Regulation of glycolytically fueled Ca2+ uptake in smooth muscle plasmalemmal vesicles by phosphorylation

A highly purified plasma membrane vesicular preparation from porcine antrum had an endogenous protein kinase activity with substrates of molecular weights of 11, 15, 20.5, 25, 35, 44, 155, and 230 x 10(3). Phosphorylation of the plasma membranes by the endogenous protein kinase activity resulted in a stimulation of initial rates of Ca2+ uptake into inside-out vesicles, which was associated with an increase in the maximum velocity of the Ca2+ pump with no apparent changes in the half-maximal effective concentration for calcium. Because we have previously reported that a membrane-associated glycolytic system may preferentially provide ATP to fuel the Ca2+ pump (9), we examined the effects of phosphorylation on Ca2+ uptake when glycolysis was the sole source of ATP for the pump. We found that the stimulation of Ca2+ uptake by phosphorylation was more pronounced when Ca2+ uptake was supported by glycolysis rather than 2 mM ATP. When ATP was added at a level similar to that produced by endogenous glycolysis, the stimulation of Ca2+ uptake by phosphorylation was comparable to when glycolysis supported the Ca2+ pump. Our observations suggest that the dynamic range (up to threefold) for regulation of the plasmalemmal Ca2+ pump by phosphorylation is considerably larger than previously reported and thus likely to be of physiological significance.