Na+-Ca2+ exchange in the isolated cochlear outer hair cells of the guinea-pig studied by fluorescence image microscopy

The outer hair cell isolated from the guinea-pig was superfused in vitro and the cytosolic calcium concentration ([Ca²⁺]_i) and sodium concentration ([Na⁺]_i) were measured using fluorescence indicators. Under the resting condition, [Ca²⁺]_i and [Na⁺]_i were 91±9 nM (n = 51) and 110±5 mM (n = 12), respectively. Removal of external Na⁺ by replacing with N-methyl-D-glucamine (NMDG⁺) increased [Ca²⁺]_i by 270±79% (n = 27) and decreased [Na⁺]_i by 23±4 mM (n = 6). Both changes in [Ca²⁺]_i and [Na⁺]_i were totally reversible on returning external Na⁺ to the initial value and were inhibited by addition of 0.1 mM La³⁺ or 100 M amiloride 5-(N,N-dimethyl) hydrochloride. Elevation of external Ca²⁺ ions to 20 mM reversibly decreased [Na⁺]_i by 8±6 mM (n = 5). Moreover, the chelation of the intracellular Ca²⁺ with 1,2-bis (2-aminophenoxy) ethane-N,N,N,N-tetraacetic acid (BAPTA) exerted an inhibitory action on the NMDG⁺-induced reduction in [Na⁺]_i. Exposure to 5 mM NaCN for 2 min significantly and reversibly increased [Ca²⁺]_i by 290±37% (n = 5), but did not affect the [Ca²⁺]_i elevation induced by the NMDG⁺ solution. The rise in [Ca²⁺]_i induced by the NMDG⁺ solution was not enhanced by ouabain pretreatment. Addition of ouabain did not alter the [Na⁺]_i. The present results are best explained by the presence of an Na⁺-Ca²⁺ exchanger in cell membrane and indicate that the activity of Na⁺/K⁺ pump is poor in outer hair cells.