Divalent cation inhibition of barley root plasma membrane‐bound Ca2+‐ATPase activity and its reversal by monovalent cations

The inhibitory action of divalent cations on the Ca2+‐ATPase activity of a plasma membrane‐rich microsome fraction isolated from the roots of barley (Hordeum vulgare L. cv. Conquest) was investigated. Using electron paramagnetic resonance spectroscopy to measure cation‐induced changes in membrane lipid properties, it was demonstrated that certain divalent cations (Ca2+, Cd2+, UO2+2) inhibit the Ca2+ ATP‐ase by restriction of lipid polar head group mobility and not by alteration of membrane surface potential. Monovalent cations which stimulate the Ca2+‐ATPase of barley roots (Na+, K+, ethanolamine HCl) can also reverse the Ca2+‐ATPase inhibition by Cd2+. The degree of Na+ reversal of Cd2+‐induced Ca2+‐ATPase inhibition was influenced by the nature of the anion.