Effects of removal and reapplication of K(+) and Cl(-) on spontaneous electrical activity, slow wave, in the circular muscle of the guinea-pig gastric antrum

Extracellular Cl- regulates electrical slow waves and setting of smooth muscle membrane potential by interstitial cells of Cajal in mouse jejunum

NEW FINDINGS

What is the central question of this study? The aim was to investigate the roles of extracellular chloride in electrical slow waves and resting membrane potential of mouse jejunal smooth muscle by replacing chloride with the impermeant anions gluconate and isethionate. What is the main finding and its importance? The main finding was that in smooth muscle cells, the resting Cl^- conductance is low, whereas transmembrane Cl^- movement in interstitial cells of Cajal (ICCs) is a major contributor to the shape of electrical slow waves. Furthermore, the data confirm that ICCs set the smooth muscle membrane potential and that altering Cl^- homeostasis in ICCs can alter the smooth muscle membrane potential. Intracellular Cl^- homeostasis is regulated by anion-permeable channels and transporters and contributes to excitability of many cell types, including smooth muscle and interstitial cells of Cajal (ICCs). Our aims were to investigate the effects on electrical activity in mouse jejunal muscle strips of replacing extracellular Cl^- (Cl^-_o ) with the impermeant anions gluconate and isethionate. On reducing Cl^-_o , effects were observed on electrical slow waves, with small effects on smooth muscle membrane voltage (E_m ). Restoration of Cl^- hyperpolarized smooth muscle E_m proportional to the change in Cl^-_o concentration. Replacement of 90% of Cl^-_o with gluconate reversibly abolished slow waves in five of nine preparations. Slow waves were maintained in isethionate. Gluconate and isethionate substitution had similar concentration-dependent effects on peak amplitude, frequency, width at half peak amplitude, rise time and decay time of residual slow waves. Gluconate reduced free ionized Ca²⁺ in Krebs solutions to 0.13 mm. In Krebs solutions containing normal Cl^- and 0.13 mm free Ca²⁺ , slow wave frequency was lower, width at half peak amplitude was smaller, and decay time was faster. The transient hyperpolarization following restoration of Cl^-_o was not observed in W/W^v mice, which lack pacemaker ICCs in the small intestine. We conclude that in smooth muscle cells, the resting Cl^- conductance is low, whereas transmembrane Cl^- movement in ICCs plays a major role in generation or propagation of slow waves. Furthermore, these data support a role for ICCs in setting smooth muscle E_m and that altering Cl^- homeostasis in ICCs can alter smooth muscle E_m .

[Cellular mechanism of the generation of spontaneous activity in gastric muscle]

In gastric smooth muscles, interstitial cells of Cajal (ICC) might be the pacemaker cells of spontaneous activities since ICC are rich in mitochondria and are connected with smooth muscle cells via gap junctions. Several types of ICC are distributed widely in the stomach wall. A group of ICC distributed in the myenteric layer (ICC-MY) were the pacemaker cells of gastrointestinal smooth muscles. Pacemaker potentials were generated in ICC-MY, and the potentials were conducted to circular smooth muscles to trigger slow waves and also conducted to longitudinal muscles to form follower potentials. In circular muscle preparations, interstitial cells distributed within muscle bundles (ICC-IM) produced unitary potentials, which were conducted to circular muscles to form slow potentials by summation. In mutant mice lacking inositol trisphosphate (IP(3)) receptor, slow waves were absent in gastric smooth muscles. The generation of spontaneous activity was impaired by the inhibition of Ca(2+)-release from internal stores through IP(3) receptors, inhibition of mitochondrial Ca(2+)-handling with proton pump inhibitors, and inhibition of ATP-sensitive K(+)-channels at the mitochondrial inner membrane. These results suggested that mitochondrial Ca(2+)-handling causes the generation of spontaneous activity in pacemaker cells. Possible involvement of protein kinase C (PKC) in the Ca(2+) signaling system was also suggested.

