COMPARISON OF THE CARDIOVASCULAR AND NEURAL ACTIVITY OF ENDOTHELIN-1, ENDOTHELIN-2, ENDOTHELIN-3 AND RESPECTIVE PROENDOTHELINS - EFFECTS OF PHOSPHORAMIDON AND THIORPHAN

1 In the anaesthetized, ganglion-blocked rat, intravenous boluses of endothelin-1, endothelin-2 and endothelin-3 induced a transient hypotensive effect followed by a potent long lasting pressor response (ED50 mmHg: 0.72 +/- 0.05, 1.8 +/- 0.2 and 2.7 +/- 0.3 nmol kg-1, respectively). The maximal effect for the three peptides was of a similar order of magnitude (DELTAMAP: 84 to 89 mmHg). Neither of these effects was influenced by phosphoramidon or thiorphan (10 mg kg-1, i.v.). 2 Intravenously administered big-endothelin-1 and -2 induced a transient (1-2 min) hypotension followed by a potent long lasting (> 25 min) vasopressor effect (ED50 mmHg: 1.8 +/- 0.2 and 6.7 +/- 0.4 nmol kg-1, respectively), with a similar maximal activity (DELTA MAP: 85 +/- 4 and 81 +/- 2.4 mmHg, respectively). The onset of the big-endothelin-1 vasopressor effect was more rapid (5 - 6 min) than that of big-endothelin-2 (10-13 min). Big-endothelin-3 was found to induce only a potent, long lasting (> 35 min) hypertension, with a maximal effect of 75 +/- 4.6 mmHg at 10 nmol kg-1 and an ED50 mmHg of 6.5 +/- 0.4 nmol kg-1. The onset of this effect was much slower (20-25 min) than that of the other proendothelins. Pressor responses induced by big-endothelin-1, -2 and -3 (3, 15 and 10 nmol kg-1, respectively) were markedly reduced (60, 80 and 100%) in the presence of phosphoramidon (10 mg kg-1, i.v.). Thiorphan (10 mg kg-1, i.v.) did not inhibit the effects of big-endothelin-1, -2 and -3. 3 In the electrically stimulated rat vas deferens, endothelin-1 and -2 were found to be equipotent enhancers of the twitch response (EC100%: 4.0 +/- 0.4 nm and 7.9 +/- 4.8 nm, respectively), both about 3-4 fold as active as endothelin-3 (EC100%: 19 +/- 2.5 nm). Endothelin-1 and -3 showed a comparable maximal stimulatory effect (E(max): 296 +/- 30 and 262 +/- 24%) while endothelin-2 was less active (E(max): 194 +/- 30%). 4 Big-endothelin-1 and -2 were potent enhancers of the twitch reponse too (EC100%: 10.0 +/- 2.6 nm and 21.6 +/- 3.2 nm, respectively), with a comparable maximal stimulatory effect (E(max): 254 +/- 22 and 264 +/- 24%). Big-endothelin-3 was found to be less potent (EC100%: 275 +/- 21 nm), but retained a marked potentiating effect (E(max): 200 +/- 38%). Phosphoramidon, but not thiorphan, concentration-dependently (10 and 100 muM) reduced big-endothelin-1 (58 and 86% respectively) and big-endothelin-2 (21 and 56%) -mediated responses. Conversely, the big-endothelin-3 effect was reduced by phosphoramidon only at 100 muM (-70%), while thiorphan acts concentration-dependently (31 and 71% at 10 and 100 muM respectively); thus, in the rat vas deferens, big-endothelin-1 and -2 were as potent as their corresponding endothelins, while big-endothelin-3 was about 20 times less potent than endothelin-3. 5 The increasing effect of endothelin-2 (194 +/- 30% over baseline) was significantly enhanced by either 10 muM phosphoramidon (277 +/- 42%) or thiorphan (318 +/- 15%). The endothelin-1 and endothelin-3-mediated twitch enhancement was not affected by the two protease inhibitors (10 muM). 6 These results suggest that in vivo big-endothelin-1, -2 and -3, are processed through a similar phosphoramidon-sensitive enzymatic pathway although with different apparent affinity. This enzymatic process is probably attributable to a neutral endoprotease, distinct from neutral-endopeptidase 24.11 (NEP). On the other hand, a NEP-like enzymatic activity may be involved, in the rat vas deferens, in the activation of big-endothelin-3 to endothelin-3 and in the metabolism of endothelin-2, but not of endothelin-I or endothelin-3.