MEDIATION BY PROSTAGLANDINS OF THE STIMULATORY EFFECT OF SUBSTANCE-P ON CYCLIC-AMP PRODUCTION IN DOG IRIS SPHINCTER SMOOTH-MUSCLE

The purpose of the present study was to examine the mechanism of the s timulatory effect of substance P (SP) on cyclic AMP (cAMP) accumulatio n in dog iris sphincter. We found that: (1) SP increased cAMP accumula tion in a time- and concentration-dependent manner, the T-1/2 and EC(5 0) values being 1.2 min and 44 nM, respectively. SP has no effect on i nositol trisphosphate and muscle contraction in this tissue. (2) SP-st imulated cAMP formation was inhibited by quinacrine, a non-specific ph ospholipase A(2) inhibitor (IC50 = 9.5 mu M), and by indomethacin (Ind o), a cyclooxygenase inhibitor (IC50 = 3.5 nM), in a concentration-dep endent manner, suggesting that SP induces cAMP accumulation via an Ind o-sensitive pathway. (3) SP-induced arachidonic acid release and SP-in duced prostaglandin E(2) (PGE(2)) release were inhibited concentration dependently by quinacrine and Indo, with IC50 values of 11 mu M and 0 .8 nM, respectively. (4) PGE(2) (1 mu M) increased cAMP formation in t he sphincter muscle by 94%, and, furthermore, the PG, but not SP, stim ulated the activity of adenylyl cyclase in membrane fractions isolated from this tissue. (5) Indo (1 mu M) blocked the relaxing effect of SP (1 mu M) in iris sphincter precontracted with carbachol (1 mu M). (6) The inhibitory effect of Indo on SP-induced cAMP accumulation was spe cies specific. Increases in cAMP represent a mechanism by which extrac ellular SP can regulate smooth muscle function. Thus, we conclude from these studies that in dog iris sphincter SP-induced cAMP accumulation is mediated through PGs, and that in this cholinergically innervated muscle SP via cAMP could function, in part, to modulate the physiologi cal responses to muscarinic receptor stimulation.