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CLC-3, a member of the CLC family of chloride channels, mediates function in many cell types in the body. The multifunctional calcium-calmodulin-dependent protein kinase II (CaMKII) has been shown to activate recombinant CLC-3 stably expressed in tsA cells, a human embryonic kidney cell line derivative, and natively expressed channel protein in a human colonic tumour cell line T84. We examined the CaMKII-dependent regulation of CLC-3 in a smooth muscle cell model as well as in the human colonic tumour cell line, HT29, using whole-cell voltage clamp. In CLC-3-expressing cells, we observed the activation of a C1- conductance following intracellular introduction of the isolated autonomous CaMKII into the voltage-clamped cell via the patch pipette. The CaMKII-dependent C1- conductance was not observed following exposure of the cells to 1 µm autocamtide inhibitory peptide (AIP), a selective inhibitor of CaMKII. Arterial smooth muscle cells express a robust CaMKII-activated C1- conductance; however, CLC3-/- cells did not. The N-terminus of CLC-3, which contains a CaMKII consensus sequence, was phosphorylated by CaMKII in vitro, and mutation of the serine at position 109 (S109A) abolished the CaMKII-dependent C1- conductance, indicating that this residue is important in the gating of CLC-3 at the plasma membrane. [ABSTRACT FROM AUTHOR]