Effects of extracellular pH on neuronal calcium channel activation

Abstract

Previous studies have shown that extracellular pH (pH o) alters gating and permeation properties of cardiac L- and T-type channels. However, a comprehensive study investigating the effects of pH o on all other voltage-gated calcium channels is lacking. Here, we report the effects of pH o on activation parameters slope factor (S), half-activation potential (V a), reversal potential (E rev), and maximum slope conductance (G max) of the nine known neuronal voltage-gated calcium channels transiently expressed in tsA-201 cells. In all cases, acidification of the extracellular bathing solution results in a depolarizing shift in the activation curve and reduction in peak current amplitudes. Relative to a physiological pH o of 7.25, statistically significant depolarizing shifts in V a were observed for all channels at pH o 7.00 except Ca v1.3 and 3.2, which showed significant shifts at pH o 6.75 and below. All channels displayed significant reductions in G max relative to pH o 7.25 at pH o 7.00 except Ca v1.2, 2.1, and 3.1 which required acidification to pH o 6.75. Upon acidification Ca v3 channels displayed the largest changes in V as and exhibited the largest reduction in G max compared with other channel subtypes. Taken together, these results suggest that significant modulation of calcium channel currents can occur with changes in pH o. Acidification of the external solution did not produce significant shifts in observed E revs or blockade of outward currents for any of the nine channel subtypes. Finally, we tested a simple Woodhull-type model of current block by assuming blockade of the pore by a single proton. In all cases, the amount of blockade observed could not be explained in these simple terms, suggesting that proton modulation is more complicated, involving more than one site or gating modification as has been previously described for cardiac L- and T-type channels.