Nerve stimulation augments angiotensin II overflow from canine gracilis muscle in vivo.

The overflow of endogenous angiotensin-(1-8)octapeptide (angiotensin II) from blood-perfused canine gracilis muscle vasculature in situ was studied. A positive veno-arterial concentration difference for angiotensin II over the gracilis muscle was found, indicating a net generation of angiotensin II under basal conditions. Angiotensin II levels in the venous effluent were elevated during 2 Hz (4-min) nerve stimulation, suggesting enhanced local angiotensin II generation both in the presence and absence of alpha-adrenoceptor blockade. Thus, our results in this in vivo model demonstrate a local overflow of angiotensin II from the skeletal muscle vasculature which can be enhanced by nerve stimulation. Whether this overflow of angiotensin II is due to conversion of circulating angiotensin I to angiotensin II or local de novo synthesis of angiotensin II remains to be established.

Influence of the renin-angiotensin system on sympathetic neurotransmission in canine skeletal muscle in vivo.

The physiological importance of interactions between angiotensin II and sympathetic neurotransmission was studied in an in vivo model with constant flow blood perfused gracilis muscle in situ in dogs pretreated with desipramine and atropine. Sympathetic nerve stimulation-(2 and 8 Hz, 480 pulses) evoked over-flow of endogenous noradrenaline and vasoconstriction, and vasoconstrictor responses to exogenous noradrenaline (0.5 nmol, locally i.a.) were evaluated. Angiotensin converting enzyme inhibition by benazeprilat (10 mg i.v.; n = 8) reduced arterial angiotensin II levels from 26 +/- 8 to 2 +/- 1 pM and reduced mean arterial and basal muscle perfusion pressures. Subsequent resubstitution of angiotensin II (3, 30 and 90 ng kg-1 min-1 i.v.) elevated arterial angiotensin II dose-dependently (to 67 +/- 14, 622 +/- 63 and 1940 +/- 251 pM, respectively), as well as mean arterial and muscle perfusion pressures. Nerve stimulation-evoked noradrenaline overflow was unchanged following benazeprilat (-4 +/- 4 and +1 +/- 8% at 2 and 8 Hz, respectively) and during subsequent infusions of angiotensin II. Vasoconstrictor responses to nerve stimulation and exogenous noradrenaline were also uninfluenced by these treatments. Thus, angiotensin II did not enhance sympathetic neurotransmission at the postjunctional level. Another group of animals was pretreated with noncompetitive alpha-adrenoceptor blockade locally by phenoxybenzamine and benextramine (0.5 mg kg-1 i.a. of each; n = 7), which abolished vasoconstrictor responses to nerve stimulation. The effects of benazeprilat and subsequent angiotensin II infusions (3 and 30 ng kg-1 min-1 i.v.) on circulating angiotensin II levels, mean arterial and muscle perfusion pressures were similar in this group.(ABSTRACT TRUNCATED AT 250 WORDS)

Platelet activation during angiotensin II infusion in healthy volunteers.

The present study was undertaken to evaluate the effects of an intravenous infusion of angiotensin II (10 ng/kg per min) on platelet function and coagulation in vivo in 18 healthy males. The infusion increased mean arterial pressure by 23+/-2 mm Hg. Plasma beta-thromboglobulin levels, reflecting platelet secretion, increased by 66+/-24% during the infusion, as did also platelet surface expression of P-selectin as measured by flow cytometry. Platelet fibrinogen binding increased, whereas platelet aggregability, assessed by ex vivo filtragometry, was unaltered. Angiotensin II caused mild activation of the coagulation cascade with increases in plasma levels of thrombin-antithrombin complex and prothrombin fragment F1 + 2. In conclusion, angiotensin II has mild platelet-activating effects in vivo and also enhances coagulation. Markers of platelet secretion are significantly increased, whereas markers of platelet aggregability are less affected. The clinical relevance of these findings remains to be clarified.

Acute effects of angiotensin II on fibrinolysis in healthy volunteers.

