A.M. Compton

Control of regional blood flow by endothelium-derived nitric oxide.

The regional hemodynamic consequences of inhibiting vascular endothelial nitric oxide generation with NG-monomethyl-L-arginine (L-NMMA) were studied in conscious Long-Evans rats. Experiments were carried out in groups of chronically instrumented rats with intravascular catheters and pulsed Doppler probes to monitor regional blood flow. L-NMMA (0.3-300 mg/kg) caused a dose-dependent, long-lasting (5-90 minutes), and enantiomerically specific increase in mean blood pressure and also caused bradycardia. The increase in blood pressure was accompanied by a dose-dependent and long-lasting vasoconstriction in the internal carotid, mesenteric, renal, and hindquarters vascular beds that could be attenuated, in a concentration-dependent manner, by L-arginine but not by D-arginine. In contrast, L-arginine did not affect the pressor or vasoconstrictor effects of vasopressin. These results indicate that nitric oxide production by vascular endothelial cells contributes to the maintenance of blood pressure and to the control of the resting tone of different vascular beds in the conscious rat.

Regional haemodynamic changes during oral ingestion of NG‐monomethyl‐l‐arginine or NG‐nitro‐l‐arginine methyl ester in conscious Brattleboro rats

Homozygous Brattleboro (i.e. vasopressin‐deficient) rats were chronically instrumented with pulsed Doppler probes and intravascular catheters to permit continuous monitoring of regional haemodynamics. Over a 9 h period, rats drinking water showed no systematic changes in heart rate or mean arterial blood pressure although renal, mesenteric and hindquarters vascular conductances fell. These changes showed diurnal rhythyms, probably related to the nocturnal habits of rats. In separate groups of animals spontaneous oral ingestion of NG‐monomethyl‐l‐arginine (l‐NMMA; 1mg ml−1) or NG‐nitro‐l‐arginine methyl ester (l‐NAME; 0.1 mg ml−1) caused marked hypertension but no significant bradycardia. Compared to control animals, rats drinking l‐NMMA for 9 h showed significantly greater mesenteric and hindquarters vasoconstrictions, and rats drinking l‐NAME showed greater vasoconstrictions in all 3 vascular beds.

Regional and cardiac haemodynamic responses to glyceryl trinitrate, acetylcholine, bradykinin and endothelin-1 in conscious rats: effects of NG-nitro-l-arginine methyl ester

1 Conscious Long Evans rats, chronically instrumented for cardiovascular measurements, were challenged with i.v. bolus doses of glyceryl trinitrate (40 nmol kg−1), acetylcholine (1.2 nmol kg−1), bradykinin (3.2 nmol kg−1), or endothelin‐1 (0.25 nmol kg−1). Under control conditions these doses produced similar falls in mean arterial blood pressure (glyceryl trinitrate, −20 ± 3 mmHg; acetylcholine, −24 ± 2 mmHg; bradykinin, −21 ± 3 mmHg; endothelin‐1, −25 ± 3 mmHg), associated with renal, mesenteric and hindquarters vasodilatations (except for endothelin‐1 which caused mesenteric vasoconstriction). 2 In the presence of NG‐nitro‐l‐arginine methyl ester (l‐NAME, 10 mg kg−1), a potent inhibitor of nitric oxide biosynthesis and endothelium‐dependent vasorelaxation in vitro, the hypotensive responses to glyceryl trinitrate, acetylcholine, and endothelin‐1 were increased, although that to bradykinin was not. However, comparing the differences between the response to glyceryl trinitrate and that to any other agonist in the absence and presence of l‐NAME showed that there were relative attenuations of the hypotensive responses to bradykinin and endothelin‐1, but not to acetylcholine, in the presence of l‐NAME. 3 This comparative analysis showed that the renal and hindquarters vasodilator responses to bradykinin and endothelin‐1 were attenuated in the presence of l‐NAME, but the renal, mesenteric and hindquarters vasodilator responses to acetylcholine were not. However, when l‐NAME was administered in the presence of pentolinium, captopril and the vasopressin V1‐receptor antagonist, d(CH2)5[Tyr‐(Et)]DAVP, (to abolish baroreflex and neurohumoral mechanisms), there was attenuation of the renal and mesenteric vasodilator effects of acetylcholine relative to those seen with glyceryl trinitrate. Under those conditions only the renal vasodilator effects of bradykinin and endothelin‐1 were attenuated. 4 In separate experiments in conscious Long Evans rats, direct measurement of cardiac haemodynamics showed that the hypotensive responses to glyceryl trinitrate, acetylcholine, bradykinin and endothelin‐1 were entirely attributable to rises in total peripheral conductance since both in the absence and presence of l‐NAME there were no reductions in cardiac index in response to these substances. 5 The results indicate that measurement of systemic arterial blood pressure alone in conscious rats does not permit reliable quantitation of the influence of l‐NAME on regional vasodilator responses to glyceryl trinitrate, acetylcholine, bradykinin or endothelin‐1. Furthermore, these substances exert effects in different vascular beds that may be differentially influenced by baroreflex mechanisms, neurohumoral mechanisms, or both. Moreover, except in the case of the renal vasodilator response to endothelin‐1 (which was abolished in the presence of l‐NAME), even when l‐NAME caused attenuation of the vasodilator effects of acetylcholine or bradykinin (relative to glyceryl trinitrate), substantial responses remained. It is feasible that such responses in vivo are nitric oxide‐independent.

