PH Modification of Human T-Type Calcium Channel Gating

Abstract

External pH (pH o) modifies T-type calcium channel gating and permeation properties. The mechanisms of T-type channel modulation by pH remain unclear because native currents are small and are contaminated with L-type calcium currents. Heterologous expression of the human cloned T-type channel, α1H, enables us to determine the effect of changing pH on isolated T-type calcium currents. External acidification from pH o 8.2 to pH o 5.5 shifts the midpoint potential ( V 1/2) for steady-state inactivation by 11 mV, shifts the V 1/2 for maximal activation by 40 mV, and reduces the voltage dependence of channel activation. The α1H reversal potential ( E rev) shifts from +49 mV at pH o 8.2 to +36 mV at pH o 5.5. The maximal macroscopic conductance ( G max) of α1H increases at pH o 5.5 compared to pH o 8.2. The E rev and G max data taken together suggest that external protons decrease calcium/monovalent ion relative permeability. In response to a sustained depolarization α1H currents inactivate with a single exponential function. The macroscopic inactivation time constant is a steep function of voltage for potentials < −30 mV at pH o 8.2. At pH o 5.5 the voltage dependence of τ inact shifts more depolarized, and is also a more gradual function of voltage. The macroscopic deactivation time constant ( τ deact) is a function of voltage at the potentials tested. At pH o 5.5 the voltage dependence of τ deact is simply transposed by ∼40 mV, without a concomitant change in the voltage dependence. Similarly, the delay in recovery from inactivation at V rec of −80 mV in pH o 5.5 is similar to that with a V rec of −120 mV at pH o 8.2. We conclude that α1H is uniquely modified by pH o compared to other calcium channels. Protons do not block α1H current. Rather, a proton-induced change in activation gating accounts for most of the change in current magnitude with acidification.