PHOSPHATE-TRANSPORT INTO THE SARCOPLASMIC-RETICULUM OF SKINNED FIBERSFROM RAT SKELETAL-MUSCLE

The rate, magnitude and pharmacology of inorganic phosphate (P-i) tran sport into the sarcoplasmic reticulum were estimated in single, mechan ically skinned skeletal muscle fibres of the rat. This was done, indir ectly, by using a technique that measured the total Ca2+ content of th e sarcoplasmic reticulum and by taking advantage of the 1:1 stoichiome try of Ca2+ and P-i transport into the sarcoplasmic reticulum lumen du ring Ca-P-i precipitation-induced Ca2+ loading. The apparent rate of P , entry into the sarcoplasmic reticulum increased with increasing myop lasmic [P,1 in the 10 mM-50 mM range at a fixed, resting myoplasmic pC a of 7.15, as judged by the increase in the rate of Ca-P-i precipitati on-induced sarcoplasmic reticulum Ca2+ uptake. At 20 mM myoplasmic [P- i] the rate of P-i entry was calculated to be at least 51 mu Ms(-1) wh ile the amount of P-i loaded appeared to saturate at around 3.5 mM (pe r fibre volume). These values are approximations due to the complex ki netics of formation of different species of Ca-P-i precipitate formed under physiological conditions. Phenylphosphonic acid (PhPA, 2.5 mM) i nhibited P-i transport by 37% at myoplasmic pCa 6.5 and also had a sma ll, direct inhibitory effect on the sarcoplasmic reticulum Ca2+ pump ( 16%). In contrast, phosphonoformic acid (PFA, 1 mM) appeared to enhanc e both the degree of P-i entry and the activity of the sarcoplasmic re ticulum Ca2+ pump, results that were attributed to transport of PFA in to the sarcoplasmic reticulum lumen and its subsequent complexation wi th Ca2+ Thus, results from these studies indicate the presence of a P- i transporter in the sarcoplasmic reticulum membrane of mammalian skel etal muscle fibres that is (1) active at physiological concentrations of myoplasmic P-i and Ca2+ and (2) partially inhibited by PhPA. This P -i transporter represents a link between changes in myoplasmic [P-i] a nd subsequent changes in sarcoplasmic reticulum luminal [P-i]. It migh t therefore play a role in the delayed metabolic impairment of sarcopl asmic reticulum Ca2+ release seen during muscle fatigue, which should occur abruptly once the Ca-P-i solubility product is exceeded in the s arcoplasmic reticulum lumen.