J. Biol. Chem., Vol. 259, Issue 6, 3596-3604, Mar, 1984
Regulation of hepatic glycogenolysis by glucagon in male and female rats. Role of cAMP and Ca2+ and interactions between epinephrine and glucagon
RK Studer, KW Snowdowne and AB Borle
The effects of 10(-10) to 10(-7) M glucagon on cAMP, phosphorylase a, cell
calcium, and glucose production, and glucagon interactions with epinephrine
were studied in isolated hepatocytes from adult male and female rats. At
physiological concentrations (10(-10) - 10(-9) M), glucagon activated
phosphorylase by increasing cAMP and not by raising the cytosolic free
calcium. At supra-physiologic concentrations (and in the male only),
glucagon slightly increased the cytosolic free calcium, the fractional
efflux of calcium, and, after 2 h, decreased the cell calcium content.
Exposure of hepatocytes to the simultaneous administration of 10(-9) M
glucagon and 10(-7) M epinephrine resulted in a prolongation of the
activation of phosphorylase a and a greater release of glucose from
glycogen stores than exposure to either agonist alone. In the male, the
effects of low concentrations of the two hormones on phosphorylase a
activity were additive. Cytosolic free calcium was increased by 10(-6) M
epinephrine from 280 to 500 nM while physiological concentrations of
glucagon did not change it. In these intact cells, there was no evidence of
an alpha 2-adrenergic inhibition of adenyl cyclase and no indication that
cAMP depresses the rise in cell calcium induced by alpha-adrenergic
stimuli.
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