Regulation of Connexin Hemichannels by Monovalent Cations

Opening of connexin hemichannels in the plasma membrane is highly regulated. Generally, depolarization and reduced extracellular Ca²⁺ promote hemichannel opening. Here we show that hemichannels formed of Cx50, a principal lens connexin, exhibit a novel form of regulation characterized by extraordinary sensitivity to extracellular monovalent cations. Replacement of extracellular Na⁺ with K⁺, while maintaining extracellular Ca²⁺ constant, resulted in >10-fold potentiation of Cx50 hemichannel currents, which reversed upon returning to Na⁺. External Cs⁺, Rb⁺, NH₄⁺, but not Li⁺, choline, or TEA, exhibited a similar effect. The magnitude of potentiation of Cx50 hemichannel currents depended on the concentration of extracellular Ca²⁺, progressively decreasing as external Ca²⁺ was reduced. The primary effect of K⁺ appears to be a reduction in the ability of Ca²⁺, as well as other divalent cations, to close Cx50 hemichannels. Cx46 hemichannels exhibited a modest increase upon substituting Na⁺ with K⁺. Analyses of reciprocal chimeric hemichannels that swap NH₂- and COOH-terminal halves of Cx46 and Cx50 demonstrate that the difference in regulation by monovalent ions in these connexins resides in the NH₂-terminal half. Connexin hemichannels have been implicated in physiological roles, e.g., release of ATP and NAD⁺ and in pathological roles, e.g., cell death through loss or entry of ions and signaling molecules. Our results demonstrate a new, robust means of regulating hemichannels through a combination of extracellular monovalent and divalent cations, principally Na⁺, K⁺, and Ca²⁺.