EFFECTS OF CHRONIC PROPRANOLOL TREATMENT ON HEPATIC ADENYLATE CYCLASE SYSTEM IN THE RAT

The biochemical aspects of hepatic β-adrenergic receptors and adenylate cyclase activity in male adult rats were examined during chronic treatment of a β-adrenergic antagonist, propranolol. The blockade of β-adrenergic nervous systems for 7 to 10 days produced a considerable elevation of basal, glucagon, sodium fluoride, and 5′-guanylylimidodiphosphate, Gpp (NH)p-stimulated enzyme activity with a negligible response to a β-adrenergic agonist, isoproterenol. There was no alteration in the density or the affinity of β-adrenergic receptors for the agonist during the treatment. Guanine nucleotides have failed to induce a transformation of the higer affinity to the lower affinity state of β-adrenergic receptors of the hepatic membrane derived either from control or the propranolol-treated animals. The activity of stimulatory guanine nucleotide regulatory proteins (Ns)in the enzyme, assessed by ADP-ribosylation was also not altered by the antagonist. These results suggest that the mechanism of the observed sensitization of adenylate cyclase induced by chronic β-adrenergic blockade involves facilitation of Ns interaction with the catalytic subunit of the enzyme with no change in the β-adrenergic receptor functions.