The circadian clock within the heart: potential influence on myocardial gene expression, metabolism, and function

Dependence of heart rate variability on heart period in disease and aging

The aim of this study was to compare the dependence of heart rate variability (HRV) on heart period (RR interval length) under different physiological and pathological states in order to detect changes in HR modulation. The dependence of HRV on the RR interval length in healthy elderly subjects, congestive heart failure (CHF) patients and one patient with a transplanted heart (T) was compared with healthy young subjects. Spectral powers, sample entropy (SampEn) and short-term fractal scaling exponent (alpha1) were determined from 24 h free-running recordings. For the same HR, HRV measures were different in different groups. In healthy subjects HRV measures depended on RR interval length and all spectral powers were highly correlated, although reduced in elderly subjects. SampEn at high HR was the most sensitive quantity to changes induced by aging. In disease, CHF and T, an achievable HR range was decreased, all spectral powers were reduced, but correlated, and the dependence of HRV measures on RR was lost. There was an evident difference in the dependence of nonlinear on linear measures between young subjects and all the other studied groups. In disease the reduction in autonomic control was associated with the decrease in short-range correlation and regularity in RR series. We have concluded that the analysis of HRV measures as functions of RR interval length can reveal important aspects of HR control that might be lost in averaging.

Reciprocal relationships between insulin resistance and endothelial dysfunction: molecular and pathophysiological mechanisms

Endothelial dysfunction contributes to cardiovascular diseases, including hypertension, atherosclerosis, and coronary artery disease, which are also characterized by insulin resistance. Insulin resistance is a hallmark of metabolic disorders, including type 2 diabetes mellitus and obesity, which are also characterized by endothelial dysfunction. Metabolic actions of insulin to promote glucose disposal are augmented by vascular actions of insulin in endothelium to stimulate production of the vasodilator nitric oxide (NO). Indeed, NO-dependent increases in blood flow to skeletal muscle account for 25% to 40% of the increase in glucose uptake in response to insulin stimulation. Phosphatidylinositol 3-kinase-dependent insulin-signaling pathways in endothelium related to production of NO share striking similarities with metabolic pathways in skeletal muscle that promote glucose uptake. Other distinct nonmetabolic branches of insulin-signaling pathways regulate secretion of the vasoconstrictor endothelin-1 in endothelium. Metabolic insulin resistance is characterized by pathway-specific impairment in phosphatidylinositol 3-kinase-dependent signaling, which in endothelium may cause imbalance between production of NO and secretion of endothelin-1, leading to decreased blood flow, which worsens insulin resistance. Therapeutic interventions in animal models and human studies have demonstrated that improving endothelial function ameliorates insulin resistance, whereas improving insulin sensitivity ameliorates endothelial dysfunction. Taken together, cellular, physiological, clinical, and epidemiological studies strongly support a reciprocal relationship between endothelial dysfunction and insulin resistance that helps to link cardiovascular and metabolic diseases. In the present review, we discuss pathophysiological mechanisms, including inflammatory processes, that couple endothelial dysfunction with insulin resistance and emphasize important therapeutic implications.

Central and peripheral clocks in cardiovascular and metabolic function

The molecular circadian clock entrains biological rhythms to a 24-hour schedule. Aspects of cardiovascular physiology and, indeed, the incidence of myocardial infarction and stroke are also subject to diurnal variation. The use of rodent models of disrupted clock function has begun to elucidate the role of the molecular clock in the pathophysiology of cardiovascular and metabolic disease.