Different role of insulin in GLUT-1 and-4 regulation in heart and skeletalmuscle during perinatal hypothyroidism

Two groups of hypothyroid rats were used; one group was given 2-mercapto-1- methylimidazole (MMI) treatment in the drinking water of the mothers and wa s killed at 2 and 4 days of life, and the other group was given similar MMI treatment and then was thyroidectomized at 5 days of life and killed at 8 or 20 days. Serum insulin, growth hormone (GH), and insulin-like growth fac tor I (IGF-I) were decreased in MMI-treated rats but increased in MMI-treat ed plus thyroidectomized rats. No significant reduction of thyroid hormones was observed in 2-day-old MMI rats. Protein and mRNA expression of GLUT-1 increased, and those of GLUT-4 decreased, in the heart in all populations i ndependent of changes in insulin, GH, and IGF-I levels. However, GLUT-4 pro tein and mRNA expression in quadriceps and gastrocnemius skeletal muscles d ecreased at 4 days and increased at 8 and 20 days of life in parallel with insulin, GH, and IGF-I levels. GLUT-1 in the skeletal muscles seemed regula ted posttranscriptionally and presented a decrease of mRNA expression in al l stages studied. A differential sensitivity to insulin regulation of GLUT- 1 and GLUT-4 glucose transporters seems to be one of the causes for the tis sue-specific regulation of these glucose transporters in heart and skeletal muscles during the perinatal period.