Tetrahydrobiopterin (BH4), a cofactor for nNOS, restores gastric emptying and nNOS expression in female diabetic rats

<jats:p> Gastroparesis is a debilitating disease predominantly affecting young women. Recently, dysregulation of neuronal nitric oxide synthase (nNOS) in myenteric plexus neurons has been implicated for delayed solid gastric emptying/gastroparesis in diabetic patients. In this study, we have explored the role of tetrahydrobiopterin (BH4), a major cofactor for nNOS activity and NO synthesis in diabetic gastroparesis. Diabetes was induced with single injection of streptozotocin (55 mg/kg body wt, ip) in female rats, with experiments performed on week 3 or 9 following induction, with or without 3-wk BH4 supplementation. Gastric pyloric BH4 levels were significantly decreased in diabetic female rats compared with control (18.6 ± 1.45 vs. 31.0 ± 2.31 pmol/mg protein). In vitro studies showed that 2,4-diamino-6-hydroxypyrimidine (DAHP), an inhibitor of BH4 synthesis, significantly decreased gastric NO release and nitrergic relaxation. Three-week dietary supplementation of BH4 either from day 1 or week 6 significantly attenuated diabetes-induced delayed gastric emptying for solids (3 wk: BH4, 67 ± 6.7 vs. diabetic, 36.05 ± 7.09; 9 wk: BH4, 57 ± 8.45 vs. diabetic, 33 ± 9.91) and diabetes-induced reduction in pyloric nNOS-α protein expression in female rats. Supplementation of BH4 significantly restored gastric nNOS-α dimerization in 9-wk-old diabetic female rats. In addition, BH4 treatment reversed (17.23 ± 5.81 vs. 42.0 ± 2.70 mmHg × s) the diabetes-induced changes in intragastric pressures (IGP) and gastric pyloric nitrergic relaxation (−0.62 ± 0.01 vs. −0.22 ± 0.07). BH4 deficiency plays a critical role in diabetes-induced alterations including delayed solid gastric emptying, increased IGP, reduced pyloric nitrergic relaxation, and nNOS-α expression in female rats. Supplementation of BH4 accelerates gastric emptying by restoring nitrergic system in diabetic female rats. Therefore, BH4 supplementation is a potential therapeutic option for female patients of diabetic gastroparesis. </jats:p>