Modulation of calcium currents in mouse ventral horn neurons by extracellular pH

Two sets of amino acids of the domain I of Cav2.3 Ca(2+) channels contribute to their high sensitivity to extracellular protons

Extracellular acidification decreases Ca(2+) current amplitude and produces a depolarizing shift in the activation potential (Va) of voltage-gated Ca(2+) channels (VGCC). These effects are common to all VGCC, but differences exist between Ca(2+) channel types and the underlying molecular mechanisms remain largely unknown. We report here that the changes in current amplitude induced by extracellular acidification or alkalinisation are more important for Cav2.3 R type than for Cav2.1 P/Q-type Ca(2+) channels. This difference results from a higher shift of Va combined with a modification of channel conductance. Although involved in the sensitivity of channel conductance to extracellular protons, neither the EEEE locus nor the divalent cation selectivity locus could explain the specificity of the pH effects. We show that this specificity involves two separate sets of amino acids within domain I of the Cavα subunit. Residues of the voltage sensor domain and residues in the pore domain mediate the effects of extracellular protons on Va and on channel conductance, respectively. These new insights are important for elucidating the molecular mechanisms that control VGCC gating and conductance and for understanding the role of extracellular protons in other channels or membrane-tethered enzymes with similar pore and/or voltage sensor domains.

Effects of extracellular pH on neuronal calcium channel activation

Previous studies have shown that extracellular pH (pHo) alters gating and permeation properties of cardiac L- and T-type channels. However, a comprehensive study investigating the effects of pHo on all other voltage-gated calcium channels is lacking. Here, we report the effects of pHo on activation parameters slope factor (S), half-activation potential (Va), reversal potential (Erev), and maximum slope conductance (Gmax) 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 pHo of 7.25, statistically significant depolarizing shifts in Va were observed for all channels at pHo 7.00 except Cav1.3 and 3.2, which showed significant shifts at pHo 6.75 and below. All channels displayed significant reductions in Gmax relative to pHo 7.25 at pHo 7.00 except Cav1.2, 2.1, and 3.1 which required acidification to pHo 6.75. Upon acidification Cav3 channels displayed the largest changes in Vas and exhibited the largest reduction in Gmax compared with other channel subtypes. Taken together, these results suggest that significant modulation of calcium channel currents can occur with changes in pHo. Acidification of the external solution did not produce significant shifts in observed Erevs 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.

Rapid pH and PO2changes in the tissue recording chamber during stoppage of a gas-equilibrated perfusate: effects on calcium currents in ventral horn neurons

In vitro studies often use bicarbonate-buffered saline solutions to mimic the normal extracellular environment of tissues. These solutions are typically equilibrated with gaseous O2 and CO2, the latter interacting with bicarbonate ions to maintain a physiological pH. In vitro tissue chambers, like those used for electrophysiology, are usually continually perfused with the gassed buffer, but stopping the perfusion to add expensive chemicals or acquire imaging data is a common practice. The present study demonstrates that this procedure leads to rapid (< 30 s) increases in pH and decreases in PO2 of the detained solution in the tissue chamber. During the first 200 s, pH increased by 0.4 units and resulted in a 25% PO2 reduction of the detained solution. The rates of these changes were dependent on the volume of solution in the chamber. In experiments using acute transverse slices from the lumbar spinal cord of neonatal (postnatal day 0-10) mice, perfusion stoppage of the same duration was accompanied by a 34.7% enhancement of the peak voltage-gated calcium current recorded from ventral horn neurons. In these cells both low voltage-activated and high voltage-activated currents were affected. These currents were unaffected by decreasing PO2 when a CO2-independent buffer was used, suggesting that changes in pH were responsible for the observed effects. It is concluded that the procedure of stopping a bicarbonate/CO2-buffered perfusate results in rapid changes in pH and PO2 of the solution detained in the tissue chamber, and that these changes have the potential to covertly influence experimental results.