pH modification of human T-type calcium channel gating.
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, alpha1H, 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 alpha1H 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 alpha1H 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 alpha1H 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 tau(inact) shifts more depolarized, and is also a more gradual function of voltage. The macroscopic deactivation time constant (tau(deact)) is a function of voltage at the potentials tested. At pH(o) 5.5 the voltage dependence of tau(deact) is simply transposed by approximately 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 alpha1H is uniquely modified by pH(o) compared to other calcium channels. Protons do not block alpha1H current. Rather, a proton-induced change in activation gating accounts for most of the change in current magnitude with acidification.
Delisle
B P
BP
Department of Physiology, University of Kentucky College of Medicine, Lexington, Kentucky 40536-0298, USA. bpdeli00@pop.uky.edu
Satin
J
J
D055592:Biophysical Phenomena; D001703:Biophysics; D020747:Calcium Channels, T-Type / Q000235:genetics / Q000378:metabolism*; D002460:Cell Line; D006801:Humans; D006863:Hydrogen-Ion Concentration; D066298:In Vitro Techniques; D015640:Ion Channel Gating; D008564:Membrane Potentials; D009206:Myocardium / Q000378:metabolism; D011994:Recombinant Proteins / Q000235:genetics / Q000378:metabolism; D014162:Transfection
D020747:Calcium Channels, T-Type; D011994:Recombinant Proteins