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© 1995 Oxford University Press

RESEARCH-ARTICLE

Activities of Ca2+ pump and low affinity Ca2+/H+ antiport in plasma membrane vesicles of corn roots

Joaquim A.F. Vicente1,3 and M. Graça P. Vale2

1Departamento de Botânica, Centro de Biologia Celular, Faculdade de Ciências, Universidade de Coimbra 3049 Coimbra, Portugal
2Departamento de Zoologia, Centro de Biologia Celular, Faculdade de Ciências, Universidade de Coimbra 3049 Coimbra, Portugal

3To whom correspondence should be addressed. Fax: + 351 39 20780.

ATP-dependent Ca2+-uptake was investigated in sealed plasma membrane vesicles isolated from corn roots (Zea mays L. cv. Hybrid-3352/Palma-Pioneer). In a chloride-containing medium, at high calcium concentrations, about 30% of the total Ca2+ accumulation ({small tilde}4 nmol Ca2+ mg–1 protein) was shown to be protonophore-sensitive and corresponded to the fraction of Ca2+ not accumulated in a sulphate-containing medium. Furthermore, vesicles in the presence of nitrate, which stimulates H+ transport, or vesicles preloaded with H+, take up Ca2+ more rapidly, suggesting that, at high calcium concentrations, there is a mechanism for Ca2+ transport which depends on the magnitude of the proton gradient across the membrane. The fraction of Ca2+ uptake shown to be sensitive to the protonophore CCCP increased by about 150–200% as the Ca2+ concentration in the medium increased from 50µM to 250µM. Under the same conditions, the CCCP-insensitive fraction of Ca2+ accumulated was reduced by about 25–30% suggesting that different Ca2+ affinities exist in the two Ca2+ uptake processes. Although calmodulin stimulation was not observed, the sensitivity to Ca2+ and external pH indicates that H+ gradient-independent Ca2+ accumulation reflects activity of the Ca2+–pump. These results indicate that the plasma membrane of corn roots contain two distinct mechanisms of Ca2+ transport: a high Ca2+ affinity, proton gradient-independent Ca2+ pump and a low Ca2+ affinity, proton gradient-dependent Ca2+/H+ antiport, which have greatest activity at concentrations of Ca2+ below and above 50+M, respectively.

Key words: Ca2+/H+ antiport, Ca2+ pump, plasmalemma, roots, Zea mays L.


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