Abstract

In awake normotensive and spontaneously hypertensive rats as well as pentobarbital-anesthetized normotensive rats, endothelin-1 (ET-1, 0.063-0.5 nmol/kg i.v.) produced rapidly appearing, transient, dose-related falls in mean carotid artery blood pressure followed by slowly developing small pressor responses. In the latter preparation, the hypotension was due to a decrease in systemic vascular resistance inasmuch as cardiac output increased slightly. Bilateral vagotomy, BW 755c, glibenclamide, idazoxan, propranolol, methylatropine, methysergide or promethazine pretreatment failed to modify the hypotension induced by ET-1 (0.25 nmol/kg i.v.), but this effect was blocked entirely when ET-1 was injected 8 min after starting an i.v. infusion of ET-1 (0.1 nmol/kg/min for 10 min). In pithed rats, ET-1 (0.125-1.0 nmol/kg i.v.) produced sustained pressor responses which were accompanied by reductions in cardiac output. This peptide (0.25 nmol/kg i.v.) did not affect renal vascular resistance significantly but increased (200%) mesenteric resistance substantially more (3-fold) than systemic or hindquarter resistance. The pressor effects of ET-1 were reduced by diltiazem, nitrendipine, verapamil or cromakalim and unchanged after BW 755c, desipramine, enalapril, indomethacin, methysergide, phentolamine or SK&F 100273. The sustained pressor response evoked by an i.v. infusion of ET-1 (0.25 nmol/kg/min/60 min) was also antagonized markedly by nitrendipine and cromakalim. In pithed rats with vasopressin-supported blood pressure, ET-1 produced a short-lasting hypotension which faded entirely after three successive injections of the peptide. Finally, ET-1 (0.4-0.8 nM) evoked greater contractile responses in rat aortic rings deprived of a functional endothelium than in intact preparations. However, in the latter preparation precontracted with norepinephrine, ET-1, in contrast to acetylcholine, failed to evoke vasorelaxation. In aortic rings, the sustained contractile effects of ET-1 (3.2 nM) were reduced moderately by nitrendipine (50 nM) and markedly by cromakalim (0.8 microM). In contrast, the latter compounds antagonized strongly the contractile response to KCl (25 mM). In conclusion, ET-1 appears to produce active vasorelaxation and vasoconstriction via stimulation of specific receptors on blood vessels. The tolerance to the hypotensive effect of ET-1 may indicate that either the receptor site for ET-1 becomes refractory or, alternatively, it is coupled to easily depletable endogenous hypotensive mediators. Finally, inasmuch as the vasoconstrictor effects of ET-1 can be easily counteracted by calcium antagonists under in vivo but not in vitro conditions, the membrane coupling mechanism for ET-1 may not be exactly the same in conductance or resistance vessels.