Serum concentrations of immunoactive and bioactive luteinizing hormone (LH) in gilts. I. Biopotency of LH increases during prepubertal development.

The patterns of secretion of bioactive and immunoactive LH during prepubertal development were examined in four groups of 12 gilts each at approximately 3, 4, 5, and 6 mo of age. The rat interstitial cell testosterone assay was validated for the determination of bioactive LH (BLH) in porcine serum. Serum concentrations of immunoactive LH (ILH) were determined by RIA. Blood samples were collected via indwelling jugular catheters every 20 min for a 6-h period of spontaneous secretion. GnRH (0.5 microgram/kg BW) was injected i.v. and blood samples were taken for an additional 4 h at 20-min intervals. Mean serum and baseline concentrations of ILH and of BLH were greater (p < 0.05) in 3-mo-old gilts than in gilts of other ages. Three-month-old gilts also secreted more ILH and more BLH in response to GnRH than 6-mo-old gilts (p < 0.05), but the biopotency of GnRH-induced LH was reduced (p < 0.05) in 3-mo-old gilts. Elevated serum concentrations of ILH and of BLH in 3-mo-old gilts may be inherently necessary to give follicles the maximum opportunity to respond to LH. During the period of prepubertal development, estimates of ILH accurately reflected BLH concentrations except when LH release was stimulated by exogenous GnRH. The age-related increase observed in the biopotency of LH released in response to GnRH may reflect a maturational process whereby the biopotency of LH increases as gilts approach puberty.