Na+/Ca2 + Exchange and Pacemaker Activity of Interstitial Cells of Cajal

Abstract

Interstitial cells of Cajal (ICC) are pacemaker cells that generate electrical slow waves in gastrointestinal (GI) smooth muscles. Slow waves organize basic motor patterns, such as peristalsis and segmentation in the GI tract. Slow waves depend upon activation of Ca 2+-activated Cl − channels (CaCC) encoded by Ano1. Slow waves consist of an upstroke depolarization and a sustained plateau potential that is the main factor leading to excitation-contraction coupling. The plateau phase can last for seconds in some regions of the GI tract. How elevated Ca 2+ is maintained throughout the duration of slow waves, which is necessary for sustained activation of CaCC, is unknown. Modeling has suggested a role for Na + /Ca 2+ exchanger (NCX) in regulating CaCC currents in ICC, so we tested this idea on murine intestinal ICC. ICC of small and large intestine express NCX isoforms. NCX3 is closely associated with ANO1 in ICC, as shown by immunoprecipitation and proximity ligation assays (PLA). KB-R7943, an inhibitor of NCX, increased CaCC current in ICC, suggesting that NCX, acting in Ca 2+ exit mode, helps to regulate basal [Ca 2+ ] i in these cells. Shifting NCX into Ca 2+ entry mode by replacing extracellular Na + with Li + increased spontaneous transient inward currents (STICs), due to activation of CaCC. Stepping ICC from −80 to −40 mV activated slow wave currents that were reduced in amplitude and duration by NCX inhibitors, KB-R7943 and SN-6, and enhanced by increasing the NCX driving force. SN-6 reduced the duration of clustered Ca 2+ transients that underlie the activation of CaCC and the plateau phase of slow waves. Our results suggest that NCX participates in slow waves as modeling has predicted. Dynamic changes in membrane potential and ionic gradients during slow waves appear to flip the directionality of NCX, facilitating removal of Ca 2+ during the inter-slow wave interval and providing Ca 2+ for sustained activation of ANO1 during the slow wave plateau phase.

Bicarbonate ion transport by the electrogenic Na+/HCO3− cotransporter, NBCe1, is required for normal electrical slow‐wave activity in mouse small intestine

“…Voltage‐gated Na + currents, while present in human intestinal ICC, have not been found in rodent smooth‐muscle cells 11,58 and even if they were present, they are freely permeable to Li + , so replacement of Na + should not impact any contribution of these channels to slow‐wave generation. However, Li + is not carried by either NCX or NKCC1 activity, 48 and both of these transporters play a role in generation of Ca 2+ transients that underlie pacemaker activity in ICC from both mouse intestine and rabbit urethra 28,29,59 . The absence of a further effect when we removed HCO 3 − from Li + containing solutions was consistent with NBCe1 activity being the major contributor to HCO 3 − transport in ICC‐MY and overall our results support a role for plasma membrane Na + transport in ICC function.…”

“…The process is dependent on release of Ca 2+ into the cytosol from inositol 1,4,5‐trisphosphate (IP3)‐sensitive intracellular stores, 20 which is coupled to influx of Ca 2+ through voltage‐gated Ca 2+ channels 21,22 and non‐selective cation channels including Ca 2+ release activated channels such as orai, 23 followed by activation of Anoctamin‐1 (Ano1, TMEM16A) Ca 2+ ‐sensitive Cl − channels 24–26 . Mitochondrial buffering of cytosolic Ca 2+ , 27 the activity of Na + /Ca 2+ exchangers (NCX), 28 and reuptake of Ca 2+ into the endoplasmic reticulum 27 alter the shape and duration of the Ca 2+ transients and the associated electrical slow wave. Plasma membrane transporters of Cl − ions including NKCC1 (Slc12a2) set the Cl − reversal potential (E Cl ), and E Cl determines the voltage to which the ICC‐membrane potential depolarizes during the plateau phase of the electrical slow wave 29 .…”

Changes in the interstitial cells of Cajal in the gallbladder of guinea pigs fed a lithogenic diet

“…ICCs are pacemaker cells that generate electrical slow waves in the gastrointestinal tract ( 23 ). ICCs have also been shown to serve a key regulatory role in controlling smooth muscle contractions in the urinary tract ( 24 ).…”

Insights on gastrointestinal motility through the use of optogenetic sensors and actuators

“…This has now been dispelled by high magnification Ca 2+ imaging of ICC‐MY and ICC‐SM in Kit‐GCaMP mice. In these ICC, Ca 2+ transients cluster asynchronously across multiple intracellular sites at the onset of slow waves (Baker, Hwang et al., 2021; Baker, Leigh et al., 2021; Drumm et al., 2017; Zheng et al., 2020). These Ca 2+ transients are brief, lasting only a few hundred milliseconds, and may occur multiple times during the plateau of a slow wave.…”

“…In neurons, due to passive distribution of Cl − , activation of halorhodopsins leads to neural silencing (Zhang et al., 2007). In ICC, however, Cl − ions are actively accumulated (Zhu et al., 2016; Zheng et al., 2020) resulting in an E Cl of ∼ 10 mV. Thus, activation of halorhodopsins in ICC with yellow light would lead to Cl − efflux, yielding a depolarizing influence on the SIP syncytium (Wiegert et al., 2017).…”