Ionic mechanism for the photoreceptor potential of the retina of Bufo marinus

Abstract

1. Membrane potentials were recorded from single rods in the isolated retina of Bufo marinus while the ionic composition of the extracellular medium was rapidly changed. Substitution of 2 mM aspartate(-) for Cl(-) produced a prompt depolarization of horizontal cells, but no modification of either resting potential or response to light in receptor cells. This implies that feed-back from horizontal cells to receptor cells was not active.2. During substitution of choline(+) or Li(+) for Na(+), and during isosmotic substitution of sucrose for NaCl, the resting potential of receptor cells became more negative and responses to light were abolished. During exposure to K(+)-free medium, the resting potential became slightly more negative and the responses to light became larger and developed small after-depolarizations. Exposure to [K(+)](out) of four times normal resulted in permanent diminution of response magnitude and permanent change of response waveshape. Removal of Mg(2+), four times normal [Mg(2+)](out) or substitution of methylsulphate(-) for Cl(-) had no effect on resting potential or responses to light. With the exception of the small effects seen with altered [K(+)](out) these results are consistent with the receptor potential being generated by a light-induced decrease of membrane conductance to Na(+).3. Exposure to decreased [Ca(2+)](out) caused both a depolarization of the receptor membrane in the dark and an increase in the magnitude of the maximal response that could be evoked by a test stimulus. The magnitude of the increase in response equalled the magnitude of the depolarization. Exposure to increased [Ca(2+)](out) or steady background light caused both a steady hyperpolarization and a decrease in the magnitude of the maximal response that could be evoked by a test stimulus. For steady hyperpolarizations greater than 3.5 mV, whether caused by elevated [Ca(2+)](out) or steady background light, the decrease in response magnitude exceeded the magnitude of the hyperpolarization. These results imply that externally applied Ca(2+) ions mimic the effects of steady background lights, but the applied Ca(2+) ions must do more than merely decrease membrane conductance to Na(+).