| Abstract: | Cyclic GMP (cGMP) has been proposed to be involved in mediating negative inotropic responses to muscarinic agonists in the presence of cyclic AMP (cAMP)-generating agents in the heart. In order to investigate this hypothesis, the effects of the novel cGMP lowering agent, LY83583, on carbachol-induced increases in cGMP levels and decreases in tension were measured in rabbit isolated left atria and right ventricular papillary muscles, in the presence and absence of the adenylate cyclase activator, forskolin. In vehicle-treated preparations, negative inotropic responses to 3 microM carbachol in the presence of 3 microM forskolin were accompanied by significant increases in cGMP levels. Carbachol had no significant effect on forskolin-induced increases in cAMP levels. LY83583 (10 microM) reduced basal tension and basal cGMP levels, and completely abolished carbachol-induced increases in cGMP both in left atria and in papillary muscles. The LY83583 significantly reduced the magnitude of the negative inotropic responses of papillary muscles to carbachol in the presence of forskolin, but had no effect on these responses in left atria. Although a causal relationship has not been established, these data suggest that cGMP may be involved in negative inotropic responses to muscarinic stimulation in the presence of cAMP-generating agonists in ventricular muscle, but not in atria. |
