Salapatek A M, Lam A, Daniel E E
Department of Biomedical Sciences, McMaster University, Faculty of Health Sciences, Hamilton, Ontario L8N 3Z5, Canada.
J Pharmacol Exp Ther. 1998 Oct;287(1):98-106.
Tonic contraction of the lower esophageal sphincter (LES) prevents gastroesophageal reflux. LES tone is produced both by cholinergic nerve and myogenic activities. The Ca++ sources for LES tone and carbachol-induced contraction in canine LES strips were determined from the effect on contractile activity of extracellular Ca++ level modulation, Ca++ entrance blockade or enhancement with nifedipine or BayK8644 respectively, and/or inhibition of Ca++ store refilling using the sarcoplasmic reticulum Ca++ pump inhibitor, cyclopiazonic acid. LES tone disappeared when a Ca++-free physiological saline solution or nifedipine was applied. Sustained Ca++ free contractions to carbachol were prevented/abolished by nifedipine or increased Ca++ chelation and enhanced by BayK8644. Inhibition of sarcoplasmic reticulum Ca++ pumps by cyclopiazonic acid reduced Ca++ free contractions to carbachol; BayK8644 restored cyclopiazonic acid-reduced Ca++ free contractions to carbachol. Therefore, some Ca++ stores can be refilled by mechanisms not requiring activity of the sarcoplasmic reticulum Ca++ pump. A preferred pathway may exist whereby Ca++ enters stores directly through L-Ca++ channels. The proposed Ca++ store refilling mechanism involves continuous Ca++ entry through L-Ca++ channels from sites not equilibrated with external Ca++. Therefore, diverse Ca++ stores exist in canine LES which are dependent on Ca++ influx through L-Ca++ channels.
