Regulation of Lens rCx46-formed Hemichannels by Activation of Protein Kinase C, External Ca2+ and Protons
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Rodent lens connexin46 (rCx46) formed active voltage-dependent hemichannels when expressed in Xenopus oocytes. Time-dependent macroscopic currents were evoked upon depolarization. The observed two activation time constants were weakly voltage-dependent and in the order of hundreds of milliseconds and seconds, respectively. Occasionally, the macroscopic steady-state current and the corresponding current-voltage curve showed inactivation at high depolarizing voltages (>+50 mV). To account for the fast recovery from inactivation (<2 msec) favored by hyperpolarization, a four-state kinetic model (C 1 closed ↔C 2 closed ↔O open ↔I inactivated ) is proposed. In the absence of inactivation, the macroscopic conductance decreased and inactivation became visible at voltages positive of +50 mV when the rCx46-expressing oocytes were treated with the protein-kinase-C-activators OAG or TPA, high external concentrations of Ca2+ or H+. However, the underlying mechanisms of OAG, H+ or Ca2+ action were different. While OAG did not alter the voltage-dependent activation of the rCx46-hemichannels, an increase in the external Ca2+ or H+ level shifted the voltage threshold for activation to more positive voltages. In contrast to Ca2+, protons were not effective in the physiological concentration range. We propose that under physiological conditions only external Ca2+ and intracellular PKC-dependent processes regulate rCx46 in the lens.
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