Is ATP required for K⁺ channel activation in Vicia guard cells?

In vivo, K⁺ entry into guard cells via inward-rectifying K⁺ channels is indirectly driven by ATP via an H⁺-ATPase that hyperpolarizes the membrane potential. However, whether activation of the K⁺ channels of guard cells requires ATP remains unknown. In the present study, both whole-cell and single-channel patch-clamp techniques were used to address this question. Exogenous ATP, ADP, and adenosine-5′-O-(3-thiotriphosphate) applied to the cytoplasm had no effect on whole-cell K⁺ currents of Vicia faba L. guard cells. Azide, an inhibitor of oxidative phosphorylation, also had no effect. However, an ATP-scavenging system, glucose plus hexokinase, inhibited whole-cell inward K⁺ currents by 30 to 40%. Single-channel results acquired from cytoplasm-free inside-out membrane patches showed definite activation of inward K⁺ channels by ATP. Other nucleotides, such as ADP, adenosine-5′-O-(3-thiotriphosphate), and GTP, did not increase channel activity in the membrane patches. Inward K⁺ channel activity in membrane patches preactivated by exogenous ATP was inhibited by glucose plus hexokinase. These results suggest that a low concentration of ATP is required for activation of the inward K⁺ channels of the guard-cell plasma membrane. The issue of how ATP as a signal regulates these K⁺ channels is discussed.