Effects of stretch or distention on phenylephrine-induced constriction of human coronary artery bypass grafts.

OBJECTIVE: To determine the effects of grafting saphenous veins into the arterial circulation and to compare the responsiveness of saphenous veins and mammary arteries to vasoconstrictors (phenylephrine or potassium) and a vasodilator (the calcium antagonist isradipine). DESIGN: Prospective, controlled, in vitro study. SETTING: Laboratory facility in a university teaching hospital. PARTICIPANTS: Small excess segments of internal mammary arteries or saphenous veins obtained from patients undergoing coronary artery bypass graft surgery. INTERVENTIONS: Vessel segments were cut into rings to measure isometric tension development in isolated tissue chambers. The law of LaPlace for a cylinder was applied to determine tensions in vitro corresponding with arterial or venous tensions in vivo or distending pressures ex vivo. MEASUREMENTS AND MAIN RESULTS: Stretching saphenous vein rings from venous to arterial tensions reduced maximal phenylephrine-induced constriction but did not alter their dose response to phenylephrine, potassium, or isradipine. At arterial tensions, potassium, but not phenylephrine, was more potent in constricting mammary artery than saphenous vein; isradipine was more potent as a vasodilator of potassium-constricted mammary artery than saphenous vein. Maximal phenylephrine-induced or potassium-induced constriction was no different for either vessel at arterial tensions; however, prior distention of veins to tensions corresponding with pressures of 200 or 300 mmHg significantly (p < 0.01, Dunnett's test) reduced subsequent constriction. CONCLUSION: Phenylephrine may be more likely to constrict native internal mammary arteries than distended autogenous saphenous vein grafts in vivo because high-pressure distention of veins markedly inhibits their vasoreactivity.

Multiple linear regression analysis of blood pressure, hypertrophy, calcium and cadmium in hypertensive and non-hypertensive states.

High blood pressure causes heart disease and remains a major public health issue. This paper expands mathematically and mechanically on environmental heavy metal exposure and heart disease. In rabbits, mean blood pressure was measured by direct puncture of the middle ear artery. Measurements of calcium and cadmium levels were made by flame atomic absorption spectrophotometry in tissue from hypertensive and non-hypertensive rabbits. Relationships between blood pressure, hypertrophy, calcium and cadmium were tested using multiple regression analysis. Multiple linear relationships occurred showing the dependence of high blood pressure on hypertrophy, calcium and cadmium: hypertrophy on calcium, cadmium and high blood pressure; and calcium on cadmium, high blood pressure and hypertrophy. These data provide insight into the mechanism of elevated blood pressure on heavy metal and calcium accumulation, and cardiac hypertrophy.

Porcine gastroepiploic artery as an in vitro experimental model to study vasodilators in microsurgery.

Arterial vasospasm is a common problem in microsurgery. This pharmacological study compares seven vasodilators-lidocaine, papaverine, nicardipine, verapamil, diltiazem, sodium nitroprusside, and hydralazine-for their efficacy and potency in an experimental model of vasospasm. Porcine gastroepiploic arteries were cut into rings to measure isometric tension development in vitro. The arteries were preconstricted with endothelin-1, a stable thromboxane A2 analogue, norepinephrine, or potassium, and then exposed to increasing concentrations of each vasodilator. Every vasodilator except hydralazine and sodium nitroprusside was efficacious in producing near-maximal relaxation of arteries preconstricted with any vasospastic substance. The five efficacious vasodilators differed markedly in potency, as reflected in the concentrations producing half-maximal relaxation. The order of potency was nicardipine < or = verapamil or diltiazem < papaverine < lidocaine. This study suggests that nicardipine would be the most potent vasodilator for systemic or direct intra-arterial administration. Papaverine and lidocaine, in concentrations employed clinically, were both efficacious as topical vasodilators.

The effects of N-methyl-D-aspartate agonists and antagonists on isolated bovine cerebral arteries.

