Energy profile of the calcium channel in the membrane of mollusc neurons

Abstract

The current-voltage characteristics of the inward calcium and barium currents at different concentrations of these ions in the extracellular solution have been measured on isolated neurons of the snailHelix pomatia intracellularly perfused with potassium-free solution containing 10mm EDTA. On the basis of these characteristics the energy profile of the calcium channel has been calculated using a model based on the absolute reaction rate theory developed by Eyring. The effect of changes of the near-membrane concentration of the penetrating ions due to the existence of fixed charges on the outer side of the membrane has been taken into account. A satisfactory description of the concentration-and potential-dependence of the calcium inward currents has been obtained based on a three-barrier model for the energy profile of the calcium channel. Calculated dissociation constants for the complexes of Ca2+ ions with the binding sites of the calcium channel have the following values:K out=10mm andK in=2.5mm; and for the complexes of Ba2+ ions,K out=91mm andK in=1.5mm. The outer binding site corresponds to the acidic group with pK=5.8. Comparison between these data and the values of pK for divalent cation complexes with different anionic groups of amino acids allowed us to suggest that the outer binding site contains only one carboxylic group. It was shown that the strength of cation binding to this group determines the conductance of the calcium channel.