EFFECTS OF INHIBITION OF MYOINOSITOL TRANSPORT ON MDCK CELLS UNDER HYPERTONIC ENVIRONMENT

To investigate the role of myo-inositol under hypertonic conditions, w e examined the effects of inhibition of myo-inositol transport in Madi n-Darby canine kidney (MDCK) cells using an analog of myo-inositol, 2- O,C-methylene-myo-inositol (MMI). We first characterized the inhibitor y effects of MMI on myo-inositol transport in MDCK cells. The Na+-depe ndent component of [H-3] myo-inositol uptake was inhibited by MMI in a concentration-dependent manner, although MMI did not inhibit the acti vities of the betaine transporter and system A neutral amino acid tran sporter. We found decreased affinity for myo-inositol in the presence of MMI, whereas the maximal velocity (V-max) of the transporter did no t change. Thus MMI behaves as a competitive inhibitor of myo-inositol transport with a relatively high inhibition constant (K-i) value (1.6 mM). Myo-inositol content in hypertonic MDCK cells was markedly reduce d in the presence of 5 mM MMI, but MMI itself did not accumulate in th ese cells. The hypertonic cells began to detach in the presence of MMI 3 days after increasing medium osmolality, whereas MMI did not affect the cells in isotonic medium. We also examined the effects of MMI on colony-forming efficiency of MDCK cells. MMI decreased colony-forming efficiency in a concentration-dependent manner, and addition of myo-in ositol returned the efficiency to the value without MMI. Addition of b etaine also increased colony-forming efficiency in the presence of MMI . These results indicate that myo-inositol plays an important role in survival and growth under hypertonic environment.