The effects of beta-adrenergic agonists and antagonists on cyclic AMP (cAMP) accumulation and parathyroid hormone (PTH) release from isolated bovine parathyroid cells have been determined. Beta-adrenergic agonists markedly stimulate cAMP production and PTH release with an order of potency (-) isoproterenol greater than (-)epinephrine greater than greater than (-) norepinephrine, suggesting a beta2-type adrenergically mediated process. Both effects are blocked by the beta-blocker propranolol with the strict stereospecificity expected for a beta-adrenergic response. Low calcium concentrations also stimulate cAMP accumulation, but the cyclic nucleotide response under these conditions is only 3% of that obtained with isoproterenol, raising the possibility that factors other than cAMP may control low calcium-mediated PTH release. The release of PTH by low calcium is also not blocked by propranolol, confirming the independence of the response to low ambient calcium from the beta-adrenergic receptor. These studies substantiate further the utility of the isolated parathyroid cell preparation for studying secretagogue-mediated alterations in cyclic nucleotides and hormone secretion. Isolated cells also also make feasible the direct identification of beta-adrenergic receptors in parathyroid cell membranes and whole cells.

The radioiodinated beta-blocker iodohydroxybenzylpindolol ([125I]HYP) has been used to identify directly and characterize beta-adrenergic receptors in isolated bovine parathyroid cells. [125I]HYP was bound rapidly and reversibly to isolated bovine parathyroid cell membranes. Scatchard analysis revealed a single class of binding sites with high affinity (4 X 10(10M-1) and low capacity (0.7 pmol/mg). Saturation analysis of [125I]HYP binding to intact bovine parathyroid cells suggested a site with similar affinity on whole cells and with a binding capacity of 5000-10,000 sites/cell. True dissociation constants (Kd's) for beta-adrenergic agonists and antagonists were in good agreement with activation constants (KA'S) and inhibition constants (KI'S) for effects on adenylate cyclase in membrane preparations. These constants also were in reasonable agreement with KA'S and KI'S previously shown for effects of agonists and antagonists on cAMP accumulation and PTH release in whole cells. This study shows by direct analysis that beta-adrenergic receptors exist on isolated bovine parathyroid cells, and that there is close coupling between receptor binding, effects on cAMP and hormonal release. This represents still another system in which [125I]HYP has been successfully used to study beta adrenergic receptors in membrane as well as intact cell preparations.

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.