Functional and molecular analysis of L-type calcium channels in human esophagus and lower esophageal sphincter smooth muscle

Excitation of human esophageal smooth muscle involves the release of Ca(2+) from intracellular stores and influx. The lower esophageal sphincter (LES) shows the distinctive property of tonic contraction; however, the mechanisms by which this is maintained are incompletely understood. We examined Ca(2+) channels in human esophageal muscle and investigated their contribution to LES tone. Functional effects were examined with tension recordings, currents were recorded with patch-clamp electrophysiology, channel expression was explored by RT-PCR, and intracellular Ca(2+) concentration was monitored by fura-2 fluorescence. LES muscle strips developed tone that was abolished by the removal of extracellular Ca(2+) and reduced by the application of the L-type Ca(2+) channel blocker nifedipine (to 13 +/- 6% of control) but was unaffected by the inhibition of sarco(endo)plasmic reticulum Ca(2+)-ATPase by cyclopiazonic acid (CPA). Carbachol increased tension above basal tone, and this effect was attenuated by treatment with CPA and nifedipine. Voltage-dependent inward currents were studied using patch-clamp techniques and dissociated cells. Similar inward currents were observed in esophageal body (EB) and LES smooth muscle cells. The inward currents in both tissues were blocked by nifedipine, enhanced by Bay K8644, and transiently suppressed by acetylcholine. The molecular form of the Ca(2+) channel was explored using RT-PCR, and similar splice variant combinations of the pore-forming alpha(1C)-subunit were identified in EB and LES. This is the first characterization of Ca(2+) channels in human esophageal smooth muscle, and we establish that L-type Ca(2+) channels play a critical role in maintaining LES tone.