Down regulation of the plasmalemma H+ pump activity by nicotine-induced intracellular alkalinization. A balance between base accumulation, biochemical pH-stat response and intracellular pH increase.

Nicotine was used to induce an intracellular alkalinization in Elodea densa leaves in order to study the regulation of the plasmalemma H+ pump activity by alkaline intracellular pH values. Nicotine was found to enter the cells rapidly in the uncharged form and to induce a significant intracellular pH increase, measured either directly as cell sap pH or as vacuolar and cytoplasmic pH by calculation from the distribution at equilibrium of labelled pH probes. The nicotine-induced alkalinization was associated with a progressive decrease in K+ uptake. A strong inhibition of net H+ efflux was also evident in the presence of K+ in the external medium, whereas no nicotine effect on net H+ efflux was detected in the absence of K+ (in spite of the larger accumulation of nicotine in the tissue) in agreement with a down-regulation of the activity of the K+-dependent plasmalemma H+-ATPase by alkaline intracellular pH values. The increase in vacuolar pH resulting from nicotine accumulation was small compared to the base load calculated from the vacuolar buffer capacity and the intracellular dissociation of nicotine. Conversely, the nicotine-induced increase in cytoplasmic pH was considerably larger than expected on the basis of the cytoplasmic buffer capacity and of the theoretical accumulation of nicotine in the experimental conditions adopted. A balance sheet between nicotine accumulation, intracellular alkalinization and malate system response was drawn up, and the seeming discrepancies observed were discussed.