Regulation of Cx45 hemichannels mediated by extracellular and intracellular calcium.

Connexin45 (Cx45) hemichannels (HCs) open in the absence of Ca and close in its presence. To elucidate the underlying mechanisms, we examined the role of extra- and intracellular Ca on the electrical properties of HCs. Experiments were performed on HeLa cells expressing Cx45 using electrical (voltage clamp) and optical (Ca imaging) methods. HCs exhibit a time- and voltage-dependent current ( I), activating with depolarization and inactivating with hyperpolarization. Elevation of [Ca] from 20 nM to 2 μM reversibly decreases I, decelerates its rate of activation, and accelerates its deactivation. Our data suggest that [Ca] modifies the channel properties by adhering to anionic sites in the channel lumen and/or its outer vestibule. In this way, it blocks the channel pore and reversibly lowers I and modifies its kinetics. Rapid lowering of [Ca] from 2 mM to 20 nM, achieved early during a depolarizing pulse, led to an outward I that developed with virtually no delay and grew exponentially in time paralleled by unaffected [Ca]. A step increase of [Ca] evoked by photorelease of Ca early during a depolarizing pulse led to a transient decrease of I superimposed on a growing outward I; a step decrease of [Ca] elicited by photoactivation of a Ca scavenger provoked a transient increase in I. Hence, it is tempting to assume that Ca exerts a direct effect on Cx45 hemichannels. [ABSTRACT FROM AUTHOR]