Relationship between hCG Receptors Disappearance and Steroid Accumulation During Long Term hCG Stimulation in Porcine Leydig Cell Cultures

Abstract

LH-hCG receptors and steroidogenesis can be maintained for more than a week in porcine Leydig cell culture (chemically defined medium). Free receptors and steroid accumulation were measured following long time exposure to hCG. As expected, the percent of receptor disappearance increases with the hCG concentration in the incubation medium. However, the steroidogenesis was also proportional to hCG concentration for 24 hours as well as 48 hours. A 50 % receptor disappearance was obtained for 0.5 ng hCG/ml which led to a steroid accumulation of 50 % of the maximal accumulation obtained with 100 ng/ml. A very significative positive correlation (ρ) was observed between receptor occupancy (internalized and/or irreversively occupied receptors) and steroid accumulation (T and DHAS) during a 24 or 48 hour period (ρ = 0.97 and 0.98 for T, and ρ = 0.96 and 0.94 for DHAS). Following 24 or 48 hour exposure to increasing hCG concentration, the cells were acutely stimulated by 25 ng hCG/ml for 4 hours. The steroidogenic response obtained to this acute stimulation was inversely proportional to the concentration of hCG during the previous period. A negative correlation was found between receptor occupancy and acute steroidogenic response (ρ : - 0.96 and - 0.98 for T). These results indicate that during long term stimulation as well as during acute restimulation the absolute number of occupied receptors conditioned the steroidogenic response. These results do not fit into the concept of spare receptors and suggest that in these immature cells all receptors are potentially active in the regulation of steroidogenesis.