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During pregnancy, in women and the rat, there is a resetting of the plasma osmolality-arginine vasopressin relationship (P(osmol)/PAVP) such that a decrease in P(osmol) is maintained without suppression of PAVP. This occurs at a time when relaxin is detectable in plasma. The hypothesis tested here was that relaxin could alter the P(osmol)/PAVP in the non-pregnant rat. One group of ovariectomized rats (n = 15) was treated for 7 days with intravenous synthetic human relaxin (10 micrograms/h) in 10 microliters 0.9% (w/v) NaCl. Controls were two groups of rats either with no treatment (n = 15) or treated with vehicle alone (n = 15). One-third of each group received hypertonic saline (0.4 mol NaCl/l, 2 ml/100 g body weight i.p.) on day 7, and one-third were deprived of water for the final 24 h. All rats were killed by decapitation and blood was collected rapidly (< 40 s) for hormone and osmolality assays. The P(osmol) in all relaxin-treated rats was significantly (P < 0.001) lower than that in both control groups, but the PAVP was unchanged. The log PAVP/P(osmol) regression line was significantly shifted in elevation (P < 0.001) but not in slope. Thus treatment of ovariectomized rats with relaxin caused changes in fluid balance which mimic those occurring in normal pregnancy.

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