Zinc inhibition of chloride efflux from skeletal muscle ofRana pipiens and its modification by external pH and chloride activity

Abstract

Efflux of 36Cl- from frog sartorius muscles equilibrated in depolarizing solutions was measured. Cl- efflux consists of a component present at low pH and a pH-dependent component which increases as external pH increases. In depolarized muscles from Rana pipiens, the pH-dependent Cl- efflux has an apparent pKa near 6.4. The reduction of Cl- efflux by external Zn2+ was determined at different external pHs and chloride activities. The effect of external chloride activity on the pH-dependent Cl- efflux was also examined. At pH 6.5 and a membrane potential of -22 mV, increasing external Cl- activity from 0.108 to 0.28 M decreased inhibition of the pH-dependent Cl- efflux at all activities of Zn2+. The Zn2+ activity needed to reduce Cl- efflux by half increased from 0.39 x 10(-3) to 2.09 x 10(-3) M. By contrast, external Cl- activity had no measurable effect on the apparent pKa of the pH-dependent efflux. At constant Cl- activity less than 0.21 M, increasing external pH from 6.5 to 7.5 decreased inhibition by low Zn2+ activities with either a slight increase or no change in the Zn2+ activity producing half-inhibition. In other words, for relatively low Cl- activities, protection against inhibition of Cl- efflux by low Zn2+ activities was obtained by raising, not lowering, external pH; this is not what is expected if H+ and Zn2+ ions compete at the same site to produce inhibition of Cl- efflux. We conclude that Zn2+ and low pH inhibit Cl- efflux by separate and distinct mechanisms.(ABSTRACT TRUNCATED AT 250 WORDS)

Ph-Dependent Inhibition of Voltage-Gated H+ Currents in Rat Alveolar Epithelial Cells by Zn2+ and Other Divalent Cations

“…Lowering pH o decreases the effectiveness of ZnCl 2 . Competition between Zn 2+ and H + has been noted previously for other channels, including Cl − (Hutter and Warner 1967; Spalding et al 1990; Rychkov et al 1997) and K + (Spires and Begenisich 1992, Spires and Begenisich 1994). We consider whether the pH o dependence indicates that (a) the active form is not Zn 2+ but ZnOH + , (b) Zn 2+ and H + compete for the same binding site, or (c) there is noncompetitive inhibition; i.e., protonated channels have a lower affinity for Zn 2+ .…”

“…The absolute concentration of ZnOH + is a small fraction of the total, and >90% of ZnCl 2 is divalent at pH < 8.0, hence [Zn 2+ ] remains relatively constant (Baes and Mesmer 1976). Spalding et al 1990 concluded that ZnOH + was the active form for Cl − currents in muscle. The “consensus” potency sequence for inhibiting H + currents by divalent metal cations, Cu < Zn > Ni > Cd > Co > Mn > Ba, Ca, Mg < 0 (DeCoursey 1998), is intriguingly similar to the tendency of these cations to hydrolyze (Perrin and Dempsey 1974) (indicated by their pK a ): Cu (8.0) > Co (8.9), Zn (9.0) > Ni, Cd (9.9) > Mn (10.6) > Mg (11.4) > Ca (12.6) > Ba (13.4).…”

Distinct voltage‐dependent gating behaviours of a swelling‐activated chloride current in human epithelial cells.

“…10), thus confirming this recent finding. Skeletal muscle chloride channels are known to be blocked by external zinc (Spalding, Taber, Swift & Horowicz, 1990;Hille, 1992), and this effect is reported to be voltage independent (Woll, Leibowitz, Neumcke & Hille, 1987). In the case of the swelling-activated chloride channel, it is possible that divalent metals also act externally, in such a voltageinsensitive manner, to 'facilitate' an inherent voltagedependent inactivation process of the channel.…”

Cation regulation of anion current activated by cell swelling in two types of human epithelial cancer cells.

“…Cations can block membrane Cl-channels. Zn2+ has long been known to inhibit Cl-efflux from skeletal muscle relaxation over the duration of the voltage clamp pulse (Stanfield, 1970;Spalding et al 1990), as do H+ ions (Spalding et al 1990;Ackerman et al 1994), but other divalent cations, specifically Mg2+, seem to be without extracellular action (Spalding et al 1990 (Lauf, Erdmann & Adragna, 1994). Intracellular Mg2+ also regulated the appearance of cell-swelling-activated Cl-channels in neutrophils (Stoddard et al 1993).…”

“…In other systems, other mechanisms for current decay include cation block and ion-dependent inactivation. Zinc ions have long been known to block Clchannels in skeletal muscle (Stanfield, 1970;Spalding, Taber, Swift & Horowicz, 1990), and the trivalent cations La3+ and Gd3+ also block cell-swelling-activated Clchannels (Ackerman, Wickman & Clapham, 1994). Mg2+ ions are well known to block a wide variety of cation channels.…”