The subcellular localisation of neuronal nitric oxide synthase determines the downstream effects of NO on myocardial function
Carnicer R., Suffredini S., Liu X., Reilly S., Simon JN., Surdo N., Zhang YH., Lygate CA., Channon KM., Casadei B.
Aim: In healthy hearts, the neuronal nitric oxide synthase (nNOS) is predominantly localised to the sarcoplasmic reticulum (SR), where it regulates the ryanodine receptor Ca2+ release channel (RyR2) and phospholamban (PLB) phosphorylation, and to a lesser extent to the sarcolemmal membrane where it inhibits the L-type Ca2+ current (ICa). However, in failing hearts, impaired relaxation and depressed inotropy are associated with a larger proportion of nNOS being localised to the sarcolemmal membrane. Whether there is a causal relationship between altered myocardial function and subcellular localisation of nNOS remains to be assessed. Methods and Results: Adenoviruses (AdV) encoding for a human nNOS.eGFP fusion protein or eGFP were injected into the left ventricle (LV) of nNOS-/- mice. nNOS.eGFP localised to the sarcolemmal and t-tubular membrane and immunoprecipitated with syntrophin and caveolin-3 but not with RyR2. Myocardial transduction of nNOS.eGFP resulted in a significantly increased NOS activity (10-fold, P<0.01), a 20% increase in myocardial tetrahydrobiopterin (BH4) (P<0.05), and a 30% reduction in superoxide production (P<0.001). LV myocytes transduced with nNOS.eGFP showed a significantly lower basal and -adrenergic stimulated ICa, [Ca2+]i transient amplitude and cell shortening (vs. eGFP). All differences between groups were abolished after NOS inhibition. By contrast, nNOS.eGFP had no effect on RyR nitrosylation, PLB phosphorylation or the rate of myocardial relaxation and [Ca2+]i decay. Conclusion: Our findings indicate that nNOS-mediated regulation of myocardial excitation-contraction (E-C) coupling is exquisitely dependent on nNOS subcellular localisation and suggests a partially adaptive role for sarcolemmal nNOS in the human failing myocardium.