Role of external calcium in homeostasis of intracellular pH in the cyanobacterium Anabaena sp. strain PCC7120 exposed to low pH

The role of external Ca2+ in the homeostasis of intracellular pH (pHi) of A nabaena sp. strain PCC7120 in response to a decrease in the external pH(pHe x) has been studied in cell suspensions. Increase in cytoplasmic DH after a cid shock is dependent on the presence of Ca2+ in the medium. The observed Ca2+-mediated alkalization of the cytoplasm depends on the extent of the sh ift in external pH. Acid pH shifts resulted in an increased permeability of the cytoplasmic membrane to protons, which could be reversed by increasing the concentration of Ca2+ in the medium. Thus, the ability of Ca2+ to incr ease cytoplasmic pH might be correlated with an inhibition of net proton up take by increasing concentrations of external Ca2+ under these conditions. This combined response resulted in the generation and maintenance of a larg er pH gradient (Delta pH) at acid external pH values. All Ca2+ channel bloc kers tested, such as verapamil and LaCl3, inhibited the observed Ca2+-media ted response. On the other hand, the Ca ionophore calcimycin (compound A231 87) was agonistic, and stimulated both cytoplasmic alkalization and inhibit ion of net proton uptake. The protonophorous uncoupler carbonylcyanide m-ch lorophenyl hydrazone, inhibited this Ca2+-mediated response, whereas monens in, an inhibitor of thr Na+/H+ antiporter, had no significant effect. The r esults of the present study suggest that apr influx of Ca2+ from the extrac ellular space is required for the regulation of cytoplasmic pH in Anabaena sp. strain PCC7120 exposed to low external pH values.