Mechanism of Action of Inhibin

Cultured anterior pituitary cells have been used to study the mechanism of action of ovine testicular inhibin proteins. Exposure of cells to inhibin for 48 or 72 h produced dose-dependent inhibition of FSH but not LH content, whereas shorter incubation times with inhibin did not reduce FSH. These changes were the result of inhibition of FSH production by the cell cultures. Despite the time delay before content was suppressed, FSH- and LH-stimulated release (10 nM LHRH) and FSH spontaneous release (no LHRH) were significantly reduced within 6 h of incubation with inhibin. This effect on release was not observed with a more active semipurified inhibin fraction derived from rete testis fluid. The results suggest that 1) the action of inhibin is specifically on FSH synthesis and does not influence LH production; and 2) crude inhibin-containing proteins of testicular origin may also cause inhibition of FSH release both in the presence and absence of LHRH, an effect independent of the primary action of inhibin on FSH production. The practical implications of these observations are that change of FSH content most accurately reflects the biological action of inhibin, and studies attempting the detection and characterization of inhibin should measure this in preference to release.