Effect of orthophosphate on the rate of calcium uptake by red and white muscle sarcoplasmic reticulum.

In the presence of added orthophosphate (Pi) there is a period during which sarcoplasmic reticulum vesicles (SR) accumulate calcium at a constant rate. This constant rate is increased and is reached sooner when the added Pi concentration is increased. A double reciprocal plot of rate and Pi concentration gives a straight line. The Pi concentration required for half-maximum rate of calcium accumulation is the same for SR preparations from red and white rabbit muscles, although the maximum rates are widely different. During the ageing of SR preparations the Pi concentration required for half-maximum rate increases, but the maximum rate remains the same.