The subcellular localization of neuronal nitric oxide synthase determines the downstream effects of NO on myocardial function

<span class="paragraphSection"><div class="boxTitle">Aims</div>In healthy hearts, the neuronal nitric oxide synthase (nNOS) is predominantly localized 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 (<span style="font-style:italic;">I</span>_Ca). However, in failing hearts, impaired relaxation and depressed inotropy are associated with a larger proportion of nNOS being localized to the sarcolemmal membrane. Whether there is a causal relationship between altered myocardial function and subcellular localization of nNOS remains to be assessed.<div class="boxTitle">Methods and results</div>Adenoviruses (AdV) encoding for a human nNOS.eGFP fusion protein or eGFP were injected into the left ventricle (LV) of nNOS^−/− mice. nNOS.eGFP localized 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, <span style="font-style:italic;">P</span> < 0.01), a 20% increase in myocardial tetrahydrobiopterin (BH4) (<span style="font-style:italic;">P</span> < 0.05), and a 30% reduction in superoxide production (<span style="font-style:italic;">P</span> < 0.001). LV myocytes transduced with nNOS.eGFP showed a significantly lower basal and β-adrenergic stimulated <span style="font-style:italic;">I</span>_Ca, [Ca²⁺]_i transient amplitude and cell shortening (vs. eGFP). All differences between groups were abolished after NOS inhibition. In contrast, nNOS.eGFP had no effect on RyR nitrosylation, PLB phosphorylation or the rate of myocardial relaxation and [Ca²⁺]_i decay.<div class="boxTitle">Conclusion</div>Our findings indicate that nNOS-mediated regulation of myocardial excitation–contraction (E–C) coupling is exquisitely dependent on nNOS subcellular localization and suggests a partially adaptive role for sarcolemmal nNOS in the human failing myocardium.</span>

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