Calcium-activated K+ channel (KCa3.1) activity during Ca2+ store depletion and store-operated Ca2+ entry in human macrophages

STIM1 'senses' decreases in endoplasmic reticular (ER) luminal Ca2+ and induces store-operated Ca2+ (SOC) entry through plasma membrane Orai channels. The Ca2+/calmodulin-activated K+ channel KCa3.1 (previously known as SK4) has been implicated as an 'amplifier' of the Ca2+-release activated Ca2+ (CRAC) current, especially in T lymphocytes. We have previously shown that human macrophages express KCa3.1, and here we used the whole-cell patch-clamp technique to investigate the activity of these channels during Ca2+ store depletion and store-operated Ca2+ influx. Using RT-PCR, we found that macrophages express the elementary CRAC channel components Orai1 and STIM1, as well as Orai2, Orai3 and STIM2, but not the putatively STIM1-activated channels TRPC1, TRPC3-7 or TRPV6. In whole-cell configuration, a robust Ca2+-induced outwardly rectifying K+ current inhibited by clotrimazole and augmented by DC-EBIO could be detected, consistent with KCa3.1 channel current (also known as intermediate-conductance IK1). Introduction of extracellular Ca2+ following Ca2+ store depletion via P2Y2 receptors induced a robust charybdotoxin (CTX)- and 2-APB-sensitive outward K+ current and hyperpolarization. We also found that SOC entry induced by thapsigargin treatment induced CTX-sensitive K+ current in HEK293 cells transiently expressing KCa3.1. Our data suggest that SOC and KCa3.1 channels are tightly coupled, such that a small Ca2+ influx current induces a much large KCa3.1 channel current and hyperpolarization, providing the necessary electrochemical driving force for prolonged Ca2+ signaling and store repletion.