Temporal fluctuations of plasma LH and testosterone in Charolais bull calves during the first year of life
Reproduction nutrition development (Print)
Plasma LH and testosterone have been measured by radioimmunoassay in samples collected weekly from 10 Charolais male calves from birth till one year of age. Plasma LH increased from the lst to the 5th week of age, then fluctuated widely from the 5th to the 20th week (3-5 ng/ml) and finally decreased to about 2 ng/ml and remained steady up to one year of age. Plasma testosterone increased very slowly from the Ist to the 20th week of age and then increased more rapidly with large fluctuations up
... ge fluctuations up to one year. Thus an intense LH release between the Ist and the 5th month of life was not followed by a clear testosterone release suggesting that testicular sensitivity to LH was not yet established. Conversely, after the sixth month of age discrete LH variations were able to induce large releases of testosterone into the blood. It is suggested that high pituitary activity during the first month of life is a signal for puberty. Despite the number of recent studies devoted to plasma LH and testosterone in calves, a comprehensive view concerning the relationship between these hormones during the first year of age is still lacking. Thus plasma LH is said not to vary between the third and twelfth months of age (Odell, Hescox and Kiddy, 1970 ; Karg et a/., 1976) to increase between the first and seventh months (Mori et al., 1974) or to increase from seven months of age to adulthood (Gombe et al.,1973) . Similarly although plasma testosterone was found to increase with age, this increase was said to be erratic (Rawlings, Hafs and Swanson, 1972) or multiphasic (Secchiari et al., 1976) . Some of these discrepancies could be due in part to a pulsatile pattern of relaese which has been described for LH both in adult (Katongole, Naftolin and Short, 1971) and young bulls (Gombe et al., 1973 ; Thibier, 1975) . They could arise also from insufficient blood samplings or number of animals studied. This confusing situation in the bull contrasts with our knowledge in the sheep in which the prepubertal period is associated with a pulsatile pattern of LH followed by a decrease in the level after puberty, whereas testosterone increases linearly (Foster, 1974 ; Courot, de Reviers and Pelletier, 1975) . Thus, in an attempt to clarify the endocrine changes around puberty the concurrent patterns of plasma LH and testosterone were followed in Charolais calves from birth till one year of age.