Thomas Wannenburg

Growth hormone, insulin-like growth factor-1 and the aging cardiovascular system

There is a large body of evidence that biological aging is related to a series of long-term catabolic processes resulting in decreased function and structural integrity of several physiological systems, among which is the cardiovascular system. These changes in the aging phenotype are correlated with a decline in the amplitude of pulsatile growth hormone secretion and the resulting decrease in plasma levels of its anabolic mediator, insulin like growth factor-1 (IGF-1). The relationship between growth hormone and biological aging is supported by studies demonstrating that growth hormone administration to old animals and humans raises plasma IGF-1 and results in increases in skeletal muscle and lean body mass, a decrease in adiposity, increased immune function, improvements in learning and memory, and increases in cardiovascular function. Since growth hormone and IGF-1 exert potent effects on the heart and vasculature, the relationship between age-related changes in cardiovascular function and the decline in growth hormone levels with age have become of interest. Among the age-related changes in the cardiovascular system are decreases in myocyte number, accumulation of fibrosis and collagen, decreases in stress-induced cardiac function through deterioration of the myocardial conduction system and beta-adrenergic receptor function, decreases in exercise capacity, vessel rarefaction, decreased arterial compliance and endothelial dysfunction leading to alterations in blood flow. Growth hormone has been found to exert potent effects on cardiovascular function in young animals and reverses many of the deficits in cardiovascular function in aged animals and humans. Nevertheless, it has been difficult to separate the effects of growth hormone deficiency from age-related diseases and associated pathologies. The development of novel animal models and additional research are required in order to elucidate the specific effects of growth hormone deficiency and assess its contribution to cardiovascular impairments and biological aging.

Decreased Myocyte Tension Development and Calcium Responsiveness in Rat Right Ventricular Pressure Overload

BACKGROUND The contractile dysfunction observed in end-stage myocardial hypertrophy has at its base an abnormality in myocyte function. However, whether depressed contractile function is related to an alteration in contractile protein function is presently unknown. METHODS AND RESULTS Contractile force, tension, and calcium responsiveness were measured in single-skinned myocytes isolated from rats with right ventricular hypertrophy (RVH) and control rats. RVH was induced by pulmonary artery constriction for 36 weeks and was associated with significant myocyte hypertrophy. Myocytes were attached to micropipettes that extended from a force transducer and motor. Isometric force was measured over a wide range of calcium concentrations at two sarcomere lengths (SLs). Maximal force was increased in the RVH group: 1.20 +/- 0.10 versus 1.62 +/- 0.13 mg at SL = 2.0 microns and 1.33 +/- 0.10 versus 1.84 +/- 0.15 mg at SL = 2.3 microns (P < .05). Maximal tension, however, was reduced in the RVH group: 24.3 +/- 1.91 versus 37.5 +/- 2.92 mN/mm2 at SL = 2.0 microns and 27.4 +/- 1.78 versus 41.8 +/- 3.19 mN/mm2 at SL = 2.3 microns (P < .01). The concentration of calcium ions required for half-maximal activation was increased in the RVH group: 2.64 +/- 0.13 versus 3.47 +/- 0.22 mumol/L at SL = 2.0 microns and 2.23 +/- 0.15 versus 2.86 +/- 0.18 mumol/L at SL = 2.3 microns (P < .01). The slope of the force-calcium relationship (Hill coefficient) was decreased in the RVH group at SL = 2.0 microns (4.3 +/- 0.4 versus 3.1 +/- 0.2, P = .04) but not at SL = 2.3 microns (3.8 +/- 0.2 versus 3.6 +/- 0.2, P = NS). CONCLUSIONS These results suggest that the depressed cardiac function of end-stage myocardial hypertrophy may be due, in part, to altered contractile protein function.

Angiotensin II Type 1 Receptor Blockade Prevents Alcoholic Cardiomyopathy

Background— Activation of the renin-angiotensin system (RAS) may contribute to the development of alcoholic cardiomyopathy. We evaluated the effect of angiotensin II (Ang II) type 1 receptor (AT1) blockade on the development of alcoholic cardiomyopathy. Methods and Results— We serially evaluated left ventricular (LV) and cardiomyocyte function and the RAS over 6 months in 3 groups of instrumented dogs. Eight animals received alcohol (once per day orally, providing 33% of total daily caloric intake); 6 received alcohol and irbesartan (5 mg · kg−1 · d−1 PO); and 8 were controls. Compared with controls, alcohol ingestion caused sustained RAS activation with progressive increases in plasma levels of Ang II, renin activity, LV angiotensin-converting enzyme activity, and LV myocyte Ang II AT1 receptor expression. The RAS activation was followed by a progressive fall in LV contractility (EES, alcohol-fed dogs 3.9±0.8 versus control dogs 8.1±1.0 mm Hg/mL); reductions in the peak velocity of myocyte shortening (78.9±5.1 versus 153.9±6.2 &mgr;m/s) and relengthening; and decreased peak systolic Ca2+ transient ([Ca2+]iT) and L-type Ca2+ current (ICa,L; P<0.05). Irbesartan prevented the alcohol-induced decreases in LV and myocyte contraction, relaxation, peak [Ca2+]iT, and ICa,L. With alcohol plus irbesartan, plasma Ang II, cardiac angiotensin-converting enzyme activity, and AT1 remained close to control values. Conclusions— Chronic alcohol consumption produces RAS activation followed by progressive cardiac dysfunction. The cardiac dysfunction is prevented by AT1 receptor blockade.

