Responses of porcine corpora lutea to somatotropin administration during pregnancy

The effects of somatotropin (ST) on functions of porcine corpora lutea (CL) during pregnancy were investigated. Twenty-four crossbred (Yorkshire/Landrace) gilts from d 30 to 43 of pregnancy were injected daily with 5 mg of recombinant porcine somatotropin (rpST; n = 12) or 1 mL of saline (control, n = 12). Blood was collected on d 30, 37, and 43 for analyses of plasma progesterone. Gilts were killed on d 44 of pregnancy, and mRNA were isolated from CL, ovary, and liver. Messenger RNA expression for LH receptor, FSH receptor, ST receptor, 3 beta-hydroxysteroid dehydrogenase (3 beta-HSD), and cytochrome P450 side-chain cleavage enzyme (SCC) were measured. Liver, CL, and ovary contained a 4.7-kb mRNA of ST receptor, but the liver contained more mRNA for ST receptor than did CL or ovary (.97 +/- .18, .47 +/- .04, and .25 +/- .04 units, respectively). There were two variants of LH receptor mRNA in CL (6.8 and 4.4 kb). The CL also contained a 1.8-kb mRNA of SCC and a 1.7-kb mRNA of 3 beta-HSD. No FSH receptor mRNA was detected in CL of the pig. The rpST treatment did not affect the mRNA level of ST receptor, 3 beta-HSD, SCC, or 4.4-kb mRNA of the LH receptor. The 6.8-kb mRNA for the LH receptor was decreased (P < .05) by rpST (.56 +/- .04 vs .78 +/- .05 units). Furthermore, concentrations of plasma progesterone decreased (P < .001) in gilts treated with rpST. Decreased luteal function was associated with decreased expression of LH receptor in rpST-treated gilts. The luteotropic effects of ST observed in vitro do not necessarily occur in vivo when gilts are administered rpST during pregnancy.