Seasonal profiles of plasma luteinizing hormone, testosterone and estradiol in the ram

Plasma profiles of luteinizing hormone (LH), testosterone (T), and estradiol (E2) have been determined in five mature rams during the primary breeding season (September) and again when breeding activity was low (May). Blood samples collected every half h for 24 h during September or May showed that LH was released into the general circulation as distinct peaks. Although this secretory pattern was not the same for all rams and seasonal differences were noted in certain individuals, it is suggested that an inherent rhythm persists within each ram. With the exception of one ram in September, obvious T peaks were preceded by LH peaks greater than 2 ng/ml (LH-T interval less than 60 min). Season had no effect on the number and magnitude of LH peaks or on the mean concentrations; however, basal levels of LH were higher (P less than 0.01) in September than in May. Seasonal differences in peripheral T levels were more dramatic with mean, baseline, and peak concentrations elevated (p less than 0.01) during the fall. The number of T peaks, like those of LH, was not affected by season. Large variations in circulating levels of E2 made any possible relationship between this hormone and LH or T difficult to identify. Furthermore, a diurnal or seasonal rhythm could not be established for E2 in this study. It is concluded that seasonal differences occur in secretory patterns of LH and T and that a cause and effect relationship between these two hormones exists. Under conditions of this study, neither a consistent relationship between E2 and LH or T, nor a seasonal difference in mean E2 levels, was apparent.