The Two Types of Ca2+ Currents Observed in Horizontal Cells Dissociated From Goldfish Retina

Abstract

Abstract: : Purpose: Goldfish retinal horizontal cell has been well studied with regard to its electrical properties. It has also been known that Ca²⁺ channel was found in there, which elicited one type of Ca²⁺ current with voltage–dependent activation. However, the two types of inward Ca²⁺ currents were observed in our study. In order to examine this unconventional Ca²⁺ current, voltage–clamp technique was applied to horizontal cells dissociated from goldfish retina. Methods: Horizontal cells were dissociated from goldfish retina by mechanical and enzymic treatment. When dissociated, the cell was mostly separated into somata and axon–terminals.Using each cell, membrane current was recorded under the whole–cell patch–clamping mode. The cell was held at –40 mV and step pulses were given from –90 to +50 mV in 5 mV increments. Results: Two components of inward Ca²⁺ currents could be identified during depolarizing step voltage ; tansient and sustained. The transient Ca ²⁺current was activated by sufficient pre–inactivation time beyond 300ms and reached a maximal value near –25 mV. On the other hand, the sustained Ca²⁺ current occurred even though pre–inactivation time less than 10ms was applied. Its activation started near –10 mV and peak amplitude was at +20 mV. Therefore current–voltage relationship of two types of Ca²⁺currents appeared to be a clear bi–phasic form when plotted . The majority of H2 somata and axon–terminals had two components of Ca²⁺currents, while both H1 and H3 types possessed either one. Co²⁺(2mM) suppressed both of two components, but nifedipine(0.1mM), L–type Ca²⁺channel antagonist, blocked only the sustained current. The replacement of external Ca²⁺ with Ba²⁺ enhanced the sustaned current up to more than 3–fold. On the basis of present responses of voltage–dependent Ca²⁺currents, the types of goldfish retinal horizontal cells can be distinguished as three groups: (1) First group has two types of Ca²⁺ channels that elicit evident currents in same amplitude. (2) Second has Ca²⁺ channels with one prominent type. (3) Third has only sustained current. Conclusions: In result, tansient type might be considered as T–type Ca²⁺channel,while sustained type as L–type Ca ²⁺channel. This sudy suggests that Ca ²⁺would play an important role in synaptic mechanism of horizontal cells, as well as other cells in goldfish retina, owing to various Ca²⁺channels.

Keywords: horizontal cells • calcium • electrophysiology: non-clinical