CYCLIC GMP-INDEPENDENT RELAXATION AND HYPERPOLARIZATION WITH ACETYLCHOLINE IN GUINEA-PIG CORONARY-ARTERY

1 The effects of acetylcholine (ACh) on membrane potential, relaxation and cyclic GMP levels were compared to the NO donor L-nitrosocysteine (Cys-NO) in segments of guinea-pig coronary artery. 2 ACh and Cys-NO produced concentration-dependent relaxations of muscles contracted wit h the H-1 receptor agonist, 2-(2-aminoethyl)pyridine (AEP, 0.35 mM). T he relaxation to ACh was unchanged in the presence of N(G)-monomethyl- L-arginine (L-NMMA; 350 muM) or indomethacin (3 muM). 3 Oxyhaemoglobin (HbO; 20 muM) alone or in combination with L-NMMA increased the EC50 for ACh-induced relaxation whereas relaxation with Cys-NO was almost c ompletely abolished with HbO. 4 Scorpion venom (SV; 8.7 mug ml-1) incr eased the EC50 for relaxation with ACh but not Cys-NO. Combined L-NMMA , HbO and SV produced nearly complete abolition of ACh-induced relaxat ions. 5 Basal cyclic GMP levels (i.e., 20 pmol mg-1 protein) were sign ificantly increased following addition of either ACh (190 pmol mg-1 pr otein) or Cys-NO (240 pmol mg-1 protein). L-NMMA significantly reduced the rise of cyclic GMP with ACh but not Cys-NO. In contrast, SV did n ot significantly reduce the rise in cyclic GMP produced with ACh. In t he combined presence of L-NMMA and HbO neither ACh nor Cys-NO produced a significant increase in cyclic GMP levels. 6 ACh gave rise to signi ficantly greater membrane hyperpolarization than Cys-NO both in the pr esence and absence of AEP. Combined L-NMMA and HbO did not reduce the amplitude of hyperpolarization with ACh. 7 These data indicate that so me but not all of the actions of ACh in the coronary artery can be mim icked by the NO donor, Cys-NO, suggesting that ACh releases NO as well as a second hyperpolarizing factor (i.e., EDHF). Release of NO result s in a large increase in tissue cyclic GMP levels and minimal change i n membrane potential whereas release of EDHF results in a large membra ne hyperpolarization which is independent of changes in tissue cyclic GMP levels. Both of these pathways appear to contribute to relaxation throughout the entire ACh concentration-relaxation relationship.