The pivotal role of neuronal nitric oxide synthase (nNOS) in regulating cardiac function has only recently been unveiled. Notably, others have shown that responsiveness to β-adrenergic stimulation is dependent on nNOS activity. In a cellular model, we showed that the Ca
2+
/calmodulin-dependent nNOS activity is reduced by overexpression of isoform 4b of the plasma membrane Ca
2+
/Calmodulin-dependent Ca
2+
-pump (PMCA4b), which binds to nNOS. We demonstrated that PMCA4b overexpression in the heart reduced β-adrenergic responsiveness
in vivo
via an nNOS dependent mechanism (Oceandy et al, Circulation 2007). Here we investigated the cellular mechanisms of the regulation of the β-adrenergic response by PMCA4b. We used an adenoviral system to overexpress PMCA4b (PMCA4b cells) or LacZ (control, C) in neonatal rat cardiomyocytes. PMCA4b cells showed an 18±5% and 24±5% reduction in nitric oxide (DAF-FM fluorescence) and cGMP levels, respectively (n=6, p<0.05 each) compared to C demonstrating the regulation of NO production by the PMCA4b in this system. Since nNOS has been shown to regulate phospholamban (PLB) phosphorylation, we examined phosphorylation of PLB at Ser16. PMCA4b cells showed a significant increase in Ser16-PLB at baseline (66±17%, p<0.05) compared to C. As a result of increased baseline Ser16-PLB in PMCA4b cells, β-adrenergic stimulation of PMCA4b cells using 2μM isoproter-enol (IP) showed reduced relative induction in Ser16-PLB (23±10% vs. 78±19% in C; n=5, p<0.05). Further analysis in adult cardiomyocytes isolated from our PMCA4b transgenic mice (PMCA4b TG) demonstrated that PMCA4b TG showed 3-fold higher Ser16-PLB phosphorylation at baseline compared to wild type (WT) myocytes and the relative response following β-adrenergic stimulation was significantly reduced (1.2±0.2 fold induction after IP treatment in PMCA4b TG, vs. 3.1±0.7 in WT, n=5, p<0.05). Thus, PMCA4b regulates NO production from nNOS, which in turn modulates cGMP levels and PLB phosphorylation. These findings provide mechanistic insight into the regulation of the β-adrenergic response in the heart by PMCA4b and place this Ca
2+
-pump upstream of the recently described pathway linking nNOS and Ser16-PLB phosphorylation and downstream of the β-adrenergic receptor(s).
Synthesis and enzymatic evaluation of 2- and 4-aminothiazole-based inhibitors of neuronal nitric oxide synthase