Localization of Tissue Transglutaminase in Human Carotid and Coronary Artery Atherosclerosis: Implications for Plaque Stability and Progression

Although atherosclerosis progresses in an indolent state for decades, the rupture of plaques creates acute ischemic syndromes that may culminate in myocardial infarction and stroke. Mechanical forces and matrix metalloproteinase activity initiate plaque rupture, whereas tissue inhibitors of metalloproteinases have an important (albeit indirect) role in plaque stabilization. In this paper, an enzyme that could directly stabilize the plaque is described. Tissue transglutaminase (TG) catalyzes the formation of ε(γ-glutamyl)lysine isopeptide bonds that are resistant to enzymatic, mechanical, and chemical degradation. We performed immunohistochemistry for TG in atherosclerotic human coronary and carotid arteries. TG was most prominent along the luminal endothelium and in the medium of the vessels with a distribution mirroring that of smooth muscle cells. Variable, often prominent, immunoreactivity for TG was also seen in the intima, especially in regions with significant neovascularization. Additionally, TG was detected in fibrous caps and near the “shoulder regions” of some plaques. A monoclonal antibody to the transglutaminase product ε(γ-glutamyl)lysine isopeptide demonstrated co-localization with TG antigen. Transglutaminase activity was found in 6 of 14 coronary artery atherectomy samples. Cross-linking of TG substrates such as fibrinogen, fibronectin, vitronectin, collagen type I, and protease inhibitors stabilized the plaque. Furthermore, the activation of transforming growth factor–beta-1 by TG might be an additional mechanism for the promotion of plaque stabilization and progression by increasing the synthesis of extracellular matrix components.

The Frank-Starling mechanism is not mediated by changes in rate of cross-bridge detachment

We tested the hypothesis that the Frank-Starling relationship is mediated by changes in the rate of cross-bridge detachment in cardiac muscle. We simultaneously measured isometric force development and the rate of ATP consumption at various levels of Ca2+ activation in skinned rat cardiac trabecular muscles at three sarcomere lengths (2.0, 2.1, and 2.2 μm). The maximum rate of ATP consumption was 1.5 nmol ⋅ s-1 ⋅ μl fiber vol-1, which represents an estimated adenosinetriphosphatase (ATPase) rate of ∼10 s-1 per myosin head at 24°C. The rate of ATP consumption was tightly and linearly coupled to the level of isometric force development, and changes in sarcomere length had no effect on the slope of the force-ATPase relationships. The average slope of the force-ATPase relationships was 15.5 pmol ⋅ mN-1 ⋅ mm-1. These results suggest that the mechanisms that underlie the Frank-Starling relationship in cardiac muscle do not involve changes in the kinetics of the apparent detachment step in the cross-bridge cycle.We tested the hypothesis that the Frank-Starling relationship is mediated by changes in the rate of cross-bridge detachment in cardiac muscle. We simultaneously measured isometric force development and the rate of ATP consumption at various levels of Ca2+ activation in skinned rat cardiac trabecular muscles at three sarcomere lengths (2.0, 2.1, and 2.2 microns). The maximum rate of ATP consumption was 1.5 nmol.s-1.microliter fiber vol-1, which represents an estimated adenosinetriphosphatase (ATPase) rate of approximately 10 s-1 per myosin head at 24 degrees C. The rate of ATP consumption was tightly and linearly coupled to the level of isometric force development, and changes in sarcomere length had no effect on the slope of the force-ATPase relationships. The average slope of the force-ATPase relationships was 15.5 pmol.mN-1.mm-1. These results suggest that the mechanisms that underlie the Frank-Starling relationship in cardiac muscle do not involve changes in the kinetics of the apparent detachment step in the cross-bridge cycle.

Anomalous intracavitary right coronary artery shown by cardiac CT: A potential hazard to be aware of before various interventions

Intracavitary right coronary arteries (RCAs) are uncommon (incidence of 0.09%-0.1%), having previously been reported nearly exclusively in autopsy series. However, more recently this entity has been detected prospectively by noninvasive cardiac computed tomography. Because many interventional procedures, including pacemaker placement and atrial flutter ablation, may be influenced by the presence of an intracavitary RCA, this entity is important to recognize. We report two cases of intracavitary right coronary artery discovered prospectively by cardiac computed tomography. In one of these cases, interventional management was altered based on our findings.

IDIOPATHIC LEFT ANTERIOR FASCICULAR TACHYCARDIA PRESENTING AFTER AORTIC VALVE REPLACEMENT

Idiopathic left fascicular ventricular tachycardia (VT) is the most common VT originating from the left ventricle. Left anterior fascicular tachycardia (LAFT) is an uncommon subset of idiopathic left fascicular VT which may respond to verapamil. ![Figure][1] A 59 year-old male with