α-Adrenergic Inhibition of Adenosine 3′,5′-Monophosphate Accumulation and Parathyroid Hormone Release from Dispersed Bovine Parathyroid Cells*

Abstract

The possibility of alpha-adrenergic modulation of cAMP accumulation and parathyroid hormone (PTH) release was investigated in dispersed bovine parathyroid cells. cAMP accumulation due to the mixed alpha- and beta-adrenergic agonists, (-)epinephrine and (-)norepinephrine, was significantly enhanced by the alpha-adrenergic inhibitor phentolamine; that due to the "pure" beta-adrenergic agonist, (-)isoproterenol, was not altered significantly. Direct inhibition of agonist-stimulated cAMP accumulation was effected by adding increasing concentrations of (-)epinephrine to concentrations of (-)isoproterenol maximally stimulating cAMP accumulation. A 50-75% inhibition of cAMP was observed which was specifically blocked by phentolamine. This inhibition was not specific for beta-adrenergic stimulation, as (-)epinephrine also inhibited dopamine-stimulated cAMP accumulation. The inhibition of (-)isoproterenol-stimulated cAMP accumulation by (-)epinephrine was unaffected by ambient calcium concentration. Stimulation of PTH release by (-)epinephrine and (-)norepinephrine was potentiated by phentolamine and inhibited by the beta-adrenergic blocker, (-)propranolol, demonstrating alpha-adrenergic modulation of hormone release and confirming the close relationship between cAMP accumulation and PTH release previously shown in this system. These results demonstrate the presence of an alpha-adrenergic receptor in dispersed bovine parathyroid cells which inhibits agonist-stimulated cAMP accumulation and PTH release by a mechanism independent of extracellular calcium.