Recent studies have suggested that angiotensin II may inhibit fibrinolysis. In order to further test this hypothesis, we investigated the acute effects of angiotensin II (intravenous infusion of 10 ng/kg per min over 15-20 min) on fibrinolytic function in 18 healthy men. Time-controls (n=11) and control experiments with a placebo infusion (n = 13) were also performed. The activities of plasmin activator inhibitor-1 (PAI-1) and tissue-type plasminogen activator (t-PA), as well as t-PA antigen levels, were determined in plasma before, during and 60 min after the infusion of angiotensin II. Angiotensin II caused a clear-cut elevation in blood pressure; heart rate and plasma noradrenaline levels tended to decrease during the infusion but increased afterwards, indicating reflexogenic adjustments. Plasma t-PA activity and antigen levels increased by 81+/-11 and 14+/-3%, respectively, during angiotensin II infusion (both P < 0.001), whereas t-PA activity was unchanged and t-PA antigen decreased (P < 0.05) in placebo experiments. PAI-1 activity decreased similarly in time-controls and during angiotensin infusion (P < 0.001). Thus, short-term infusion of angiotensin II enhances fibrinolysis by elevating plasma t-PA. It is not clear whether this is a direct angiotensin-receptor-mediated effect or if it is related to the hemodynamic effects of the infusion.

Influence of angiotensin-converting enzyme inhibition on sympathetic neurotransmission: possible roles of bradykinin and prostaglandins.

Effects of angiotensin-converting enzyme (ACE) inhibitors on sympathetic neurotransmission have generally been ascribed to their ability to block angiotensin II (Ang II) formation, but they also inhibit the degradation of vasoactive kinins, such as bradykinin. The latter may, in turn, lead to enhanced prostaglandin formation. Prostaglandins have been reported to influence sympathetic neurotransmission at different sites; much less is known about the influence of bradykinin due to the lack (until recently) of specific and effective bradykinin receptor antagonists, and difficulties with measurements of true plasma or tissue levels of bradykinin. Bradykinin may modulate sympathetic activity via a central stimulatory action and via activation of sensory input to the central nervous system; however, the importance of bradykinin for central effects of ACE inhibition remain to be established. At the sympathetic neuro-effector junction, results are more conflicting. Thus, bradykinin has been reported to enhance or reduce peripheral noradrenergic transmission or even lack any effect. Possible explanations for the differing results obtained include species and/or tissue differences in the responses to bradykinin. Also, the effects of bradykinin may be influenced by enhanced formation of prostaglandins and/or endothelium-derived relaxing factor (EDRF), which may contribute to the confusion. As most studies have been performed under in vitro conditions and with high doses of bradykinin, the physiological relevance of the data may be questioned.(ABSTRACT TRUNCATED AT 250 WORDS)

D-myo-inositol-1,2,6-triphosphate (PP56) antagonizes nonadrenergic sympathetic vasoconstriction: possible involvement of neuropeptide Y.

The possibility that D-myo-inositol-1,2,6-triphosphate (PP56), which has shown some specificity for antagonism of neuropeptide Y (NPY) in vitro, may antagonize responses to sympathetic nerve stimulation was investigated in canine blood-perfused gracilis muscle in situ after pretreatment with irreversible alpha-adrenoceptor blockade by phenoxybenzamine and beta-adrenoceptor blockade by propranolol. Sympathetic nerve stimulation (10 Hz, 2 min) increased muscle perfusion pressure and evoked overflow of norepinephrine (NE) from the tissue. PP56 at 50 and 500 microM (calculated local arterial plasma concentrations) did not influence NE overflow but attenuated the late phase of the vasoconstrictor response to nerve stimulation and reduced the increase in perfusion pressure evoked by exogenous NPY. PP56 also induced vasodilatation per se, but had no effect on the vascular response to exogenous angiotensin II (AII) or the remaining vasoconstrictor response to exogenous NE. PP56 antagonizes late components in the nonadrenergic sympathetic vasoconstriction and vasoconstriction evoked by exogenous NPY, without influencing transmitter release. Because previous results with this and other models propose a role for NPY in mediating nonadrenergic vasoconstriction, we suggest that PP56 may antagonize effects of neuronally released NPY at the postjunctional level.

Converting enzyme inhibition modulates sympathetic neurotransmission in vivo via multiple mechanisms.

