Figure 7. Alteration of H⁺ flux by glutamate requires the presence of extracellular calcium.

Recording from a single cell bathed initially in a solution in which the 4 mm calcium normally present had been replaced by 4 mm magnesium. The first ‘R’ denotes application of 1 ml skate Ringer solution containing nominally 0 mm calcium. The addition of 300 μm glutamate now produced no change in H⁺ flux from the cell. At about 1250 s, the recording was halted and the solution in the dish exchanged for one containing normal 4 mm calcium. At the second ‘R’, 1 ml of normal skate Ringer solution was added to the dish; this was followed by a 1 ml drop of glutamate (final concentration, 300 μm), which produced the typical change in H⁺ flux expected. Asterisks denote recordings from background locations 250 μm away from the cell.