Patterns of intracellular and intercellular Ca 2+ waves in the longitudinal muscle layer of the murine large intestine In vitro

Ca 2+ wave activity was monitored in the longitudinal (LM) layer of isolated murine caecum and proximal colon at 35 °C with fluo‐4 AM and an iCCD camera. Both intracellular (within LM cells) and intercellular (also spreading from cell to cell) Ca 2+ waves were observed. Intracellular Ca 2+ waves were associated with a lack of muscle movement whereas intercellular Ca 2+ waves, which were five times more intense than intracellular waves, were often associated with localized contractions. Several intracellular Ca 2+ waves were present at the same time in individual LM cells. Waves in adjacent LM cells were not coordinated and were unaffected by TTX (1 μM) but were blocked by IP 3 receptor antagonists xestospongin‐C (Xe‐C; 2 μM) or 2‐aminoethyl diphenylborate (2‐APB; 25 μM), and by ryanodine (10 μM). Caffeine (5 m m ) restored wave activity following blockade with Xe‐C. NiCl 2 (1 m m ) blocked intracellular Ca 2+ waves, and nicardipine (2 μM) reduced their frequency and intensity, but did not affect their velocity, suggesting the sarcoplasmic reticulum may be fuelled by extracellular Ca 2+ entry. Intercellular Ca 2+ waves often occurred in bursts and propagated rapidly across sizeable regions of the LM layer and were blocked by heptanol (0.5 m m ). Intercellular Ca 2+ waves were dependent upon neural activity, external Ca 2+ entry through L‐type Ca 2+ channels, and amplification via calcium‐induced calcium release (CICR). In conclusion, intracellular Ca 2+ waves, which may reduce muscle excitability, are confined to individual LM cells. They depend upon Ca 2+ release from internal Ca 2+ stores and are likely to be fuelled by extracellular Ca 2+ entry. Intercellular Ca 2+ waves, which are likely to underlie smooth muscle tone, mixing and propulsion, depend upon neural activity, muscle action potential propagation and amplification by CICR.