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Abstract Endothelins exert potent excitatory cardiac effects by acting on specific receptors on myocytes. In this study, we have examined the signal transduction mechanism for the chronotropic effect of endothelins in guinea pig atria. A competition binding of [ 125 I]endothelin 1 ([ 125 I]ET-1) using the recently developed ET A receptor–selective antagonist BQ123 showed the presence of almost equal populations of ET A (44%) and ET B (56%) receptors in the guinea pig right atria. In a concentration-response study, endothelin 3 (ET-3), an agonist with higher affinity to ET B receptors than to ET A receptors, and sarafotoxin S6c (STXS6c), an ET B receptor–selective agonist, increased the rate of spontaneous beating at all concentrations tested (10 pmol/L to 100 nmol/L). In contrast, ET-1, a nonselective agonist, increased the heart rate at lower concentrations (10 pmol/L to 10 nmol/L) but decreased it at higher concentrations (30 to 100 nmol/L). When ET-1 (100 nmol/L) was applied in a single amount, heart rate was strongly increased; however, this increase was followed by a rapid decline in the response. ET-1 (100 nmol/L) but not ET-3 or STXS6c significantly reduced the heart rate when it was raised by isoproterenol (ISO, 300 nmol/L) either in the absence or presence of a phosphodiesterase inhibitor, 3-isobutyl-1-methylxanthine (IBMX). Correspondingly, ET-1 significantly reduced the ISO-induced elevation of cAMP accumulation (19.1±1.7 pmol/mg protein [n=8] and 12.6±1.2 pmol/mg protein [n=7] in the absence and presence of ET-1, respectively; P A receptors are involved in an inhibitory cardiac action of endothelins, which is coupled to a pertussis toxin–sensitive G protein/adenylate cyclase inhibition pathway. This ET A receptor–mediated inhibitory action gives new insights into understanding physiological and pathophysiological modulations of cardiac functions by endothelins.

Endothelin-l (ET-I) induces persistent vasoconstriction via a sustained increase in intracellular free Ca 2+ concentrations ([Ca 2+ ] i ). The mechanisms of the elevation of [Ca 2+ ] i operating at physiologically low concentrations of ET-1 are controversial. Here we report that both native ET A receptors in vascular smooth-muscle cells and recombinant ET A receptors expressed in mouse fibroblasts (Ltk cells) are functionally coupled with a nonselective cation channel, which is permeable to Ca 2+ and is blocked by mefenamic acid. The channel is persistently activated by a low concentration of ET-1 (10 -10 M) without stimulation of inositol triphosphate (IP 3 ) formation and mediates sustained vasoconstriction.

Pharmacologic responses to endothelins (ETs) are mediated by two subtypes of G-protein-coupled receptors, termed ETA and ETB. A chimeric receptor that has the transmembrane domains (TMDs) IV-VI with the adjacent loop regions from ETB embedded in the remaining regions from ETA exhibits specific binding to the N-terminally truncated ETB agonists 125I-BQ3020 and 125I-IRL1620, to the same level as that of wild-type ETB receptor. Furthermore, the ETA-selective antagonist BQ123 competed for the binding of these ETB-selective radioligands to this chimeric receptor, with Ki values similar to those determined by using wild-type ETA receptor and 125I-ET-1. These findings indicated that the endothelin systems consist of two distinct parts, both on the ligand and receptor sides. The N-terminal loop structure of the agonists and the TMDs IV-VI with adjoining loops of the receptors determine the isopeptide/subtype selectivity. On the other hand, the C-terminal linear portion of the isopeptides and the TMDs I-III and VII plus adjacent loops of the receptors are probably involved in ligand-receptor binding itself. This scheme can be explained by the classic "message-address" concept proposed for a number of peptidergic ligand families.