Some observations on the behaviour of chloride current–voltage relations in Xenopus muscle membrane in acid solutions

Abstract

Chloride current--voltage relations in Xenopus laevis muscle membrane have been investigated in phosphate-buffered solution (pH 5.2--5.4) using a three-microelectrode voltage clamp. Resting chloride conductance in these conditions is about 10(-4) S/cm2, approximately 1/10th that at pH 8.8. When the membrane potential is stepped from the holding (resting) potential to a more negative voltage the current rises from the initial to the steady state. The instantaneous current--voltage relation is linear and the steady-state relation shows inward-going rectification. As hyperpolarization appears to "activate" the chloride conductance, the "availability" of chloride current has been measured at the beginning of a voltage step to a standard test potential following conditioning at a variety of potentials. The relationship between the test current and the conditioning voltage is sigmoid. The normalized sigmoid curve has the same slope (absolute value) but opposite sign to that obtained in the same experiment at pH 8.8. In mildly acidic solutions (pH 6.4) the current wave form is diphasic: current initially falls then rises to the steady state. This combination of transients militates against the idea that transients are due solely to accumulation--depletion effects in restricted spaces ("unstirred layers") and a hypothesis is qualitatively outlined in which pH-and voltage-dependent effects are ascribed to a single type of channel whose orientation in the membrane is unconstrained.

Nitrate and chloride ions have different permeation pathways in skeletal muscle fibers ofRana pipiens

“…The currents had not reached a steady level at the end of the 600-msec pulses. This kind of CI-current behavior has been extensively studied by several investigators in other muscle preparations (Warner, 1972;Loo et al, 1980;Vaughan et al, 1980), and our results demonstrate that it occurs in lumbricalis fibers as well. Table 2.…”

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

“…Further, salicylate and other aromatic anions appear to stimulate exchange diffusion of Ct-in frog muscle (Venosa, Ruarte & Horowicz, 1972). On the other hand, there is also evidence for a measurable, though small, C1-conductance in acid solutions (Hurter & Warner, 1967c;Vaughan & Fong, 1978;Loo, McLarnon & Vaughan, 1981). The proportion of C1-efflux through each of these pathways is not well determined.…”

The pH dependence of chloride net flux in skeletal muscle fibres of Rana temporaria.

“…Figure 1 also shows that the apparent point of switch from activation of PC1 at low pH and inactivation at high pH occurs at a pH value in the range between 6-4 and pH DEPENDENCE OF CG-NET FLUXES 241 7-2, i.e. a higher value than that (pH 6A4-6 5) of a similar switch for the response of the steady state gcl to large hyperpolarizing pulses in normally polarized muscle (Warner, 1972;Loo et al 1981). …”

“…Kinetic analyses of this behaviour suggest (i) that changes of permeability rather than transport number effects due to accumulation or depletion of ions underlie the changes of gcl (e.g. Vaughan, McLarnon & Loo, 1980), (ii) that the same gating mechanism is involved in both the activation at low pH and the inactivation at high pH (Loo, McLarnon & Vaughan, 1981), and (iii) that the state of the chloride conductance mechanism is related to the chloride equilibrium potential, Vcl eq' rather than to the membrane potential, Vm, itself (Vaughan et al 1980;Vaughan & Trotter, 1982). One event by which a perturbation from Vci,eq might provide energy to affect the system is the onset of Cl-current.…”