SERUM CONCENTRATIONS OF IMMUNOACTIVE AND BIOACTIVE LUTEINIZING-HORMONE (LH) IN GILTS .1. BIOPOTENCY OF LH INCREASES DURING PREPUBERTAL DEVELOPMENT

The patterns of secretion of bioactive and immunoactive LH during prep ubertal development were examined in four groups of 12 gilts each at a pproximately 3, 4, 5, and 6 mo of age. The rat interstitial cell testo sterone 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 jugula r catheters every 20 min for a 6-h period of spontaneous secretion. Gn RH (0.5 mug/kg BW) was injected i.v. and blood samples were taken for an additional 4 h at 20-min intervals. Mean serum and baseline concent rations of ILH and of BLH were greater (p < 0.05) in 3-mo-old gilts th an in gilts of other ages. Three-month-old gilts also secreted more IL H 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 gil ts may be inherently necessary to give follicles the maximum opportuni ty to respond to LH. During the period of prepubertal development, est imates of ILH accurately reflected BLH concentrations except when LH r elease was stimulated by exogenous GnRH. The age-related increase obse rved 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.