Regional differences in L-type Ca²⁺channel expression in feline lower esophageal sphincter

Abstract

In humans and cats, muscle from the lower esophageal sphincter (LES) circular region exhibits greater spontaneous tone than LES sling muscle, whereas the sling muscle is much more responsive to cholinergic stimulation. Despite physiological and pharmacological evidence for the presence of L-type Ca2+ channel current (ICa,L) activity in LES circular muscle, the identity of this channel has not been demonstrated biochemically or electrophysiologically fingerprinted. Furthermore, there is no information on the channel's presence and role in the sling region of the LES. We hypothesized that regional differences in the expression of ICa,L between LES circular and sling muscles, if present, could contribute to the functional asymmetry observed within the LES. ICa,L expression was compared between circular and sling regions of the LES by Western blot analysis. The patch-clamp technique was used to study ICa,L. Muscle strip studies assessed ICa,L contribution to contractile activity. We found both protein expression of ICa,L and ICa,L density to be greater in LES circular muscle than sling muscle. ICa,L voltage- and time-dependent activation and inactivation curves were similar in cells from both regions. ICa,L blockade with nifedipine inhibited spontaneous tone and ACh-induced contractions only in circular muscle but was able to abolish depolarization (KCl)-induced contractions in both sling and circular muscles. In contrast, La3+ inhibited tone and ACh-induced contractions in muscles from both regions. Therefore, regional myogenic differences in ICa,L expression within the LES circular and sling muscle exist and provide one explanation for the differential contribution of sling and circular muscle to LES contractility.

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

“…Ca 2ϩ currents with similar properties and pharmacology to those in the present study have been reported in feline LES smooth muscle (21). However, other sources of Ca 2ϩ entry exist in esophageal muscle, including nonselective cation channels of the transient receptor potential C (TRPC) family (39).…”

Feline lower esophageal sphincter sling and circular muscles have different functional inhibitory neuronal responses

“…In achalasia, absence of the dominant nitrergic inhibition in the high tone circular muscle (14,25) results in LES pressures equalizing around the sphincter circumference to the higher level of the sling where cholinergic excitation is dominant (45). Reducing LES pressure in vivo with the administration of nifedipine would in large part be directed to the circular portion, because calcium entry in that muscle is largely through L-type calcium channels, whereas that of the sling is largely through non-L-type channels (28). Reduction of LES tone by blockade of ACh release with botulinum toxin would presumably involve primarily the cholinergic innervation of the sling muscle (35) and suggests that the injection should be directed mainly to the left.…”

“…Reduction of LES tone by blockade of ACh release with botulinum toxin would presumably involve primarily the cholinergic innervation of the sling muscle (35) and suggests that the injection should be directed mainly to the left. The regional differences between sling and circular muscles also provide different options for the surgical management of achalasia (17,23,28,45,51). Cutting the circular muscle may be all that is necessary to relieve the functional obstruction while leaving the cholinergic sling activity will protect against reflux.…”

“…The circular muscle has significant spontaneous tone, whereas the sling muscle has little tone and is more responsive to cholinergic stimulation (30,37,38,47,51). Furthermore, these two muscles demonstrate differences in resting membrane potential, voltage-gated K ϩ and Ca 2ϩ channel current densities, and calcium handling (28,29,43). These differences provide for the likelihood that neural mediation of relaxation as well as maintenance of resting tone would also differ in sling and circular muscles.…”

Mechanisms controlling function in the clasp and sling regions of porcine lower oesophageal sphincter

“…The macroscopic arrangement of LOS circular smooth fibres of the inner muscle in humans and pigs is similar, with short transverse muscle clasps on the lesser curve side (clasp fibres) and long oblique gastric sling fibres in the angle of His of the greater curvature on the left side 4,5 . Differences in function between clasp and sling fibres have been attributed to regional specialization relating to smooth muscle cell physiology 1,6,7 , distribution and function of enteric motor neurones (EMNs) 8 -10 , and extrinsic vagal input 11 .…”