Influence of external pH on two types of low-voltage-activated calcium currents in primary sensory neurons of rats

The influence of extracellular pH (pH(o)) on low-voltage-activated calcium channels of acutely isolated DRG neurons of rats was examined using the whole cell patch-clamp technique. It has been found that in the neurons of middle size with capacitance C=60+/-4.8 pF (mean+/-S.E., n=8) extracellular acidification from pH(o) 7.35 to pH(o) 6.0 significantly and reversibly decreased LVA calcium current densities by 75+/-3.7%, shifted potential for half-maximal activation to more positive voltages by 18.7+/-0.6 mV with significant reduction of its voltage dependence. The half-maximal potential of steady-state inactivation shifted to more positive voltages by 12.1+/-1.7 mV (n=8) and also became less voltage dependent. Dose-response curves for the dependence of maximum values of LVA currents on external pH in neurons of middle size have midpoint pK(a)=6.6+/-0.02 and hill coefficient h=0.94+/-0.04 (n=5). In small cells with capacitance C=26+/-3.6 pF (n=5), acidosis decreased LVA calcium current densities only by 15.3+/-1.3% and shifted potential for half-maximal activation by 5.5+/-1.0 mV with reduction of its voltage dependence. Half-maximal potential of steady-state inactivation shifted to more positive voltages by 10+/-1.6 mV (n=4) and also became less voltage dependent. Dose-response curves for the dependence of maximum values of LVA currents on external pH in neurons of small size have midpoint pK(a)=7.9+/-0.04 and hill coefficient h=0.25+/-0.1 (n=4). These two identified types of LVA currents besides different pH sensitivity demonstrated different kinetic properties. The deactivation of LVA currents with weak pH sensitivity after switching off depolarization to -30 mV had substantially longer decay time than do currents with strong pH sensitivity (tau(d) approximately 5 ms vs. 2 ms respectively). It was found that the prolongation of depolarization steps slows the subsequent deactivation of T-type currents in small DRG neurons. Deactivation traces in these neurons were better described by the sum of two exponentials. Thus, we suppose that T-type channels in small DRG neurons are presented mostly by alpha1I subunit. We suggest that these two types of LVA calcium channels with different sensitivity to external pH can be differently involved in the origin of neuropathic changes.