Hormonal stimulation of cyclic AMP accumulation and glycogen phosphorylase activity in calcium-depleted hepatocytes from euthyroid and hypothyroid rats

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Abstract
Activation of glycogen phosphorylase by hormones was examined in hepatocytes isolated from euthyroid and hypothyroid female rats and incubated in Ca2+-free buffer containing 1 mM ethyleneglycol-bis(.beta.-aminoethyl ether) N,N,N'',N''-tetraacetic acid. Basal glycogen phosphorylase activity was decreased in Ca2+-free buffer. The activation of hepatocyte glycogen phosphorylase, in the absence of extracellular Ca2+, in response to adrenaline [epinephrine, E], glucagon or phenylephrine was slightly lower; that by vasopressin was abolished. The activation of glycogen phosphorylase by phenylephrine, E or isoproterenol (isoprenaline) in hepatocytes from euthyroid rats incubated in the absence of Ca2+ was not accompanied by any detectable increase in total cyclic[c]AMP. The log-dose/response curves for activation of phosphorylase by phenylephrine or low E concentrations were the same in hepatocytes from hypothyroid as compared with euthyroid rats; the response to isoproterenol was greater in hepatocytes from hypothyroid rats. The increases in total cAMP accumulation caused by E or isoproterenol were greater in hepatocytes from hypothyroid rats than in hepatocytes from euthyroid rats. The increases in cAMP accumulation caused by E or isoproterenol in Ca2+-depleted hepatocytes from hypothyroid rats were blocked by propranolol, a .beta.-adrenergic antagonist. Propranolol was only partially effective as an inhibitor of the activation of glycogen phosphorylase by phenylephrine or E in hepatocytes from hypothyroid rats and was ineffective on phosphorylase activation in cells from euthyroid rats. The .alpha.-adrenergic activation of glycogen phosphorylase apparently is not affected by the absence of extracellular Ca2+, and the extent to which total cAMP was increased by adrenergic amines did not correlate with glycogen phosphorylase activation.

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