Modulation of the mdr-1b gene in the kidney of rats subjected to dehydration or a high-salt diet

The localization of the multidrug resistance gene (mdr-1b) messenger ribonu cleic acid (mRNA) alone the rat nephron and its regulation was investigated under two different experimental situations: dehydration and high-Na+ diet . The mdr-1b mRNA was detected in glomeruli, proximal tubule segments, cort ical and medullary thick ascending limbs, inner medullary collecting ducts and thin limbs of Henle's loop. Using the ribonuclease (RNase) protection a ssay (RPA), the abundance of mdr-1b mRNA was shown to de 35% less in renal cortex than in medulla. The mdr-1b mRNA expression in dehydrated rats in co rtex or medulla did not differ from control. However. after 5 or 14 days on a high-Na+ diet, mdr-1b expression had decreased significantly in both cor tex and medulla. There was no change in protein expression in dehydrated ra ts but a significant decrease occurred in rats fed the high-salt diet, conf irming the results obtained with RPA. Our results suggest that the mdr-1b p roduct is involved in extracellular volume regulation in rats.