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

Abstract

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 '). 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 Vj-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-l 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 r...

Regional coronary haemodynamic effects of two inhibitors of nitric oxide synthesis in anaesthetized, open‐chest dogs

“…If this were true, then it would be expected that the vasodilatation induced by acetylcholine administered as a bolus injection would not be markedly affected by L-arginine analogues, since the vasodilatation induced in these conditions is very short lasting and thus probably does not rely on de novo synthesis of nitric oxide form L-arginine. Interestingly, in conscious rats, the inhibitory effect of L-NAME on acetylcholine-induced dilatation is more marked when acetylcholine is administered as a slow infusion (Gardiner et al, 1991), than when administered as a bolus injection (Gardiner et al, 1990c).…”

“…For example, in anaesthetized guinea-pigs (Aisaka et al, 1989) and rats (Rees et al, 1990), as well as in conscious rats (Gardiner et al, 1990ab,c), administration of L-NMMA or L-NAME consistently induced hypertension, but did not markedly affect the maximal hypotensive response to intravenously administered acetylcholine, although the L-arginine analogues tended to reduce the duration of this hypotensive phase. Furthermore, in conscious rats, the regional (renal, mesenteric and hindquarter) vasodilator effects of acetylcholine were not markedly affected by L-NAME (Gardiner et al, 1990c). Several hypotheses were advanced to explain this lack of inhibition in vivo; one possibility is that, in vivo, the vasodilator response to acetylcholine, as well as that to other endothelium-dependent vasodilators (e.g.…”

“…This is in contrast with previous experiments, in which systemically administered acetylcholine consistently induced marked hypotension and reflex tachycardia. In these conditions, it is likely that the vasodilator response to acetylcholine is the result of various influences, including baroreflex, neurohumoral and autoregulatory mechanisms (Gardiner et al, 1990c), all of which are probably unaffected by L-NMMA or L-NAME. Our experiments show that, when acetylcholine is administered locally in the absence of systemic modifications, the regional vasodilator response obtained is indeed largely mediated through the L-arginine-nitric oxide pathway.…”

“…Several hypotheses were advanced to explain this lack of inhibition in vivo; one possibility is that, in vivo, the vasodilator response to acetylcholine, as well as that to other endothelium-dependent vasodilators (e.g. bradykinin), is partly nitric oxide-independent (Gardiner et al, 1990c), or is mediated through the release of nitric oxide from intracellular stores that do not rely on mobilization of L-arginine (Aisaka et al, 1989). In this regard, the fact that, in the presence of L-NMMA or L-NAME, the peak hypotensive response to acetylcholine was not affected, but the duration of acetylcholine-induced hypotension was reduced (Aisaka et al, 1989;Rees et al, 1990), would suggest that acetylcholineinduced vasodilatation is the result of two events: (1) an initial vasodilatation which depends on the release of NO or an NO-like molecule (e.g.…”

“…Furthermore, in conscious rats, the regional vasodilator effects of acetylcholine in different vascular beds are virtually unaffected by L-NAME (Gardiner et al, 1990c). In situations in which acetylcholine induces marked hypotension, the regional responses obtained are likely to be the consequence of both a direct vasodilator effect and indirect influences such as neurohumoral and (at least in the conscious animals) baroreflex mechanisms that occur in response to acetylcholine-induced hypotension (Gardiner et al, 1990c), and these indirect influences are probably unaffected by L-arginine analogues. To avoid such confounding factors, it is essential to evaluate the effects of the inhibitors on the response to acetylcholine administered locally at doses that do not markedly affect blood pressure.…”