Plasma membrane phosphate transport and extracellular phosphate concentration in the regulation of cellular respiration in isolated perfused rat heart

Abstract

The role of extracellular P i and transmembrane fluxes across the sarcolemma in the regulation of cellular respiration was studied in isolated Langendorff-perfused rat hearts. Extracellular phosphate did not significantly affect the oxygen consumption or cellular phosphorylation potential of the myocardium. K +-induced arrest was used to change the mechanical work load of the heart. Arresting the heart caused a rapid decrease in the unidirectional efflux of phosphate determined by in vitro prelabelling of the intracellular phosphate compounds with 32P and determining the specific radioactivity of the γ-P of ATP, and the label appearance into the perfusion medium. At normal or elevated perfusate phosphate concentration there was a fairly slow net uptake of phosphate. The decrease in phosphate fluxes upon the K +-induced arrest was probably not due to a decrease in the transmembrane Na + or K + gradients because a further increase in the perfusate K + concentration caused an increase in the K + efflux to the levels observed in contracting hearts. The use of higher than normal concentrations of phosphate necessitated a lowering of the extracellular Ca 2+ concentration, which caused a diminution of the oxygen consumption, accompanied by mitochondrial flavoprotein oxidation in the heart. This finding suggested that the extracellular Ca 2+ concentration may be involved in the substrate level regulation of mitochondrial metabolism.