This pharmacologic study examines the direct cerebrovascular effects of N-methyl-D-aspartate (NMDA) receptor agonists and antagonists to determine whether large cerebral arteries have NMDA receptors. Bovine middle cerebral arteries were cut into rings to measure isometric tension development in vitro. Two competitive agonists, L-glutamate and NMDA, each had negligible effects on ring tension in the absence of exogenous vasoconstrictors. L-glutamate (in high concentrations) produced direct relaxation of potassium (K+)-constricted arteries, but the relaxation was not selective for L-glutamate, D-glutamate, or mannitol. Relaxation with L-glutamate was abolished when it was isosmotically substituted in the K(+)-rich medium. NMDA (in the absence or presence of glycine) and two competitive antagonists, 2-amino-5-phosphopentanoic acid (AP5) and (+/-)-3-(s-carboxypiperazin-4-yl)-propyl-1-phosphonic acid (CPP), each had little effect on the tone of arteries preconstricted with potassium or the stable thromboxane A2 analog U-46,619. Three noncompetitive antagonists (S(+)-ketamine, dizocilpine, and dextrorphan) and their steroeisomers (R(-)-ketamine, (-)MK-801, and levorphanol) each produced dose-dependent relaxation of K(+)- or U-46,619-constricted arteries; relaxation was not selective for the (+) or (-) stereoisomers. These results suggest that large cerebral arteries lack NMDA receptors mediating constriction or relaxation. All noncompetitive antagonists dilated cerebral arteries, but by mechanisms that were not stereospecific.

Calcium accumulation in experimental hypertension.

At present, the significance of calcium accumulation in the aetiology of coronary artery disease (CAD) in humans is not known, except only to exacerbate the condition. In an attempt to understand ionic disturbances in vasculature derived from cardiovascular abnormalities, soft tissues from hypertensive models were examined. The study hypothesis was to see if basic cardiovascular regulatory sites in hypertensive models accumulated greater amounts of Ca2+. Calcium levels were measured by flame atomic absorption spectrophotometry in tissue derived from two types of hypertensive rabbits. Both models of hypertension developed mean arterial pressures of at least 50 mm Hg greater than those of controls over a 5-wk period. Significant increases in calcium levels were found in left ventricle and aorta derived from both hypertensive groups compared with controls. Renal cortex and medulla were not significantly different among any of the groups. These levels corroborate the findings of others which show increased calcium levels in cardiovascular tissues in experimental hypertension in rabbits. Although there have been several studies that have shown the relationship between calcium, hypertension and CAD, this is the first study to look at calcium accumulation rather that the effects of calcium channel blockers. The link between hypertension and calcium accumulation in a number of tissues may be important in the development of CAD and other cardiac dysfunction.

Multiple linear regression analysis of hypertrophy, calcium and cadmium in hypertensive and non-hypertensive states.

Heart disease remains a major public health issue. In this study we aimed to achieve a greater mathematical and mechanistic understanding of the relationship between exposure to heavy metals and heart disease. Measurements of calcium and cadmium levels were made by flame atomic absorption spectrophotometry in tissue from hypertensive and non-hypertensive rabbits. Relationships between hypertrophy, calcium and cadmium were tested using multiple regression analysis. Multiple linear relationships occurred that showed the dependence of hypertrophy on calcium and cadmium levels, and of calcium accumulation on cadmium and hypertrophy. These data provide an insight into the mechanisms of heavy metal accumulation and the development of cardiovascular hypertrophy.

Ketamine directly dilates bovine cerebral arteries by acting as a calcium entry blocker.

