Alpha-Adrenergic activation of phosphorylase in liver cells involves mobilization of intracellular calcium without influx of extracellular calcium.

Abstract

Addition of the a-adrenergic antagonist phenoxybenzamine rapidly terminates a-adrenergic stimulation of phosphorylase in isolated hepatocytes and enhances reaccumulation of Ca2+. a-Adrenergic receptors and effects in hepatocytes are not desensitized after phenylephrine treatment since a second addition of the agonist at 30 min, when phosphorylase activity and Ca2+ content have returned to basal levels, activates phosphorylase and elicits Ca2+ efflux to the same extent as seen after the first addition. Phosphorylase activation in hepatocytes by maximally effective doses of epinephrine and vasopressin is consistently observed at 5 s, with maximum effects at 20 s. Efflux of Ca2+ from perfused livers or of 45Ca2+ from hepatocytes is also very rapidly increased by aadrenergic stimulation. Agents which affect mitochondrial function such as 2,4-dinitrophenol, carbonyl cyanide m-chlorophenylhydrazone, valinomycin, and oligomycin also activate phosphorylase and elicit Ca2+ efflux in hepatocytes at rates imilar to those seen with a-agonists and vasopressin. When a-agonists are combined with vasopressin or mitochondrial inhibitors at maximally effective concentrations, their effects on Ca2+ efflux or phosphorylase are nonadditive implying common mechanisms. Rapid chelation of extracellular Ca2+ with ethylene glycol bis (P-aminoethyl ether)-N,N,N’,I?”tetraacetic acid to lower the concentration of free ca2+ to 0.1 pM does not influence the ability (rate and magnitude) of a-agonists, vasopressin, A23187, and uncouplers of oxidative phosphorylation to stimulate Ca2+ efflux and phosphorylase activation in hepatocytes. However, when cells are depleted of Ca2+ by prolonged ethylene glycol bis(P-aminoethyl ether)-N,N,iV’,N”tetraacetic acid treatment, the actions of vasopressin and uncouplers are severely impaired. It is concluded from these studies that the activation of hepatic phosphorylase by a-adrenergic agonists, vasopressin, and mitochondrial inhibitors does not involve an influx of external Ca2+ and that the mobilization of intracellular Ca2+ pools such as mitochondria and perhaps endoplasmic reticulum and plasma membrane, is sufficiently rapid to account for the metabolic