Down-regulation of L-type calcium channels in inflamed circular smooth muscle cells of the canine colon

Background & Aims: Circular smooth muscle phasic contractions and tone are suppressed during colonic inflammation, but the contributing factors are poorly understood. This study investigated if the expression level of voltage-gated long-lasting (L-type) Ca²⁺ channel protein and functional Ca²⁺ channel current are down-regulated in the circular muscle cells of the inflamed canine colon. Methods: L-type Ca²⁺ channel expression was compared between normal and inflamed smooth muscle cells by Western immunoblots using an antibody directed against the pore-forming α1C-subunit, and patch-clamp methods were used to evaluate Ca²⁺ channel current density. Results: The expression of the L-type Ca²⁺ channel protein was significantly reduced in inflamed compared with normal circular smooth muscle cell membranes, and this finding was associated with suppressed levels of Ca²⁺ channel current in patch-clamped cells. The L-type Ca²⁺ channel current in normal and inflamed cells increased proportionately in response to Bay K 8644, but the maximal current density was still lower in the inflamed cells. Acetylcholine increased the L-type Ca²⁺ channel current in normal but not in inflamed cells. Conclusions: The expression level of L-type Ca²⁺ channels is down-regulated in the circular smooth muscle cell membranes of the inflamed colon, which may result in reduced Ca²⁺ influx. The functional and pharmacologic properties of the channels seem normal. Although some Ca²⁺ channels are still present in the inflamed cells, acetylcholine does not activate these channels, which may be caused by additional upstream defects in the receptor signaling cascade. The down-regulation of L-type Ca²⁺ channel expression may suppress circular smooth muscle contractions in the inflamed colon and contribute to the abnormalities in motility and digestion observed during inflammatory disorders.