Comparison of ANP and BNP Granular Density in Atria of Rats After Physiological and Pathological Hypertrophy.

Atrial natriuretic peptide (ANP) and brain natriuretic peptide (BNP) are cardiac hormones located in atria granules. Both peptides respond to cardiac pressure and volume dynamics and accordingly serve as translation biomarkers for the clinical treatment of heart failure. Serum ANP and BNP play central secretary roles in blood pressure and cardiac output regulation and have proven utility as differential biomarkers of cardiovascular proficiency and drug-induced maladaptation, yet both peptides are impervious to exercise-induced hypertrophy. We employed immunoelectron microscopy to examine the effects of 28 days of chronic swim exercise or administration of a PPARγ agonist on atrial granules and their stored natriuretic peptides in Sprague Dawley rats. Chronic swimming and drug treatment both resulted in a 15% increase in heart weight compared with controls, with no treatment effects on perinuclear granule area in the left atria (LAs). Drug treatment resulted in larger size granules with greater BNP density in the right atria. Comparing swimming and PPARγ agonist treatment effects on ANP:BNP granule density ratios between atrial chambers revealed a shift toward a greater proportion of ANP than BNP in LAs of swim-trained rats. These data suggest a distinction in the population of ANP and BNP after chronic swim or PPARγ that makes it a novel metric for the differentiation of pathological and physiological hypertrophy.

Serum Natriuretic Peptides as Differential Biomarkers Allowing for the Distinction between Physiologic and Pathologic Left Ventricular Hypertrophy.

Given the proven utility of natriuretic peptides as serum biomarkers of cardiovascular maladaptation and dysfunction in humans and the high cross-species sequence conservation of atrial natriuretic peptides, natriuretic peptides have the potential to serve as translational biomarkers for the identification of cardiotoxic compounds during multiple phases of drug development. This work evaluated and compared the response of N-terminal proatrial natriuretic peptide (NT-proANP) and N-terminal probrain natriuretic peptide (NT-proBNP) in rats during exercise-induced and drug-induced increases in cardiac mass after chronic swimming or daily oral dosing with a peroxisome proliferator-activated receptor γ agonist. Male Sprague-Dawley rats aged 8 to 10 weeks were assigned to control, active control, swimming, or drug-induced cardiac hypertrophy groups. While the relative heart weights from both the swimming and drug-induced cardiac hypertrophy groups were increased 15% after 28 days of dosing, the serum NT-proANP and NT-proBNP values were only increased in association with cardiac hypertrophy caused by compound administration. Serum natriuretic peptide concentrations did not change in response to adaptive physiologic cardiac hypertrophy induced by a 28-day swimming protocol. These data support the use of natriuretic peptides as fluid biomarkers for the distinction between physiological and drug-induced cardiac hypertrophy.

Mechanical function, glycolysis, and ultrastructure of perfused working mouse hearts following thoracic aortic constriction.

BACKGROUND: Glycolytic flux in the mouse heart during the progression of left ventricular hypertrophy (LVH) and mechanical dysfunction has not been described. METHODS: The main objectives of this study were to characterize the effects of thoracic aortic banding, of 3- and 6-week duration, on: (1) left ventricular (LV) systolic and diastolic function of perfused working hearts quantified by analysis of pressure-volume loops; (2) glycolytic flux in working hearts expressed as the rate of conversion of (3)H-glucose to (3)H(2)O, and (3) ultrastructure of LV biopsies assessed by quantitative and qualitative analysis of light and electron micrographs. RESULTS: Results revealed that (1) indexes of systolic function, including LV end-systolic pressure, cardiac output, and rate of LV pressure development and decline, were depressed to similar degrees at 3 and 6 weeks post-banding; (2) diastolic dysfunction, represented by elevated LV end-diastolic pressure and volume, was more severe at 6 than at 3 weeks, consistent with a transition to failure; (3) a progressive decline in glycolytic flux that was roughly half the control rate by 6 weeks post-banding; and (4) structural derangements, manifested by increases in interstitial collagen content and myocyte Z-band disruption, that were more marked at 3 weeks than at 6 weeks. CONCLUSION: The results are consistent with the view that myocyte damage, fibrosis, and suppressed glycolytic flux represent maladaptive structural and metabolic remodeling that contribute to the development of failure in high pressure load-induced LVH in the mouse.

Optimizing cardiac material parameters with a genetic algorithm.

Determining the unknown material parameters of intact ventricular myocardium can be challenging due to highly nonlinear material behavior. Previous studies combining a gradient-search optimization procedure with finite element analysis (FEA) were limited to two-dimensional (2D) models or simplified three-dimensional (3D) geometries. Here we present a novel scheme to estimate unknown material parameters for ventricular myocardium by combining a genetic algorithm (GA) with nonlinear finite element analysis. This approach systematically explores the domain of the material parameters. The objective function to minimize was the error between simulated strain data and finite element model strains. The proposed scheme was validated for a 2D problem using a realistic material law for ventricular myocardium. Optimized material parameters were generally within 0.5% of the true values. To demonstrate the robustness of the new scheme, unknown material parameters were also determined for a realistic 3D heart model with an exponential hyperelastic material law. When using strains from two material points, the algorithm converged to parameters within 5% of the true values. We conclude that the proposed scheme is robust when estimating myocardial material parameters in 2D and 3D models.

