Influence of pituitary growth hormone on DNA synthesis in rat tissues

DNA synthesis was compared in seven tissues of hypophysectomized rats and normally growing controls. Liver, heart, kidney, and three skeletal muscles of hypophysectomized rats incorporated[3H]thymidine into DNA more slowly than tissues of normal rats, measured either in intact rats or upon in vitro incubation of tissue slices. However, removal of the pituitary did not alter incorporation into DNA of brain. Administration of ovine growth hormone to hypophysectomized rats stimulated total body growth and increased incorporation into DNA of diaphragm within 6h. The response of liver to growth hormone was less marked and slower in onset than that of diaphragm. Incubation of diaphragm in vitro with growth hormone, however, did not affect [3H]thymidine incorporation. Hypophysectomy reduced incorporation into nuclear DNA of liver, kidney, and muscle, but did not suppress incorporation into mitochondrial DNA. These results suggest that growth hormone only affects synthesis of nuclear DNA. Autoradiographic studies indicate that DNA synthesis is stimulated in distinct cell types in the different organs, including hepatocytes in liver, epithelia of renal tubules, and fibroblasts within muscle.