SERUM GONADOTROPIN ISOFORMS BECOME MORE BASIC AFTER AN EXOGENOUS CHALLENGE OF GONADOTROPIN-RELEASING-HORMONE IN CHILDREN UNDERGOING PUBERTAL DEVELOPMENT

Recent clinical studies have questioned whether there is a qualitative change in the circulating isoforms of LH and FSH after stimulation by GnRH. The present study investigated the median charge of serum gonad otropin isoforms before and after an exogenous challenge of 100 mu g G nRH in 10 girls and 10 boys undergoing pubertal development. All of th e children had basal serum levels of LH and FSH and responses 30 and 6 0 min after GnRH treatment that were considered normal for puberty. LH and FSH in serum and eluates after electrophoresis in 0.10% agarose s uspension were measured with sandwich fluoroimmunoassays. The increase s in serum gonadotropin concentrations were generally similar for both sexes, but girls had significantly (P < 0.05) higher LH levels at 30 and 60 min than boys and a larger (P < 0.05) relative increase in seru m FSH levels after GnRH treatment. In terms of the median charge of se rum isoforms, the girls had significantly less negative (i.e. more bas ic) isoforms of LH (P < 0.05) and FSH (P < 0.001) than the boys. There was a change to more basic isoforms of both LH and FSH in all childre n 30 min after GnRH administration. For LH, the charge had returned to pretreatment values by 90 min after GnRH, but for FSH, the charge was still significantly (P < 0.05) more basic at this time (n = 4/sex). W hen the LH isoforms were more acidic before GnRH treatment, the change in median charge was larger than when the isoforms were more basic be forehand. A similar relationship was not found for FSH. Conversely, th ere was for FSH, but not for LH, a significant (P < 0.001) relationshi p between the relative increase in serum concentrations and the change in charge of the isoforms 60 min after GnRH treatment. These findings show that the circulating forms of LH and FSH become more basic after an exogenous challenge of GnRH in children undergoing pubertal develo pment and suggest that the differences in the responses of LH and FSH isoforms may be due to differing degrees of selective secretion and/or survival.