Virtually every mammalian cell, including cardiomyocytes, possesses an intrinsic circadian clock. The role of this transcriptionally based molecular mechanism in cardiovascular biology is poorly understood. We hypothesized that the circadian clock within the cardiomyocyte influences diurnal variations in myocardial biology. We, therefore, generated a cardiomyocyte-specific circadian clock mutant (CCM) mouse to test this hypothesis. At 12 wk of age, CCM mice exhibit normal myocardial contractile function in vivo, as assessed by echocardiography. Radiotelemetry studies reveal attenuation of heart rate diurnal variations and bradycardia in CCM mice (in the absence of conduction system abnormalities). Reduced heart rate persisted in CCM hearts perfused ex vivo in the working mode, highlighting the intrinsic nature of this phenotype. Wild-type, but not CCM, hearts exhibited a marked diurnal variation in responsiveness to an elevation in workload (80 mmHg plus 1 M epinephrine) ex vivo, with a greater increase in cardiac power and efficiency during the dark (active) phase vs. the light (inactive) phase. Moreover, myocardial oxygen consumption and fatty acid oxidation rates were increased, whereas cardiac efficiency was decreased, in CCM hearts. These observations were associated with no alterations in mitochondrial content or structure and modest mitochondrial dysfunction in CCM hearts. Gene expression microarray analysis identified 548 and 176 genes in atria and ventricles, respectively, whose normal diurnal expression patterns were altered in CCM mice. These studies suggest that the cardiomyocyte circadian clock influences myocardial contractile function, metabolism, and gene expression.

The transcriptional coactivator peroxisome proliferator-activated receptor-gamma coactivator-1β (PGC-1β) has been implicated in important metabolic processes. A mouse lacking PGC-1β (PGC1βKO) was generated and phenotyped using physiological, molecular, and bioinformatic approaches. PGC1βKO mice are generally viable and metabolically healthy. Using systems biology, we identified a general defect in the expression of genes involved in mitochondrial function and, specifically, the electron transport chain. This defect correlated with reduced mitochondrial volume fraction in soleus muscle and heart, but not brown adipose tissue (BAT). Under ambient temperature conditions, PGC-1β ablation was partially compensated by up-regulation of PGC-1α in BAT and white adipose tissue (WAT) that lead to increased thermogenesis, reduced body weight, and reduced fat mass. Despite their decreased fat mass, PGC1βKO mice had hypertrophic adipocytes in WAT. The thermogenic role of PGC-1β was identified in thermoneutral and cold-adapted conditions by inadequate responses to norepinephrine injection. Furthermore, PGC1βKO hearts showed a blunted chronotropic response to dobutamine stimulation, and isolated soleus muscle fibres from PGC1βKO mice have impaired mitochondrial function. Lack of PGC-1β also impaired hepatic lipid metabolism in response to acute high fat dietary loads, resulting in hepatic steatosis and reduced lipoprotein-associated triglyceride and cholesterol content. Altogether, our data suggest that PGC-1β plays a general role in controlling basal mitochondrial function and also participates in tissue-specific adaptive responses during metabolic stress.

To investigate the role of insulin signaling on postnatal cardiac development, physiology, and cardiac metabolism, we generated mice with a cardiomyocyte-selective insulin receptor knockout (CIRKO) using cre/loxP recombination. Hearts of CIRKO mice were reduced in size by 20–30% due to reduced cardiomyocyte size and had persistent expression of the fetal β-myosin heavy chain isoform. In CIRKO hearts, glucose transporter 1 (GLUT1) expression was reduced by about 50%, but there was a twofold increase in GLUT4 expression as well as increased rates of cardiac glucose uptake in vivo and increased glycolysis in isolated working hearts. Fatty acid oxidation rates were diminished as a result of reduced expression of enzymes that catalyze mitochondrial β-oxidation. Although basal rates of glucose oxidation were reduced, insulin unexpectedly stimulated glucose oxidation and glycogenolysis in CIRKO hearts. Cardiac performance in vivo and in isolated hearts was mildly impaired. Thus, insulin signaling plays an important developmental role in regulating postnatal cardiac size, myosin isoform expression, and the switching of cardiac substrate utilization from glucose to fatty acids. Insulin may also modulate cardiac myocyte metabolism through paracrine mechanisms by activating insulin receptors in other cell types within the heart

