The influence of MgCl2 and APB on the light-induced potassium changes and the ERG b-wave of the isolated superfused rat retina

The ERG and the extracellular potassium concentration, [K+](0), of the isolated superfused rat retina were measured in a physiological solution and in solutions containing 10 mM MgCl2 or 100 mu M APE. MgCl2 nearly abolished the b-wave, but the light-induced distal [K+](0), increase was enlarged from 0.13 +/- 0.05 to 0.28 +/- 0.08 mM. There was also an increase in the light-induced [K+](0), in the proximal retina. APE abolished the b-wave completely, and the distal light-induced [K+](0), increase was then replaced by a [K+](0), decrease, Upon return to the control solution, there was a larger transitory [K+](0) increase than under control conditions, and this occurred before the b-wave had returned. Under these experimental conditions, the distal [K+](0) increase could not be correlated with the b-wave, and so the Muller cells are unlikely to be the main source of the rising phase of the b-wave, More probable sources of the b-wave are the on-bipolar cells with their metabotropic and ionotropic receptors, with only the latter apparently being blocked by MgCl2.