Expression of the Regulated Isoform of the Electrogenic Na+/HCO3- Cotransporter, NBCe1, is Enriched in Pacemaker Interstitial Cells of Cajal.

Interstitial cells of Cajal (ICC) generate electrical slow waves, which are required for normal gastrointestinal motility. The mechanisms for generation of normal pacemaking are not fully understood. Normal gastrointestinal contractility and electrical slow wave activity depend on the presence of extracellular HCO3-. Previous transcriptional analysis identified enrichment of mRNA encoding the electrogenic Na+/HCO3- cotransporter (NBCe1) gene (Slc4a4) in pacemaker myenteric ICC in mouse small intestine. We aimed to determine the distribution of NBCe1 protein in ICC of the mouse gastrointestinal tract, and to identify the transcripts of the Slc4a4 gene in mouse and human small intestinal tunica muscularis. We determined the distribution of NBCe1-immunoreactivity (NBCe1-IR) by immunofluorescent labeling in mouse and human tissues. In mice, NBCe1-IR was restricted to Kit-positive myenteric ICC of the stomach and small intestine and sub-muscular ICC of the large intestine; that is the slow wave generating subset of ICC. Other sub-types of ICC were NBCe1-negative. Quantitative real time PCR identified >500 fold enrichment of Slc4a4‑207 and Slc4a4‑208 transcripts (IP3-receptor binding protein released by IP3" (IRBIT) regulated isoforms) in Kit expressing cells isolated from KitcreERT2/+, Rpl22tm1.1Psam/Sj mice and from single GFP-positive ICC from Kittm1Rosay mice. Human jejunal tunica muscularis ICC were also NBCe1-positive and SLC4A4‑201 and SLC4A4‑204 RNAs were >300 fold enriched relative to SLC4A4‑202. In summary, NBCe1 protein expressed in ICC with electrical pacemaker function is encoded by Slc4a4 gene transcripts that generate IRBIT regulated isoforms of NBCe1. In conclusion Na+/HCO3- cotransport through NBCe1 contributes to the generation of pacemaker activity in subsets of ICC.