Study of the renal tubular interactions of thyrocalcitonin, cyclic adenosine 3',5'-monophosphate, 25-hydroxycholecalciferol, and calcium ion.

Acute clearance studies were performed in thyroparathyroidectomized animals to determine the actions and interactions of thyrocalcitonin (TCT), cyclic adenosine 3'5'-monophosphate (cAMP), 25-hydroxycholecalciferol (25HCC), and calcium ion on the reabsorption of phosphate, calcium, sodium, and potassium by the kidney. The infusion of 25HCC in a dosage of 60 U/h to moderately saline-expanded animals (2.5% body weight) induced a fall in the excretion of all of the ions under study after 90-120 min similar to that observed in previous experiments from this laboratory. Mean decrements in fractional excretion were: phosphate, 42.0% (P < 0.005); calcium, 25.0% (P < 0.005); sodium, 23.4% (P < 0.001); and potassium, 14.7% (P < 0.005). The superimposition of either porcine or salmon TCT (1-100 MRC U/h for 2 h) resulted in no further alterations in electrolyte excretion. However, the infusion of TCT during steady-state saline expansion, before the administration of 25HCC, obviated the renal transport effects of the vitamin D metabolite. Both in the latter studies, as well as those in which similar doses of TCT were given to hydropenic animals, the hormone itself failed to induce any consistent alteration in electrolyte excretion. Cyclic AMP (50 mg/h) caused an increase in the excretion of phosphate, sodium, and potassium and no change in calcium excretion. Like TCT, the nucleotide blocked the action of 25HCC on the kidney. Raising the mean level of serum ultrafilterable calcium to 3.02+/-0.25 mEq/liter from 1.62+/-0.17 mEq/liter likewise prevented enhanced ionic reabsorption due to 25HCC.