Activation of neural circuitry and Ca2+ waves in longitudinal and circular muscle during CMMCs and the consequences of rectal aganglionosis in mice

Abstract

In mammals that develop rectal aganglionosis, the aganglionic segment still exhibits spontaneous phasic contractions that contribute to dysmotility and pseudoobstruction in this region. However, almost nothing is known about the mechanisms that generate these myogenic contractions or the effects of aganglionosis on the generation of Ca2+ waves that underlie contractions of the longitudinal muscle (LM) and circular muscle (CM). In a mouse model of Hirschsprung's disease [endothelin type B receptor-deficient ( Ednrb s−l/ Ednrb s−l) mice], the Ca2+ indicator fluo-4 was used to simultaneously monitor the temporal activation and spread of intercellular Ca2+ waves in the LM and CM during spontaneous colonic motor activities. During the intervals between colonic migrating motor complexes (CMMCs) in control mice, Ca2+ waves discharged asynchronously between the LM and CM. However, in these same mice, during CMMCs, a burst of discreet Ca2+ waves fired simultaneously in both muscle layers, where the propagation velocity of Ca2+ waves significantly increased, as did the rate of initiation and number of collisions between Ca2+ waves. Hexamethonium (300 μM) or atropine (1 μM) prevented synchronized firing of Ca2+ waves. In the aganglionic distal colon of Ednrb s−l/ Ednrb s−l mice, not only were CMMCs absent, but Ca2+ waves between the two muscle layers fired asynchronously, despite increased propagation velocity. The generation of CMMCs in control mice involves synchronized firing of enteric motor nerves to both the LM and CM, explaining the synchronized firing of discreet Ca2+ waves between the two muscle layers. Aganglionosis results in a sporadic and sustained asynchrony in Ca2+ wave firing between the LM and CM and an absence of CMMCs.