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

Abstract

SUMMARY1. Double-barrel micropipettes were used to pass pulses of current in darkness into single rods in the isolated, perfused retina of the toad, Bufo marines.2. In normal Ringer solution, current pulses evoked non-linear changes in membrane potential which varied as a function of current amplitude and of time. Responses to currents of both polarities showed slow relaxations toward the base line during the pulse, and the steady-state I-V curve exhibited a prominent outward rectification.3. In Ringer containing 12 mM-TEA, the slow relaxation of voltage during outward current pulses was diminished, and the outward rectification was markedly reduced. In contrast Co2+, at a concentration in excess of that required to block Ca2+ spikes in rods, increased the receptor input resistance but did not reduce either the amplitude of the slow relaxation or the extent of outward rectification.4. These experiments indicated that the outward rectification of rods is predominantly due to a conductance which is gated by voltage rather than by entry of Ca2+. 5. Long-lasting after-potentials followed the termination of outward current pulses. In normal Ringer the after-potentials were hyperpolarizing and were accompanied by an increase in input conductance. In TEA, the afterpotentials were depolarizing and were also accompanied by an increase in input conductance. The after-depolarizations in TEA were enhanced by Sr2+ and blocked by Co2+. These experiments suggest that the hyperpolarizing and depolarizing afterpotentials are produced by different mechanisms, the hyperpolarizing by an increase in K+ conductance, and the depolarizing by an increase in Ca2+ conductance.

Calcium spikes in toad rods.

“…Rather, the regenerative potentials seem to be produced indirectly by TEA, as a result of its effect on the rod K conductance. More evidence for this is given in the following paper (Fain & Quandt, 1980). Our experiments show that the amplitude of regenerative potentials recorded from rods exposed to TEA depends upon extracellular Ca2+ concentration.…”

“…The results in this paper and the one which follows (Fain & Quandt, 1980) show that the rod membrane is not passive, as is often assumed, but contains at least two kinds of voltage-gated conductances, which appear to resemble closely the Ca2+ and delayed K+ conductances of neurones and muscles. Preliminary results of these experiments have been published (Fain, Quandt & Gerschenfeld, 1977).…”

“…Membrane potentials were estimated from the change in voltage recorded during the insertion or removal of the micropipette. When impalement occurred just after the pipette had reached the retina, the membrane potential was consistently between -30 and -45 mV (see for example Fain & Quandt, 1980) and is given in the Figure legends. When impalement occurred after the pipette had traversed part of the retina, membrane potentials varied greatly, perhaps because the pipette entered one cell from another, poorly penetrated cell, or because the tip potential of the pipette changed.…”

“…Nomenclature. Throughout this paper and the one which follows (Fain & Quandt, 1980), we shall use the phrase 'regenerative potential' to refer to the spike-like potential changes which we have recorded from rods perfused with TEA and other K channel blocking agents. We call these potential changes 'regenerative' solely because of the resemblance of their wave form to action potentials and other regenerative responses which have been recorded from neurones and muscle fibres.…”

“…When the retina was perfused with Ringer containing 12 mM-TEA, the amplitude of the anode break response increased and its After-potentials from a single rod to 0 7 nA, 2-5 sec pulses of negative current. The hyperpolarization during the step was not measured for this cell but can be estimated to have been 50-55 mV from the current-voltage curve of toad rods (Fain & Quandt, 1980). Experimental sequence is from left to right.…”