Effect of nitric oxide synthase inhibitor on diaphragmatic function after resistive loading

We studied the effect of a nitric oxide synthase inhibitor, N(ω)- Nitro-L-arginine-methyl-ester (L-NAME), on in vitro diaphragmatic function both at rest (control) or after inspiratory resistive loading (IRL). Sprague- Dawley rats were anesthetized, instrumented, and then the following experimental groups: (1) controls; (2) L-NAME (100 mg/kg/body weight intravenously alone); (3) IRL alone; and (4) L-NAME + IRL. The IRL protocol consisted of applying a variable resistor to the inspiratory limb of a two- way valve at 70% of maximal airway pressure until apnea. After the experiment, the animals were sacrificed and diaphragmatic strips were obtained for activity of constitutive nitric oxide synthase (cNOS) and measurements of in vitro contractile properties: tetanic (Po) and twitch tensions (Pt). cNOS activity was significantly decreased in the L-NAME and L- NAME + IRL groups (P ≤ 0.05) as compared with control and IRL groups. L- NAME alone did not affect Pc or Pt. However, in both IRL groups, with and without was a significant decrease in Pc and Pt. This reduction was comparable in both groups. In summary, our data showed that L-NAME resulted in a significant decrease cNOS activity, but in vitro contractility was impaired.