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 flux 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.