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

Summary

The effects of pH on the permeability and conductance of the membranes to nitrate and to chloride of semitendinosus and lumbricalis muscle fibers were examined.

Membrane potential responses to quick solution changes were recorded in semitendinosus fibers initially equilibrated in isotonic, high K₂SO₄ solutions. External solutions were first changed to ones in which either Rb⁺ or Cs⁺ replaced K⁺ and then to solutions containing either NO ⁻₃ or Cl⁻ to replace SO ²⁻₄ . The hyperpolarizations produced by Cl⁻ depend on external pH, being smaller in acid than in alkaline solutions. By contrast, hyperpolarizations produced by NO ⁻₃ were independent of external pH over a pH range from 5.5 to 9.0.

In addition, voltage-clamp measurements were made on short lumbricalis muscle fibers. Initially they were equilibrated in isotonic solutions containing mainly K₂SO₄ plus Na₂SO₄. KCl or KNO₃ were added to the sulfate solutions and the fibers were equilibrated in these new solutions. When finally equilibrated the fibers had the same volume they had in the sulfate solutions before the additions. Constant hyperpolarizing voltage pulses of 0.6-sec duration were applied when all external K⁺ was replaced by TEA⁺. For these conditions, inward currents flowing during the voltage pulses were largely carried by Cl⁻ or NO ⁻₃ depending on the final equilibrating solution. Cl⁻ currents during voltage pulses were both external pH and time dependent. By contrast, NO ⁻₃ currents were independent of both external pH and time.

The voltage dependence of NO ⁻₃ currents could be fit by constant field equations with aP _{NO 3} of 3.7·10⁻⁶ cm/sec. The voltage dependence of the initial or “instantaneous” Cl⁻ currents at pH 7.5 and 9.0 could also be fit by constant field equations with P_Cl of 5.8·10⁻⁶ and 7.9·10⁻⁶ cm/sec, respectively. At pH 5.0, no measurable “instantaneous” Cl⁻ currents were found.

From these results we conclude that NO ⁻₃ does not pass through the pH, time-dependent Cl⁻ channels but rather passes through a distinct set of channels. Furthermore, Cl⁻ ions do not appear to pass through the channels which allow NO ⁻₃ through. Consequently, the measured ratio ofP _Cl/P _{NO 3} based on membrane potential changes to ionic changes made on intact skeletal muscle fibers is not a measure of the selectivity of a single anion channel but rather is a measure of the relative amounts of different channel types.