Coupling of the type A endothelin receptor to multiple responses in adult rat cardiac myocytes.
In adult rat cardiac myocytes, endothelin (ET) receptors couple to multiple signaling pathways, including stimulation of phosphoinositide hydrolysis (pertussis toxin insensitive) and inhibition of adenylyl cyclase via Gi. We have used ET-1 and congeners to characterize the subtypes of ET receptors on isolated rat myocytes. The rank orders of potency for stimulating phosphoinositide hydrolysis, inhibiting hormone-sensitive adenylyl cyclase, and competing with 125I-ET-1 for binding to myocytes are the same and show the pattern characteristic of an ETA receptor interaction, i.e., ET-1 approximately ET-2 > sarafotoxin 6b > ET-3; the corresponding EC50 values for the effects of ET on signal transduction are approximately 0.5 nM (ET-1), 0.7 nM (ET-2), 7 nM (sarafotoxin 6b), and 60 nM (ET-3). The ETA receptor antagonist BQ-123 abolishes the cellular responses to ET-1 and competes fully for 125I-ET-1 binding in a concentration-dependent manner. Sarafotoxin 6c, an ETB-specific agonist, does not diminish the responses to ET-1 or compete for 125I-ET-1 binding; no specific binding of the ETB-specific ligand 125I-IRL-1620 is detectable on myocytes. Myocytes express approximately 4 x 10(5) ET-1 binding sites/cell. The association of 125I-ET-1 with myocytes is largely irreversible, as are the biochemical responses to ET-1; thus, constants derived from analyses that assume reversible equilibria are in error. We conclude that the effects of ET on transmembrane signaling in rat ventricular myocytes result from occupation of ETA receptors and that the responses are likely to be long lived, compared with those of the readily dissociable neurotransmitters released by the autonomic nervous system.