Characterization of a nicotinamide–adenine dinucleotide‐dependent cation channel in the CRI‐G1 rat insulinoma cell line

Open Access
Publisher Website
Google Scholar
Abstract
Cell‐free excised membrane patches were used to examine the properties of a novel nicotinamide–adenine dinucleotide (β‐NAD+)‐activated ion channel in the rat insulin‐secreting cell line, CRI‐G1. In inside‐out recordings, β‐NAD+ (0.05–1.0 mm) induced the appearance of a channel characterized by extremely slow kinetics, with mean open times in the range of seconds. The estimated EC50 for activation was 114 μm. Channel activity declined with time (run‐down) following activation by β‐NAD+ in excised patches and this was not prevented by intra‐cellular application of trypsin. The single channel current–voltage relationship was linear with a conductance of 74 pS in symmetrical NaCl. The channel appears equally permeable to Na+, K+ and Cs+, exhibits an appreciable permeability to Ca2+, Mg2+ and Ba2+, but excludes anions. The channel displays an unusual voltage sensitivity, with an abrupt increase in open‐state probability at depolarized voltages. Channel opening, in the presence of β‐NAD+, required both Ca2+ and Mg2+ to be present at the internal side of the membrane. Activation by Ca2+ required a concentration of at least 10 μm and was maximal at 0.1 mm. Ba2+ did not substitute for Ca2+ in inducing channel activity nor did it inhibit activation by Ca2+. Increasing the concentration of intracellular Mg2+ stabilized the open state of NAD+‐activated channels. The non‐selective cation channel reported here differs in its gating and modulatory characteristics from non‐selective cation channels described in other tissues. This channel may play a role in the pathophysiological reponses of β‐cells to oxidative stress.