LOCALIZATION AND RAPID REGULATION OF NA+ MYO-INOSITOL COTRANSPORTER IN RAT-KIDNEY/

myo-Inositol, a major compatible osmolyte in renal medulla, is accumul ated in several kinds of cells under hypertonic conditions via Na+/myo -inositol cotransporter (SMIT). To investigate the physiological role of the SMIT, we sought to determine its localization by in situ hybrid ization and its acute regulation by NaCl and furosemide administration , Northern analysis demonstrated that SMIT is strongly expressed in th e medulla and at low levels in the cortex of kidney, Intraperitoneal i njection of NaCl rapidly induced SMIT mRNA in both the cortex and medu lla, and furosemide completely abolished this induction, In situ hybri dization revealed that SMIT is predominantly present in the medullary and cortical thick ascending limbs of Henle's loop (TALH) and macula d ensa cells, Less intense signals were seen in the inner medullary coll ecting ducts (IMCD), NaCl loading increased the signals throughout the TALH, and furosemide reduced the signals, SMIT in the IMCD is less se nsitive to these kinds of acute regulation, Thus, the distribution pat tern of SMIT does not correspond to the corticomedullary osmotic gradi ent, and SMIT in the TALH and macula densa cells is regulated very rap idly, These results suggest that SMIT expression in TALH may be regula ted by intracellular and/or peritubular tonicity close to the basolate ral membrane, which is supposed to be proportional to the magnitude of NaCl reabsorption.