L-type Ca(2+) channel expression along feline smooth muscle oesophagus

Generation of Spontaneous Tone by Gastrointestinal Sphincters

An important feature of the gastrointestinal (GI) muscularis externa is its ability to generate phasic contractile activity. However, in some GI regions, a more sustained contraction, referred to as "tone," also occurs. Sphincters are muscles oriented in an annular manner that raise intraluminal pressure, thereby reducing or blocking the movement of luminal contents from one compartment to another. Spontaneous tone generation is often a feature of these muscles. Four distinct smooth muscle sphincters are present in the GI tract: the lower esophageal sphincter (LES), the pyloric sphincter (PS), the ileocecal sphincter (ICS), and the internal anal sphincter (IAS). This chapter examines how tone generation contributes to the functional behavior of these sphincters. Historically, tone was attributed to contractile activity arising directly from the properties of the smooth muscle cells. However, there is increasing evidence that interstitial cells of Cajal (ICC) play a significant role in tone generation in GI muscles. Indeed, ICC are present in each of the sphincters listed above. In this chapter, we explore various mechanisms that may contribute to tone generation in sphincters including: (1) summation of asynchronous phasic activity, (2) partial tetanus, (3) window current, and (4) myofilament sensitization. Importantly, the first two mechanisms involve tone generation through summation of phasic events. Thus, the historical distinction between "phasic" versus "tonic" smooth muscles in the GI tract requires revision. As described in this chapter, it is clear that the unique functional role of each sphincter in the GI tract is accompanied by a unique combination of contractile mechanisms.

Effects of Unoprostone and Endothelin 1 on L-Type Channel Currents in Human Trabecular Meshwork Cells

BACKGROUND

The trabecular meshwork (TM) is a smooth muscle-like tissue with contractile properties and by this mechanisms involved in the regulation of aqueous humor outflow. Isopropyl unoprostone (Rescula, Novartis Ophthalmics), a synthetic docosanoid, reduces intraocular pressure in glaucoma patients and normal subjects. In isolated TM strips, unoprostone reduces TM contractility in the presence of endothelin 1 (ET-1). However, the signal transduction pathway of unoprostone still remains unclear. Since L-type channel currents are known to influence the contractility of TM, we examined the effects of unoprostone and ET-1 on L-type channel currents of TM cells.

METHODS

The effects of unoprostone, ET-1 and the tyrosine kinase inhibitor herbimycin A on L-type channel currents of cultured human TM cells were investigated using the perforated patch configuration of the patch-clamp technique.

RESULTS

Application of ET-1 had no effect on L-type channel currents. Unoprostone led to a dose-dependent reduction of control currents. The effect of unoprostone is independent of ET-1. After preincubation of cells with herbimycin A, unoprostone had no effect on the L-type channel current amplitude. Human TM cells preincubated with herbimycin A showed a reduced current density compared with control cells. Both substances, unoprostone and herbimycin A, increased the inactivation time constant of L-type channel currents.

CONCLUSION

We conclude that unoprostone reduces the activity of L-type Ca2+ channels. This effect seems to be independent of ET-1. The signal transduction pathway seems to be mediated by tyrosine kinases.

Regional differences in cholinergic regulation of potassium current in feline esophageal circular smooth muscle

Potassium channels are important contributors to membrane excitability in smooth muscles. There are regional differences in resting membrane potential and K(+)-channel density along the length of the feline circular smooth muscle esophagus. The aim of this study was to assess responses of K(+)-channel currents to cholinergic (ACh) stimulation along the length of the feline circular smooth muscle esophageal body. Perforated patch-clamp technique assessed K(+)-channel responses to ACh stimulation in isolated smooth muscle cells from the circular muscle layer of the esophageal body at 2 (distal)- and 4-cm (proximal) sites above the lower esophageal sphincter. Western immunoblots assessed ion channel and receptor expression. ACh stimulation produced a transient increase in outward current followed by inhibition of spontaneous transient outward currents. These ACh-induced currents were abolished by blockers of large-conductance Ca(2+)-dependent K(+) channels (BK(Ca)). Distal cells demonstrated a greater peak current density in outward current than cells from the proximal region and a longer-lasting outward current increase. These responses were abolished by atropine and the specific M(3) receptor antagonist 4-DAMP but not the M(1) receptor antagonist pirenzipine or the M(2) receptor antagonist methoctramine. BK(Ca) expression along the smooth muscle esophagus was similar, but M(3) receptor expression was greater in the distal region. Therefore, ACh can differentially activate a potassium channel (BK(Ca)) current along the smooth muscle esophagus. This activation probably occurs through release of intracellular calcium via an M(3) pathway and has the potential to modulate the timing and amplitude of peristaltic contraction along the esophagus.

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

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.