood pressure decrease changed from pure vasodilation to a decrease in cardiac output, i.e., cardiac depression. Verapamil but not PN 200-110 prolonged the PQ interval, especially in animals who had received beta blockade. Most differences in the interaction were attributable to differences between the 2 calcium antagonists; the differences between the beta blockers were small and in favor of pindolol.

Comparison of cardiodepressant and vasodilator effects of PN 200-110 (isradipine), nifedipine and diltiazem in anesthetized rabbits.

Cardiodepressant and vasodilator effects of PN 200-110 (isradipine) were investigated in anesthetized open-chest rabbits and compared with those of nifedipine, diltiazem or placebo. In order to eliminate compensatory reflexes that may mask negative inotropic and chronotropic effects of blood pressure-lowering agents, the animals were vagotomized and pretreated with 1 mg/kg propranolol. Dose-response curves were obtained with 4 doses of each drug in 6 animals. The doses were chosen to cause comparable decreases in blood pressure. PN 200-110 had the highest efficacy with respect to peripheral vasodilatation (increase in total peripheral conductance) and coronary blood flow, measured with radioactive-labeled microspheres. It did not decrease myocardial contractile force (measured with a strain gauge). The 2 other compounds caused biphasic changes in coronary blood flow and cardiac output. Their efficacy with respect to peripheral vasodilatation was limited by cardiodepression. Their dose-response curve for cardiodepression was flat at the 2 smaller doses and became steeper at the 2 larger doses. Nifedipine did not change heart rate; PN 200-110 decreased it minimally and diltiazem had a dose-dependent negative chronotropic effect. After eliminating autonomic influences on the heart, direct myocardial effects of calcium antagonists become demonstrable in whole animals and can be compared with the vascular effects measured in the same animals.

Selective effects of PN 200-110 (isradipine) on the peripheral circulation and the heart.

PN 200-110 (isradipine) is a new dihydropyridine calcium antagonist with selective actions on the heart as well as the peripheral circulation. It selectively inhibits the sinus node but not atrioventricular conduction and its negative inotropic action is minimal, about 20 times weaker than its negative chronotropic effect. This in vitro pattern also expresses itself in vivo: partial suppression of the reflex tachycardia induced by its peripheral vasodilatation and no effect on the P-Q interval on the electrocardiogram even at large doses. The presence of first- or second-degree heart block should therefore not limit its use, whereas the sick sinus syndrome might. PN 200-110 does not decrease myocardial contractile force even in vagotomized animals with full beta blockade. PN 200-110 nevertheless lowers myocardial oxygen consumption mainly by its action on afterload. It should therefore be useful in angina pectoris. PN 200-110 is a powerful peripheral vasodilator. It preferentially dilates coronary, cerebral and skeletal muscle vasculature. Its long lasting (24 to 48 hours) antihypertensive action is not accompanied by tachycardia in spontaneously hypertensive rats and it enhances sodium and water excretion in normotensive rats. It should be useful in the treatment of hypertension, and, considering its pattern of cardiac actions, perhaps also as an after-load-reducing agent for the treatment of heart failure. Antiarteriosclerotic effects in conscious rabbits were found at reasonably small doses, suggesting that such effects might occur in man at therapeutic doses.

Modification of vasopressin- and angiotensin II- induced changes by calcium antagonists in the peripheral circulation of anaesthetized rabbits.

