Calcium spikes in toad rods.

Abstract

SUMMARY1. When the retina of the toad, Bufo marines, was superfused with 6-12 mMtetraethylammonium 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; whereas, 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-BaCl2.2. 4. The regenerative potentials were blocked by 25 /SM-Cd2+, 50-100 /LM-Co2+j 5mM-Mg2+, and 100 ,tM-D-600. They were unaffected by 2 /LM-TTX or 2-5 mM-Na aspartate.5. 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 aspartame.6. We conclude that the membrane of toad rods contains 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.

Characterization of a voltage-gated K+ channel that accelerates the rod response to dim light

“…As a consequence, I Kx figures prominently in setting the rod resting potential, providing a hyperpolarizing influence that stabilizes the membrane. We know that Ba 2+ or TEA induces oscillations of the membrane potential (Brown and Flaming, 1978;Fain et al, 1980;Owen and Torre, 1984), and our results suggest that this instability could be caused by Ba 2+ or TEA blocking I Kx at the dark resting potential. It is tempting to speculate that modulation of I Kx would be an effective means by which to influence the resting potential and membrane stability of rods (and possibly cones), and hence affect their output to second-order cells.…”

“…In addition, Ba 2+ is known to prolong the light response in rods (Brown and Flaming, 1978;Fain et al, 1980) and to delay dark adaptation (Walter et al, 1982). Ba 2+ also may be concentrated normally in rods of some species (Bellhorn and Lewis, 1976).…”

The effects of tetraethylammonium and cobalt ions on responses to extrinsic current in toad rods.

“…In the preceeding paper (Fain, Gerschenfeld & Quandt, 1980), we showed that Ca2+-dependent spikes and regenerative potentials can be recorded from rods superfused with Ringer containing tetraethylammonium chloride (TEA), 4-aminopyridine, Ba2+, and other agents known to block K+ conductances in nerve and muscle cells (Narahashi, 1974;Meves & Pichon, 1977;Hagiwara, Fukuda & Eaton, 1974). In rods, as in arthropod muscle (Fatt & Katz, 1953) and presynaptic membrane (Katz & Intracellular recording and current passing.…”

“…During extrusion, the two barrels (and the capillary tubing) fused together, and the final outside diameters of the barrels were approximately 1-0 and 0-8 mm. Double-barrel micropipettes were pulled from this glass as for single barrel electrodes (Fain et al 1980). They were filled by the method of Barrett & Barrett (1976).…”

“…We also recorded the e.r.g. and receptor light responses at regular intervals, since these also could give information about the progress of drug action (see Fain et al 1980). …”