Decrease in Adenylyl Cyclase V/VI and Increase in Phosphodiesterase 4D5 Expression and Activities in Smooth Muscle lead to Decrease in cAMP levels, Smooth Muscle Relaxation and Gastrointestinal Motility in Diabetes
Authors: Mahavadi Sunila , Kendig Derek , Grider John R , Murthy Karnam S 
Published: November 2023

In gastrointestinal (GI) smooth muscle, inhibitory transmitters (e.g., vasoactive intestinal peptide (VIP)) that are coupled to activation of Gs protein mediate relaxation via generation of cAMP and activation of cAMP‐dependent protein kinase (PKA). Cyclic AMP levels are regulated by the activities of adenylyl cyclase (AC) and phosphodiesterases (PDEs). Previous studies in gastrointestinal muscle showed selective expression of ACV/VI and cAMP‐specific PDE4D5. Diabetes‐associated GI motility disorders are common and are correlated with the decrease in neuronal nitric oxide synthase expression and activity, and density of interstitial cells of Cajal. Changes in the smooth muscle AC/cAMP/PKA pathway leading to decrease in relaxation could also contribute to decreased motility. However, diabetes‐induced changes in the expression and activities of ACV/VI and PDE4D5 in the smooth muscle are not known.

Aim

To test the hypothesis that downregulation of ACV/VI and upregulation of PDE4D5 expression in smooth muscle contribute to decreased relaxation and motility in diabetes.

Methods

Gastric and colonic smooth muscle from control and diabetic (db/db and ob/ob) mice were used to measure the expression of ACV/VI and PDE4D5, cAMP levels, and muscle relaxation. Expression of ACV/VI and PDE4D5 was measured by western blot. cAMP levels in response to forskolin, VIP or isoproterenol were measured by radioimmunoassay. Muscle relaxation in response to forskolin, VIP or isoproterenol was measured in muscle strips (organ bath studies) and isolated muscle cells (scanning micrometry). Gastric emptying and colonic pellet propulsion were measured as indices of gastrointestinal motility.

Results

Smooth muscle isolated from the stomach and the colon of db/db and ob/ob mice showed a decrease in ACV/VI expression and an increase in PDE4D5 expression compared to the stomach and colon of control mice. Similar changes in the expression of ACV/VI and PDE4D5 were obtained in control muscle treated with 30 mM glucose for 48 h. Forskolin‐induced cAMP levels, measured in the presence or absence of 3‐isobutyl‐1‐methylxanthine, were significantly reduced in gastric and colonic smooth muscle of db/db and ob/ob mice compared to control. A similar decrease in cAMP levels in diabetic mice was obtained with Gs‐coupled receptor agonists, VIP, and isoproterenol. Forskolin‐induced cAMP levels were also decreased in control muscle treated with 30 mM glucose. Consistent with the decrease in cAMP levels, muscle relaxation in response to forskolin, VIP or isoproterenol was decreased in gastric and colonic smooth muscle of db/db and ob/ob mice compared to control. Both gastric emptying and the velocity of propulsion of fecal pellets in colonic segments were decreased in diabetic mice suggesting a decrease in gastrointestinal motility in diabetes.

Conclusion

In diabetic stomach and colon, downregulation of ACV/VI expression and upregulation of PDE4D5 expression in smooth muscle causes a decrease in cAMP levels and muscle relaxation leading to a decrease in gastrointestinal motility.

Support or Funding Information

R01DK28300