Antagonistic effect of human α-CGRP [8–37] on the in vivo regional haemodynamic actions of human α-CGRP

Abstract In conscious rats, infusion of human α-CGRP [8–37] (30 nmol/kg/min) caused small, reversible reductions in hindquarters flow and vascular conductance only, whereas at a dose of 300 nmol/kg/min there was a tachycardia and an increase in mean arterial blood pressure, together with renal, mesenteric and hindquarters vasoconstrictions. Human α-CGRP (0.03 nmol/kg/min) caused tachycardia, hypotension, and transient renal, but sustained hindquarters, vasodilatation; these changes were accompanied by mesenteric vasoconstriction. Infusion of human α-CGRP [8–37] (30 nmol/kg/min) during administration of human α-CGRP (0.03 nmol/kg/min) abolished the effects of the latter but these re-appeared when the human α-CGRP [8–37] infusion was stopped. This dose of human α-CGRP [8–37] did not affect cardiovascular responses to isoprenaline. These results indicate that human α-CGRP [8–37] is an effective antagonist of the cardiovascular actions of human α-CGRP in vivo .

Selective impairment of hindquarters vasodilator responses to bradykinin in conscious Wistar rats with streptozotocin-induced diabetes mellitus

1 Male, Wistar rats were treated with streptozotocin (STZ, 70 mgkg−1, i.p.) or saline and chronically instrumented with pulsed Doppler probes and intravascular catheters (implanted under sodium methohexitone anaesthesia) to allow assessment of haemodynamics in the conscious state 28 days later. 2 Control and STZ‐treated rats received bolus doses of glyceryl trinitrate (10–80 nmol kg−1), acetylcholine (0.1–5 nmol kg−1) and bradykinin (0.3–30 nmol kg−1). 3 Although, as reported previously, STZ‐treated rats had normal mean arterial blood pressure together with renal and mesenteric vasodilatations and hindquarters vasoconstriction relative to control rats, both groups showed similar hypotensive and regional haemodynamic responses to glyceryl trinitrate and acetylcholine. However, while the depressor effects of bradykinin were similar in control and STZ‐treated rats, the former showed a hindquarters vasodilator response to bradykinin that was absent in the STZ‐treated rats. 4 A loss of bradykinin‐mediated vasodilatation in the hindquarters vascular bed in STZ‐treated rats in the presence of normal, hindquarters vasodilator responses to other agents and normal bradykinin‐mediated vasodilator responses in other vascular beds is consistent with existing evidence that the vasodilatation elicited by bradykinin in the hindquarters vascular bed is particularly dependent on nitric oxide synthesis and that this is impaired selectively in STZ‐treated rats.

Regional haemodynamic effects of endothelin-1 and endothelin-3 in conscious Long Evans and Brattleboro rats.

