The role of CNP-mediated PKG/PKA-PLCβ pathway in diabetes-induced gastric motility disorder.

Our previous work demonstrated that the C-type natriuretic peptide (CNP)/cyclic guanosine monophosphate (cGMP)/cyclic adenosine monophosphate (cAMP) pathway in gastric antrum smooth muscle of rats with diabetes was upregulated and played an important role in the development of diabetic gastric dysmotility. Our goal for this study was to explore the downstream signaling pathways of CNP. We found that the expressions of protein kinase G (PKG) and protein kinase A (PKA) in gastric smooth muscle tissue of rats with diabetes were significantly upregulated. The expressions of β-type phospholipase C 3(PLCβ3) and β-type phospholipase C 1(PLCβ1) protein were reduced, whereas Phosphor-PLCβ3Ser1105 (P-PLCβ3Ser1105) was increased. The inhibitory effect of CNP on gastric antral smooth muscle in diabetic rats was significantly greater than in the normal group. The content of trisphosphate inositol (IP3) in the gastric antral smooth muscle of rats with diabetes was significantly lower than that of the normal group. After blocking PKA with N-[2-(p-Bromocinnamylamino)ethyl]-5-isoquinolinesulfonamide dihydrochloride (H-89, a blockage PKA), the effect of CNP on the production of IP3 was decreased, while blocking PKG with KT5823 (a blockage PKG) simultaneously, and CNP can no longer reduce the IP3 production. CNP promoted the phosphorylation of PLCβ3Ser1105, thereby inhibiting the activity of PLCβ3 in gastric smooth muscle tissue of rats with diabetes; this effect can be abolished by blocking PKA and PKG. These results suggested that CNP can decrease IP3 level in gastric smooth muscle cells and thus inhibit gastric smooth muscle contraction through PKG/PKA-PLCβ pathway, which may play an important role in the development of diabetic gastroparesis.