RENAL NA+ H+ EXCHANGER ISOFORMS AND THEIR REGULATION BY THYROID-HORMONE/

Na+ crosses the luminal membrane of the proximal tubule primarily via Na+/H+ exchange (NHE), and NHE activity is influenced by thyroid statu s. Pharmacological, immunological, and kinetic studies indicate multip le isoforms of NHE, and four full-length cDNAs have been cloned to dat e. The aims of this study were to determine which NHE mRNAs (NHE1, -2, -3, and -4) were expressed in the rat proximal tubule, the relative a bundance of each in the renal cortex, and the effect of thyroid status on their expression. By blot hybridization of poly(A)(+) RNA, all NHE isoform mRNAs were detected in the rat renal cortex; NHE1, -2, and -3 in the proximal tubule; and NHE1 and -3 in LLC-PK1 cells. NHE3 mRNA a bundance was fourfold higher than the other three isoforms in renal co rtex. The effect of thyroid status was assessed in renal cortex from e uthyroid, hypothyroid, and hyperthyroid rats. Although none of the NHE mRNA levels was altered in the transition from euthyroid to hypothyro id states, both NHE2 and NHE3 mRNA levels increased 1.5-fold in the tr ansition from hypo- to hyperthyroidism. NHE3 protein, measured by immu noblot with the use of an NHE3-specific antibody, was detected at 83-8 5 kDa in renal cortex and codistributed on sorbitol gradients with the brush-border marker alkaline phosphatase. No significant difference i n NHE3 protein abundance was detected between hypothyroid and hyperthy roid rats. In conclusion, in the renal cortex, the NHE3 isoform predom inates at the mRNA level, is expressed in apical membranes, and increa ses at the mRNA but not the protein levels in response to thyroid horm one treatment, suggesting parallel changes in synthesis and turnover o f NHE3 by thyroid hormone.