Quantification of the functional expression of the Ca 2+ ‐activated K + channel K Ca 3.1 on microglia from adult human neocortical tissue

The K Ca 3.1 channel ( KCNN4 ) is an important modulator of microglia responses in rodents, but no information exists on functional expression on microglia from human adults. We isolated and cultured microglia (max 1% astrocytes, no neurons or oligodendrocytes) from neocortex surgically removed from epilepsy patients and employed electrophysiological whole‐cell measurements and selective pharmacological tools to elucidate functional expression of K Ca 3.1. The channel expression was demonstrated as a significant increase in the voltage‐independent current by NS309, a K Ca 3.1/K Ca 2 activator, followed by full inhibition upon co‐application with NS6180, a highly selective K Ca 3.1 inhibitor. A major fraction (79%) of unstimulated human microglia expressed K Ca 3.1, and the difference in current between full activation and inhibition (ΔK Ca 3.1) was estimated at 292 ± 48 pA at −40 mV ( n  = 75), which equals at least 585 channels per cell. Serial K Ca 3.1 activation/inhibition significantly hyperpolarized/depolarized the membrane potential. The isolated human microglia were potently activated by lipopolysaccharide (LPS) shown as a prominent increase in TNF‐α production. However, incubation with LPS neither changed the K Ca 3.1 current nor the fraction of K Ca 3.1 expressing cells. In contrast, the anti‐inflammatory cytokine IL‐4 slightly increased the K Ca 3.1 current per cell, but as the membrane area also increased, there was no significant change in channel density. A large fraction of the microglia also expressed a voltage‐dependent current sensitive to the K Ca 1.1 modulators NS1619 and Paxilline and an inward‐rectifying current with the characteristics of a K ir channel. The high functional expression of K Ca 3.1 in microglia from epilepsy patients accentuates the need for further investigations of its role in neuropathological processes. GLIA 2016;64:2065–2078