Displacement of Ca2+ by Na+ from the Plasmalemma of Root Cells

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Abstract
A microfluorometric assay using chlorotetracycline (CTC) as a probe for membrane-associated Ca2+ in intact cotton (Gossypium hirsutum L. cv Acala SJ-2) root hairs indicated displacement of Ca2+ by Na+ from membrane sites with increasing levels of NaCl (0 to 250 millimolar). K+(86Rb) efflux increased dramatically at high salinity. An increase in external Ca2+ concentration (10 millimolar) mitigated both responses. Other cations and mannitol, which did not affect Ca2+-CTC chelation properties, were found to have no effect on Ca2+-CTC fluorescence, indicating a Na+-specific effect. Reduction of Ca2+-CTC fluorescence by ethyleneglycol-bis-(β-aminoethyl ether) N,N′-tetraacetic acid, which does not cross membranes, provided an indication that reduction by Na+ of Ca2+-CTC fluorescence may be occurring primarily at the plasmalemma. The findings support prior proposals that Ca2+ protects membranes from adverse effects of Na+ thereby maintaining membrane integrity and minimizing leakage of cytosolic K+.