Effects of the external pH on Ca channels: experimental studies and theoretical considerations using a two-site, two-ion model.

Abstract

Some effects of the external pH on Ca channels were studied in a hybridoma cell line (mAb-7B), by using the whole-cell configuration of the patch-clamp technique. As the pH was lowered, both the activation and the inactivation curves shifted toward less negative membrane potentials, suggesting a pH-induced decrease of an external negative surface potential, sensed by the mechanism of gating. The potential for half-activation, V1/2, and that for half-inactivation, Vh, were related by a straight line with a slope of one. The inward current varied exponentially with V1/2, as would be expected if the field inside the channel and the Ca2 concentration at the entrance were sensitive to the surface potential. However, the reversal potential and the outward current were unaltered by changes in the pH. Under the hypothesis that the channel senses the surface potential, all these results, as well as the nernstian behavior of the reversal potential with respect to Ca2+, observed in previous studies, are accounted for by a three-barrier, two-ion model for a channel, provided it is assumed that the potential in the channel drops almost entirely across the barrier adjacent to the external solution.Voltage-gated Ca channels are found in hybridoma cell lines constructed by fusion of mouse myeloma cell line S194 and mouse splenic B lymphocytes (1). Voltage-clamp type analyses revealed that the current through the channel is carried in the inward direction by external Ca ions and in the outward direction by internal alkali cations. No other types of voltage-gated ion channels were found in these cells (2).In the present experiments, the pH of the external solution was changed keeping all other external and internal ion concentrations constant. The gating kinetics of the membrane current shifted in the positive direction along the voltage axis when the external pH was reduced. This indicates that the surface potential is altered by the external H+ concentration and that the potential changes are sensed by the gating mechanism. The aim of this work is to investigate whether or not a similar surface potential is also sensed by the channel itself. In studies of Ca channels some investigators have simply assumed that surface potential changes alter the field in the channel as well as in the surrounding lipid area, whereas others have asserted that only gating is altered, not the channel conductance. In most cases the assumptions had no clear experimental support. The same question has also been raised for the Na channel, leading to different conclusions depending on the type of preparation (3, 4). The conclusion that the Na channel in the node of Ranvier does not sense the surface potential (4) has been deduced from the behavior of the outward current, which was found to be insensitive to the ionic strength and the external pH. Difficulties in isolating the outward current have been the major obstacle to performing equally significant experiments with Ca channels.However, in the present preparation we can record clear outward ...

Hydrogen ion modulation of Ca channel current in cardiac ventricular cells. Evidence for multiple mechanisms.

“…Furthermore, increases in current upon alkalization are not unique to cardiac Ca channels. Iijima et al (1986) have reported the same observation for Ca channels in a mouse hybridoma cell line and Hille et al (1975) found the same effect for Na channels. Presumably there is a site that is only partially titrated at pH 7.4.…”

“…A recent study by Iijima et al (1986) investigated the effects of extracellular H ions on Ca currents in a mouse hybridoma cell line. They came to the conclusion that gating shifts and surface concentration changes were responsible for the increase or decrease of the Ca current magnitude as a function of pHo.…”

Regulation of Postsynaptic Ca2+Influx in Hippocampal CA1 Pyramidal Neurons via Extracellular Carbonic Anhydrase

“…Among these, the low threshold channels may have been partially inhibited by benzolamide (Gottfried and Chesler, 1995), but, in the somata and proximal dendrites, high threshold Ca 2ϩ channels would predominate (Christie et al, 1995;Magee and Johnston, 1995). There is evidence that a small rise in pH e can enhance high threshold Ca 2ϩ currents (Iijima et al, 1986;Barnes and Bui, 1991;Chen et al, 1996;Tombaugh and Somjen, 1996). It is therefore plausible that benzolamide, by increasing the alkaline transients, may have augmented Ca 2ϩ influx through both NMDARs and voltage-gated Ca 2ϩ channels.…”

Endogenous Alkaline Transients Boost Postsynaptic NMDA Receptor Responses in Hippocampal CA1 Pyramidal Neurons

“…These Ca 2ϩ channels could be involved because of their own sensitivity to external pH (Iijima et al, 1986;Barnes and Bui, 1991;Chen et al, 1996) or because of depolarization-mediated, mutually reinforcing interactions with NMDARs.…”

“…L-type Ca 2ϩ channels are sensitive to extracellular pH near the physiological range (Iijima et al, 1986;Barnes and Bui, 1991;Chen et al, 1996) and might therefore be influenced by the enhanced buffering caused by XCA. With the L-type channel antagonist D-600 in the patch pipette (Schiller, 2004), there was no effect on the baseline ADP, and the addition of XCA still caused a significant curtailment of the ADP in seven cells.…”