Gating of retinal horizontal cell hemi gap junction channels by voltage, Ca2+, and retinoic acid

Hemi gap junction (HGJ) channels, precursors of gap junctional channels, are functionally expressed in retinal horizontal cells where they may play roles in osmoeregulation and ephaptic regulation of synaptic feedback to photoreceptors. In this study we examined mechanisms of gating of these channels by transmembrane voltage, Ca2+ and retinoic acid (RA).

Experiments were performed on cultured bass horizontal cells using the conventional whole cell patch clamp configuration.

HGJ currents in isolated bass horizontal cells, revealed by perfusion with Ca2+ free media, were opened by positive holding potentials and inhibited by negative holding potentials. These currents were also inhibited by external application of either Ca2+ or RA. Using a rapid perfusion system, the latency of 2 mM Ca2+ to begin channel closure was unmeasurably brief, whereas the latency for 30 microM RA action was 177+/-9 ms (mean+/-standard error of the mean). The total inhibition of HGJ channel currents by coapplication of 0.3 microM RA and 100 microM Ca2+ was less than the sum of inhibition by RA alone and Ca2+ alone suggesting that the actions of RA and Ca2+ were not independent. In the presence of 0.3 microM RA, the half maximal concentration for Ca2+ inhibition was increased from a control value of 192 microM to 375 microM without affecting maximal inhibition. Similarly, the half maximal concentration for RA inhibition was increased from a control value of 0.44 microM to 1.1 microM without affecting maximal inhibition in the presence of 100 microM Ca2+.

These results suggest that horizontal cell HGJ channels are closed by the normal negative resting potentials of these cells. Extracellular Ca2+ and the retinal neuromodulator RA also act to close HGJ channels through mechanisms or sites which are not independent.