Effects of α-adrenergic stimulation and blockade on parathyroid hormone responses to β-adrenergic agonists in cattle

Abstract

Abstract. Experiments were designed to investigate the effect of α-adrenergic stimulation on parathyroid hormone (PTH) responses to isoproterenol (ISO). In addition, PTH responses to adrenaline (A) and hypocalcaemia were studied with and without α-adrenergic blockade by phentolamine. Infusions of phenylephrine (a 'pure' α-adrenergic agonist) did not modify basal PTH levels and PTH responses to ISO infusions, whereas the heart rate response to ISO was reduced (P < 0.05). The results demonstrate that α-adrenergic stimulation by phenylephrine does not reduce PTH responses to β-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 < 0.05), but not to hypocalcaemia, were enhanced. The potentiation of PTH responses to A in the presence of phentolamine may have been due to inhibition of α-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.