In vivo bioactivities and kinetic parameters of rat luteinizing hormone components: discrepancy between in vitro and in vivo assays.

Rat pituitary LH shows an electrical charge heterogeneity owing to their carbohydrate moiety. The biological potencies of these isoelectric components increase with increasing pI when examined by in vitro testosterone production in isolated rat Leydig cells. The present study was carried out to clarify if this tendency was also true with of in vivo action and to estimate their kinetic parameters. Seven isoelectric components of LH, i.e. X (pI 7.9), A (8.4), B (8.8), C (9.1), D (9.3), E (9.6) and F (9.8), were administered to cannulated adult male rats. Blood samples were serially collected over a 5 h period from the conscious animal, and rat LH and testosterone were assayed by RIA. The doses of a single component showed a linear relationship with the areas under the curve (AUC) of the blood LH. The testosterone AUC also correlated well with LH AUC. It was found that the components with lower pIs had longer half-lives and larger LH AUC, and smaller total body clearance rates with highly significant correlation coefficients. The in vivo bioactivities of the components expressed as testosterone AUC increased with decreasing pIs, indicating that the in vivo bioactivity was much influenced by the total body clearance rate. The intrinsic activities of the components were calculated as ratios of testosterone AUC to LH AUC where the effect of the clearance rate (or biological half-life) was eliminated. The intrinsic activities of components were not significantly different among the components except component X which showed a smaller activity. These data indicated that, with the exception of component X, an LH component with a lower pI has a longer biological half-life, resulting in a higher in vivo potency, quite contrary to the tendency of in vitro potency.