This in vitro study was performed to determine the role of calcium in ketamine-induced cerebral vasodilation. Isolated bovine middle cerebral arteries were cut into rings to measure isometric tension development or into strips to measure radioactive 45Calcium (45Ca) uptake. Ketamine produced direct relaxation of arterial rings; the relaxation was attenuated in Ca(2+)-deficient media. Ketamine produced dose-related relaxation of arteries preconstricted with potassium, a stable thromboxane A2 analogue, or endothelin. Endothelial stripping with Triton X-100 had no effect on subsequent ketamine-induced relaxation. In Ca(2+)-deficient media containing potassium or the stable thromboxane A2 analogue, ketamine produced competitive inhibition of subsequent Ca(2+)-induced constriction. Ketamine blocked potassium- and thromboxane A2-stimulated 45Ca uptake in a dose-dependent manner, but had no effect on basal 45Ca uptake, the externally bound 45Ca content, or the volume of the 3H-sorbitol space. These results indicate that ketamine can directly dilate cerebral arteries by acting as a calcium channel antagonist; ketamine inhibits 45Ca uptake through both potential-operated (potassium) and receptor-operated (thromboxane A2) channels in cerebrovascular smooth muscle.

Divalent cations in hypertension with implications to heart disease: calcium, cadmium interactions.

Heart disease still pervades all socioeconomic classes within the United States. Understanding the effects of environmentally-related pathogenesis (e.g., heavy metal accumulation) may aid in developing novel treatments for the prevention of heart dysfunction. The aim of this report was to use experimental investigation in an attempt to expand upon the multivariate importance of divalent cation interactions in the development of heart disease. Calcium and cadmium levels were measured by flame atomic absorption spectrophotometry in various tissues derived from two types of hypertensive rabbit models. Both models of hypertension developed mean arterial pressures of at least 50 mm mercury greater than those of controls over a 5 week period. Interrelationships between calcium and cadmium levels were found to exist for both hypertensive groups in the left ventricle, aorta, and renal medulla. The renal cortex showed no such interrelationship for the renal hypertensive group. Multiple interrelationships between calcium and cadmium levels and hypertrophy were also observed. These studies related the importance of endogenous or exogenous (viz, environmental) factors governing cadmium and calcium accumulation in hypertensive rabbits. The resulting relationships between divalent cations and hypertrophy were presented to draw attention to areas which remain unexplored with perspective to cardiovascular disease.

Effects of prostaglandin F2 alpha and thromboxane A2 analogue on bovine cerebral arterial tone and calcium fluxes.

We determined sources of activator calcium for prostanoid-induced cerebrovascular constriction by measuring isometric tension and calcium-45 (45Ca) fluxes in bovine middle cerebral arteries. Constriction induced by prostaglandin F2 alpha or the stable thromboxane A2 analogue SQ-26,655 was near-maximally inhibited in calcium-deficient solutions but only partially inhibited by calcium antagonists (10(-5) M verapamil or 3.3 x 10(-7) M nifedipine). Studies of 45Ca binding at different external Ca2+ concentrations showed that cerebral arteries possess two calcium binding sites, a high-affinity site and a low-affinity site. Each prostanoid significantly increased low-affinity 45Ca uptake (external Ca2+ concentration = 1.2 mmol/l) during 5 minutes of 45Ca loading; for prostaglandin F2 alpha 45Ca uptake increased from 69 to 108 nmol/g and for SQ-26,655, from 78 to 141 nmol/g. The prostanoid-induced increases in low-affinity 45Ca uptake were completely abolished by pretreatment with verapamil or nifedipine. Prostaglandin F2 alpha, SQ-26,655, verapamil, and nifedipine had no effect on high-affinity 45Ca uptake (external Ca2+ concentration = 45 mumol/l) or 45Ca efflux (after 60 minutes' preincubation in calcium-deficient media). Prostaglandin F2 alpha and SQ-26,655 each appear to constrict cerebral arteries by two mechanisms: first, by promoting calcium uptake from low-affinity binding sites through receptor-operated channels sensitive to the calcium antagonists, and second, by releasing calcium from depletable internal stores.

