EVIDENCE THAT SYSTEMIC RELAXIN PROMOTES MODERATE WATER-CONSUMPTION DURING LATE PREGNANCY IN RATS

The protein hormone relaxin is secreted by the ovaries throughout the second half of the 23 day pregnancy in the rat. We recently reported t hat neutralization of endogenous relaxin with monoclonal antibodies fo r rat relaxin decreases water consumption during the daily Light perio d during the second half of pregnancy in rats. The apparent effects of relaxin on water consumption, however, were extremely modest. One exp lanation for the failure to observe a greater relaxin-dependent effect on water consumption is failure of the monoclonal antibody for rat re laxin to neutralize all circulating relaxin. A second explanation is t hat circulating relaxin has only slight effects on water consumption. This investigation was conducted with an experimental model in which c irculating relaxin was removed in order to re-examine the effects of r elaxin on water consumption during the daily light period in late preg nancy in rats. On day 9 (D9) of pregnancy, before the presence of rela xin (R) in the circulation, primiparous pregnant rats were ovariectomi zed (O) or sham ovariectomized (C). Throughout the remainder of pregna ncy, rats were treated with combinations of either progesterone (P) an d estrogen (E, group OPE) or progesterone, estrogen and porcine relaxi n (group OPER) in doses that restore physiological parameters to value s similar to those that occur during the second half of pregnancy in i ntact rats. Progesterone and estrogen were administered by Silastic tu bing implants and porcine relaxin was administered via miniature osmot ic pump. Sham-ovariectomized animals received either the hormone vehic les (group SC) or no implants (group IC). Water consumption was measur ed daily from D4 to D20 at both 0700 and 2100 h which was when the lig hts went on and off respectively.Water consumption increased as pregna ncy continued from D10 to D20 during the daily 10 h dark periods (P<0. 01), but not during the 14 h light periods for all four groups. Daily water consumed by rats in group OPE was significantly lower (P<0.05) t han that consumed by sham-ovariectomized rats from D17 to D20 and lowe r than that consumed by rats in group OPER on D20. During the dark per iod there was no difference in water consumption among groups. During the light period, however, group OPE consumed significantly less water (P<0.05) than group C from D18 to D22. Moreover, there was a consiste nt tendency (P<0.13) for the water consumption to be greater in rats i n group OPER than in those in the relaxin-deficient group OPE during t he daily light period from D11 to D20 of pregnancy. We conclude that t he increase in water consumption that occurs during the daily dark per iods during the second half of pregnancy is not attributable to circul ating relaxin. Circulating relaxin promotes only modest increases in w ater consumption during the daily light periods during late pregnancy in the rat.