We investigated the mechanism(s) by which angiotensin-converting enzyme (ACE) inhibition influences peripheral sympathetic neurotransmission. Thus effects of the angiotensin II (ANG II) receptor antagonist losartan (Du Pont 753) were compared with those of the ACE inhibitor benazeprilat on sympathetic neurotransmission in canine gracilis muscle in situ, with alpha-adrenoceptors either intact or irreversibly blocked by phenoxybenzamine. Furthermore, effects of the bradykinin receptor antagonist HOE 140 and the prostaglandin synthesis inhibitor diclofenac were studied after ACE inhibition. Losartan reduced the vasoconstrictor response to exogenous ANG II by 76 +/- 4% at the dose used and lowered muscle perfusion pressures. ACE inhibition by benazeprilat reduced plasma ANG-(1-8) octapeptide levels (from 8 +/- 2 to 2 +/- 1 pM), mean arterial pressure, and muscle perfusion pressures. After ACE inhibition, both HOE 140 (at a dose that reduced the vasodilatory response to exogenous bradykinin by 80 +/- 3%) and diclofenac elevated basal perfusion pressures. Losartan reduced the nerve stimulation-evoked overflow of endogenous norepinephrine (NE) (-14 +/- 6%) and vasoconstrictor responses (alpha-adrenoceptors intact). ACE inhibition increased NE overflow when alpha-adrenoceptors were intact (+12 +/- 5%) and tended to reduce it when alpha-adrenoceptors were blocked (-12 +/- 4%). During ACE inhibition, HOE 140 reduced and diclofenac enhanced the evoked NE overflow. In the absence of ACE inhibition, neither HOE 140 nor diclofenac influenced NE overflow. Our findings indicate that ACE inhibition influences sympathetic neurotransmission via reduced ANG II formation and enhanced bradykinin and prostaglandin accumulation. The effects of ANG II on sympathetic neurotransmission are, however, small under these in vivo conditions.

Participation of prostaglandins and bradykinin in the effects of angiotensin II and converting enzyme-inhibition on sympathetic neurotransmission in vivo.

We investigated the mechanism(s) by which angiotensin converting enzyme (ACE)-inhibition and angiotensin (Ang) II influence peripheral sympathetic neurotransmission in canine gracilis muscle in situ, with alpha-adrenoceptors either intact or irreversibly blocked by phenoxybenzamine. ACE-inhibition by ramiprilat reduced, and subsequent infusion of Ang II (30 ng kg-1 min-1 i.v.) markedly increased arterial plasma Ang-(1-8)octapeptide levels, basal muscle perfusion pressures and mean arterial pressure. Local intra-arterial bolus injection of Ang II caused marked vasoconstriction followed by vasodilation. This vasoconstrictor response was enhanced and the ensuing vasodilation was abolished following prostaglandin synthesis inhibition by diclofenac. The vasoconstrictor response to low frequency (0.5 Hz) sympathetic nerve stimulation was also enhanced by diclofenac. The nerve stimulation-evoked noradrenaline (NA) overflow was reduced by ramiprilat when alpha-adrenoceptors were blocked (-11 +/- 3%, P < 0.05), but increased when alpha-adrenoceptors were intact (+28 +/- 14%, P < 0.05). During ACE-inhibition, effective bradykinin receptor antagonism by HOE 140 reduced stimulation-evoked NA overflow irrespective of alpha-adrenoceptor blockade (i.e. by 25 +/- 5 and 20 +/- 3% in the absence and presence of alpha-adrenoceptor blockade, respectively, P < 0.01). Diclofenac increased stimulation-evoked NA overflow in the absence of alpha-adrenoceptor blockade (+ 19 +/- 4%, P < 0.05). IV infusion of Ang II failed to enhance stimulation-evoked NA overflow both before and after diclofenac.(ABSTRACT TRUNCATED AT 250 WORDS)

Angiotensin II overflow from canine skeletal muscle in vivo: importance of plasma angiotensin I.

