Ca2+-Counteracted Changes of Membrane Functions Induced by Calmodulin Antagonists in Red Beet Storage Tissue, in Pea and Maize Tissues Treated with Optimal Fusicoccin Concentrations

Abstract

Previous work shows that the two calmodulin antagonists trifluoperazine (TFP) and chlorpromazine (CP), applied in vivo to isolated segments from Pea stems maize coleoptiles and roots, enhance acid secretion and hyperpolarize the transmembrane electric potential difference (PD) (maize roots) in the absence and, more markedly, in the presence of a fusicoccin (FC) concentration inducing half maximal stimulation of electrogenic H+ extrusion. The present data show that, when the two drugs are supplied in the presence of optimal FC concentrations, their stimulating effect on acid secretion disappears and in the roots becomes inhibitory and is associated with a depolarization of the PD. A similar inhibition of acid secretion and depolarization of PD is induced by TFP and CP in red beet root disks. TFP and CP induce an increase of the leakage of ninhydrin-reacting substances in Pea stem and maize root segments and of betacyanine in red beet root disks both in the absence and in the presence of FC. These responses to the calmodulin antagonists are partially counteracted by 0.5 mM CaSO4. No correlation is observed between the changes in the leakage of amino acids induced by the drugs and the simultaneous effects of these drugs on acid secretion under the different conditions. These results suggest that the two calmodulin antagonists, when applied with PC at optimal concentration, inhibit electrogenic H+ extrusion. The question remains open, however, whether this effect depends on the inactivation of the Ca2+-calmodulin complex or on consequent changes of free Ca2+ concentration or on non-specific modifications of the membrane structure.