REGULATION OF ANP SECRETION BY ENDOTHELIN-1 IN CULTURED ATRIAL MYOCYTES - DESENSITIZATION AND RECEPTOR SUBTYPE

We examined endothelin-1 (ET-1) binding and ET-1-regulated atrial natriuretic peptide (ANP) secretion in primary cultures of adult rat atrial myocytes. ET-1 binding was analyzed as a reversible bimolecular reaction, with bimolecular association rate constant = 1.9 x 10(9) M(-1).h(-1), dissociation rate constant = 0.028 h(-1), and a dissociation constant, calculated from these values = 0.015 nM. ET-1 increased ANP secretion with a one-half effective concentration (EC(50)) of 0.62 nM, which correlated with EC(50) receptor occupancy under equivalent experimental conditions (0.75 nM). The secretory response rapidly desensitized (half-time = 10 min at 10 nM ET-1). The time courses for ET-1 binding, ET-1-stimulated secretion, and desensitization were all comparable. Recovery from desensitization was slow and paralleled the recovery of I-125-labeled ET-1 binding. The ET(A) receptor subtype-selective antagonist, BQ-123, inhibited I-125-ET-1 binding and ET-1-activated ANP secretion with high affinity, whereas the ET(B)-selective agonists, endothelin-3 and sarafotoxin S6c, inhibited binding with low affinity and did not effectively stimulate ANP secretion. We conclude that 1) ET-1 can stimulate ANP secretion by direct action on the atrial myocytes; 2) primary cultures of adult rat atrial myocytes express only the ET(A) receptor subtype; 3) the ANP secretory response to ET-1 desensitizes rapidly but recovers slowly; and 4) occupation of the ET(A) receptors by ET-1 initiates the unidirectional sequence of receptor activation, signal transduction, ANP secretion, and finally, desensitization.