imally invasive aortic banding in mice: effects of altered cardiomyocyte insulin signaling during pressure overload. Am J Physiol Heart Circ Physiol 285: H1261-H1269, 2003. First published May 8, 2003 10.1152/ajpheart.00108. 2003.-We developed a minimally invasive method for producing left ventricular (LV) pressure overload in mice. With the use of this technique, we quickly and reproducibly banded the transverse aorta with low surgical morbidity and mortality. Minimally invasive transverse aortic banding (MTAB) acutely and chronically increased LV systolic pressure, increased heart weight-to-body weight ratio, and induced myocardial fibrosis. We used this technique to determine whether reduced insulin signaling in the heart altered the cardiac response to pressure overload. Mice with cardiac myocyte-restricted knockout of the insulin receptor (CIRKO) have smaller hearts than wild-type (WT) controls. Four weeks after MTAB, WT and CIRKO mice had comparably increased LV systolic pressure, increased cardiac mass, and induction of mRNA for ␤-myosin heavy chain and atrial natriuretic factor. However, CIRKO hearts were more dilated, had depressed LV systolic function by echocardiography, and had greater interstitial fibrosis than WT mice. Expression of connective tissue growth factor was increased in banded CIRKO hearts compared with WT hearts. Thus lack of insulin signaling in the heart accelerates the transition to a more decompensated state during cardiac pressure overload. The use of the MTAB approach should facilitate the study of the pathophysiology and treatment of pressureoverload hypertrophy.hypertrophy; contractility; fibrosis THE ABILITY TO MANIPULATE gene expression in mice has contributed greatly to the study of many diseases. In terms of the heart, the response of genetically altered animals to increased cardiac work (e.g., pressure overload) is of particular interest. Rockman and colleagues (33) pioneered a model of transverse aortic constriction in the mouse. The use of this model has provided significant insight into the cellular and molecular pathways responsible for the development of left ventricular (LV) hypertrophy (LVH). Although transverse aortic constriction in mice is now done routinely by a number of groups, the technical difficulty of the surgical procedure has limited the availability of this model.Previously published methods for the creation of transverse aortic constriction in mice require microsurgical skills and the ability to provide mechanical ventilation when the thorax is entered. The requirement for tracheal intubation and low-volume, high-rate mechanical ventilation mandates additional time and expense associated with these procedures. Moreover, inflammatory reactions within the chest may complicate the analyses of cardiac function and pathology. Herein, we report a minimally invasive transverse aortic banding (MTAB) procedure in mice that obviates the need for providing mechanical ventilation because the pleural space is not entered. The procedure can be performed rapidly and...

The kinetics of contractions and Ca(2+) transients are slowed in myocytes from failing hearts. The mechanisms accounting for these abnormalities remain unclear. Myocardial infarction (MI) was produced by ligation of the circumflex artery in rabbits. We used confocal microscopy to record spatially resolved Ca(2+) transients during field stimulation in left ventricular (LV) myocytes from control and infarcted hearts (3 weeks). Compared with controls, Ca(2+) transients in myocytes adjacent to the infarct had lower peak amplitudes and prolonged time courses. Control myocytes showed relatively uniform changes in [Ca(2+)] throughout the cell after electrical stimulation. In contrast, in MI myocytes [Ca(2+)] increased inhomogeneously and localized increases in [Ca(2+)] occurred throughout the rising and falling phases of the Ca(2+) transient. Ca(2+) content of the sarcoplasmic reticulum did not differ between MI and control myocytes. Peak L-type Ca(2+) current density was reduced in MI myocytes. The macroscopic gain function was not different in control and MI myocytes when calculated as the amplitude of the Ca(2+) transient/peak I:(Ca). However, when calculated as the peak rate of rise of the Ca(2+) transient/peak I:(Ca), the gain function was modestly decreased in the MI myocytes. Application of isoproterenol (100 nmol/L) improved the synchronization of Ca(2+) release in MI myocytes at both 0.5 and 1 Hz. The poorly coordinated production of Ca(2+) sparks in myocytes from infarcted rabbit hearts likely contributes to the diminished and slowed macroscopic Ca(2+) transient. These abnormalities can be largely overcome when phosphorylation of Ca(2+) cycling proteins is enhanced by ss-adrenergic stimulation.