The release of phosphate from contracting atria as a parameter of (Na+ + K+)-ATPase activity. Effect of ouabain

An experimental situation that permits simultaneous evaluation of the mechanical activity and inorganic phosphate (Pi) release by beating atria was developed. It concerned cell membrane integrity and compartmentalization of the cells. The Pi released by isolated rat and guinea pig atria was measured in a physiological salt solution. The preparations were incubated under preload for the simultaneous recording of dF/dt and Pi. The substrate of this reaction was the endogenous ATP. Pi release by atria increased with time. It was stimulated at high K+ concentrations (30 mmol/l) and inhibited at very low ones (1.2 mmol/l). The lack of Mg2+ in the incubation media did not affect it. Ouabain induced different effects upon Pi release; at low concentrations it was increased while at higher ones it was inhibited. The relationship between ouabain concentrations, mechanical responses and liberation of Pi by isolated atria from rat and guinea pig showed that at low concentrations ouabain simultaneously increased dF/dt and Pi release. On the contrary, at high concentrations of ouabain dF/dt decreased, contracture occurred and the Pi release decreased. Isoproterenol, norepinephrine and calcium, at concentrations that increased dF/dt, did not modify Pi release. Different concentrations of potassium and ouabain also modulated the 32P Pi efflux in a fashion similar to that with the Pi released by beating atria. We propose that Pi release by living cells appears to be correlated with manipulations which either stimulate or inhibit (Na+ + K+)-ATPase activity.