Renal electrolyte excretion and renin release during calcium and parathormone infusions in conscious rabbits

Following a random block experimental design in each case, three repeated measurement studies were carried out in three different groups of conscious rabbits, to investigate the renal effects of increasing doses of intravenous calcium chloride (CaCl2) and bovine parathyroid hormone (PTH). In the first study, each rabbit received either CaCl2 (0.15, 0.3, 0.5 or 1.0 mg kg-1 min-1) or vehicle alone (control) for 160 min. In the second study, rabbits were given either PTH (0.15 microgram kg-1 min-1), CaCl2 (1.0 mg kg-1 min-1), PTH plus CaCl2 (0.15 microgram kg-1 min-1 and 1.0 mg kg-1 min-1, respectively) or vehicle alone; PTH was infused for just over 60 min. In the third study, a much smaller dose (0.05 mg kg-1 min-1) of CaCl2 was infused for 100 min. CaCl2 infusion produced a striking fall in fractional excretion of sodium of at least 50% (P less than 0.01), but this was not dose related, being almost maximal at the smaller doses infused. Although this effect was evident in the absence of any changes in total plasma calcium concentration at the lower doses of CaCl2, renal calcium excretion was increased between 2- and 20-fold (P less than 0.01) at all doses infused. Fractional excretion of chloride doubled at the two higher doses of CaCl2 (P less than 0.01), but potassium excretion was unchanged. There were no consistent alterations in mean arterial blood pressure, effective renal plasma flow, glomerular filtration rate or plasma renin activity (PRA); total plasma calcium concentration was consistently elevated only during infusion of the high dose by just under 1 mmol l-1. PTH infusion had no measured effect on fractional excretion of sodium or renal calcium excretion, but doubled fractional potassium excretion (P less than 0.05). Heart rate and PRA increased (P less than 0.01 and less than 0.05, respectively), the latter by 50%, but systemic pressure and renal haemodynamics were not significantly affected. By contrast, PTH infused with CaCl2 produced a 4-fold rise in fractional sodium excretion and although renal calcium excretion remained increased, it was reduced by ca. 80% when compared with renal calcium excretion during infusion of CaCl2 alone. Infusion of PTH alone increased PRA, but when PTH and CaCl2 were infused together, PRA did not change.(ABSTRACT TRUNCATED AT 400 WORDS)