Parathyroid hormone response to dopamine in cattle.

The effects of dopaminergic agonists on plasma levels of parathyroid hormone (PTH) have been studied in cattle. PTH increased within minutes in a dose-dependent manner during intravenous infusions of dopamine (DA) or epinine. Bromocriptine, in amounts that lowered plasma prolactin, inconsistently elevated PTH. Pimozide suppressed PTH responses to DA, whereas propranolol, phentolamine, phenylephrine, and atropine were ineffective. Plasma calcium and magnesium remained unaltered during DA and epinine infusions. Therefore DA appears to stimulate PTH secretion directly. After prolonged DA infusions, the secretion of PTH became resistant to both the administration of DA and isoproterenol, whereas the parathyroid glands remained responsive to a hypocalcemia. On the other hand, calcium suppressed PTH responses to DA. The results suggest that DA, beta-adrenergic agonists, and low calcium stimulate PTH secretion by separate but closely related mechanisms. Based on biochemical and histochemical observations on DA occurring in high amounts in bovine parathyroid glands, it is conceivable that this amine might stimulate PTH secretion at the level of the gland.