Calcium potentiates the cyclic nucleotide and phosphaturic response to parathyroid hormone infusion

To determine whether calcium modulates the action of PTH, we measured the cyclic nucleotide and phosphaturic response to PTH following a 4-h infusion of glucose (day 1) and calcium (day 2). The 12 subjects were selected to provide a range of low, normal, and high endogenous PTH function. PTH stimulated nephrogenous cAMP [185 +/- 31 nmol/100 ml glomerular filtrate (GF)], cyclic guanosine monophosphate (0.44 +/- 0.09 mumol/g creatinine), and phosphate (367 +/- 59 mg P/g creatinine) excretion. Calcium infusion stimulated nephrogenous cAMP excretion in the hypoparathyroid subjects (1.42 +/- 0.35 nmol/100 ml GF) but reduced it in subjects with normal parathyroid function (-2.22 +/- 0.46 nmol/100 ml GF). Calcium infusion stimulated cGMP (0.64 +/- 0.1 mumol/g creatinine) and phosphate (113 +/- 48 P/g creatinine) excretion in all subject groups. Calcium infusion led to a 2-fold increase in the cyclic nucleotide and phosphaturic response to PTH in the normal and hypoparathyroid subjects, but had little effect on the PTH response in hyperparathyroid subjects. The extent to which calcium potentiated the ability of PTH to stimulate nephrogenous cAMP excretion correlated negatively with the basal nephrogenous cAMP excretion (r = -0.685, P less than 0.01). We conclude that calcium potentiates the acute effects of PTH on renal cyclic nucleotide and phosphate excretion. This effect is modified by the basal levels of PTH stimulation of the kidney such that it is reduced in magnitude when basal PTH stimulation is increased.