Excitation of skinned muscle fibers by imposed ion gradients. II. Influence of quercetin and ATP removal on the Ca2+-insensitive component of stimulated 45Ca efflux

Ionic gradients imposed by choline Cl replacement of K methanesulfonate (Mes) at constant [K][Cl] product stimulate 45Ca efflux from skinned muscle fibers; a small, sustained Ca2+-insensitive efflux component, observed in EGTA, appears to grade a much larger Ca2+-dependent component responsible for contractile activation and is likely to reflect intermediate steps in excitation-contraction coupling. The present studies examined ATP-related effects on the Ca2+-insensitive stimulation. 45Ca efflux was measured on segments of frog semitendinosus muscle skinned by microdissection, with isometric force monitored continuously. The Ca2+-insensitive component was potentiated by quercetin, a flavonoid thought to inhibit the sarcoplasmic reticulum (SR) Ca pump by stabilizing a phosphorylated intermediate. Quercetin increased the stimulated net 45Ca release in the absence of EGTA, as expected from inhibition of reaccumulation, but its effectiveness in EGTA indicated potentiation of unidirectional efflux as such. Quercetin also increased unstimulated (control) 45Ca efflux in EGTA, to a smaller extent; potentiation appeared to be a function of efflux, with stimulation above control loss increased approximately 2.6-fold. ATP removal before stimulation, which led to rigor force and increased stiffness, prevented all quercetin effects in EGTA. ATP removal by itself inhibited ionic stimulation of the Ca2+-insensitive component, with little residual increase above the parallel control loss. Addition of the nonhydrolyzable ATP analogue AMP-PCP ([adenylyl-beta,gamma- methylene]diphosphate) (0.8 mM) after ATP removal gave similar results to ATP-free solution, which suggests that adenine nucleotide binding alone does not support stimulation by choline Cl. These results imply a fundamental role for ATP in the excitation of skinned fibers by imposed diffusion potentials; they also suggest that ATP regulates the SR Ca efflux channel, in a manner that could provide the positive feedback in Ca2+-dependent Ca release.