Phosphorylation and the control of calcium fluxes.

Cell activation, e.g. stimulus-contraction or stimulus-secretion coupling, is brought about by a 100-fold increase in cytosolic free Ca2+ concentration from 0.1 to 10 microM, upon release of Ca2+ from intrareticular or extracellular stores along the concentration gradient. A return to steady state is achieved by either Na+-Ca2+ exchange or ATP-dependent Ca2+ transport against the concentration gradient. Both processes, Ca2+ influx and Ca2+ efflux, are regulated by sophisticated covalent mechanisms. The positive inotropic effect of adrenalin is mediated by the cyclic-AMP-dependent phosphorylation of cardiac sarcolemmal proteins, among which calciductin is the major phosphate acceptor. Upon cyclic-AMP-dependent phosphorylation, the slow Ca2+ channel is activated 3.5 time above its basal low-conductance state, and retains its characteristics, competition by divalent metals, inhibition by La3+ and Ca2+ entry blockers. The adrenalin-induced abbreviation of systole is also explained in terms of the dual phosphorylation of the cardiac sarcoplasmic reticulum calcium pump activator, phospholamban, by cyclic-AMP-dependent protein kinase on the one hand and Ca2+-calmodulin-dependent phospholamban kinase on the other. Calciductin and phospholamban are closely similar acidic proteolipids. A phospholamban-like protein is also found in platelet Ca2+-accumulating vesicles, where its cyclic-AMP-dependent phosphorylation doubles the rate of Ca2+ efflux. These observations raise the possibility that calcium fluxes are regulated by phosphorylation of membrane-bound proteolipids. More generally, phosphorylation modulates K+, Na+ and Ca2+ fluxes through membranes, i.e. the general excitability properties of the cell.