ELECTROPHYSIOLOGY OF THE SALT-GLANDS OF AVICENNIA GERMINANS

Microelectrode techniques were used on epidermal peels of Avicennia germinans to characterize functional aspects of secretion in salt glands. Increasing the concentrations of K+, Na+, or Mg++ in the bath medium decreased the measured transcellular voltage and resistance. The velocity and magnitude of these transcellular depolarizations varied: K+ > Na+ > Mg++ >> Cl-. Increasing the pH of the incubation medium from 5.5 to 7.0 reduced both the rate and the magnitude of voltage decrease when Na+ concentration was changed, tissue incubated at pH 8.0 did not form secretion droplets. Transcellular depolarizations resulting from concentration shifts of K+ from 10 to 100 mM were partially suppressed in the presence of La+++. The voltage decrease accompanying changes in K+ concentration was partially blocked by tetraethyl ammonium chloride (TEA) but not in Na+, Niflumic acid reduced the magnitude of the transcellular voltage with increased K+ concentration but did not affect the velocity. The small voltage decrease accompanying changes in Cl- concentration was not affected by either TEA or niflumic acid (NA). 2-4-Dinitrophenol (DNP), sodium azide, and vanadate markedly reduced the rapidity and the magnitude of voltage decrease accompanying increases in concentration of K+, Na+, or Mg++. Ion concentrations in secretion droplets indicated that a considerable cation Bur occurred across epidermal peels of, A. germinans in regions where there were salt glands. Salt secretion is best explained by a modified chemiosmotic hypothesis where cation channels and!or permeases work in concert with the electrochemical proton gradient generated by the plasma membrane H+/ATPase.