ACUTE EFFECT OF PARATHYROID-HORMONE ON URINE CONCENTRATION IN THE RAT

1. It has been demonstrated that parathyroid hormone can increase aden ylate cyclase activity in the rat papilla, produce a small antidiureti c effect and in vitro can interfere with the action of arginine vasopr essin on water transport, Clearance studies were performed in the anae sthetized water diuretic thyroparathyroidectomized rat to evaluate fur ther the effect of parathyroid hormone on urine concentration in tbe p resence and absence of arginine vasopressin. 2. A maximal phosphaturic concentration of rat parathyroid hormone (2 mu g/kg) reduced urine fl ow from 125+/-7 to 81+/-9 mu l/min within 10 min (P<0.01). Addition of a maximal antidiuretic concentration of arginine vasopressin (100 ng/ kg) produced a delayed and diminished antidiuretic response when compa red with a group of rats not pretreated with parathyroid hormone (47+/ -5 compared with 27+/-5 mu l/min; P<0.01), However, a supramaximal arg inine vasopressin concentration (1000 ng/kg) produced a maximal antidi uretic effect in the presence of parathyroid hormone. 3. To evaluate f urther the inhibitory effect of parathyroid hormone on arginine vasopr essin-induced antidiuresis, parathyroid hormone (2 mu g/kg) was admini stered to one group of rats and a minimally effective arginine vasopre ssin concentration (7.5 ng/kg) to another group, which produced a simi lar antidiuretic effect, However, the subsequent effect of a maximal a ntidiuretic arginine vasopressin concentration (100 ng/kg) was again s ignificantly blunted in the group pretreated with parathyroid hormone. 4. Parathyroid hormone produced only a small increase in mean plasma calcium concentration, and glomerular filtration rate was not altered by either hormone. 5. These results demonstrate that high physiologica l concentrations of parathyroid hormone do have a significant antidiur etic effect and can interfere with the action of arginine vasopressin, This suggests that parathyroid hormone may act as a partial agonist t o arginine vasopressin in the collecting system.