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Relaxin alters the plasma osmolality-arginine vasopressin relationship in the rat
R S Weisinger1, P Burns, L W Eddie
1Howard Florey Institute of Experimental Physiology and Medicine, University of Melbourne, Parkville, Victoria, Australia.
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.
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.