Calcium spikes in toad rods.

Abstract

When the retina of the toad, B. marinus, was superfused with 6-12 mM tetraethylammonium chloride (TEA), intracellular recordings from rods showed large, depolarizing regenerative potentials. For brief exposures to TEA, these potentials occurred during the recovery phase of the light responses; during longer exposures, they were spontaneous in darkness but suppressed during illumination. Similar regenerative potentials were observed during perfusion with 3-10 mM 4-aminopyridine and 1-2 mM BaCl2. The amplitude of the regenerative potentials depended on the extracellular Ca concentration ([Ca2+]0). Lowering [Ca2+]0 decreased their amplitude and in zero [Ca2+]0 they were reversibly abolished. Increasing [Ca2+]0 by 1.5-2 times produced a small hyperpolarization of membrane potential and a large augmentation in regenerative response amplitude. Larger increases in [Ca2+]0 produced large membrane hyperpolarizations and reversibly suppressed the regenerative responses. High concentrations of Sr2+ in TEA also enhanced regenerative activity but did not affect the rod resting membrane potential. The amplitude of regenerative potentials increased continuously with increasing [Sr2+]0, and in 28 mM Sr2+ the rods generated 60-70 mV action potentials, even in the absence of extracellular Na+. The regenerative potentials were blocked by 25 .mu.M Cd2+, 50-100 .mu.M Co2+, 5 mM Mg2+ and 100 .mu.M D-600 [methoxyverapamil]. They were unaffected by 2 .mu.M TTX or 2-5 mM Na aspartate. In Ringer containing 12 mM TEA, large anode break responses could be recorded from rods at the termination of inward current pulses. These anode break responses were also suppressed by Co2+ and unaffected by TTX or Na aspartate. The membrane of toad rods may contain a conductance normally selective for Ca2+, which is activated by depolarization. In normal Ringer, the inward current through this conductance produces little effect, since it is balanced by a large outward current, probably carried by K+. TEA and other agents appear to block this outward current, permitting the Ca2+ current to become regenerative.