Energy-Dependent Ca²⁺ Efflux from the Cells of Nostoc calcicola Bréb: Role of Modifying Factors

Abstract.

Energy-dependent Ca²⁺ efflux and its regulation from the diazotrophic cyanobacterium Nostoc calcicola Bréb has been investigated. Like Ca²⁺ uptake, Ca²⁺ efflux pattern also reflected a rapid phase for the first 10 min followed by a slower one lasting up to 1 h with a total of 80 nmol Ca²⁺ mg⁻¹ protein (31% of the Ca²⁺ concentration taken in by such cells at 1 h). Ca²⁺ efflux kinetics remained hyperbolic with a K _m of 1.9 mM and V_max 5.5 nmol mg⁻¹ protein min⁻¹. Ca²⁺ efflux to a major extent depended on photosynthetic energy generation as the cells facing dark incubation and addition of 3-(3,4-dichlorophenyl)-1-dimethyl urea (DCMU) to light-grown cells showed significant reduction in Ca²⁺ extrusion. The strong inhibition in Ca²⁺ efflux by addition of metabolic inhibitors like carbonyl cyanide-p-nitrofluoromethoxylphenyl hydrazone (FCCP) and N,N,-dicyclohexylcarbo-diimide (DCCD) suggested the vital role of membrane potential and ATP hydrolysis in driving this process. Verapamil (Ca²⁺ antagonist) had insignificant effect on Ca²⁺ efflux, whereas the addition of Calmodulin antagonists like trifluoroperazine, W-7 and compound 48/80 resulted in the enhancement in Ca²⁺ efflux over control sets, thus suggesting that this increase may be owing to the additional extrusion of intracellular free calcium that was unable to bind with calmodulin in the presence of these antagonists.