Fluorescence imaging of cardiac propagation: spectral properties and filtering of optical action potentials.

Fluorescence imaging using voltage-sensitive dyes is an important tool for studying electrical propagation in the heart. Yet, the low amplitude of the voltage-sensitive component in the fluorescence signal and high acquisition rates dictated by the rapid propagation of the excitation wave front make it difficult to achieve recordings with high signal-to-noise ratios. Although spatially and temporally filtering the acquired signals has become de facto one of the key elements of optical mapping, there is no consensus regarding their use. Here we characterize the spatiotemporal spectra of optically recorded action potentials and determine the distortion produced by conical filters of different sizes. On the basis of these findings, we formulate the criteria for rational selection of filter characteristics. We studied the evolution of the spatial spectra of the propagating wave front after epicardial point stimulation of the isolated, perfused right ventricular free wall of the pig heart stained with di-4-ANEPPS. We found that short-wavelength (<3 mm) spectral components represent primarily noise and surface features of the preparation (coronary vessels, fat, and connective tissue). The time domain of the optical action potential spectrum also lacks high-frequency components (>100 Hz). Both findings are consistent with the reported effect of intrinsic blurring caused by light scattering inside the myocardial wall. The absence of high-frequency spectral components allows the use of aggressive low-pass spatial and temporal filters without affecting the optical action potential morphology. We show examples where the signal-to-noise ratio increased up to 150 with <3% distortion. A generalization of our approach to the rational filter selection in various applications is discussed.

Optical action potential upstroke morphology reveals near-surface transmural propagation direction.

The analysis of surface-activation patterns and measurements of conduction velocity in ventricular myocardium is complicated by the fact that the electrical wavefront has a complex 3D shape and can approach the heart surface at various angles. Recent theoretical studies suggest that the optical upstroke is sensitive to the subsurface orientation of the wavefront. Our goal here was to (1) establish the quantitative relationship between optical upstroke morphology and subsurface wavefront orientation using computer modeling and (2) test theoretical predictions experimentally in isolated coronary-perfused swine right ventricular preparations. We show in numerical simulations that by suitable placement of linear epicardial stimulating electrodes, the angle phi of wavefronts with respect to the heart surface can be controlled. Using this method, we developed theoretical predictions of the optical upstroke shape dependence on phi. We determined that the level VF* at which the rate of rise of the optical upstroke reaches the maximum linearly depends on phi. A similar relationship was found in simulations with epicardial point stimulation. The optical mapping data were in good agreement with theory. Plane waves propagating parallel to myocardial fibers produced upstrokes with VF*<0.5, consistent with theoretical predictions for phi>0. Similarly, we obtained good agreement with theory for plane waves propagating in a direction perpendicular to fibers (VF*>0.5 when phi<0). Finally, during epicardial point stimulation, we discovered characteristic saddle-shaped VF* maps that were in excellent agreement with theoretically predicted changes in phi during wavefront expansion. Our findings should allow for improved interpretation of the results of optical mapping of intact heart preparations.

Epicardial fiber organization in swine right ventricle and its impact on propagation.

Fiber organization is important for myocardial excitation and contraction. It can be a major factor in arrhythmogenesis and current distribution during defibrillation shocks. In this study, we report the discovery of a previously undetected thin epicardial layer in swine right ventricle (RV) with distinctly different fiber orientation, which significantly affects epicardial propagation. Experiments were conducted in isolated coronary-perfused right ventricular free wall preparations (n=8) stained with the voltage-sensitive dye di-4-ANEPPS. Optical signals were recorded from the epicardium with a CCD video camera at 800 fps. Preparations were sectioned parallel to the epicardial surface with a resolution of 50 mum or better. To link the histological data with the observed activation patterns, resulting fiber angles were introduced into a 3D computer model to simulate the electrical activation and voltage-dependent optical signals. In all preparations, we detected a thin epicardial layer with almost no depth-dependent fiber rotation. The thickness of this layer (z(0)) varied from 110 to 930 microm. At the boundary of this layer, we observed an abrupt change in fiber angle by 64+/-13 degrees followed by a gradual fiber rotation in the underlying layers. In preparations with z(0) <700 microm, optical mapping during epicardial stimulation revealed unusual diamond- and rectangular-shaped activation fronts with two axes of fast conduction. Computer simulations accurately predicted the features of the experimentally recorded activation fronts. The free wall of swine RV has a thin epicardial layer with distinctly different fiber orientation, which can significantly affect propagation and give rise to unusually shaped activation fronts. This is important for understanding electrical propagation in the heart, and further refines the existing knowledge of myocardial fiber architecture.