Regulation of Cx45 hemichannels mediated by extracellular and intracellular calcium.
Connexin45 (Cx45) hemichannels (HCs) open in the absence of Ca(2+) and close in its presence. To elucidate the underlying mechanisms, we examined the role of extra- and intracellular Ca(2+) on the electrical properties of HCs. Experiments were performed on HeLa cells expressing Cx45 using electrical (voltage clamp) and optical (Ca(2+) imaging) methods. HCs exhibit a time- and voltage-dependent current (I(hc)), activating with depolarization and inactivating with hyperpolarization. Elevation of [Ca(2+)](o) from 20 nM to 2 μM reversibly decreases I(hc), decelerates its rate of activation, and accelerates its deactivation. Our data suggest that [Ca(2+)](o) 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(hc) and modifies its kinetics. Rapid lowering of [Ca(2+)](o) from 2 mM to 20 nM, achieved early during a depolarizing pulse, led to an outward I(hc) that developed with virtually no delay and grew exponentially in time paralleled by unaffected [Ca(2+)](i). A step increase of [Ca(2+)](i) evoked by photorelease of Ca(2+) early during a depolarizing pulse led to a transient decrease of I(hc) superimposed on a growing outward I(hc); a step decrease of [Ca(2+)](i) elicited by photoactivation of a Ca(2+) scavenger provoked a transient increase in I(hc). Hence, it is tempting to assume that Ca(2+) exerts a direct effect on Cx45 hemichannels.
Department of Physiology, University of Bern, Bühlplatz 5, 3012, Bern, Switzerland.
Patrick
P
Bader
Robert
R
Weingart
Marcel
M
Egger
D000818:Animals; D002118:Calcium / Q000032:analysis / Q000494:pharmacology*; D017630:Connexins / Q000378:metabolism* / Q000502:physiology; D003600:Cytosol / Q000737:chemistry; D045604:Extracellular Fluid / Q000737:chemistry; D006367:HeLa Cells; D006801:Humans; D015640:Ion Channel Gating / Q000187:drug effects; D051379:Mice; D010775:Photic Stimulation
D017630:Connexins; C082920:connexin 45; D002118:Calcium