Opposing effects of calcium and barium in vertebrate rod photoreceptors

Abstract

Intracellular recording in outer segments of red rods of the toad [Bufo marinus] retina showed that increasing Ca2+ concentration in the perfusate mimicked certain aspects of light adaptation. Light sensitivity was reduced, the amplitude of light responses was reduced, the time course of light responses was altered by shortening the delay to the peak and increasing the decay rate and the resting membrane potential was generally increased. Apparently Ca2+ acts as an internal transmitter subject to light-induced release from the rod saccules. The addition of Ba2+ to the perfusate mimicked dark adaptation by having effects opposite to all those described for Ca2+. The Ba2+ effects were specific, Mg2+ and Sr2+ being ineffective in similar or greater concentration. Ba2+ gave clear effects at concentrations as low as 0.1 mM. Probably Ba2+ enters the cell and reduces the uptake of Ca2+ into the rod saccules, because the various effects of Ba2+ upon light responses all seem explicable from this hypothesis. Ba2+ also reduced the resting membrane potential, probably by reducing the membrane conductance for K+. The demonstrated effects of Ba2+ indicate an important role for Ba2+ in analyzing the ionic aspects of transduction in vertebrate photoreceptors. They also suggest a critical physiological role for Ba2+ in controlling both the resting properties and light-induced responses of vertebrate rods, because Ba2+ has been reported to be concentrated in outer and inner segments of cat photoreceptors.