Rhythmic spontaneous contractions in the rat proximal colon

C-kit immunoreactive cells are known to be interstitial cells of Cajal (ICCs), and they generate pacemaker activity of the gastrointestinal tract. Recently a large number of special smooth muscle cells corresponding to c-kit immunoreactive cells were found in the proximal colon of the guinea pig. We learned that the rat proximal colon showed tetrodotoxin-insensitive regular rhythmic spontaneous contractions (RSCs) and hypothesized that RSCs are generated and/or regulated by ICCs. To prove our hypothesis, we investigated whether RSCs are absent in homozygous Ws/Ws mutant rats, since c-kit positive ICCs along the submucosal surface of the circular muscle (ICC(SM)) and myenteric plexus (ICC(MY)) are lacking. In contrast to our hypothesis, we found that RSCs were still present in the proximal colon of the Ws/Ws mutant rats. A recent study has reported that c-kit negative ICC(SM) remains in Ws/Ws mutant rats. Taken together, RSCs may be generated by c-kit negative ICC(SM) in the rat proximal colon. The blockade of sarcoplasmic reticulum Ca(2+)-ATPase by cyclopiazonic acid (CPA) (10(-6)M) or by thapsigargin (10(-6)M) increased the frequency of RSCs. The increasing effects of CPA on the frequency of RSCs were more prominent in Ws/Ws mutant rats than in +/+ rats. We concluded that the functional coordination between c-kit negative ICC(SM) and other mutationally impaired c-kit positive ICC(MY) and ICC(SM) may be required for moderate regulation in the frequency of spontaneous activity.

Voltage dependency of the frequency of slow waves in antrum smooth muscle of the guinea-pig stomach

The effects of membrane depolarization on the frequency of spontaneous activities were investigated in circular smooth muscle of the guinea-pig antrum attached with (intact tissue) or without longitudinal muscles (circular tissue). Both types of tissue were spontaneously active; the intact tissues generated slow wave and circular tissues generated regenerative potential. The latter but not the former was abolished by caffeine. Increasing K(+) concentrations depolarized the membrane and reduced the amplitude and interval between spontaneous activities in both tissues; the amplitude was reduced linearly with depolarization and disappeared at about -35 mV; the interval was reduced successively with depolarization and reached a stable value (about 8 s) at about -45 mV. The depolarization and reduction in amplitude and interval of spontaneous activities induced by high K(+) solution were not altered by atropine, nitroarginine, or apamin in either tissue, suggesting that these changes did not involve the effects of neurotransmitters. The depolarization of the membrane by electrical stimulation also reduced the amplitude and interval of spontaneous activities in both tissues, in a potential-dependent way. The absolute refractory period for generation of the evoked regenerative potential was about 8 s, and the relative refractory period was 8--12 s. The results indicate that the frequency of slow waves increases with a depolarization of the membrane up to -45 mV, irrespective of the presence of caffeine-insensitive components. A depolarization of the membrane above -45 mV does not further increase the frequency of slow waves, possibly because of the refractory period for the generation of slow waves.

Effects of removal of Na(+) and Cl(-) on spontaneous electrical activity, slow wave, in the circular muscle of the guinea-pig gastric antrum

In the circular muscle of the guinea-pig gastric antrum, a decrease in the external Na(+) to less than 20 mM produced depolarization of the membrane with transient prolongation of the slow wave. This was followed by a high rhythmic activity. The activity was inhibited by reapplication of Na(+) before recovery. The depolarization in Na(+)-deficient solution was prevented and rhythmic activity continued at about 5/min for at least 6 min by simultaneous removal of K(+), Ca(2+), or Cl(-). After exposure to a Na(+)- and Cl(-)-deficient solution for a few minutes, reapplication of the Na(+) in Cl(-)-deficient solution inhibited generation of the slow wave until Cl(-) reapplication. Similar results were obtained when Na(+) and Cl(-) were reapplied in the absence of K(+) after exposure to a Na(+)-, K(+)-free, and Cl(-)-deficient solution, although the inhibition was weaker than Na(+) reapplication in a Cl(-)-deficient solution. In the presence of furosemide or bumetanide, a strong inhibition of activity was produced by the reapplication of Na(+) and Cl(-) after exposure to an Na(+)- and Cl(-)-deficient solution. A hypothesis is presented that intracellular Ca(2+) concentration ([Ca(2+)](i)) is the most important factor determining the generation and frequency of the slow wave and that [Ca(2+)](i) is regulated by the Na(+) concentration gradient across the plasma membrane. The recovery of the Na(+) concentration gradient by Na(+) reapplication after removal of Na(+) and Cl(-) is mainly controlled by a Na(+)-K(+)-Cl(-) co-transport.