Cloning and functional expression of a liver isoform of the small conductance Ca²⁺-activated K⁺ channel SK3

Abstract

Small conductance Ca2+-activated K+ (SK) channels have been cloned from mammalian brain, but little is known about the molecular characteristics of SK channels in nonexcitable tissues. Here, we report the isolation from rat liver of an isoform of SK3. The sequence of the rat liver isoform differs from rat brain SK3 in five amino acid residues in the NH3 terminus, where it more closely resembles human brain SK3. SK3 immunoreactivity was detectable in hepatocytes in rat liver and in HTC rat hepatoma cells. Human embryonic kidney (HEK-293) cells transfected with liver SK3 expressed 10 pS K+ channels that were Ca2+ dependent (EC(50) 630 nM) and were blocked by the SK channel inhibitor apamin (IC(50) 0.6 nM); whole cell SK3 currents inactivated at membrane potentials more positive than -40 mV. Notably, the Ca2+ dependence, apamin sensitivity, and voltage-dependent inactivation of SK3 are strikingly similar to the properties of hepatocellular and biliary epithelial SK channels evoked by metabolic stress. These observations raise the possibility that SK3 channels influence membrane K+ permeability in hepatobiliary cells during liver injury.

Calcium Mobilization Evoked by Hepatocellular Swelling Is Linked to Activation of Phospholipase Cγ

“…Coverslips were washed three times with TBS and then mounted on slides with Aqua Polymount (Polysciences). Prepared slides were analyzed on a Bio-Rad MRC 1024 ES laser scanning confocal microscope as previously described (12).…”

Design and Characterization of a Highly Selective Peptide Inhibitor of the Small Conductance Calcium-activated K+Channel, SkCa2

“…Opening in response to an increase in cytosolic [Ca 2ϩ ] i in the 200 -500 nM range (2), SK Ca channels modulate the firing pattern of neurons by generating slow membrane after hyperpolarizations (3)(4)(5). In the liver, they are believed to play a role in metabolic stress responses (6), and in human Jurkat T cells, they promote calcium entry in response to a mitogenic stimulus (7). SK Ca channels have also been implicated in fasciculations in denervated skeletal muscle (8) and in myotonic dystrophy (9).…”

SK3-1C, a Dominant-negative Suppressor of SKCa and IKCa Channels

“…Small conductance Ca 2ϩ -activated K ϩ channels (SK Ca ) 1 tightly couple calcium signaling and regulation of membrane excitability in excitable cells and influence membrane potential in non-excitable cells (1)(2)(3)(4)(5)(6). Three closely related genes, SK1, SK2, and SK3, known also as K Ca 3.1-3.3, KCNN1-KCNN3, and SKCa1-SKCa3, encode SK Ca proteins (7)(8)(9).…”

“…Expression of SK3 has been reported in the brain, skeletal muscle, endothelium, uterus, and hepatobiliary cells (6,7,(13)(14)(15). Recent quantitative RT-PCR studies have identified SK3 transcripts in other human tissues including the trachea, thyroid, spleen, and thymus (16).…”

“…In endothelium, SK3 has been reported to contribute to membrane hyperpolarization mediated by the endothelium-derived hyperpolarization factor, which is necessary for vasodilation (15). The channel has also been suggested to regulate membrane K ϩ permeability of hepatobiliary cells following liver injury (6). Several lines of evidence have implicated SK3 in schizophrenia (9,(22)(23)(24)(25)(26)(27), but these results are controversial because other studies have failed to confirm this disease association (28 -35).…”