Permeabilities of teleost and elasmobranch gill apical membranes

Hill, W. G.; Mathai, J. C.; Gensure, R. H.; Zeidel, J. D.; Apodaca, G.; Sáenz, J. P.; Kinne-Saffran, E.; Kinne, R.; Zeidel, M. L. (2004) American Journal of Physiology. Cell Physiology, 287(1), C235–242. DOI: 10.1152/ajpcell.00017.2004

Summary

This study measured the permeabilities of gill apical membranes from teleost (winter flounder) and elasmobranch (dogfish shark) species to water, urea, and ammonia. Using stopped-flow fluorometry on isolated apical membrane vesicles, the authors found that while gill apical membranes have low permeabilities, they are not low enough to fully account for the barrier function observed in vivo. The results indicate that additional mechanisms beyond the lipid bilayer — such as tight junctions or mucus layers — must contribute to maintaining osmotic and concentration gradients across gill epithelia.

Key Findings

• Gill apical membrane water permeabilities were low (~7 x 10^-4 cm/s) but similar between teleost and elasmobranch species
• Basolateral membranes from shark gill showed approximately twofold higher water permeability than apical membranes
• Apical membrane permeabilities alone cannot account for the low water loss (teleosts) and urea loss (elasmobranchs) measured in vivo
• Additional barrier mechanisms beyond lipid bilayer composition must exist in gill epithelia

Our Contribution

Sáenz contributed to membrane vesicle preparation and permeability measurements as an undergraduate researcher at the Mount Desert Island Biological Laboratory.

Significance

This early work on membrane barrier function established that lipid bilayer properties alone do not explain epithelial permeability barriers, a concept that connects to the broader question of how membrane composition governs biophysical function.