Sex steroids steroid receptors and body composition of lambs : A thesis submitted in partial fulfilment of the requirements for the degree of Master of Science in the University of Canterbury

Abstract

Thirty Coopworth x Suffolk lambs were used to study the effects of changing the endogenous concentrations of testosterone and oestradiol on the growth rates and body composition of lambs. To manipulate the endogenous testosterone and oestradiol concentrations , seven male and eight female lambs were randomly selected and gonadectomized at two weeks of age. All the Jambs were slaughtered on reaching liveweights of 40 kg. Muscle and adipose oestrogen and androgen receptors were also characterized to determine whether there was a relationship between the binding characteristics of the receptors and body composition of the lambs since oestrogens and androgens are thought to influence protein accretion and fat deposition through their receptors. There were differences in the growth rates and body composition of the male and female lambs . The female lambs grew faster (P<0.001) and had a greater proportion of carcass and whole body fat (P<0.05) than the male lambs. The female lambs also had less carcass and body water than the male lambs (P<0.01). Removal of the gonads influenced the rates of bodyweight gain, protein deposition and ash content. Gonadectomy in the males tended to decrease fat deposition in the carcass although the results obtained were not significant (P<0.10). Following gonadectomy. the males demonstrated a retardation in growth rate whilst the males and females showed significant reductions in carcass protein (P<0.01), total body protein (P<0.05) and carcass ash (P<0.05). Characterization of the steroid receptors showed that there was one population of oestradiol receptors and two populations of androgen receptors in the gastrocnemius muscles of all the lambs. Gonadectomy increased the concentration (P<0.05) and binding affinity (P<0.05) of the high affinity androgen receptor in the males , and decreased the androgen receptor concentration in the females (P < 0. 00 l). Gonadectomy increased the amount of oestradiol receptors in the male lambs (P < 0.05) but did not affect the binding capacity or the binding affinity of the oestradiol receptors in female lambs . These results suggest that oestrogen and testosterone act directly through their receptors on skeletal muscle. The observed low-affinity receptors are probably due to excess hormone binding to non-specific receptor sites . No proge tin receptors were detected in the muscle tissue of the lambs. Similarly , no oestrogen , androgen or progestin receptor sites were detected in adipose tissue using the assay procedures described in the text. Castration lowered the plasma concentrations of testosterone in the males to the extent that the concentrations were similar to those in the females. Plasma progesterone concentrations were not altered by gonaclectomy in the male or female lambs . Oestradiol concentrations were not measured because of the lack of a reliable method. Neither receptor binding characteristics nor plasma sex steroid hormone levels showed significant correlations to the body composition of the lambs. A strong correlation existed, however. between plasma testosterone concentrations and androgen receptor binding capacity in the rams . Apart from this. plasma sex hormone levels were not related to the binding characteristics of their specific receptors.