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Extracellular space and diffusion barriers in muscle fibres from Megabalanus psittacus (Darwin)
Abstract:
1. Muscle fibres from Megabalanus psittacus (Darwin) were used to measure the exchange of Na+ and Ca2+ between intracellular and extracellular compartments. 2. The size of the extracellular space was evaluated directly from electron micrographs at 6.1 +/- 0.5% (percentage of the fibre volume). The more conventional estimates using Na+ and 134Cs+ as space markers gave higher values, namely, 8.9 +/- 0.7 and 9.5 +/- 1.5%. 3. An average value of 9.2% was used to correct the total Na+ and K+ in the muscle fibres and to estimate the intracellular concentrations of Na+ and K+ as 39 +/- 4 and 202 +/- 11 mM respectively. 4. Na+ (and similar Ca2+) washout curves could be described using a three compartments diffusion model. The data analysed in terms of this model enabled us to estimate membrane permeabilities as PNa+ = 2.7 x 10(-7) and PCa2+ = 2.7 x 10(-7) cm/sec.
Insights
This study quantifies ion exchange in Megabalanus psittacus muscle fibers, revealing intracellular sodium (Na+) and calcium (Ca2+) concentrations and membrane permeabilities. Findings provide insights into ion transport mechanisms in marine invertebrates.
Area of Science:
- Marine biology
- Cellular physiology
- Biophysics
Background:
- Understanding ion transport is crucial for cellular function.
- Marine invertebrates possess unique physiological adaptations to their environment.
- Megabalanus psittacus, a large barnacle, offers a model for studying ion dynamics.
Purpose of the Study:
- To measure the exchange of sodium (Na+) and calcium (Ca2+) ions between intracellular and extracellular compartments in Megabalanus psittacus muscle fibers.
- To determine intracellular ion concentrations and membrane permeabilities.
- To evaluate methods for estimating extracellular space in muscle tissue.
Main Methods:
- Utilized muscle fibers from Megabalanus psittacus for ion exchange measurements.
- Employed electron microscopy to directly assess extracellular space.
- Applied Na+ and 134Cs+ as space markers for conventional extracellular space estimation.
- Analyzed ion washout curves using a three-compartment diffusion model.
Main Results:
- Direct electron microscopy estimated extracellular space at 6.1% ± 0.5% of fiber volume.
- Conventional methods yielded higher extracellular space estimates (8.9%–9.5%).
- Intracellular concentrations were estimated at Na+: 39 ± 4 mM and K+: 202 ± 11 mM.
- Estimated membrane permeabilities for Na+ (PNa+) and Ca2+ (PCa2+) were both 2.7 x 10^-7 cm/sec.
Conclusions:
- Established intracellular ion concentrations and extracellular space in Megabalanus psittacus muscle.
- The three-compartment diffusion model effectively describes ion washout kinetics.
- Quantified membrane permeabilities provide a basis for understanding ion flux regulation in this species.