42K efflux has been measured from small strips of turtle sinus venosus tissue in order: (a) to characterize further the pharmacology of the acetylcholine response and (b) to test whether cyclic guanosine 3':5'-monophosphoric acid (cyclic GMP) is the intracellular mediator of this response. 2. The 42K wash-out curves show that the fractional escape rate (FER) of 42K efflux is nearly constant after 60-80 min, indicating that after this time period 42K FER is controlled by barrier-limited diffusion from a single intracellular compartment. 3. The threshold of the dose-response relationship for the acetylcholine-induced increase in 42K FER is about 10(-8) M and the Km is 2.75 x 10(-7) in non-eserinized preparations. 4. This acetylcholine response is completely blocked by atropine; but nicotinic blockers produce no detectable reduction of it. 5. Exogenous application of lipid-soluble analogues of cyclic GMP (dibutyryl or 8-bromo-cyclic GMP applied at 2-3 mM for 30 min) failed to mimic the acetylcholine-induced augmentation of 42K FER. 6. Experiments in which sodium nitroprusside (5 x 10(-4) M for 30 min) was applied in order to stimulate the guanylate cyclase and hence produce a large, maintained increase in intracellular cyclic GMP failed to show a significant increase in 42K FER. 7. When acetylcholine (10(-6)M) was applied in the presence of O[Ca2+]0 (in an attempt to inhibit the guanylate cyclase) there was no significant reduction in the acetylcholine-induced increases in 42K FER. 8. Hence, these three indirect tests indicate that the muscarinic acetylcholine-induced increase in 42K FER in cardiac pace-maker tissue is unlikely to be mediated entirely by changes in the levels of intracellular cyclic GMP.
