Differences in immunological and biological activity of serum luteinizing hormone during the bovine estrous cycle

Abstract

Although immunological luteinizing hormone (LH) profiles have been extensively characterized in several species, an awareness has developed that these immunological LH measurements may not adequately describe the biological activity or stimulation of the target tissue. The objective of this study was to compare serum LH at different stages of the bovine estrous cycle as measured by both bioassay and radioimmunoassay. Blood samples were collected from 5 cows at 10 min intervals for 10 hr on days 3, 10 and 19 of the estrous cycle; for 24 hr at estrus on day 0 (natural preovulatory LH surge, NS); and for 8 hr following administration of luteinizing hormone-releasing hormone (induced LH surge) on day 10 of that cycle. Immunological activity of LH was determined by a previously validated radioimmunoassay. A sensitive and specific in vitro mouse Leydig cell bioassay in which testosterone production was an index of added LH was used to assess biological activity of LH. The same preparation (NIAMDD-bLH4) served as the standard for each assay system. Patterns of biological active LH closely followed those of immunological LH, although biological LH values tended to be lower. Although the pulse frequency (pulses/hr) of immunological LH was lower (P<.05) on day 10 ( .48 ± .07; x ± SE ) than day 3 or 19 (.88 ± .09 and .88 ± .15, respectively), no changes in biological LH pulse frequency were noted (P>.05). The mean biological to immunological (B:I) ratios were greater (P<.05) on days 3 and 10 (0.91 ± .16 and 0.93 ± .12, respectively) than day 19 (0.64 ± .05) and day 0 (0.43 ± .07). The B:I ratios of the natural LH surge and induced LH surge were not different from each other (P>.05). These observations demonstrate that biological LH exhibits a pattern similar to, but not identical to, immunological LH. Furthermore, as evidenced by changing B:I ratios, the pituitary releases LH which is not of the same biological activity at all stages of the estrous cycle. These data suggest that both quality and quantity of LH released may be important in regulating LH action at the target tissue.