Receptor Ca current and Ca-gated K current in tonic electroreceptors of the marine catfish Plotosus.

The tonic electroreceptors of the marine catfish Plotosus consist of a cluster of ampullae of sensory epithelia, each of which is an isolated receptor unit that is attached to the distant skin with only a long duct. The single-cell layered sensory epithelium has pear-shaped receptor cells interspersed with thin processes of supporting cells. The apical border of the receptor cells is joined to the supporting cells with junctional complexes. Single ampullae were excised and electrically isolated by an air gap. Receptor responses were recorded as epithelial current under voltage clamp, and postsynaptic potentials (PSP) were recorded externally from the afferent nerve in the presence of tetrodotoxin. The ampulla showed a DC potential of -19.2 +/- 6.5 mV (mean +/- SD, n = 18), and an input resistance of 697 +/- 263 K omega (n = 21). Positive voltage steps evoked inward currents with two peaks and a positive dip, associated with PSPs. The apical membrane proved to be inactive. The inward current was ascribed to Ca current, and the positive dip to Ca-gated transient K current, bot in the basal membrane of receptor cells. The Ca channels proved to have ionic selectivity in the order of Sr2+ greater than Ca2+ greater than Ba2+, and presumably they also passed outward current nonselectively. Double-pulse experiments further revealed a current-dependent inactivation for a part of the Ca current.