Parathyroid hormone response to hypocalcaemia following hypercalcaemia

Experiments were designed to study the ability of calcium infusions to suppress parathyroid hormone (PTH) responses to hypocalcaemia, induced by iv infusions of ethylene-glycol-bis(beta-aminoethylether)N,N'-tetraacetate (EGTA). When plasma calcium (Ca) was raised for 60 min to levels higher then 2.6 mmol/l, PTH responses to a subsequent fall of Ca due to EGTA infusions for 7 min were significantly reduced (P < 0.02) or abolished when compared to PTH responses during 7-min EGTA infusions in normocalcaemic animals. CaCl2 infusions lasting for 10 min only, however, did not cause a significant reduction of PTH responses to subsequent 7-min EGTA infusions. The results demonstrate, that raised Ca reduced PTH available for immediate release. This effect was time-dependent, since it was only seen, when CaCl2 was infused for more than 10 min prior to the administration of EGTA.