281 THE SUPRA ADDITIVE EFFECTS: TRIIODOTHYRONINE (T3) AND CORTISONE ON JEJUNAL Na-K ATPASE IN DEVELOPING RAT

Abstract

We determined the effect of T3 and cortisone (cort) on jejunal Na-K ATPase in the developing rat. Na-K ATPase in jejunal homo-genates increases two-fold at weaning (21 days). Since serum T3, increases at weaning (20-24 days), we investigated the effect ot T3 on Na-K ATPase. One dose of T3 (0.1 ug/gm body wt) administered at 17 days of age increases Na-K ATPase in suckling 18 day old pups to postwean levels. This activation does not occur in jejunum of 14 and 16 day old or postwean pups. Since endogenous glucocorticoids increase on days 16-18 of age, these data suggest that jejunum only respond to T3 after exposure to glucocorticoids. To test this we treated pups at ages 10-13 days with (cort) (15 ug/gm body wt) and on day 15 with one dose of T3. Na-K ATPase was measured on day 16. Treatment with (cort) increased Na-K ATPase in 16 day old pup jejunum. The addition of T3 and (cort) further stimulated Na-K ATPase to postwean levels in suckling 16 day old pups. These data suggest that the increase in jejunal Na-K ATPase of developing pups is mediated by T3 and that the T3 is only effective in enterocytes exposed to glucocorticoids.