The influence of MgCl sub(2) 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 super(+)] sub(o), of the isolated superfused rat retina were measured in a physiological solution and in solutions containing 10 mM MgCl sub(2) or 100 mu M APB. MgCl sub(2) nearly abolished the b-wave, but the light-induced distal [K super(+)] sub(o) increase was enlarged from 0.13 plus or minus 0.05 to 0.28 plus or minus 0.08 mM. There was also an increase in the light-induced [K super(+)] sub(o) in the proximal retina. APB abolished the b-wave completely, and the distal light-induced [K super(+)] sub(o) increase was then replaced by a [K super(+)] sub(o) decrease. Upon return to the control solution, there was a larger transitory [K super(+)] sub(o) increase than under control conditions, and this occurred before the b-wave had returned. Under these experimental conditions, the distal [K super(+)] sub(o) increase could not be correlated with the b-wave, and so the Mueller 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 MgCl sub(2). The extracellular [K super(+)] sub(o) changes, however, had an influence upon the slow potentials of the ERG.