INFLUENCE OF PORCINE SOMATOTROPIN ON ENDOCRINE AND HISTOLOGICAL VARIABLES IN GILTS

Thirty-six crossbred gilts (71.7 +/- .9 kg and 140.1 +/- .9 d) were assigned to one of three doses (0, 35, or 70 mug.kg BW-1.d-1) of recombinant porcine somatotropin (rpST). The doses of rpST were adjusted weekly, and i.m. injections continued until d 50. Gilts were pen fed (six gilts/pen) a 17% CP corn-soybean diet (1.2% lysine and 3.2 Mcal of ME/kg). At d 50, feed intake, feed:gain ratios (P = .02), and blood urea nitrogen were decreased ( P < .001) by increasing rpST doses, whereas ADG was increased (P = .04) by increasing rpST doses. Injections of rpST did not affect (P > .05) conception rate, age, or weight at puberty. Numbers of blastocysts or corpora lutea observed at d 10.4 +/- 1.5 of gestation (42.2 d after final rpST injection) were unaffected (P > .05) by rpST treatment. Anterior and posterior pituitary weights were increased (P less-than-or-equal-to .003) linearly with rpST dose. However, liver, adrenal, and heart weights were unaffected ( P > .0 5) by rpST. After a withdrawal period of 42.2 +/- 2.0 d, rpST increased (P less-than-or-equal-to .02) the estimated percentage of lean by 5.8% and longissimus muscle area by 10.4%. Eighteen hours before the initial rpST injection (d 0), 10 gilts per rpST dose were catheterized. Catheterizations were repeated on d 40. The rpST or diluent was given i.m. in the extensor muscle of the neck 1 h after initiation of blood collection. Adrenocorticotropic hormone (1.4 IU/kg BW) was administered through the catheter 3 h after initiation of blood collection. Mean serum concentrations of growth hormone and IGF-I increased linearly (P < .01) with rpST dose on d 0 and 40 of treatment. Mean area under the cortisol curve after both rpST and ACTH was similar (P = .84) among treatment groups on both d 0 and 40. Mean serum concentrations of LH and FSH were similar among treatment groups on both d 0 and 40, but they were higher on d 0 than on d 40 (P = .03 and P = .003 for LH and FSH, respectively). Percentages of acidophils, basophils, and chromophobes in the anterior pituitary gland were similar (P > .15) among treatments 42 d after rpST withdrawal. However, the percentage of somatotrophs increased (P < .001) and the percentage of lactotrophs decreased (P < .001) linearly with increasing rpST. Width of the glomerulosa and fasciculata zones of the adrenal cortex tended (P < .10) to differ among treatments. Overall, positive nutritional advantages for replacement gilts in both growth and carcass traits were found in gilts that received rpST without any detrimental effects on reproduction. In addition, chronic rpST increased pituitary somatotrophs after withdrawal and tended to increase the width of glomerulosa and fasciculata zones in the adrenal cortex without affecting basal or induced release of cortisol.