Question:

Gastrointestinal diseases such as gastroparesis or achalasia originate from impaired gastrointestinal smooth muscle (SM) relaxation. Regulation of gastrointestinal motility involves nitric oxide (NO) which activates NO-sensitive guanylyl cyclase (NO-GC) and leads to production of the second messenger cGMP. Apart from smooth muscle cells (SMC), interstitial cells of Cajal (ICC) and fibroblast-like cells (FLC) are implicated in nitrergic relaxation of the gut. In this study, we attended to elucidate the function of NO-GC regarding nitrergic relaxation of the murine GI tract.

Methods:

Immunohistochemistry, isometric force studies and membrane potential recordings were conducted in tissues from mice that lack NO-GC ubiquitously (GCKO), specifically in SMC (SM-GCKO), ICC (ICC-GCKO) or FLC (FLC-GCKO) as well as a double knock out of NO-GC in SMC and ICC (SM/ICC-GCKO). Gut transit time was measured to monitor the consequences of NO-GC deletion on gut motility in vivo.

Results:

NO-dependent relaxation of gastrointestinal SM was abolished in GCKO mice. In SM-GCKO, NO still led to partial relaxation whereas ICC-GCKO and FLC-GCKO showed a WT-like phenotype. Only in SM/ICC-GCKO we observed lack of nitrergic relaxation similar to that seen in GCKO mice. Inhibitory junction potentials were lost in GCKO and reduced in SM-GCKO. Gut transit time was prolonged in GCKO but, surprisingly, reduced in FLC-GCKO.

Conclusions:

NO-GC in gastrointestinal SMC, ICC or FLC is dispensable for motility and survival. Lack of NO-GC in at least two cell types, though, abolishes nitrergic signaling. Our data show that the mechanism of nitrergic relaxation via NO-GC is more complex than previously thought.