Are acetylcholine‐induced increases in 42K efflux mediated by intracellular cyclic GMP in turtle cardiac pace‐maker tissue?

Abstract

SUMMARY1. 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 acetylcholineinduced 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+]o (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 acetylcholineinduced increase in 42K FER in cardiac pace-maker tissue is unlikely to be mediated entirely by changes in the levels of intracellular cyclic GMP.

Cyclic guanosine 3',5'‐monophosphate regulates the calcium current in single cells from frog ventricle.

“…In fact, ACh is known to act on a variety of different systems: it not only decreases ICaw but it also increases a K+ current (IK(Ach)) and stimulates dephosphorylation of phospholamban, troponin I, and C-protein (for references see Loffelholz & Pappano, 1985;Hartzell & Simmons, 1987;Hartzell, 1987). In mammalian atrium IK(Ach) may be more sensitive than ICa to ACh (Ten Eick, Nawrath, McDonald & Trautwein, 1976; Jijima, Irisawa & Kameyama, 1985) and IK(ACh) is not regulated by cyclic GMP (Nawrath, 1977;Fleming, Giles & Lederer, 1981;Trautwein et al 1982;Soejima & Noma, 1984;Pfaffinger, Martin, Hunter, Nathanson & Hille, 1985). Thus, in atrial cells, in particular, one would expect decreases in contractile force to result from a shortening of action potential duration due to increased IK(ACh) in the absence of changes in cyclic GMP levels.…”

Changes in 42K efflux produced by alterations in transmembrane calcium movements in turtle cardiac pace‐maker tissue.

“…They show quite clearly that in the presence of calcium there exists a very substantial A23187-induced release of acetylcholine. Presumably this acetylcholine is released from endogenous nerve varicosities, and then enhances 42K FER in the manner which is described in detail in the previous paper (Fleming et al 1981). The ability ofthis ionophore to produce calcium-dependent release ofneural transmitters is well known (cf.…”

“…The methods used in this study were quite similar to those of the preceding paper (Fleming, Giles & Lederer, 1981). Hence, only a brief outline will be given.…”

“…A preliminary account of some of these data has been presented previously (Fleming & Giles, 1980). METHODS The methods used in this study were quite similar to those of the preceding paper (Fleming, Giles & Lederer, 1981). Hence, only a brief outline will be given.…”

“…Individual terms in the above expression are as defined in the previous paper (Fleming et al 1981). The FER represents the fraction of the remaining 42K content which escaped from the sinus tissue during a particular sampling period.…”