Calcium‐dependent chloride currents in isolated cells from rat lacrimal glands.

Abstract

Isolated cells from rat lacrimal glands were studied with the tight‐seal whole‐cell recording technique. Cells were dialysed with K‐free solutions containing a high concentration of Ca2+ buffer in order to record Ca‐dependent Cl‐ currents at a Ca2+ level fixed between 0.1 and 10 microM. HEDTA was preferable to EGTA as a Ca2+ buffer, even under conditions of equivalent equilibrium buffering power. After replacement of all internal K+ with Na+, the cells displayed a small conductance component which could be abolished by removal of external K+ or by external application of 2 mM‐tetraethylammonium. It is suggested that this conductance is due to Ca‐dependent K+ channels. The main part of the cell current was Cl selective. The Cl‐ conductance was negligible at 0.5 microM‐Ca2+, and fully activated at 2 microM‐Ca2+. The dose‐response curve relating Cl‐ currents to the internal Ca2+ concentration, [Ca]i, was steeper than predicted by a simple binding isotherm reaction. Relaxations observed in response to voltage jumps could, in most cases, be fitted with single exponentials. At [Ca]i 0.5 microM, the curve relating the relaxation time constant, tau, to the membrane potential, displayed a maximum near +20 mV and 250 ms. At hyperpolarized potentials, tau varied by an e‐fold factor in 130 mV. At [Ca]i 1 microM, tau decreased from 100 ms at ‐120 mV to 60 ms at +60 mV. Relaxation analysis gave an estimate of the variation of the channel open state probability, Po, with potential. At [Ca]i 0.5 microM, Po varied by an e‐fold factor in 50‐70 mV at hyperpolarized potentials, and saturated above +60 mV. At [Ca]i 1 microM, Po varied e‐fold in 100 to 110 mV at hyperpolarized potentials, and saturated near +20 mV. External Cl‐ was substituted with various anions. From reversal potential measurements, the following permeability sequence was obtained: I‐ greater than NO3‐ greater than Br‐ greater than Cl‐ greater than F‐ greater than isethionate, methanesulphonate greater than glutamate. The corresponding normalized permeability coefficients were 2.7, 2.4, 1.6, 1, 0.2, 0.1, 0.1, 0.05. Replacement of external Cl‐ with Br‐, isethionate, methanesulphonate or glutamate did not alter current kinetics as obtained during or after a depolarizing voltage jump.(ABSTRACT TRUNCATED AT 400 WORDS)