The overflows (i.e., veno-arterial concentration differences multiplied by plasma flow) of angiotensin-(1-10) decapeptide (ANG I) and angiotensin-(1-8) octapeptide (ANG II) from blood-perfused canine gracilis muscle in situ were studied. Special precautions were taken to minimized ex vivo generation and/or degradation of angiotensins in the sampled blood. ANG I was found to be generated in the catheter system supplying the gracilis muscle with arterial blood, but plasma renin activity and ANG II levels were uninfluenced by the catheter system. A positive venoarterial concentration difference over the muscle itself was found for ANG II but not for ANG I under basal conditions. Isoprenaline elicited vasodilatation, reduced ANG I overflow, and tended to increase ANG II overflow, whereas beta-adrenoceptor blockade by propranolol had no effect on these variables. In conclusion, we found no evidence for a local de novo synthesis of ANG II from the gracilis muscle vasculature in vivo. The net overflow of ANG II was most likely caused by local conversion in the tissue of ANG I artifactually generated in the arterial catheter system. beta-Adrenoceptor stimulation enhanced the local conversion of ANG I to ANG II, probably by exposing a greater endothelial surface containing angiotensin-converting enzyme activity.

Influence of angiotensin II, alpha- and beta-adrenoceptors on peripheral noradrenergic neurotransmission in canine gracilis muscle in vivo.

Interactions between angiotensin II and adrenoceptor-mediated effects on peripheral sympathetic neurotransmission were investigated in constant flow blood-perfused canine gracilis muscle in situ, without and with pretreatment by non-competitive alpha-adrenoceptor blockade. Angiotensin converting enzyme (ACE)-inhibition by benazeprilat increased nerve stimulation (2 Hz, 4 min)-evoked noradrenaline (NA) overflow (+ 21 +/- 5%) with alpha-adrenoceptors intact, but reduced NA overflow (- 18 +/- 6%) when alpha-adrenoceptors were blocked. Vasoconstrictor responses were slightly reduced by benazeprilat. Subsequent infusion of angiotensin II (Ang II, 20 and 500 ng kg-1 min-1 i.v., raising arterial concentrations from 0.6 +/- 0.2 pM to 1390 +/- 240 and 25,110 +/- 3980 pM, respectively) failed to increase NA overflow or to enhance stimulation-evoked vasoconstriction. Adrenaline (0.4 nmol kg-1 min-1 i.v.) did not change evoked NA overflow before or after benazeprilat, either with or without alpha-adrenoceptor blockade, despite high concentrations (approximately 10 nM) in arterial plasma. Following benazeprilat, propranolol reduced NA overflow (- 24 +/- 3%) only if the alpha-adrenoceptors were blocked. In conclusion, benazeprilat reduced evoked NA overflow in the presence of alpha-adrenoceptor blockade to a similar degree as previously shown in the presence of neuronal uptake inhibition in this model. However, contrasting to our previous findings, benazeprilat enhanced NA overflow and reduced the post-junctional response to nerve stimulation in the absence of alpha-adrenoceptor blockade. This could be related to bradykinin accumulation during ACE-inhibition, in addition to the reduction of Ang II generation. Our data are not compatible with facilitation of NA release by circulating Ang II even at pharmacological dose levels. Although activation of prejunctional beta-adrenoceptors may facilitate evoked NA overflow in this model, circulating adrenaline is ineffective under physiological conditions even after alpha-adrenoceptor blockade. Also, beta-adrenoceptor-mediated prejunctional effects do not seem to involve Ang II in canine skeletal muscle in vivo.

Circulatory effects and pharmacology of clevidipine, a novel ultra short acting and vascular selective calcium antagonist, in hypertensive humans.

The pharmacokinetics of clevidipine, a potent short-acting vascular-selective calcium antagonist, was investigated during steady state and the postinfusion period in patients with mild to moderate hypertension. Furthermore, the dose-effect and blood concentration-effect relations and the tolerability of the drug were studied. Twenty patients were randomized to clevidipine intravenously at target dose rates of 0.18, 0.91, 2.74, and 5.48 microg/kg/min, respectively, or placebo. Each patient received in random order three infusion rates of clevidipine or placebo during three separate study days. Dose-dependent reduction in blood pressure and a modest increase in heart rate were noted. The extremely high clearance value and the small volume of distribution resulted in short half-lives of clevidipine, 2.2 and 16.8 min, respectively. The blood concentration and dose rate producing half the maximal effect (i.e. EC50 and ED50) were approximately 25 nM and 1.5 microg/kg/min, respectively. There was a linear relation between blood concentration and dose rate in the range studied. Clevidipine was safe and generally well tolerated; one patient was excluded because of adverse events at 2.74 microg/kg/min. In conclusion, clevidipine is a high-clearance calcium antagonist that may become a valuable contribution to the drugs used in conditions in which precise and rapid control of blood pressure is needed.