Investigations into the site of vasodilator and antivasoconstrictor activity of calcium antagonists previously performed in cats were extended to a second species, barbiturate-anaesthetized rabbits, and a second vasoconstrictor agent, vasopressin. The dihydropyridine derivative darodipine (code name PY 108-068; 10, 30 and 100 micrograms kg-1 i.v.) showed systemic haemodynamic effects comparable to those seen in cats at half these doses. Darodipine effected regional vasodilatation (measured with tracer microspheres) in the heart, brain and skeletal muscles as in cats. Only the vessels of the adrenals (dilated in rabbits but not in cats), and the kidneys and skin (constricted in rabbits but not in cats) responded differently to darodipine. Angiotensin II (A II; 0.15 and 1.5 micrograms kg-1 min-1) constricted the same vascular beds in rabbits as in cats, namely the heart, kidneys, small intestine, pancreas, spleen, skin and arterio-venous shunts (inferred from microspheres reaching the lungs), the only exceptions being the vessels of the stomach and liver (constriction only in cats) and the adrenals (constriction only in rabbits). Darodipine (30 and 100 micrograms kg-1) attenuated the A II-induced vasoconstriction in the same vascular beds in rabbits as in cats including the kidneys, which were constricted after administration of the antagonist alone. These results indicate surprisingly small species differences for the vasodilator effects of darodipine as well as the attenuation of the vasoconstrictor effects of A II. Lysine-vasopressin (2 and 50 mu kg-1 min-1) did not increase blood pressure in anaesthetized rabbits but dose-dependently lowered heart rate, cardiac output, total peripheral conductance and myocardial contractile force (measured with a strain gauge). Vasopressin constricted all peripheral vascular beds dose-dependently, except for those of the kidney and liver. The effects of vasopressin persisted in the animals infused with placebo solution. Darodipine (30 and 100 micrograms kg-1), but not verapamil (300 and 1000 micrograms kg-1) reversed the vasopressin-induced cardiac depression and decrease in cardiac output. This probably also explains most of the apparent differences between the effects of the two calcium antagonists on the peripheral circulation. Both calcium antagonists diminished the vasopressin constriction in most vascular beds except those of the spleen, skin and arterio-venous shunts. Most of the effects were dose-related but not strictly competitive, as far as this can be judged based on two doses of agonist and antagonist. 9 As with A II the effects of vasopressin were diminished in vascular beds not normally dilated by calcium antagonists. 10 Calcium antagonists display two typical patterns of activity. The vasodilator pattern consists of dilatation of the vesels of the heart, brain and, to a degree varying with the agents, skeletal muscle. The antivasoconstrictor effects occur in some but not all of the vessels constricted by the constrictor agent, vasoconstriction of the spleen, skin and arterio-venous shunts being resistant to the action of calcium antagonists. The pattern of antivasoconstrictor activity appears to depend on the constrictor compound used, inasmuch as such agents constrict different vascular beds.

Vasopressin induced myocardial depression in neurally mediated and not due to impaired coronary blood flow.

The mechanism of the cardiodepressant effect of vasopressin was studied by measuring simultaneously myocardial contractile force and coronary blood flow (with tracer microspheres) in anaesthetized open-chest rabbits. Lysine-vasopressin administered at two dose levels (10 and 100 mu kg-1 infused in 2 min with a maintenance dose of 2 mu kg-1 min-1 between these two loading doses) to a group of 6 rabbits caused dose-dependent myocardial depression and also severely decreased coronary blood flow in a dose-dependent manner. Blood pressure remained almost unchanged but heart rate, cardiac output and total peripheral conductance were also decreased dose-dependently. In another group of 6 rabbits treated in the same way with lysine-vasopressin, darodipine (PY 108-068, 30 and 100 micrograms kg-1) was infused intravenously. It reversed the vasopressin-induced coronary constriction and cardiodepression. The high dose of vasopressin brought back cardiac depression but did not reduce coronary blood flow below baseline values. Myocardial depression could therefore not be adequately explained by the changes in coronary blood flow. In a further group of rabbits which had been subjected to cervical vagotomy and beta-adrenoceptor blockade (propranolol 1 mg kg-1 i.v.) before the experiment, vasopressin still caused coronary constriction which was reversed by darodipine, but had no effect on myocardial contractile force and heart rate. The cardiodepressant effect of vasopressin can thus be explained fully by effects on the autonomic nervous system which are reversed by lowering blood pressure, whereas the severe reduction of coronary flow did not contribute to the vasopressin-induced myocardial depression.

Calcium antagonist and the peripheral circulation: differences and similarities between PY 108-068, nicardipine, verapamil and diltiazem.

1 The effects of two dihydropyridines, PY 108-068 (PY) and nicardipine (N), and two other calcium antagonists, verapamil (V) and diltiazem (D), on regional blood flow were measured in open-chest cats, anaesthetized with chloralose-urethane.2 Each substance was infused at 3 different dose rates, each for 10 min. The total doses given were 5 plus 10 plus 35 (total of 50) mug/kg for PY, 10 plus 70 (total of 100) mug/kg for N and 100 plus 200 plus 700 (total of 1000) mug/kg for V and D.3 All substances lowered blood pressure and increased total peripheral conductance. Heart rate was lowered only by V, D and PY. Cardiac output was markedly increased only by the diyhydroypridine derivatives; D had small and V almost no effects.4 All substances increased coronary flow and redistributed it in favour of the subepicardial layer. All substances also increased blood flow to the brain. The effects of verapamil were comparatively small.5 Skeletal muscle flow was increased strongly by the two dihydropyridine derivatives. D and V had negligible effects.6 Blood flow to stomach and small intestine was only slightly increased. Flow to the kidneys increased slightly in diltiazem-treated animals but did not change with all other treatments. Flow to the liver, the adrenals, and the spleen remained unchanged or showed a tendency to decrease.7 The organ conductances which reflect the active changes in vascular tone better than blood flow values, showed that there was a tendency towards vasodilatation even in most organs where blood flow tended to decrease.8 Results obtained in an earlier series of experiments with nifedipine were very similar to those described here for N, except that nifedipine was about twice as potent.9 Calcium antagonists were thus neither general peripheral vasodilators nor did they show a uniform pattern of preferential sites of action. The most important common features were increases in coronary and cerebral blood flow and the most important differences the divergent effects of the dihydropyridines on one side and V and D on the other side on skeletal muscle flow. The size of this vascular bed may help to explain why dihydropyridines appear to be particularly potent as peripheral vasodilators.