1 The regional haemodynamic effects of bolus doses (4 and 40 pmol) and infusions (12 and 120 pmol h−1) of endothelin‐1 and endothelin‐3 were assessed in conscious, Long Evans and Brattleboro (i.e. vasopressin‐deficient) rats, chronically‐instrumented with pulsed Doppler flow probes. 2 In both strains of rat the lower bolus dose of endothelin‐1 caused only a slight pressor effect, but there were marked renal and mesenteric vasoconstrictions and hindquarters vasodilatation. 3 The lower bolus dose of endothelin‐3 did not affect blood pressure significantly, although the changes in regional haemodynamics were qualitatively similar to those seen following endothelin‐1 in Long Evans and Brattleboro rats. 4 The higher dose of endothelin‐1 caused an initial hypotension accompanied by substantial hindquarters vasodilatations in Long Evans and Brattleboro rats. Subsequently, in both strains, there was a rise in blood pressure accompanied by renal, mesenteric and hindquarters vasoconstrictions. 5 The higher bolus dose of endothelin‐3 caused initial hypotension and hindquarters vasodilatation similar to those seen with endothelin‐1. However, the subsequent pressor effect was less with endothelin‐3, as was the renal vasoconstriction, and it did not cause any increase in hindquarters vascular resistance. 6 Infusion of endothelin‐1 at the lower rate (12 pmol h−1) caused renal and mesenteric vasoconstrictions in both strains of rat, whereas endothelin‐3 at this rate caused only mesenteric vasoconstriction. 7 Infusion of endothelin‐1 at the higher rate (120 pmol h−1) caused progressive hypertension and vasoconstrictions in all three vascular beds studied; these were similar in both strains of rat. Endothelin‐3 had a smaller pressor effect and a lesser constrictor action on the renal and mesenteric vascular beds; it did not constrict the hindquarters vascular bed. 8 These results show, that in conscious Long Evans and Brattleboro rats, the initial depressor effects of the higher bolus doses of endothelin‐1 and −3 were similar, and, hence, not influenced by the absence of endogenous vasopressin. Endothelin‐1 and −3 appear equipotent in their initial hyperaemic vasodilator effects in the hindquarters vasculature in both strains, making it unlikely that this effect is dependent on the release of atrial natriuretic peptide (ANP), since ANP does not cause significant increases in hindquarters blood flow in Brattleboro rats. The greater delayed pressor effect of endothelin‐1 is associated with its more marked vasoconstrictor effects on renal and mesenteric vascular beds and is accentuated, relative to endothelin‐3, by the lack of a constrictor effect of endothelin‐3 in the hindquarters vasculature.

Antagonistic Effect of Human α-Calcitonin Gene–Related Peptide (8–37) on Regional Hemodynamic Actions of Rat Islet Amyloid Polypeptide in Conscious Long-Evans Rats

Rat synthetic amidated islet amyloid polypeptide (IAPP) was infused into conscious Long-Evans rats chronically instrumented for the measurement of regional hemodynamics. Rat IAPP (0.25–2.5 nmol · kg−1 · min−1) had dose-dependent tachycardiac and hypotensive effects. Renal blood flow increased at all dose levels in association with incremental rises in renal vascular conductances. Hindquarters blood flow and vascular conductance increased at the higher dose levels, but mesenteric blood flow fell with mean arterial blood pressure (i.e., there was no change in mesenteric vascular conductance). Concurrent infusion of 25 nmol · kg−1 · min−1 human α-calcitonin gene–related peptide (CGRP) (8–37) abolished the hypotensive, tachycardiac, and renal and hindquarters vasodilator effects of rat IAPP, and during administration of both peptides, there was a transient renal and sustained mesenteric vasoconstriction. When the infusion of human α-CGRP (8–37) was stopped, the effects of the continued infusion of rat IAPP were reestablished. The results indicate that the reported ability of IAPP to induce insulin resistance cannot be due to decreased skeletal muscle blood flow. In addition, human α-CGRP (8–37) is an effective antagonist of the hemodynamic actions of rat IAPP. Because it has been shown previously that human α-CGRP (8–37) antagonizes the hemodynamic effects of human α-CGRP, these results, collectively, indicate that human α-CGRP and rat IAPP might act on the same receptor at which human α-CGRP (8–37) is an effective antagonist or that the latter is a nonselective antagonist of separate receptors on which human α-CGRP and rat IAPP act.

Regional Hemodynamic Effects of Endothelin-1 in Conscious, Unrestrained, Wistar Rats

Summary Regional hemodynamic measurements were made in conscious, unrestrained, Wistar rats chronically instrumented with pulsed Doppler flow probes around left renal and superior mesenteric arteries and the distal abdominal aorta, or around left and right common carotid arteries. The cardiovascular changes with i.v. bolus doses (0.004 and 0.04 nmol) or a 20-min infusion (0.04 nmol/20 min of endothelin-1 (ET-1) were assessed. ET-1 at a dose of 0.004 nmol had no effect on mean arterial pressure (MAP), but caused reductions in renal and mesenteric blood flow accompanied by hindquarters hyperemia; there were no changes in carotid hemodynamics. The higher bolus dose (0.04 nmol) of ET-1 caused initial hypotension and tachycardia followed by hypertension and bradycardia; these changes were associated with sustained reductions in renal and mesenteric flows but a transient hindquarters hyperemia. There was an initial carotid hyperemia followed by a marked reduction in blood flow. Infusion of ET-1 caused progressive bradycardia and hypertension accompanied by reductions in renal and mesenteric flows, but no changes in hindquarters or carotid hemodynamics. These observations are consistent with the hyperemia in the latter vascular beds with high bolus doses of ET-1 due to release of an endogenous vasodilator substance(s).

