Acute decrease in progesterone and increase in estrogen secretion caused by relaxin during late pregnancy in beef heifers.

Purified porcine relaxin (3000 U/mg) was administered im (RLX-IM; 1 mg; n = 2) and in the cervical os (RLX-OS; 1 mg; n = 2) on day 273 (approximately 10 days before parturition normally occurs) of gestation to determine the profiles of immunoreactive relaxin and its effects on progesterone, estrone (E1), and 17 beta-estradiol (17 beta-E2) secretion in peripheral blood plasma of beef heifers. Controls received either 0.01 M PBS (1 ml, im; n = 2) or 0.01 M gel-PBS (gel; 1 ml, os; n = 2) in cervical os. One relaxin-treated (im) heifer calved at 4 h and 36 min after treatment; thus, data from this heifer were not included in subsequent analysis. Relaxin-treated heifers showed an acute elevation in relaxin, a precipitous decrease in progesterone, and a significant (P less than 0.05) elevation of E1 and 17 beta-E2. Plasma relaxin levels were 4.95, 1.5, and 0.24 ng/ml at 0.5 h in RLX-IM, RLX-OS, and control animals, respectively. Peripheral plasma relaxin peaked between 23-31 ng/ml 1-2.5 h before returning to less than 0.5 ng/ml 5-12 h after treatment. Relaxin administration accounted for 70%, 73%, and 58% of the progesterone, E1, and 17 beta-E2 variability between treatments, respectively. An abrupt decrease (P less than 0.01) in progesterone preceded the rises (P less than 0.05) in E1 and 17 beta-E2 at 1.5, 2-2.5, and 2-3.5 h, respectively. Maximum progesterone deviations from the pretreatment mean concentration were -5.43, -3.05, and -0.92 ng/ml for RLX-IM, RLX-OS, and controls. Progesterone rebounded from 36% to 61% and 62% to 79% of respective pretreatment means for RLX-IM and RLX-OS. Peak elevation of E1 was 407.3, 306.5, and 71.5 pg/ml and that of 17 beta-E2 was 82.2, 35.8, and 7.8 ng/ml for RLX-IM, RLX-OS, and controls, respectively. These results provide strong evidence that a pharmacological dosage of relaxin induces an acute depression of progesterone secretion beginning within 90 min in beef heifers during late pregnancy. We suggest that these early and marked luteolytic effects of relaxin on progesterone secretion in cattle could be by direct or indirect actions via mechanisms that are yet unknown.