COLFORSIN OR IMIDAZOLIDIONE POTENTIATES CAMP ELEVATION CAUSED BY ENDOTHELIN-1 IN RAT AORTA

Endothelin-1 (ET-1) caused slow-developing and stable vasoconstrictions in isolated rings of rat thoracic aortae with a pD(2) value of 7.55 +/- 0.10 compared to pD(2) values of 9.30 +/- 0.10 and 8.36 +/- 0.30 for angiotensin II and norepinephrine, respectively. Although the potency of ET-1 was somewhat lower than those of norepinephrine and angiotensin II, the maximal tension generated by ET-1 was comparable to that of norepinephrine and considerably greater than that of angiotensin II. Incubation of aortic rings in the absence of extracellular Ca2+ or in the presence of the Ca2+ channel blocker nifedipine (100 nmol . L(-1)) greatly attenuated ET-1-induced vasoconstriction. ET-1 (20 nmol . L(-1), approximately the EDS, for vasoconstrictions) also caused elevation of cAMP levels in aortic rings after 15 and 25 min of exposure. The cAMP phosphodiesterase inhibitor imidazolidione (Imi, Ro 20-1724, 100 mu mol . L(-1)) potentiated the cAMP responses to ET-1. Rings incubated for 25 min with ET-1 (20 nmol . L(-1)) showed much larger cAMP elevations caused by colforsin (Col, forskolin 1 mu mol . L(-1)), a direct adenylate cyclase activator and potentiator, than with Col or ET-1 alone. Therefore, ET-1 may utilize at least 2 signal transduction mechanisms, one involving the opening of nifedipine-sensitive Ca2+ channels and the other involving the elevation of cAMP levels, to produce the unusually slow-delevoping and stable vasoconstrictions in rat aortae.