EVIDENCE THAT STIMULATION OF PLASMA-MEMBRANE CA2-CYCLIC-AMP IN RAT HEPATOCYTES( INFLOW IS AN EARLY ACTION OF GLUCAGON AND DIBUTYRYL)

The ability of glucagon (1 nM) and of dibutyryl cyclic AMP (50 muM) to increase cytosolic free Ca2+ concentration ([Ca2+]i) in Fura-loaded r at hepatocytes was examined in a system wherein Ca2+ inflow was induce d by the re-admission of excess Ca2+ to a nominally Ca2+-free medium. An increase in [Ca2+]i did not occur in the absence of either agonist, but did so after co-addition of either agonist with Ca2+. Increasing the time between addition of dibutyryl cyclic AMP (or of glucagon) and Ca2+ led to increases in [Ca2+]i; half-maximal and maximal increases were observed at 0 s (i.e. at co-addition) and 5-7 s respectively. Dib utyryl cyclic AMP and Ca2+ each exhibited a concentration-dependence w hen their respective concentrations were changed for a fixed time inte rval between additions. Half-maximal and maximal effects were obtained with 30 muM and 50 muM dibutyryl cyclic AMP and with 0.5 mM and appro x. 1 mM Ca2+ respectively. The data demonstrate an early action of glu cagon and dibutyryl cyclic AMP on [Ca2+]i. It is argued that the agoni st-induced rise in [Ca2+], results from an increase in plasma-membrane Ca2+ inflow, an effect that appears to occur much earlier than that o n mobilization of internal stores of Ca2+.