Frog striated muscle is permeable to hydroxide and buffer anions

Area of Science:

• Cell Physiology
• Membrane Transport
• Biophysics

Background:

• Understanding ion and molecule transport across cell membranes is crucial for cellular function.
• The permeability of muscle fibers to buffer anions and hydroxide ions has not been fully elucidated.
• Previous studies have focused on cation permeability, leaving anion permeability less explored.

Purpose of the Study:

• To measure and quantify the permeabilities of hydroxide, bicarbonate, and various buffer anions in Rana pipiens semitendinosus muscle fibers.
• To investigate the contribution of buffer anion and hydroxide permeability to membrane potential changes and intracellular pH.
• To determine the relative permeabilities of different anions, including CAPS, CHES, nitrate, bicarbonate, chloride, and HEPES.

Main Methods:

• Equilibration of muscle fibers in isotonic, high K2SO4 solutions at pH 7.2, buffered with phosphate.
• Measurement of membrane potential changes in response to altered external ion concentrations, with K+ replaced by Cs+ to enhance anion effects.
• Measurement of intracellular pH changes in response to altered external ion concentrations.
• Calculation of relative or absolute permeabilities using constant field equations.

Main Results:

• Significant membrane hyperpolarization was observed with increasing external pH and buffer concentration, indicating anion permeability.
• Calculated relative permeabilities at pH 10.0: POH/PK = 890 ± 150, PCAPS/PK = 12 ± 2, PCHES/PK = 5.3 ± 0.9, PNO3/PK = 4.7 ± 0.5.
• Other measured relative permeabilities: PHCO3/PK = 0.49 ± 0.03 (at pH 9.6), PCl/PK = 18 ± 2 (at pH 8.9), PHEPES/PK = 20 ± 2 (at pH 7.1).
• Intracellular alkalinization was observed upon increasing external pH, attributed to OH- entry and buffer anion absorption of intracellular H+.

Conclusions:

• Both hydroxide ions and buffer anions exhibit significant permeability across the surface membrane of Rana pipiens semitendinosus muscle fibers.
• The measured permeabilities provide quantitative data on anion transport, essential for understanding muscle cell electrophysiology and pH regulation.
• Buffer anion structure and charge influence their respective permeabilities across the muscle fiber membrane.