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NEUROREGULATION AND MOTILITY

Calcium source diversity in feline lower esophageal sphincter circular and sling muscle

Departments of ¹Medicine and ²Physiology, University of Toronto, Toronto, Ontario, M5S 1A8; and ³Toronto Western Research Institute, University Health Network, Toronto, Ontario, Canada M5T 2S8

Submitted 10 July 2003 ; accepted in final form 20 September 2003

Within muscular equivalents of cat lower esophageal sphincter (LES), the circular muscle develops greater spontaneous tone, whereas the sling muscle is more responsive to cholinergic stimulation. Smooth muscle contraction involves a combination of calcium release from stores and of calcium entry via several pathways. We hypothesized that there are differences in the sources of Ca²⁺ used for contraction in sling and circular muscles and that these differences could contribute to functional asymmetry observed within LES. Contraction of muscle strips from circular and sling regions of LES was assessed in the presence of TTX. In Ca²⁺-free Krebs, tone was inhibited to a greater degree in circular than sling muscle. L-type Ca²⁺ channel blockade with nifedipine or verapamil inhibited tone in LES circular but not sling muscle. Sarcoplasmic reticulum (SR) Ca²⁺-ATPase inhibitor cyclopiazonic acid (CPA) caused greater increase in tone in sling than in circular muscle. The phospholipase C inhibitor U-73122 and the SR inositol 1,4,5-trisphosphate [Ins(1,4,5)P₃] receptor blocker 2-aminoethoxydiphenyl borate (2-APB) inhibited tone in circular and sling muscles, demonstrating that continuous release of Ca²⁺ from Ins(1,4,5)P₃-sensitive stores is important in tone generation in both muscles. In Ca²⁺-free Krebs, ACh-induced contractions (AChC) were inhibited to a greater degree in sling than circular muscles. However, nifedipine and verapamil greatly inhibited AChC in the circular but not sling muscle. Depletion of SR Ca²⁺ stores with CPA or inhibition of Ins(1,4,5)P₃-mediated store release with either U-73122 or 2-APB inhibited AChC in both muscles. We demonstrate that LES circular and sling muscles 1) use intracellular and extracellular Ca²⁺ sources to different degrees in the generation of spontaneous tone and AChC and 2) use different Ca²⁺ entry pathways. These differences hold the potential for selective modulation of LES tone in health and disease.

acetylcholine; tone; contractility; calcium handling; smooth muscle

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