The biphasic Ca2+-uptake by the fragmented sarcoplasmic reticulum.

The non-equilibrium dialysis has been used for kinetic studies of ATP dependent calcium uptake by the sarcoplasmic reticulum. The uptake displays two phases, which are defined as fast and slow uptake. The former is an exponential function of time, with a half-life time of approximately 15--20 sec, the latter presents the characteristics of an autocatalytic reaction. The fast uptake is characterized by its amplitude, the slow uptake by its rate. Compared with the fast uptake, the slow uptake requires higher amounts of Mg2+ or ATP, and is more sensitive to pH variations and aging. The reasons which formerly prevented the resolution of the second phase from the first are discussed. It is concluded that the fast uptake is not a simple binding reaction, and that the slow uptake is more sensitive to changes by the vesicular membrane.