Mediation of bradykinin-induced contraction in canine veins via thromboxane/prostaglandin endoperoxide receptor activation.

1. Canine jugular and femoral veins were studied to determine the possible importance of thromboxane (TXA2) and prostaglandin endoperoxides (prostaglandin H2, PGH2) in mediating bradykinin(BK)-induced contraction. 2. Isolated vein rings incubated in modified Krebs solution contracted to TXA2/PGH2 analogs SQ26655 and U44069 with potency of contraction exceeding that for BK. The potency ranking for both veins was SQ26655 greater than U44069 greater than BK greater than PGF2 alpha greater than TXB2 much greater than PGD2. 3. The cyclo-oxygenase inhibitors indomethacin (3 x 10(-7) M) and flufenamic acid (10(-5) M) reduced BK contractions without affecting those induced by noradrenaline (NA). 4. TXA2/PGH2 receptor antagonists SQ29548 (10(-8) M) and BM13177 (10(-6) M) strongly inhibited BK-induced tension. The action of antagonists was reversible with negligible influence on NA-elicited contraction. Selective removal of endothelium had no effect on BK-induced contraction or the action of the antagonists. 5. The thromboxane synthase inhibitors dazoxiben (10(-4) M) and CGS 12970 (10(-5) M) had no significant inhibitory effect on BK-induced tension. 6. These results suggest that in canine jugular and femoral vein, the action of BK is largely dependent upon stimulation of the cyclo-oxygenase pathway to produce PGH2 and possibly TXA2, which can activate a smooth muscle TXA2/PGH2 receptor to elicit vasoconstriction.

Effects of cadmium ingestion on blood pressure and ventricular mass in rabbits.

Ingestion of cadmium (Cd) acetate in deionized drinking water (1 ppm) in rabbits resulted in the development of hypertension and increased left ventricular mass similar to what was observed in experimental renal hypertension (Grollman technique). Regardless of the approach, mean arterial pressures (MAP) of at least 50 mm Hg greater than those of controls developed over a 34-day period. Increased left ventricular mass relative to body weight was found in both hypertensive groups. However, only an increase in renal mass was observed in unilaterally nephrectomized animals. The increased mass of hearts and kidneys that was detected after an approximately 1-month period attests to the severity of these forms of experimental hypertension. These results demonstrate that the ingestion of drinking water contaminated with Cd can cause hypertension and an increase in left ventricular mass over a short time period in rabbits.

Effects of calcium antagonists on isolated bovine cerebral arteries: inhibition of constriction and calcium-45 uptake induced by potassium or serotonin.

The purpose of this study was to determine the mechanisms by which organic calcium channel blockers inhibit cerebral vasoconstriction. Isolated bovine middle cerebral arteries were cut into rings to measure contractility or into strips to measure radioactive calcium (45Ca) influx and efflux. Calcium channel blockers (10(-5) M verapamil or 3.3 X 10(-7) M nifedipine) and calcium-deficient solutions all produced near-maximal inhibition of both potassium- and serotonin-induced constriction. In calcium-deficient solutions containing potassium or serotonin, verapamil and nifedipine each blocked subsequent calcium-induced constriction in a competitive manner. Potassium and serotonin significantly increased 45Ca uptake into cerebral artery strips during 5 minutes of 45Ca loading; for potassium 45Ca uptake increased from 62 to 188 nmol/g, and for serotonin from 65 to 102 nmol/g. Verapamil or nifedipine had no effect on basal 45Ca uptake but significantly blocked the increase in 45Ca uptake induced by potassium or serotonin. Potassium, and to a lesser extent serotonin, each induced a brief increase in the rate of 45Ca efflux into calcium-deficient solutions. Verapamil or nifedipine had no effect on basal or potassium-stimulated 45Ca efflux. The results demonstrate that verapamil and nifedipine block 45Ca uptake through both potential-operated (potassium) and receptor-operated (serotonin) channels in bovine middle cerebral arteries.

