ADAPTATION OF THE MEDULLARY THICK ASCENDING LIMB TO DIETARY-PROTEIN INTAKE

Since the renal growth response to a high-protein diet is characterize d by prominent hypertrophy of the medullary thick ascending limb of He nle's loop (MTAL), the functional and metabolic adaptations of this ne phron segment to dietary protein were investigated. MTAL suspensions w ere obtained from rats fed equal amounts of isocaloric food containing either 10% (LP) or 32% (HP) casein for 4-6 weeks. The results show th at intact MTAL of HP rats exhibit a blunted respiration rate, sodium p ump activity, hormone-sensitive cAMP production and leucine oxidation rate in comparison with those of LP rats. On the other hand, adenylate cyclase and leucine transaminase activities, measured on permeabilize d or homogenized MTAL, are enhanced by a HP diet. We conclude that the MTAL adapts to high dietary protein by increasing its maximal enzyme activities, but certain factors, present in intact cells, limit transp ort and metabolism in HP- more than in LP-fed rats. This reduced funct ion per unit MTAL protein in HP rats is more than compensated for by h ypertrophy of the MTAL tissue mass.