In-vitro and in-vivo responsiveness of the corpus luteum of the mare to gonadotrophin stimulation

Dispersed horse luteal cells were used to evaluate the ability of horse LH, hCG and PMSG to stimulate progesterone secretion in vitro. Morphological characterization of these cells before gonadotrophin stimulation indicated the presence of two populations of cells based on cell diameters. In luteal cells incubated as suspended cells, horse LH and hCG stimulated (P less than or equal to 0.05) progesterone production at all levels of treatment. Stimulation of progesterone secretion by hCG was greater (P less than or equal to 0.05) than by horse LH over the range of concentrations utilized. When mares (N = 7) received an intramuscular injection of 1000 i.u. hCG on Days 3, 4 and 5 after the end of oestrus, there was an increase (P less than or equal to 0.05), in peripheral progesterone concentrations beginning on Day 7 and continuing until Day 14 compared with controls (N = 7). Peripheral progesterone concentrations continued to be elevated in hCG-treated mares for Days 15-30 after oestrus in those mares that conceived. Although treatment with hCG increased progesterone concentrations, it had no influence on anterior pituitary release of LH as measured by frequency and amplitude of LH discharge. We conclude that the mare corpus luteum is responsive to gonadotrophins in vitro and that exogenous hCG can enhance serum progesterone concentrations throughout the oestrous cycle and early pregnancy.