Metabolic substrate regulation of cardiac myocyte contraction and insulin response in normal and hypertensive rats

Abstract

Fatty acid-dependent metabolism is predominant in cardiac ATP production that maintains normal contractile function of the heart. However, excessive fat or fatty acids (metabolic syndrome) are the precursors of cardiovascular complications, such as ventricular arrhythmias (sudden cardiac death), heart failure and stroke. Insulin resistance in muscle is one of the key underlying mechanisms for the adverse effects of increased metabolites. Until recently, investigation of metabolic substrates regulation of cardiac contractile function and insulin responsiveness in vitro is lacking. Therefore, we design to study whether supplementation of metabolic substrates (oleic acid 200M, palmitic acid 100M, linolic acid 100M, lactate 1 mM, pyruvate 100M and carnitine 50M) to normal tyrode (NT) perfusate (termed nutrition full, NF) affects basal and beta-adrenergic left ventricular (LV) myocyte contractility and changes insulin response in normal and angiotensin II (Ang II)-induced hypertensive rat hearts.

Our results demonstrated that basal and isoprenaline (ISO, 100 nM)-stimulated myocyte shortening (field stimulation, 2Hz, 36 ± 1 oC) were significantly increased with NF in LV myocytes from normal and hypertensive rat hearts. In NT, insulin (10 nM) abolished ISO-increase in LV myocyte contraction from normal rats. This effect was prevented by nitric oxide synthase (NOS) inhibitor, L-NG-nitroarginine methyl ester (L-NAME, 1mM) but not by neuronal NOS (nNOS) inhibition with S-methyl-L-thiocitrulline (SMTC, 100 nM), suggesting eNOS-mediated anti-adrenergic response of insulin. In NF, insulin did not change basal or ISO-stimulated myocyte contraction in either group, suggesting reduced insulin response. Furthermore, L-NAME did not affect myocyte contraction in the presence or absence of insulin in NF. On the other hand, ISO induced spontaneous contractions (arrhythmias) in NF and the percentage of arrhythmic incidence was significantly greater in hypertension. Ranolazine (10 M), inhibitor of carnitine palmitoyl transferase 1 (CPT-1), that is known to inhibit late Na+ current, did not affect NF-enhancement of myocyte contraction in normal or hypertensive hearts but significantly reduced arrhythmias in both groups. Interestingly, SMTC significantly increased arrhythmias in normal but reduced it in hypertension, suggesting contrasting roles nNOS play in the rhythmic contraction in the presence of metabolic substrates between normal and hypertension.

Taken together, our results demonstrate that metabolic substrate supplementation improves myocyte contraction but impairs insulin response and induces arrhythmias with beta-adrenergic stimulation. nNOS plays an important role in cardiac arrhythmogenesis in the presence of metabolic stress.