Role of a guanine nucleotide-binding regulatory protein in the hydrolysis of hepatocyte phosphatidylinositol 4,5-bisphosphate by calcium-mobilizing hormones and the control of cell calcium. Studies utilizing aluminum fluoride.

Treatment of isolated hepatocytes with NaF produced a concentration-dependent activation of phosphorylase, inactivation of glycogen synthase, efflux of Ca2+, rise in cytosolic free Ca2+ (Ca2+i), increase in myo-inositol-1,4,5,-P3 levels, decrease in phosphatidylinositol-4,5-P2 levels, and increase in 1,2-diacylglycerol levels. These changes were evident within 1 min and maximum at 2-5 min. Maximum effects on Ca2+ efflux, Ca2+i, glycogen synthase, and phosphorylase were observed with 15 mM NaF, whereas myo-inositol-1,4,5-P3 and 1,2-diacylglycerol levels were maximally stimulated by 50 mM NaF. The levels of intracellular cAMP were decreased by NaF (up to 10 mM) in the absence or presence of glucagon (0.1-1 nM) or forskolin (2 microM). The effects of low doses of NaF (2-15 mM) to inhibit basal or glucagon-stimulated cAMP accumulation, mobilize Ca2+, activate phosphorylase, and inactivate glycogen synthase were all potentiated by AlCl3. This potentiation was abolished by the Al3+ chelator deferoxamine. These results illustrate that AlF4- can mimic the effects of Ca2+-mobilizing hormones in hepatocytes and suggest that the coupling of the receptors for these hormones to the hydrolysis of phosphatidylinositol-4,5-P2 to myo-inositol 1,4,5-P3 is through a guanine nucleotide-binding regulatory protein. This is because AlF4- is known to modulate the activity of other guanine nucleotide regulatory proteins (Ni, Ns, and transducin).