Calcium and magnesium transport by isolated goldfish hair cells

We used electron-probe analysis (EPA) to investigate the transport of the divalent cations calcium and magnesium across the plasma membranes of hair cells. Unlike ion-sensitive fluorescent dyes, EPA detects these ions regardless of the state of chemical combination inside the cell; changes in these cell ions determined by EPA indicate net transport across the cell membrane. Raising or lowering either extracellular divalent cation within 1 mM of its control level raised or lowered its cell contents, but further increases in extracellular concentration of either ion had little additional effect on the cell content of that ion. New steady-state contents could be obtained within minutes, but the net divalent cation currents required to account for the observed changes would have been smaller than most currents recorded electrophysiologically, less than 1 pA. The effects of replacing extracellular Na+ with other ions were consistent with the presence in hair cells of exchangers for divalent cations thought to occur in other tissues: electrically neutral sodium/magnesium exchange (2 Na+ per Mg2+) and electrogenic sodium/calcium exchange (at least 3 Na+ per Ca2+). The increase in cell Ca after 1 minute of potassium-depolarization was similar to that expected from electrophysiological studies of voltage-sensitive calcium currents in goldfish hair cells. After that time in elevated potassium, however, either calcium-entry pathways were inhibited or calcium-export mechanisms were enhanced.