Lack of nitric oxide synthase depresses ion transporting enzyme function in cardiac muscle

Nitric oxide (NO^·) is produced endogenously from NOS isoforms bound to sarcolemmal (SL) and sarcoplasmic reticulum (SR) membranes. To investigate whether locally generated NO^· directly affects the activity of enzymes mediating ion active transport we studied whether knockout of selected NOS isoforms would affect the functions of cardiac SL (Na⁺ + K⁺)-ATPase and SR Ca²⁺-ATPase. Cardiac SL and SR vesicles containing either SL (Na⁺ + K⁺)-ATPase or SR Ca²⁺-ATPase were isolated from mice lacking either nNOS or eNOS or both and tested for enzyme activities. Western blot analysis revealed that absence of single or double NOS isoforms did not interrupt the protein expression of SL (Na⁺ + K⁺)-ATPase and SR Ca²⁺-ATPase in cardiac muscle cells. However lack of NOS isoforms in cardiac muscle significantly altered both (Na⁺ + K⁺)-ATPase activity and SR Ca²⁺-ATPase function. Our experimental results suggest that disrupted endogenous NO^· production may change local redox conditions and lead to an unbalanced free radical homeostasis in cardiac muscle cells which in turn may affect key enzyme activities and membrane ion active transport systems in the heart.