Regulation of mitochondrial respiration by inorganic phosphate; comparing permeabilized muscle fibers and isolated mitochondria prepared from type-1 and type-2 rat skeletal muscle

ADP is generally accepted as a key regulator of oxygen consumption both in isolated mitochondria and in permeabilized fibers from skeletal muscle. The present study explored inorganic phosphate in a similar regulatory role. Saponin permeabilized fibers and isolated mitochondria from type-I and type-II muscle from male Wistar rats were prepared. Respiration was measured while the medium P-i concentration was gradually increased. The apparent K (m) values for P-i were 607 +/- A 17 A mu M and 405 +/- A 15 mu M (P < 0.0001) for type-I and type-II fibers, respectively. For isolated mitochondria the values were significantly lower than type-1 permeabilized fibers, 338 +/- A 130 mu M and 235 +/- A 30 mu M (P < 0.05), but not different with respect to fiber type. The reason for this difference in K (m) values in the permeabilized muscle is unknown, but a similar pattern has been observed for K (m) of ADP. Our data indicate that phosphate may play a role in regulation of oxygen consumption in vitro and in vivo.