ATP as a positive effector of the sodium efflux in single barnacle muscle fibers

Abstract

A study has been made of the mechanism by which the injection of ATPNa 2 stimulates the ouabain-insensitive Na efflux in fibers from the barnacle, Balanus nubilus. The results of this study are as follows: ATPNa 2 is found to be a more potent effector of the Na efflux in unpoisoned fibers than ATPMg on an equimolar basis, but not more potent than ADPNa 2. In ouabain-poisoned fibers ATPNa 2 and ATPMg are equipotent but the former is more potent than ADPNa 2. The magnitude of the response to ATPNa 2 injection into ouabain-poisoned fibers depends on: (i) the ouabain concentration used; (ii) the concentration of ATPNa 2 injected, and (iii) the external Ca 2+ concentration. Ouabain is without effect when it is applied at the time of ATPNa 2 injection. Responsiveness to ouabain, however, is found to return if the glycoside is applied after complete decay of the response to ATP. Under these conditions, the effect of ouabain in fibers injected with ATPNa 2 is significantly less than in fibers injected with ATPMg. Preinjection of EGTA in high concentrations fails to reduce the size of the response to ATPNa 2 injection. Injection of Mg 2+ following peak stimulation by ATP almost completely reverses the response. The response to Mg 2+ is concentration-dependent. Ryanodine but not neomycin reduces the response to ATP. ATPγS is not as effective as ATPNa 2. Nor is AMP-PNP consistently as effective as ATPNa 2. Collectively, these results support the hypothesis that the response of the Na efflux to ATPNa 2 injection involves the operation of the putative Na +-Ca 2+ exchanger in the reverse mode and that a raised Ca i 2+ is not an absolute requirement. They also strongly suggest that two other governing factors are the Na + gradient across the sarcolemma and the myoplasmic pMg. Mg 2+ seems to act as an inhibitor.