Effects of alpha-adrenergic stimulation and blockade on parathyroid hormone responses to beta-adrenergic agonists in cattle.

Experiments were designed to investigate the effect of alpha-adrenergic stimulation on parathyroid hormone (PTH) responses to isoproterenol (ISO). In addition, PTH responses to adrenaline (A) and hypocalcaemia were studied with and without alpha-adrenergic blockade by phentolamine. Infusions of phenylephrine (a 'pure' alpha-adrenergic agonist) did not modify basal PTH levels and PTH responses to ISO infusions, whereas the heart rate response to ISO was reduced (P less than 0.05). The results demonstrate that alpha-adrenergic stimulation by phenylephrine does not reduce PTH responses to beta-adrenergic agonists. In response to phentolamine the heart rate increased and plasma levels of Ca and Ca++ decreased. Plasma PTH concentrations were slightly but not significantly raised and PTH responses to adrenaline (P less than 0.05), but not to hypocalcaemia, were enhanced. The potentiation of PTH responses to A in the presence of phentolamine may have ben due to inhibiton of alpha-adrenergic properties of A. It may have been also, in part, the consequence of the hypocalcaemia, which is known to augment PTH responses to A.