The calcium antagonists PY 108-068 and verapamil diminish the effects of angiotensin II: sites of interaction in the peripheral circulation of anaesthetized cats.

The sites of interaction between the vasoconstrictor angiotensin II (A II) and the calcium antagonists PY 108-068 (PY) (a dihydropyridine derivative) or verapamil (V) in different peripheral vascular beds were investigated using the microsphere method in chloralose-urethane anaesthetized open-chested cats. A II was infused intravenously into 27 cats at a rate of 0.15 microgram kg-1 min-1. Systemic haemodynamic variables and regional blood flow were measured immediately before and 10 min after the start of the infusion. While the infusion of A II continued, PY (3 micrograms kg-1 min-1), V (30 micrograms kg-1 min-1) or the vehicle was infused for 10 min into 9 cats each and the effects of this combined infusion were again measured at the end of the 10 min period. A II increased mean arterial blood pressure but decreased peripheral conductance and, to a smaller but still significant degree, cardiac output and peak acceleration of blood in the aorta (an ejection phase parameter of myocardial contractility). The calcium antagonists reversed these effects. Cardiac output and total peripheral conductance were increased even beyond the pre-A II level by PY. A II constricted the vascular beds of the kidney, small intestine, liver and skin. Arterio-venous shunt flow decreased. Vasoconstriction was also found in the stomach, spleen and in different parts of the heart with the exception of the subendocardial layer of the left ventricle, where blood flow increased and conductance remained unchanged. A II did not decrease conductance in different parts of the brain or in skeletal muscle. The vasoconstrictor effects of A II persisted or tended to be increased in most of the vascular beds of placebo treated animals. PY 108-068 and verapamil abolished the vasoconstrictor effects of A II in most of the vascular beds with the exception of the liver, the spleen, the skin and the arterio-venous shunts and caused vasodilatation in the heart. PY also induced vasodilatation in the brain and skeletal muscle, where A II had not induced vasoconstriction. The pattern of attenuation of A II effects was different from the pattern of vasodilatation induced by these and other calcium antagonists in the same cat preparation not treated with a vasoconstrictor. The sites of action of this dihydropyridine derivative (PY) on the peripheral circulation thus, appear to depend not only on the vascular bed but also on the presence of a vasoconstrictor influence at the time of investigation.

Cardiovascular effects of apamin and BRL 34915 in rats and rabbits.

1. The cardiovascular effects of apamin, a selective blocker of certain calcium-activated potassium channels, and BRL 34915, a vasodilator thought to act by opening of potassium channels, have been investigated in vivo in rats and rabbits. 2. In anaesthetized normotensive rats, apamin (0.05 and 0.15 mg kg-1, i.v.) potentiated angiotensin II pressor responses but did not modify baseline blood pressure or heart rate. 3. Apamin (0.15 mg kg-1, i.v.) was without cardiovascular effects in rabbits. 4. BRL 34915 (0.1 and 0.3 mg kg-1, i.v.) lowered blood pressure in rats dose-dependently and caused reflex tachycardia. The heart rate increase was abolished by prior administration of the beta-adrenoceptor blocker bopindolol (0.1 mg kg-1, i.v.). 5. In anaesthetized rabbits, regional blood flow measurements (with radioactive tracer microspheres) showed that BRL 34915 (3 to 30 micrograms kg-1, i.v.) caused marked vasodilatation in the stomach, with increases in flow also to the heart and small intestine. Brain blood flow also tended to increase. Blood flow to the kidneys was reduced by BRL 34915, whereas flow to skeletal muscle was unchanged. 6. Apamin pretreatment did not modify the blood pressure lowering activity of BRL 34915 in rats. The site at which BRL 34915 acts to cause vasodilatation in vivo thus appears to be apamin-insensitive.

Vasoconstrictor and vasodilator effects in normal and atherosclerotic conscious rabbits.

