THE NA+ H+ EXCHANGER PRESENT IN TROUT ERYTHROCYTE-MEMBRANES IS NOT RESPONSIBLE FOR THE AMILORIDE-INSENSITIVE NA+/LI+ EXCHANGE ACTIVITY/

The protein responsible for the Na+/Li+ exchange activity across the e rythrocyte membrane has not been cloned or isolated. It has been sugge sted that a Na+/H+ exchanger could be responsible for the Na+/Li+ exch ange activity across the erythrocyte membrane. Previously, we reported that in the trout erythrocyte, the Li+/H+ exchange activity (mediated by the Na+/H+ exchanger beta NHE) and the Na+/Li+ exchange activity r espond differently to cAMP, DMA (dimethyl-amiloride) and O-2. We concl uded that the DMA insensitive Na+/Li+ exchange activity originates fr om a different protein. To further examine these findings, we measured Li+ efflux in fibroblasts expressing the beta NHE as the only Na+/Hexchanger. Moreover, the internal pH of these cells was monitored with a fluorescent probe. Our findings indicate that acidification of fibr oblasts expressing the Na+/H+ exchanger beta NHE, induces a Na+ stimul ated Li+ efflux activity in trout erythrocytes. This exchange activity , however, is DMA sensitive and therefore differs from the DMA insensi tive Na+/Li+ exchange activity. In these fibroblasts no significant DM A insensitive Na+/Li+ exchange activity was found. These results suppo rt the hypothesis that the trout erythrocyte Na+/Li+ exchange activity is not mediated by the Na+/H+ exchanger (beta NHE) present in these m embranes.