ENDOGENOUS K+ H+ EXCHANGE ACTIVITY IN THE SF9 INSECT-CELL LINE/

The fluorescent pH indicator 2',7'-bis(carboxyethyl)-5,6-carboxyfluore scein (BCECF) was used to investigate changes in the intracellular pH (pH(i)) of individual Sf9 cells in response to changes in the composit ion of the external medium. Under standard conditions, the resting pH( i) was 0.2-0.3 unit lower than the extracellular pH (6.5). The extrace llular concentration of K+ had a major influence on pH(i). Removal of K+ from the medium resulted in a rapid but reversible acidification of the cells. The buffer capacity of the cells was a U-shaped function o f pH(i) with a minimum at about pH(i) = 6.2. In the presence of K+, a change in the pH of the medium was followed by an equivalent change in pK(i). In its absence, however, changes in the external pH had little effect on the pH of the cells. Following removal of K+ from the mediu m, the cells realkalinized at an initial rate which increased with inc reasing concentration of added K+. This cation was about 30 times more effective in promoting realkalinization of the cells than Li+, Na+, R b+, and Cs+. The apparent K-m for K+-dependent H+ efflux was about 12 mM and was slightly modulated by extracellular pH. These results stron gly suggest that, in Sf9 cells, a K+/H+ antiporter plays a key role in the movement of protons across the cell membrane.