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Membrane permeability to the molecular and ionic forms of DMO in barnacle muscle
Abstract:
The time course of the intracellular pH (pHi) of barnacle muscle fibers was followed using microelectrodes while the fibers were exposed to 15 mM of the weak acid 5,5-dimethyloxazolidine-2,4-dione (DMO) at pH 6.5. The rapid initial fall in pHi was used to determine the membrane permeability to the DMO molecule, while the much slower fall during the subsequent plateau phase yielded the permeability to the DMO ion. This experimental approach and the mathematical treatment of the data can be used to obtain the membrane permeabilities to other weak acids or bases. We found values of 1.9 X 10(-4) cm/s for the permeability to the DMO molecule and 1.5 X 10(-7) cm/s for the permeability to the ion, assuming the fiber to be a cylinder. Thus the permeability of the neutral form is about 10(3) times that of the ionized form. At commonly encountered conditions of pHi = 7.3, outside pH = 7.5, and membrane potential = -52 mV, this permeability ratio introduces an error of only -0.01 to -0.02 into the determination of pHi based on the distribution of DMO.
Insights
Barnacle muscle fibers
Area of Science:
- Cellular Physiology
- Membrane Transport
- Biophysics
Background:
- Intracellular pH (pHi) regulation is crucial for cellular function.
- Weak acids and bases are commonly used to study membrane permeability.
- 5,5-dimethyloxazolidine-2,4-dione (DMO) is a weak acid often used for pHi measurements.
Purpose of the Study:
- To determine the membrane permeability of barnacle muscle fibers to both the neutral and ionized forms of DMO.
- To establish a method for measuring membrane permeabilities of weak acids/bases using pHi changes.
- To assess the impact of DMO permeability on pHi measurements.
Main Methods:
- Microelectrode measurements of intracellular pH (pHi) in barnacle muscle fibers.
- Exposure of fibers to 5,5-dimethyloxazolidine-2,4-dione (DMO) at a controlled external pH.
- Analysis of the time course of pHi changes to differentiate between DMO molecule and ion permeability.
- Mathematical modeling to calculate membrane permeability coefficients.
Main Results:
- The membrane permeability to the neutral DMO molecule was determined to be 1.9 x 10(-4) cm/s.
- The membrane permeability to the ionized DMO form was found to be 1.5 x 10(-7) cm/s.
- The permeability of the neutral DMO form is approximately 1000 times greater than the ionized form.
Conclusions:
- The neutral form of DMO permeates barnacle muscle membranes significantly more readily than its ionized form.
- The differential permeability of DMO forms has minimal impact on pHi determination under physiological conditions.
- The described method provides a robust approach for quantifying membrane permeability to weak acids and bases.