Effects of somatostatin on parathyroid hormone levels and on responses of parathyroid hormone to beta-adrenergic agonists and hypocalcaemia.

Experiments were designed to study the effect of exogenous somatostatin (S) on basal levels or parathyroid hormone (PTH) and on PTH responses to isoproterenol (ISO), adrenaline (A) or ethyleneglycol-bis (beta-aminoethylether) N,N'-tetraacetate (EGTA) in cattle. During S infusions (6.11 nmol injected iv as a bolus and 0.82 nmol/kg/min infused iv for 30 or 60 min) plasma levels of PTH and calcium (Ca) did not change, whereas concentrations of immunoreactive insulin (IRI) decreased significantly (P less than 0.01). In response to ISO (0.09 nmol/kg/min) or EGTA (5.6 mumol/kg/min, depressing plasma Ca by 0.27 nmol/l) PTH levels were similarly increased (P less than 0.05) both in the absence and presence of S. On the other hand, S suppressed IRI responses to ISO. During 60-min infusions of EGTA (3.15 mumol/kg/min) plasma Ca fell to significantly lower levels (P less than 0.01) during simultaneous infusions of S than in the absence of exogenous S, while the magnitude of PTH responses was similar. The results do not support a role of somatostatin on basal PTH levels and on PTH responses to beta-adrenergic agonists or hypocalcaemia under conditions, in which S markedly lowers IRI levels and IRI responses to ISO. However, S apparently reduces Ca mobilisation during abrupt hypocalcaemia.