CHARACTERIZATION OF FUNCTIONAL ENDOTHELIN RECEPTORS IN THE CANINE ISOLATED-PERFUSED SPLEEN

The endothelin receptor subtypes involved in the vasoconstriction, cap sular smooth muscle contraction, prostaglandin E(2) and prostacyclin r elease induced by endothelin-l have been investigated in the canine is olated perfused spleen using both the endothelin ET(A) receptor antago nist FR 139317 and the endothelin ET(B) receptor agonist IRL 1620. The isolated canine spleen was perfused with warmed (37 degrees C) and ox ygenated (95% O-2/5% CO2) Krebs solution at constant now with continuo us recording of splenic arterial perfusion pressure and spleen weight, Samples of splenic venous effluent were collected to determine the am ounts of prostaglandin E(2) and prostacyclin, measured by radioimmunoa ssay. Endothelin-1 (4-200 mu mol) and IRL 1620 (20-1000 pmol) dose-dep endently increased splenic arterial perfusion pressure but the former was more potent on a molar basis (the molar dose ratio IRL/endothelin- 1 required to increase splenic arterial vascular resistance by 25% was approximately 33). The infusion of the nitric oxide inibitor N-omega- nitro-L-arginine methyl ester (10 mu M), but not of the enantiomer N-o mega-nitro-D-arginine methyl ester, significantly potentiated the incr ease in splenic arterial vascular resistance induced by endothelin-1. The infusion of FR 139317 (1 mu M) markedly attenuated the increased s plenic arterial perfusion pressure induced by endothelin-1 without aff ecting that evoked by IRL 1620. At the highest dose (200 pmol, endothe lin-1 induced a small but significant capsule contraction as reflected by the reduction in the spleen weight. The infusion of FR 139317 (1 m u M) abolished this contractile effect. IRL 1620 (in doses up to 1000 pmol) did not significantly affect the capsule tone. The administratio n of either endothelin-1 (20-200 pmol) or IRL 1620 (20-1000 pmol) caus ed the release of 6-oxo-prostaglandin F-1 alpha (breakdown product of prostacyclin) and prostaglandin E(2) into the splenic venous effluent. The amount of both prostanoids released by endothelin-1 was significa ntly greater than that induced by IRL 1620. FR 139317 (1 mu M) signifi cantly reduced (P < 0.05) the release of both 6-oxo-prostaglandin F-1 alpha and prostaglandin E(2) by endothelin-1 without affecting that re leased by IRL 1620. The results demonstrate that the release of prosta glandins and nitric oxide modulates the vasoconstrictor activity of en dothelin-1 in the splenic circulation. Furthermore, the vasoconstricti on and eicosanoids (prostacyclin and prostaglandin E(2)) release by en dothelin-1 are due to activation of both endothelin ET(A) and ET(B) re ceptors, although the former seems to be the predominant form. The spl enic capsule contraction is mediated by activation of endothelin ET(A) receptors only.