. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. NEUROBIOLOGY: SENSORY PHYSIOLOGY AND NEURAL NETWORKS 263 quinine and quinidine block a number of voltage-gated con- ductances in the horizontal cells, including those selective for potassium, sodium, and calcium ions. However, these alkaloids also appeared to promote an increase in membrane conductance when the horizontal cells were held at potentials more positive than 0 mV. The outward current elicited under these conditions turned off rapidly upon repolarization of the cell to a holding potential of -70 mV. Tail-current a
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. NEUROBIOLOGY: SENSORY PHYSIOLOGY AND NEURAL NETWORKS 263 quinine and quinidine block a number of voltage-gated con- ductances in the horizontal cells, including those selective for potassium, sodium, and calcium ions. However, these alkaloids also appeared to promote an increase in membrane conductance when the horizontal cells were held at potentials more positive than 0 mV. The outward current elicited under these conditions turned off rapidly upon repolarization of the cell to a holding potential of -70 mV. Tail-current analysis indicated that the alkaloid-induced current had a reversal potential very close to 0 mV. The outward current elicited by quinine and quinidine was blocked by 4 m.\f cobalt or a halothane-saturated Ringer solution, and was also reduced by acidification of the cells via the application of 25 mAI acetate. Moreover, depolarization in the presence of quinine or quinidine permitted entry into the cells of extracellularly applied Lucifer yellow. In sum, the large, apparently nonselective nature of the conductance suggested by the tail current analysis, coupled with the block by halothane. cobalt, and acetate (agents known to suppress gap-junctional communication between cells), and the influx of extracellularly applied Lucifer yellow, have led us to suggest that the current elicited by these drugs results from the opening of hemi-gap junctional channels. Several reports have indicated that large increases in internal calcium ( > 1 /^Af) lead to the closure of gap junctions and a reduction in electrical communication between pairs of elec- trically coupled cells (1, 2). On the other hand, the results of recent studies suggest that gap junctions are permeant to calcium ions when intracellular calcium concentrations are below I uAI (3, 4). We have recently undertaken experiments to examine whether the conductance opened in the presence of the cinchona alkaloids is permean
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