Ammonium inhibition of Ca(2+)-dependent inwardly rectifying K+ currents in HeLa cells

pH modulation of currents that contribute to the medium and slow afterhyperpolarizations in rat CA1 pyramidal neurones

We examined the effects of changes in pH(o) and pH(i) on currents contributing to the medium and slow afterhyperpolarizations (mI(AHP) and sI(AHP), respectively) in rat CA1 neurones. Reducing pH(o) from 7.4 to 6.7 inhibited mI(AHP) and sI(AHP) whereas increasing pH(o) to 7.7 augmented mI(AHP) and, to a greater extent, sI(AHP). The ability of changes in pH(o) to modulate mI(AHP) reflected changes in the Ca(2+)-activated K(+) current, I(AHP), and a Co(2+)- and XE991-resistant component of mI(AHP), but not the muscarine-sensitive current, I(M). In the presence of 1 microM TTX and 5 mM TEA, low pH(o)-evoked reductions in sI(AHP) were associated with reductions in Ca(2+)-dependent depolarizing potentials; because neither effect was attenuated when internal H(+) buffering power was raised by including 100 mm tricine in the patch pipette, the actions of reductions in pH(o) to inhibit sI(AHP) and, possibly, I(AHP) in large part appear to reflect a low pH(o)-dependent decrease in Ca(2+) influx. In contrast, the effects of high pH(o) to augment mI(AHP) and sI(AHP) were associated with relatively small increases in Ca(2+) potentials but were significantly attenuated by 100 mM internal tricine, indicating that a rise in pH(i) consequent upon the rise in pH(o) was largely responsible. The possibility that changes in pH(i) could act to modulate mI(AHP) and sI(AHP), independently of changes in Ca(2+) influx, was also suggested by experiments in which pH(i) was lowered at a constant pH(o) by the external application of propionate or by the withdrawal of HCO(-)(3) from the perfusing medium. Lowering pH(i) at a constant pH(o) had little effect on Ca(2+) potentials but inhibited mI(AHP) and, to a greater extent, sI(AHP), effects that were attenuated by 100 mM internal tricine. Together, the results indicate that changes in pH(o) and pH(i) modulate mI(AHP) and sI(AHP) in rat CA1 neurones and suggest that, depending on the direction of the pH(o) change, the sensitivities of the underlying currents to changes in Ca(2+) influx and/or pH(i) may contribute to the effects of changes in pH(o) to modulate mI(AHP) and sI(AHP).

NH(4)Cl inhibition of acid secretion: possible involvement of an apical K(+) channel in bullfrog oxyntic cells

This study was undertaken to determine the mechanism by which ammonium chloride (NH(4)Cl) inhibits stimulated acid secretion in the bullfrog gastric mucosa. To this end, four possible pathways of inhibition were studied: 1) blockade of basolateral K(+) channel, 2) blockade of ion transport activity, 3) neutralization of secreted H(+) in the luminal solution, or 4) ATP depletion. Addition of nutrient 10 mM NH(4)Cl (calculated NH(3) concentration = 92.5 microM and NH(4)(+) concentration = 9.91 mM) inhibited acid secretion within 30 min. Inhibition of acid secretion did not occur by blockade of basolateral K(+) channel activity or ion transport activity or by neutralization of the luminal solution. Although ATP depletion occurred in the presence of NH(4)Cl, the magnitude of ATP depletion in 30 min was not sufficient to inhibit stimulated acid secretion. By comparing the effect of NH(4)Cl on the resistance of inhibited or stimulated tissues, we demonstrate that NH(4)Cl acts specifically on stimulated tissues. We propose that NH(4)Cl blocks activity of an apical K(+) channel present in stimulated oxyntic cells. Our data suggest that the activity of this channel is important for the regulation of acid secretion in bullfrog oxyntic cells.

Swelling-activated potassium currents of Ehrlich ascites tumour cells

The K+ and Cl- currents activated by Ca2+-ionophore treatment or by hypotonic cell swelling have been studied in Ehrlich ascites tumour cells by the patch-clamp technique. A charybdotoxin-inhibitable K+ current was activated by increasing intracellular Ca2+ concentration. In contrast, the K+ current activated by cell swelling was insensitive to charybdotoxin as well as to apamin, suggesting that channels different from those sensitive to Ca2+ are responsible for regulatory volume adjustments in these cells. The magnitude of the K+ and Cl- currents activated by hypotonic challenge was markedly temperature-dependent, possibly reflecting the temperature-dependence of enzymes involved in the intracellular signalling of cell volume regulation.

Possible thiol group involvement in intracellular pH effect on low-conductance Ca(2+)-dependent K+ channels

We have studied the effect of intracellular pH (pHi) shifts on the activity of Ca(2+)-dependent, inwardly rectifying K+ channels of HeLa cells. Recordings of macroscopic currents in symmetrical 145 mK K+ and internal pH of 7.4 gave moderate inward rectification of the current. At pH 6.4, inward rectification was more marked, whereas it switched to outward rectification at pH 8.2. In excised inside-out membrane patches, similar changes in pHi did not affect the single-channel conductance of the channels underlying the Ca(2+)-dependent K+ currents. At neutral pH, the open state probability (Po) was independent of voltage in the range from -70 to 70 mV. At alkaline pH, Po became voltage dependent, decreasing at negative potentials and increasing with depolarization compared with pH 7.4. These changes accounted for the pH-dependent changes in rectification of the macroscopic current. The possibility that voltage dependence might arise from the ionization of a thiol group was tested by using thiol-directed reagents. The decrease in Po produced by intracellular alkalinization at negative potential was reverted by treatment with N-ethylmaleimide, 5,5'-dithiobis(2-nitrobenzoic acid), and 2,2'-dithiodipyridine. The effect of intracellular alkalinization is speculated to occur through ionization of a cysteine group(s) within the field of the membrane affecting gating.