Rhythmic calcium transients in smooth muscle cells of the mouse internal anal sphincter

• Published in: Neurogastroenterology and motility 2020-03, Vol.32 (3), p.e13746-n/a
• Main Authors: Cobine, Caroline A., Hannigan, Karen I., McMahon, Megan, Ni Bhraonain, Emer P., Baker, Salah A., Keef, Kathleen D.
• Format: Article
• Language: English
• Subjects: Anal Canal - physiology; Animals; Calcium - metabolism; Calcium channels; Calcium channels (voltage-gated); Calcium Channels, L-Type - metabolism; Calcium influx; Calcium Signaling - physiology; Calcium signalling; Conduction; Confocal microscopy; Female; gastrointestinal; interstitial cells of Cajal; Male; Mice; Mice, Inbred C57BL; Muscle Contraction - physiology; Myocytes, Smooth Muscle - physiology; Nifedipine; pacemaker; Pinacidil; Rhythms; slow wave; Smooth muscle; Sphincter; tone
• ISSN: 1350-1925; 1365-2982
• DOI: 10.1111/nmo.13746

Background The internal anal sphincter (IAS) exhibits slow waves (SWs) and tone that are dependent upon L‐type Ca2+ channels (CavL) suggesting that phasic events (ie, SWs) play a fundamental role in tone generation. The present study further examined phasic activity in the IAS by measuring the spatiotemporal properties of Ca2+ transients (CTs) in IAS smooth muscle cells (SMCs). Methods Ca2+ transients were recorded with spinning disk confocal microscopy from the IAS of SM‐GCaMP mice. Muscles were pinned submucosal surface up at two different lengths. Drugs were applied by inclusion in the superfusate. Key Results Ca2+ transients displayed ongoing rhythmic firings at both lengths and were abolished by nifedipine and the KATP channel activator pinacidil indicating their dependence upon CavL. Like SWs, CTs were greatest in frequency (average 70.6 cpm) and amplitude at the distal extremity and conducted proximally. Removal of the distal IAS reduced but did not abolish CTs. The time constant for clearing cytoplasmic Ca2+ averaged 0.46 seconds and basal Ca2+ levels were significantly elevated. Conclusions & Inferences The similarities in spatiotemporal and pharmacological properties of CTs and SWs suggest that SW gives rise to CTs while muscle stretch is not required. Elevated relative basal Ca2+ in the IAS is likely due to the inability of cells to clear or sequester Ca2+ between rapid frequency voltage‐dependent Ca2+ entry events, that is, conditions that will lead to tone development. The conduction of CTs from distal to proximal IAS will lead to orally directed contractions and likely contribute to the maintenance of fecal continence. The SW and CT frequency is greatest at the distal IAS where basal Ca2+ is most elevated. We propose that ICC‐IM generate SWs that give rise to CTs and that basal Ca2+ is elevated because of the inability to reduce [Ca2+]i between high frequency SWs; this contributes to IAS tone..