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

The internal anal sphincter (IAS) exhibits slow waves (SWs) and tone that are dependent upon L-type Ca²⁺ channels (Cav_L ) 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).

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.

Ca²⁺ transients displayed ongoing rhythmic firings at both lengths and were abolished by nifedipine and the K_ATP channel activator pinacidil indicating their dependence upon Cav_L . 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.

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.