Mercury- and lead-induced contraction of aortic smooth muscle in vitro.

Mercuric chloride and lead acetate caused contraction of rabbit aortic segments in vitro. Mercuric chloride was added to a Krebs Henseleit solution; lead acetate was added to a Tris-buffered medium to avoid any precipitation of lead. Cadmium (Cd++) acetate, in either medium, had no direct effect. The Hg++- and Pb++-induced contractions were significantly reduced when Ca++ was omitted from the physiological buffer or when pH decreased. The alkylated analogues of Hg++ and Pb++ salts had little direct contractile effect. However, segments exposed to these analogues lost their ability to respond to other agonists. This report provides evidence that Hg++ and Pb++ can directly induce active tension by action on processes which rely on extracellular Ca++.

Myosin light chain phosphorylation and evidence for latchbridge formation in norepinephrine stimulated canine veins.

The role of 20000 dalton myosin light chain phosphorylation in mediating venous smooth muscle contraction was studied in isolated preparations of canine jugular and femoral vein. One min 10(-5) M norepinephrine-induced contraction was accompanied by significant increases in phosphorylation (jugular - 21 to 46%; femoral - 19 to 54%) which were reversed within 10 min after agonist washout. During 40 min stimulation, phosphorylation and isometric force redevelopment rates declined to near basal levels while force was maintained. These findings implicate light chain phosphorylation as a prerequisite for initial tension development by crossbridge cycling in venous smooth muscle. However, long term tension can be maintained through a process similar to the latchbridge state in tracheal and arterial smooth muscle.

Comparative actions of pentobarbital and verapamil on canine cerebral and peripheral arteries in vitro.

Verapamil and pentobarbital were compared for their actions on isolated canine cerebral (basilar and middle cerebral) and peripheral (mesenteric) arteries of similar diameter. The two agents shared several nonselective actions on canine arteries, but differed widely in potency. Both agents produced direct relaxation of cerebral, but not peripheral, arteries, and both agents inhibited constriction of cerebral and peripheral arteries by KCl and CaCl2 (in K+-depolarizing, Ca2+-deficient media). However, 1 mM pentobarbital was required to produce the same maximal effects as 4 uM verapamil. In addition, verapamil selectively blocked constriction of cerebral arteries by a receptor-mediated agent, prostaglandin F2a, while pentobarbital was nonselective in its blockade. On the basis of their comparative actions on isolated cerebral and peripheral arteries, calcium channel blockers such as verapamil may be a more rational choice in the treatment of cerebral ischemia than the barbiturates.

The effects of hyperosmolar solutions on isolated vascular smooth muscle examined with verapamil.

Hyperosmotic sucrose solutions elicited tension from rat aortic strips in direct proportion to osmolarity. Norepinephrine-induced tension was reduced in proportion to increases in osmolarity; however, reduction of barium-induced tension by hyperosmolar solutions was minimal. Norepinephrine-induced tension is primarily dependent on intracellular calcium mobilization, while barium-induced tension is primarily dependent on extracellular calcium influx. Therefore, hyperosmolar solutions may alter vasoconstriction associated with intracellular calcium mobilization rather than that associated with extracellular calcium influx. In the presence of verapamil which blocks calcium entry into muscle, barium-induced tension was eliminated while the direct tension elicited by hyperosmolar solutions was slightly affected (6% reduction) and the inhibitory effect of hyperosmolar solutions on norepinephrine-induced tension was still observed. In contrast, the tension elicited by hyperosmolar solutions was greatly reduced by papaverine which promotes sequestration of myoplasmic calcium to cause relaxation. The vascular effects of hyperosmolar solutions may be due to alterations in the intracellular calcium rather than the extracellular calcium utilized by vasoconstricting agents.

An index for comparing the inhibitory action of vasodilators.

