The purpose of this investigation was to use an approach targeted specifically on endogenous relaxin to determine the influence of relaxin on birth in the rat. To that end, a monoclonal antibody specific for rat relaxin, designated MCA1, was used to passively neutralize endogenous relaxin in intact pregnant rats. MCAl was administered iv to intact rats daily from days 12-22 of pregnancy. Animals were observed for birth continuously from 2100 h on day 22 until 1200 h on day 24. MCAl-treated rats exhibited significantly prolonged durations of straining and litter delivery as well as reduced incidence of live pups compared with controls. Whereas approximately 20-25% of fetuses and placentae were retained in utero at 1200 h on day 24 in MCA1- treated rats, neither fetuses nor placentae were retained in control rats. Passive immunization with MCAl throughout the second half of pregnancy had no apparent influence on normal ovarian function. The time of occurrence of the antepartum decline in serum progesterone levels (functional luteolysis) as well as the time interval between the attainment of basal progesterone levels and the onset of litter delivery in MCAl-treated and control rats that delivered on day 23 were in close agreement with previous reports. In conclusion, the prolonged durations of straining and litter delivery, reduced incidence of live pups, and increased incidence of retained fetuses and placentae after passive immunization with MCAl establish the physiological need for endogenous relaxin to attain normal delivery of the young in the rat.
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