PROSTAGLANDINS MEDIATE THE STIMULATORY EFFECTS OF ENDOTHELIN-1 ON CAMP ACCUMULATION AND INOSITOL-1,4,5-TRISPHOSPHATE PRODUCTION AND CONTRACTION IN CAT IRIS SPHINCTER

We previously reported that in the iris sphincter smooth muscle, endot helin-1 (ET-1) activates both adenylyl cyclase and the phosphoinositid e cascade and that the changes in the levels of cAMP and inositol-1,4, 5-trisphosphate (IP3) produced are species specific. In the present st udy, we examined the mechanism of the ET-1 effects in cat iris sphinct er. In general, we found that ET-1 (0.1 mu M) increased prostaglandin E(2) (PGE(2)) release by 156%, cAMP accumulation by 310%, IP3 producti on by 88% and induced contraction; that PGE(2) increased cAMP accumula tion, IP3 production and contraction; and that the effects of ET-1 are inhibited by indomethacin (indo), suggesting that arachidonic acid me tabolites may mediate the responses to the peptide. Kinetic studies re vealed the following: (1) The effect of ET-1 on cAMP accumulation is r apid (within 30 sec), dose dependent (EC(50) = 5.8 nM) and completely abolished by Indo (K-i = 0.16 mu M), a cyclooxygenase inhibitor, but n ot by nordihydroguairetic acid, a lipoxygenase inhibitor, implying the involvement of PGs. (2) ET-1 dose-dependently evoked PGE(2) release ( EC(50) = 1.8 nM), IP3 production (EC(50) = 4.5 nM) and contraction (EC (50) = 5 nM) and that all of these responses were inhibited by Indo. ( 3) PGE(2) increased cAMP accumulation in a dose-dependent manner with an EC(50) of 1.5 x 10(-7) M, and PGD(2) and PGF(2 alpha) had little ef fect on the cyclic nucleotide. (4) PGE(2) (1 mu M) increased IP3 produ ction by 55% and induced muscle contraction in a dose-dependent manner (EC(50) = 40 nM). We conclude from these data that in cat iris sphinc ter PGs may mediate ET-1-induced cAMP accumulation, IP3 production and smooth muscle contraction.