The Effects of Ovine Prolactin on Water and Electrolyte Excretion in Man Are Attributable to Vasopressin Contamination

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Abstract
Prolactin is an important osmoregulatory hormone in several lower vertebrate species. The present study was undertaken to clarify the effects of prolactin, if any, on human renal function. Eight normal adult male subjects on a 150 mEq sodium (Na), 60 mEq potassium (K) diet for 5 days were studied during 12 h of oral water (H2O) loading on 2 consecutive days. On day 1, after a 6 h control period, a 1 ml normal saline placebo was given im; on day 2, 25 mg of ovine prolactin (OP) was substituted. The subjects were supine and received a constant infusion of Na and K. After OP, serum prolactin rose from 6.9 ± 0.8 ng/ml to 15.0 ± 2.5 ng/ml (P < .01) at 1 h, 27.6 ± 4.0 ng/ml (P <.002) at 2 h, 33.1 ±4.3 ng/ml (P <.001) at 3 h and remained elevated for the remaining 3 h of study. The ovine prolactin had 20– 25% of the potency of human prolactin in the human prolactin radioimmunoassay system. In response to OP, free H2O clearance (CH2o) promptly decreased from 10.1 ± .06 ml/min to 6.1 ± .05 ml/min (P <.01) at 1 h, to a nadir of 5.1 ±.3 ml/min (P <.001) at 2 h, and returned to control levels by 4 h. CH2o was unchanged after placebo, and urinary Na and K excretion, creatinine and osmolar clearance (Cosm), plasma Na, K, osmolality and aldosterone were unchanged after OP or placebo. Control plasma vasopressin was 1.0 ± 0.1 μ U/J/ml and was not changed after prolactin (1.1 ± 0.1 μ U/ml at 1 h, 1.1 ± 0.1 < μU/ml at 2 h and 1.1 ± 0.1 μU/ml at 3 h). The ovine prolactin contained 2 μU of immunoassayable vasopressin per/ng of powder. Aqueous vasopressin, 50 mU (contained in 25 mg of ovine prolactin), produced a decrease in CH2o not significantly different from prolactin in 6 water loaded subjects. Four different subjects given 100 mg of OP had decreased CH2o from 8.3 ±0.3 to 2.7 ± 0.7 ml/min at 1 h (P <.001) and to 2.8 ±0.7 ml/min at 2 h (P <.01). Control plasma osmolality was 301 ± 4 mOsm/1 and decreased to a maximum of 288 ± 5 mOsm/1 4 h after OP (P <.001). After prolactin administration, plasma vasopressin rose from 0.44 ± 0.15 to 0.80 ± 0.41 μU/ml (P = NS) at 1 h. The transient antidiuresis in response to ovine prolactin is due to contamination of the preparation with vasopressin. Prolactin does not acutely influence renal electrolyte excretion and probably does not influence water excretion in man.