Modulation by extracellular pH of bradykinin-evoked activation of Ca(2+)-activated K+ channels in endothelial cells

We used the patch-clamp technique to investigate, via the activation of Ca(2+)-activated potassium channels [K(Ca2+)channels], the effects of extracellular pH (pHo) on the bradykinin (BK)-stimulated rise in cytosolic Ca2+ concentration in bovine aortic endothelial cells (BAE). In cell-attached experiments, the external application of BK caused a transient activation of the K(Ca2+) channels. Increasing pHo from 7.3 to 9 maintained the channel activity, which was not inhibited by withdrawing the agonist. The channel-activation process could be blocked either by removing external Ca2+ or by depolarizing the cells with a high-K+ external solution. These results indicate that the Ca2+ influx triggered by BK contributes in maintaining the agonist-evoked response in high pHo. Changes in pHo produced a slight increase in the intracellular pH (pHi) measured fluorimetrically with the H+ indicator dye 2',7-bis(carboxyethyl)5(6')-carboxyfluorescein. However, increasing pHi by the external application of NH4Cl at physiological pHo caused a rapid decline and not an increase in the K(Ca2+) channel activity triggered by BK. In fura-2-loaded cells, alkaline pHo had no effect on the time course of the Ca2+ response to BK in external Ca(2+)-free conditions, suggesting that the Ca2+ extrusion process is not affected by pHo. Our results suggest that the BK-evoked Ca2+ influx, which is required to reload internal Ca2+ stores, is controlled by a mechanism depending on extracellular H+.