Neuronal nitric oxide synthase modulation of intracellular Ca handling overrides fatty acid potentiation of cardiac inotropy in hypertensive rats.

Cardiac neuronal nitric oxide synthase (nNOS) is an important molecule that regulates intracellular Ca homeostasis and contractility of healthy and diseased hearts. Here, we examined the effects of nNOS on fatty acid (FA) regulation of left ventricular (LV) myocyte contraction in sham and angiotensin II (Ang II)-induced hypertensive (HTN) rats. Our results showed that palmitic acid (PA, 100 μM) increased the amplitudes of sarcomere shortening and intracellular ATP in sham but not in HTN despite oxygen consumption rate (OCR) was increased by PA in both groups. Carnitine palmitoyltransferase-1 inhibitor, etomoxir (ETO), reduced OCR and ATP with PA in sham and HTN but prevented PA potentiation of sarcomere shortening only in sham. PA increased nNOS-derived NO only in HTN. Inhibition of nNOS with S-methyl-L-thiocitrulline (SMTC) prevented PA-induced OCR and restored PA potentiation of myocyte contraction in HTN. Mechanistically, PA increased intracellular Ca transient ([Ca]) without changing Ca influx via L-type Ca channel (I-) and reduced myofilament Ca sensitivity in sham. nNOS inhibition increased [Ca], I- and reduced myofilament Ca sensitivity prior to PA supplementation; as such, normalized PA increment of [Ca]. In HTN, PA reduced I- without affecting [Ca] or myofilament Ca sensitivity. However, PA increased I-, [Ca] and reduced myofilament Ca sensitivity following nNOS inhibition. Myocardial FA oxidation (F-fluoro-6-thia-heptadecanoic acid, F-FTHA) was comparable between groups, but nNOS inhibition increased it only in HTN. Collectively, PA increases myocyte contraction through stimulating [Ca] and mitochondrial activity in healthy hearts. PA-dependent cardiac inotropy was limited by nNOS in HTN, predominantly due to its modulatory effect on [Ca] handling.