Effects of indomethacin on the regional haemodynamic responses to low doses of endothelins and sarafotoxin

1 Regional haemodynamic responses to i.v. bolus injections of low doses (4 pmol and 40 pmol) of endothelin‐1, −2, −3 and sarafotoxin‐S6b were assessed in conscious, Long Evans rats in the absence and presence of indomethacin. 2 Both doses of endothelin‐3 and sarafotoxin‐S6b caused early renal vasodilatations that were not affected by indomethacin. Endothelin‐1 caused an initial renal vasodilatation only in the presence of indomethacin, indicating that this peptide produced concurrent release of cyclo‐oxygenase products that caused renal vasoconstriction. Neither dose of endothelin‐2 produced an increase in renal conductance. 3 The 4 pmol dose of all four peptides caused mesenteric vasoconstrictions only. With the 40 pmol dose of the peptides, none caused early mesenteric vasoconstriction except in the presence of indomethacin. Thus, in this vascular bed the primary vasoconstrictor effects of the peptides (seen with the 4 pmol dose) were offset, following the 40 pmol dose, by release of vasodilator cyclo‐oxygenase products. Indomethacin alone caused significant vasoconstriction only in the mesenteric vascular bed, indicating that in this region of the circulation, vasodilator prostanoids might be involved also in the tonic control of vascular conductance. 4 All four peptides at both doses caused early hindquarters vasodilatation. However, only the initial hypotensive and hindquarters vasodilator effects of the 40 pmol dose of sarafotoxin‐S6b were attenuated by indomethacin. Under these conditions the hindquarters vasodilator effects of sarafotoxin‐S6b were similar to those of the other peptides, indicating that the more marked effects of sarafotoxin‐S6b in the absence of indomethacin were contributed to by vasodilator cyclo‐oxygenase products in the hindquarters.

The effects of phosphoramidon on the regional haemodynamic responses to human proendothelin [1-38] in conscious rats.

1 Cardiovascular responses to human proendothelin [1–38], in the absence and presence of phosphoramidon, were studied in conscious Long Evans rats, chronically instrumented for the continuous recording of heart rate, systemic arterial blood pressure and renal, mesenteric and hindquarters blood flows. 2 A dose of 0.1 nmol kg−1 human proendothelin [1–38] caused a slight pressor effect (maximum 5 ± 2mmHg), but a clear bradycardia (maximum −29 ± 7 beats min−1). Renal haemodynamics were unchanged but there was mesenteric vasoconstriction and a vasodilatation followed by a vasoconstriction in the hindquarters. 3 A dose of 1.0 nmol kg−1 human proendothelin [1–38] caused a gradual hypertension (maximum 42 ± 4 mmHg at 10 min) and a profound bradycardia (–149 ± 10 beats min−1 at 30 min). There were gradual but marked, renal and hindquarters vasoconstrictions, whereas there was a substantial mesenteric vasoconstriction that was relatively rapid in onset. 4 In 2 animals, administration of human proendothelin [1–38] at a dose of 10 nmol kg−1 caused an initial hypotension followed by a rapidly‐developing pressor effect; there were renal and mesenteric vasoconstrictions and vasodilatation followed by vasoconstriction in the hindquarters. These changes were very similar to those seen following injection of endothelin‐1 (0.1 nmol kg−1). 5 Phosphoramidon (2 μmol kg−1) had no cardiovascular effects itself and it did not affect significantly the pressor or mesenteric vasoconstrictor effects of human proendothelin [1–38], but it reduced the bradycardia and renal and hindquarters vasoconstrictor responses. A higher dose of phosphoramidon (10 μmol kg−1) caused significant attenuation of all the responses to human proendothelin [1–38], but a substantial mesenteric vasoconstrictor response still occurred under these conditions. 6 The results are consistent with the involvement of phosphoramidon‐sensitive enzyme systems in the conversion of human proendothelin [1–38] to endothelin‐1 in vivo. In addition, considering the different patterns of responses to human proendothelin [1–38] in the effector tissues studied (heart, and renal, mesenteric and hindquarters vascular beds), and the differential degrees of inhibition of them by phosphoramidon, it is likely that the effects of human proendothelin [1–38] were due to its local (rather than systemic) conversion to endothelin‐1 by processes with differing degrees of susceptibility to phosphoramidon.