Activation of particulate guanylyl cyclase by endothelins in cultured SV-40 transformed cat iris sphincter smooth muscle cells

We investigated the effects of endothelins (ETs) on cGMP production in cultured SV-40 transformed cat iris sphincter smooth muscle (SV-CISM-2) cells. ET-3 increased cGMP formation in a concentration-dependent manner (EC₅₀ =-98nM), which was 2.5 times higher than that of ET-1. The ET(B)receptor agonists sarafotoxin-S6c and IRL 1620 also increased cGMP production, mimicking the effects of the ETs. The ETs receptor antagonist BQ 788, but not the ET(A) receptor antagonist BQ610, dose-dependently blocked ET-3-stimulated cGMP formation (IC₅₀=10nM). The phorbol ester, Phorbol 12, 13 - dibutyrate (PDBu), which inhibits particulate guanylyl cyclase in smooth muscle, dose-dependently inhibited ET-3-stimulated cGMP accumulation (IC₅₀=66nM). LY83583 and ODQ, inhibitors of soluble guanylyl cyclases, as well as inhibitors of the nitric oxide cascade and of intracellular Ca²⁺ elevation had no appreciable effect on ET-3-induced cGMP production. ET-3 markedly inhibited carbachol-induced intracellular Ca²⁺ mobilization. We conclude that ET-3 increases intracellular cGMP levels in SV-CISM-2 cells through activation of the ET(B) receptor subtype and subsequent stimulation of the membrane-bound guanylyl cyclase. Elevation of cGMP by ET and the subsequent inhibition of muscarinic stimulation of intracellular Ca²⁺ mobilization by the cyclic nucleotide could serve to modulate the contractile effects of Ca²⁺ -mobilizing agonists in the iris sphincter smooth muscle.