LH AND PROGESTERONE CONCENTRATIONS DURING DIESTRUS IN THE MARE AND THE EFFECT OF HCG

The temporal association between circulating concentrations of LH and progesterone was studied by collecting blood samples from 4 mares at 1 5-min intervals for 12h on Days 3, 8 and 13 of diestrus. Luteinizing h ormone appeared to be secreted in an episodic fashion on Days 8 and 13 , but on Day 3 of the cycle discrete pulses of LH were detected in onl y 1 mare. Episodic progesterone secretion was detected on all days of sampling. No correlation was observed between LH and progesterone conc entrations. Intravenous administration of 1,500 IU hCG to mares result ed in significantly elevated circulating concentrations of progesteron e in 3 of the 4 mares. These results suggest that exogenous LH can be luteotrophic for the mare corpus luteum but that there was little temp oral relationship between endogenous circulating LH and progesterone i n peripheral blood.