Effects of alpha- and beta-hCG subunits on the shortloop feedback control of gonadotropins.

We have previously demonstrated that, in the rabbit, both LH and FSH secretions are controlled by shortloop feedback regulatory systems which are highly specific and sensitive. FSH secretion is suppressed by FSH; LH secretion is suppressed by LH and hCG. We have now evaluated the possible role of alpha- and beta-subunits of hCG in the control of LH secretion in rabbits. Experiments were performed in unanesthetized castrated female rabbits bearing chronically implanted silastic catheters to permit repeated blood sampling without anesthesia. Intravenous injections of purified alpha- and beta-chain of hCG were administered in dose levels from 10 to 300 micrograms, and blood samples were taken from -30 to 240 min before and after the bolus injections. Peripheral levels of rabbit FSH and LH were determined by RIA systems, which do not cross-react with intact hCG or its alpha- and beta-subunits. By analysis of variance, no significant suppression of LH and FSH occurred in response to these doses of the subunits. Intact hCG, in doses of 1 microgram, produced a sharp decrease in rabbit LH levels within 10 min. These in vivo studies demonstrate that there is no intrinsic activity of the dissociated purified alpha- and beta-hCG subunits on the shortloop feedback control of gonadotropins in the rabbit.