Electrogenic proton pump in Nitella and the effect of calcium.

The hyperpolarisation of the membrane potential in Characeae above that of the diffusion potential is explained by the operation of the electrogenic proton pump. We studied the interaction of calcium with the functioning of the pump. The membrane potential was measured using the standard microelectrode technique. An increase in the calcium concentration resulted in depolarisation, its magnitude increasing with lower proton concentrations. Calcium-induced membrane potential changes, tested in the concentration range of 0.25 mmol/l to 25 mmol/l, were greatest at 0.25 mmol/l CaCl2 and decreased with the increasing calcium concentration. Light-induced initial changes in the membrane potential also showed a dependence on the presence of calcium in the external medium. We conclude that calcium has a role in the regulation of the proton pump in Nitella.