The influence of anabolic steroids on growth hormone (GH) secretion in ruminants remains unclear, perhaps because of the dynamic nature of GH secretion. In the present study, blood samples were obtained at 15-min intervals for 12 h from intact ram lambs, ram lambs castrated postpubertally, and castrated ram lambs treated with either testosterone propionate (TP) or diethylstilbestrol (DES). Intact rams exhibited GH secretory episodes of greater (P less than 0.01) amplitude than did castrated lambs. Similarly, mean base-line and mean overall GH concentrations were higher (P less than 0.01) in rams than in castrates. Treatment of castrates with either TP or DES resulted in increased base-line (P less than 0.05) and overall (P less than 0.05) GH concentrations compared to untreated castrates. Although amplitude of GH spikes was larger in TP and DES groups, this difference was not statistically significant (P greater than 0.05). Because numerous studies have demonstrated that intact rams and castrates treated with TP or DES grow faster than untreated castrates, the data in the present study support the hypothesis that the anabolic action of androgens and estrogens is due, at least in part, to their influence on GH secretion.

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An experiment was conducted to test the hypothesis that bull calves of a larger, faster growing breed of cattle exhibit greater secretory activity of anabolic pituitary hormones than bull calves of a smaller, slower growing breed. Calves of both breeds were raised under similar conditions and maintained on the same diet for approximately 4 weeks before the blood sampling period. Simmental bull calves had higher (P less than .01) average daily gains and a heavier (P less than .01) mean body weight than Hereford bull calves. Overall plasma growth hormone (GH) concentrations were higher (P less than .01) in Simmental bull calves than in Hereford bull calves. Mean baseline GH was also higher (P less than .05) in Simmentals. Overall and baseline concentrations of prolactin (PRL) were similar for both breeds. Mean amplitude of PRL secretory spikes was higher (P less than .05) for Simmental than for Hereford bull calves. There was no breed effect on any variable of thyrotropin secretion. These data suggest that representatives of a larger, faster growing breed of cattle (Simmental) exhibit greater secretory activity of GH and PRL than do representatives of a smaller, slower growing breed (Hereford).

The ontogenetic changes that occur in secretory patterns of growth hormone (GH), prolactin (Prl), luteinizing hormone (LH), and testosterone (T) in rams maintained in constant photoperiod were examined. Nine ram lambs were moved to individual pens in a controlled environment (12L: 12D cycle; 18-24 degrees C temperature) at 66 days of age. Blood samples were collected via indwelling cannulae at 15-min intervals for an 8-h period at 80, 136, 192, 248, and 304 days of age. Plasma concentrations of GH, Prl, LH, and T were quantitated and parameters of the secretory patterns determined. Mean concentration of GH tended to decline with age, probably because the amplitude of secretory peaks was significantly reduced with age. There were no age-associated changes in basal concentration of GH or incidence of GH peaks. There was an increase in Prl secretion (as estimated by mean concentration) at 136 and after 248 days of age. Significant age-associated changes occurred in all parameters of LH and T secretion. At the younger ages, testosterone concentrations were low and LH concentrations were elevated. At the older ages the relationship was reversed, with LH low and testosterone high. There were no significant correlations between frequency and magnitude of LH and T peaks. The significant correlations present among parameters of LH and T secretion were between basal concentration of LH and overall mean concentration and basal concentration of T. These results suggest that LH may not be the sole tropic stimulator of acute T secretion.