An index for comparing the inhibitory effects of vasodilators was developed to gain insight into their mechanism of action on vascular smooth muscle. Rat aortic strips were bathed in Krebs bicarbonate solution and were initially contracted to a stable tension by either phenylephrine or barium chloride. A vasodilator was then added and the remaining tension was noted; this was repeated for cumulative concentrations of vasodilator. At each concentration of vasodilator, the percent reduction in phenylephrine-induced tension (Phe) was compared to the percent reduction in barium-induced tension (Ba) and was expressed as a ratio (Phe/Ba). This ratio clearly separated verapamil and nifedipine (ratio less than 1), which block calcium influx, from papaverine (ratio = 1) which promotes calcium sequestration regardless of the source of calcium, and from dantrolene (ratio greater than 1) which interferes with intracellular calcium mobilization. This index provides a method for comparing the action of those agents presently classified as non-receptor specific vasodilators which act directly on vascular smooth muscle.

Calcium fractions in aortic segments from hypertensive rabbits.

The rate constants (-k) of Ca2+ from cellular compartments of intact smooth muscle cells of thoracic aorta of normotensive, renal hypertensive, and cadmium hypertensive rabbits were studied by 45Ca efflux and by employing norepinephrine (NE, 5.9 X 10(-7) M). Tissues from hypertensive animals showed an increased rate of release of 45Ca over a 5-h unloading period when compared to normotensive controls. Three exchangeable compartments were found to contribute to the total kinetic efflux curve for all groups. The Ca2+ fraction in compartments I and II showed an increased rate of Ca2+ release for the hypertensive subgroups. Although the rates were slowed for tissues treated with NE prior to efflux, the rates of the hypertensive subgroups were increased compared to the normotensive group. Pretreatment of aortic segments with NE caused an overall increase in 45Ca activity of the incubation medium and may explain the slowed rates attained in the efflux analyses. Influx studies showed that tissues from hypertensive animals had a greater uptake of 45Ca than did the normotensive controls. Such data may represent a larger exchangeable component in aortae from the hypertensive subgroups. These findings indicate differences in the rates of Ca2+ release from intact smooth muscle cells of hypertensive rabbits.

Role of sulfhydryl groups in aortic responses using N-ethylmaleimide.

This study examines the effects of N-ethylmaleimide (NEM) 10(-7) M on agonist-induced contraction, and the relaxation following drug washout of vascular smooth muscle (VSM) segments. 45Ca flux, cyclic 3':5'-adenosine-monophosphate (cAMP), and cyclic 3':5'-guanosine-monophosphate (cGMP) were analyzed in an attempt to understand the observed physiological modifications with NEM. Cyclic nucleotides were measured by 125I-radioimmunoassay. Contractile responses to norepinephrine (NE) 5.9 x 10(-7) M, angiotensin II (AT) 9.8 x 10(-8) M, and potassium chloride (KCl) 2.2 x 10(-2) M were significantly depressed when VSM segments were exposed to NEM prior to agonist challenge. An inhibition of relaxation occurred when NEM was added after the development of a maximal contractile response to NE, AT, or KCl. VSM relaxation was significantly inhibited when compared to controls. Altered 45Ca efflux, increased cGMP, and decreased cAMP levels were all associated with an inhibition of relaxation of VSM. These results suggest a regulatory role of sulfhydryl (SH) groups in contraction and relaxation of VSM involving both the control of Ca2+ flux and cAMP and cGMP metabolism.

Cyclic nucleotide changes in aortic segments derived from hypertensive rabbits.

Mean blood pressure increase was produced by: (1) renal constriction plus contralateral nephrectomy, and (2) cadmium acetate ingestion. Blood pressure was measured directly via the middle ear artery. Cyclic nucleotide levels were determined by 125I-radioimmunoassay. Levels of cAMP were reduced in both hypertensive subgroups when compared to controls. The altered cAMP levels suggest a relationship of this nucleotide to the maintenance of increased blood pressure.