Effects of alpha-adrenergic stimulation and blockade of plasma parathyroid hormone concentrations in cows.

Experiments were designed to investigate responses of immunoreactive parathyroid hormone (PTH) during alpha-adrenergic stimulation and blockade in cows. Alpha-adrenergic agonists (methoxamine, phenylephrine and noradrenaline, the beta-adrenergic action of which was blocked by propranolol) did not change PTH and free fatty acid levels, whereas they characteristically increased the blood pressure and decreased the heart rate. In contrast, alpha-adrenergic blockade by phentolamine progressively increased PTH levels. The elevated PTH concentrations, associated with increased plasma noradrenaline and free fatty acid levels, rising heart rate and decreasing blood pressure, indicated that all these changes can be related to a beta-adrenergic stimulatory mechanism. Beta-adrenergic stimulation was presumably responsible for the initial elevation of PTH concentrations, whereas, during the later phase of the phentolamine infusions, a concomitant hypocalcaemia probably also produced a stimulatory effect.