REGULATION OF BLOOD LEVELS OF LH IN BULLS: INFLUENCE OF AGE, BREED, SEXUAL STIMULATION AND TEMPORAL FLUCTUATIONS

Abstract

Summary. Histological studies of adenohypophyses of male Friesian and Ayrshire calves, aged 1 to 2 months, and measurements of plasma or serum LH by solid phase radioimmunoassay in Angus, Holstein, Guernsey and Jersey bulls, ranging in age from 7 months to 2 years were carried out to determine factors influencing gonadotrophin secretion. The gonadotrophin-secreting delta cells constituted 3% of the cell population of the cortical region of the adenohypophyses at 1 month of age and 4% at 2 months of age. Considerable quantities of LH (3·7±0·3 ng/ml) were present in plasma samples of Angus bulls at 7 to 9 months of age. The levels rose to 4·3±1·1 ng/ml at 12 to 14 months and, at 2 or more years of age, basal LH values were of the order of 7 to 10 ng/ml plasma or serum. No breed differences were noted either in pituitary histology or LH concentrations. Teasing and ejaculations caused no increase in serum LH values for up to 3 hr after sexual excitement, irrespective of whether a bull or an oestrous cow was the teaser. Large irregularly occurring daily fluctuations in serum LH (0 to 50 ng/ml) were found in 6-year-old Holstein bulls. Similar but smaller variations (1 to 16 ng/ml plasma) were found in the younger Angus bulls aged 7 to 14 months. A distinct 24-hr variation in serum LH was found in two Holstein and two Guernsey bulls aged 1½ to 2 years. The diurnal serum LH values were twice the nocturnal values, an abrupt change-over occurring at 18.00 hours when most of the bulls were lying down. Hourly variations were small and insignificant during the morning and afternoon.