INVOLVEMENT OF ET(A) RECEPTORS IN THE FACILITATION BY ENDOTHELIN-1 OFNONADRENERGIC NONCHOLINERGIC TRANSMISSION IN THE RAT URINARY-BLADDER

1 Endothelin-l (ET-1; 3-10nM) raised the tone of rat bladders bathed i n buffer containing atropine (1 mu M) plus guanethidine (3.4 mu M). In addition, ET-1 potentiated, in a concentration-dependent fashion (1-1 0nM), the contractions evoked by both transmural nerve stimulation and applications of exogenous adenosine 5'-triphosphate (ATP). 2 The thre shold concentration of ET-1 required to facilitate non-adrenergic non- cholinergic (NANC) transmission and potentiate ATP-induced contraction s, was about 10 fold lower than that required to increase the bladder tone (3nM). 3 The ET-l-induced increase in basal tension reached its m aximal effect within 60-90s. In contrast, the 7.8 mu M ATP-induced con tractions increased by 50% within the first minute following incubatio n with 10nM ET-1 but required about 5 min to develop the maximal effec t. 4 The ET-l-induced potentiation of NANC or ATP responses was long-l asting and persisted in spite of extensive washing. The recovery of th e bladder excitability depended on the concentration of ET-1. Followin g the application of 3nm ET-1, recovery required 30 min; applications of 10nM ET-1 required at least 60 min for full recovery. 5 The ET-1-in duced potentiation of responses was selective for ATP and related stru ctural analogues. ET-1 did not modify the contractions induced by acet ylcholine, 5-hydroxytryptamine, prostaglandin F-2 alpha or bradykinin. 6 The potency of ET-2 was similar to that of ET-1. ET-3 and ET-C-term inal hexapeptide were inactive up to 100M. Sarafotoxin S6b was 2 to 3 fold less potent than ET-1 whereas sarafotoxin S6c (100nM) was inactiv e. AGETB-9 and AGETB-89, two ET(B) receptor agonists, were also inacti ve (up to 100nM). 7 Removal of one or both disulphide bonds in ET-1 an d tryptophan-21 formylation of ET-1, resulted in inactive peptides (up to 100nM). 8 The ET-1 receptor antagonists, BE-18257B and FR139317, b locked both the ET-1-induced rise in tone and the potentiation of ATP responses in a concentration-dependent fashion. FR139317 was at least 30 fold more potent than BE-18257B. Both antagonists blocked at lower concentrations the ET-1 increase in bladder tone as compared to the AT P potentiation. The antagonism was slowly reversible. 9 Results are co nsistent with the presence of ET(A) receptors in the rat bladder, whic h mediate both actions of ET-1. The interaction of ET-1 with purinergi c mechanisms is discussed.