Transsarcolemmal Movement of Inorganic Phosphate in Glucose-perfused Rat Heart: A 31P Nuclear Magnetic Resonance Spectroscopic Study

Fluxes of orthophosphate (Pi) across the sarcolemma may be important in myocardial metabolism, yet little is known of these in the intact heart. We used 31P NMR spectroscopy to measure net Pi fluxes from changes in the concentrations of Pi, phosphocreatine (PCr), ATP and "total phosphate" ([TP]=[PCr] + 3[ATP]+[Pi]) in the isolated perfused rat heart in response to a change in extracellular [Pi]) from 2 mM to 0 mM. [Pi] decreased to 62% of control with a half-time of 6 min, while [TP] decreased with initial rate 20 mM/h, a measure of net Pi efflux. As [PCr] decreased to 83% of control, phosphorylation potential remained constant. Contractile function was unaffected. Reperfusion with 2 mM Pi reversed all changes, causing net Pi influx at 26 mM/h. Analysed according to a model of net Pi flux, these imply a sarcolemmal permeability rate constant of 13 per h. Insulin in the 2 mM Pi perfusion buffer caused a transient decrease in intracellular [Pi] to 59% of control, while [TP] increased, giving a net Pi influx of 12 mM/h and a permeability constant of 12 per h. [PCr] increased by 28% over 34 min. Thus, insulin caused Pi influx by stimulating Pi incorporation into organic phosphates, transiently decreasing intracellular [Pi] and increasing the outside-to-inside [Pi] gradient. The response to 0 mM and 2 mM Pi perfusion was unaffected by insulin. We conclude that Pi fluxes across the sarcolemma are substantial and may have implications for Pi-free tissue perfusion and clinical hypophosphatemia.