THE PRESENCE OF H-LINKED CA2+ EXTRUDING SYSTEMS IN METHANOBACTERIUM-THERMOAUTOTROPHICUM( AND NA+)

The effects of monovalent cations (Na+, K+ and choline(+)) and the unc oupler 3,3',4',5-tetrachlorosalicylanilide (TCS) were tested on Ca-4S( 2+) uptake by non-energized cells of Methanobacterium thermoautotrophi cum. Ca-45(2+) uptake was stimulated by the addition of K+ and (less) by choline(+) while Na+ slowed down and even reversed it, thereby mimi cking the energization of cells. The uncoupler agent, TCS, suppressed Ca-45(2+) uptake in non-energized cells in the presence or absence of Na+ but in cells energized in an atmosphere of CO2 + H-2 it exerted a stimulating effect. Uncoupled Ca-45(2+) efflux was measured in cells p re-loaded with Ca-45(2+) by means of the divalent ionophore A23187 fol lowing its washing out by buffer containing serum albumin. The efflux was temperature-dependent and was stimulated by external Ca-40(2+) and Na+. In the absence of Na+, the uncoupled efflux was completely inhib ited by TCS, whereas in the presence of Na+, TCS was without any effec t. The results are in agreement with the model in which the Ca2+ influ x pathway is represented by a membrane potential-driven uniport wherea s Ca2+ efflux is mediated by two transport systems - Na+/Ca2+ and H+/C a2+ antiporters whose participation in the total efflux is dependent o n the energy of the corresponding gradients of driving ions.