Luteinizing hormone secretion and corpus luteum function in cows receiving two levels of progesterone

Abstract

The objectives of this experiment were to determine if subnormal levels of progesterone (P4) indicative of luteal insufficiency influence (1) pulsatile release of luteinizing hormone (LH), (2) the interval to the preovulatory surge of LH after removal of P4, and (3) the secretion of P4 during the estrous cycle subsequent to administration of subnormal levels of P4. On Day 5 (Day = 0 day of estrus) of the estrous cycle, cows received P4-releasing intravaginal devices (PRID) to produce normal (2 PRIDs; n = 7) or subnormal (0.5 PRID; n = 6) concentrations of P4. Five cows served as controls. On Day 10, serial blood samples were collected from all cows. Collection of blood samples was again initiated on Day 17 in cows receiving PRIDs. The PRIDs were removed and blood collection continued for 78 h. Daily blood samples were collected from all animals for 42 days subsequent to estrus (estrous cycles 1 and 2, respectively). During estrous cycle 1, mean concentration of P4 was lower (p < 0.05) and frequency of pulses of LH was higher (p < 0.05) in cows receiving subnormal P4 than in cows receiving normal P4 and control cows. Plasma concentrations of estradiol (E2) were higher (p < 0.05) on Days 9-16 of estrous cycle 1 in cows receiving subnormal P4 than in cows receiving normal P4 or in control cows. Concentrations of E2 were greater (p < 0.05) at 6, 18, and 30 h following removal of PRIDs in cows receiving subnormal P4 than in cows receiving normal P4. Onset of the preovulatory surge of LH occurred 17.1 ± 2 h earlier (p < 0.05) in cows receiving subnormal P4 than in cows receiving normal P4. During estrous cycle 2, mean P4 and duration of the luteal phase were not different (p > 0.05) between cows in the three treatment groups. We conclude that consequences of luteal phase insufficiency on fertility may involve altered ovarian folliculogenesis modulated by associated alterations in secretion of LH.