1. Rabbits were fed a cholesterol-rich diet for 5 two-week intervals. Polyvinyl catheters were then implanted into the femoral artery and vein. Dose-response curves to acetylcholine (ACh), noradrenaline (NA), phenylephrine (Phen) and angiotensin II (AII), were obtained in 6 cholesterol-fed and 6 control rabbits before and after isradipine (code name PN200-110) 100 micrograms kg-1. After these experiments the animals were killed and aortic rings were suspended in an organ bath. ACh but not nitroprusside-induced relaxation was impaired in atherosclerotic but not in control preparations. 2. ACh decreased blood pressure dose-dependently in both groups of rabbits even though ACh did not relax the aortae of the same rabbits in vitro. 3. Blood pressure effects reflect mostly changes in resistance vessels. The pressor effects of NA, Phen and AII were enhanced in atherosclerotic compared with normal rabbits. 4. After a dose of 100 micrograms kg-1 isradipine the dose-response curves of all agents were shifted to the right. The differences between atherosclerotic and control rabbits disappeared, except for the AII-induced pressor response, which remained enhanced in atherosclerotic animals. The calcium antagonist thus only partly corrected the atherosclerosis-associated hyperresponsiveness to vasoconstrictor agents.

8-OH-DPAT, flesinoxan and guanfacine: systemic and regional haemodynamic effects of centrally acting antihypertensive agents in anaesthetized rabbits.

1. 8-Hydroxy-2-(di-n-propylamino) tetralin (8-OH-DPAT) and flesinoxan, agents which show high affinity and selectivity for 5-HT1A receptors, were administered intravenously in doses of 0.003 to 0.1 and 0.01 to 0.3 mg kg-1 respectively to 5 rabbits each. Their effects were compared with those of the centrally acting agent and alpha 2-adrenoceptor agonist, guanfacine, 0.01-0.3 mg kg-1, administered to a group of 5 rabbits. Five further rabbits were used as controls and treated with the vehicle of the active agents. 2. Both flesinoxan and 8-OH-DPAT induced similar systemic and regional haemodynamic changes. Both lowered mean arterial pressure and heart rate. The principal blood pressure lowering mechanism was vasodilatation; cardiac output changed minimally despite the falls in heart rate and myocardial contractile force. 3. With guanfacine the maximal fall of blood pressure was comparable to that obtained with the 5-HT1A receptor ligands; however, in contrast to the latter, the dose-response curve was U-shaped, the highest dose eliciting a pressor effect with reversal of the vasodilatation. 4. Widespread peripheral vasodilatation was found with all the agents in the splanchnic circulation and also in the brain and skeletal muscle. A weak tendency towards vasodilatation was found in the kidneys where the dose-response curve was bell-shaped for guanfacine. 5. This spectrum of activity is different from that of peripheral vasodilators, such as calcium antagonists, potassium channel activating agents or hydralazine; it is, however, consistent with the putative mechanism of action of these compounds to reduce peripheral sympathetic tone by a central mechanism of action.

Cardioprotection by the calcium antagonist PN 200-110 in the absence and presence of cardiodepression.

The globally-ischaemic Langendorff rabbit heart model has been used to study the cardioprotective effects of the dihydropyridine PN 200-110 (PN) at two doses, one having no negative inotropic effect and a higher dose causing a 62 +/- 5% reduction in contractility. Following 45 min no-flow global ischaemia, recovery was monitored for a period of 90 min reperfusion. Hearts were paced at a constant rate throughout experiments. Contractile force and coronary flow were recorded continuously. Tracer microspheres were injected at regular intervals to assess regional flow distributions, drill biopsies were taken to determine tissue high energy phosphate content, and enzyme leakage in the coronary effluent measured during the first 15 min of reperfusion. Untreated hearts recovered 21 +/- 2% of their initial contractile force and flow to all heart regions was reduced. In particular, endocardial flow fell to 20% of its pre-ischaemic level, with the ratio of flow to the endocardium (endo)/epicardium (epi) decreasing from ca. 1.0 to 0.4. Hearts treated with 2 X 10(-8)M PN (included in the perfusate from 30 min before ischaemia until 30 min after ischaemia) recovered 49 +/- 2% of their initial, pretreatment contractile force, and following the ischaemia the endo/epi ratio was not significantly changed from the pre-ischaemic value. The lower PN dose (3 X 10(-10)M) afforded a lesser degree of protection, contractility recovering to 29 +/- 4% of the initial level, with an endo/epi ratio of 0.7 after 90 min reperfusion. 6 The two PN doses afforded a similar degree ofprotection against enzyme leakage which was in both cases significantly less than in untreated hearts. 7 Myocardial ATP and creatine phosphate content was markedly reduced by the ischaemic episode. Neither PN dose modified this depletion. 8 These results suggest that whilst cardiodepression may well offer protection against ischaemic damage, this is not the sole mechanism wherby PN (and possibly other calcium antagonists) can protect the heart. Preservation of blood flow to the inner layers of the left ventricular wall is likely to be one of the major factors underlying the enhanced recovery shown by PN.