Perinatal ontogeny of porcine growth hormone receptor gene expression is modulated by thyroid status

The ontogeny of growth hormone receptors (GHR) represents a critical stage in growth and metabolism. We have investigated the perinatal ontogeny of hepatic and skeletal muscle GHR gene expression in piglets, and its modulation by GH and thyroid hormones. Test piglets were rendered hypothyroid in late gestation by feeding the sow a high-glucosinolate rapeseed meal. Plasma and tissue samples were obtained from test and control piglets at various ages between 80 days of fetal life (80F) and 2 days postnatally. Plasma hormone levels were determined by radioimmunoassay and GHR mRNA by RNase protection assays. In controls, plasma thyroxine (T4) and 3,5,3'-triiodothyronine (T3) levels increased between 80F and birth and the early postnatal period was characterized by a marked surge in plasma T3. Test piglets were hypothyroid at 110F with total T4, total T3 and free T3 levels being reduced by 28, 53 and 33% respectively. By contrast, the postnatal increase in T3 was more marked in test than in control animals. Plasma GH levels decreased over the perinatal period and there was no effect of treatment. Hepatic GHR mRNA was at the lower limit of detection at 80F but by 110F was expressed in both groups of animals. However, fetal hypothyroidism at 110F resulted in a marked 70% decrease in hepatic GHR mRNA (p < 0.01). The higher postnatal rise in T3 in test piglets was accompanied by a recovery of hepatic GHR mRNA levels. By contrast with liver, skeletal muscle (longissimus dorsi) expressed high levels of GHR mRNA at 80F and hypothyroidism induced a 68% increase in GHR mRNA (p < 0.001). The present results suggest that thyroid hormones may modulate the perinatal ontogeny of GHR gene expression, in addition to other hormonal factors, and that this modulation is tissue-specific.