Disparate effects of early pressure overload hypertrophy on velocity-dependent and force-dependent indices of ventricular performance in the conscious baboon

Transesophageal Echocardiography in Healthy Young Adult Male Baboons (Papio hamadryas anubis): Normal Cardiac Anatomy and Function in Subhuman Primates Compared to Humans

Implantable, viable tissue engineered cardiovascular constructs are rapidly approaching clinical translation. Species typically utilized as preclinical large animal models are food stock ungulates for which cross species biological and genomic differences with humans are great. Multiple authorities have recommended developing subhuman primate models for testing regenerative surgical strategies to mitigate xenotransplant inflammation. However, there is a lack of specific quantitative cardiac imaging comparisons between humans and the genomically similar baboons (Papio hamadryas anubis). This study was undertaken to translate to baboons transesophageal echocardiographic functional and dimensional criteria defined as necessary for defining cardiac anatomy and function in the perioperative setting. Seventeen young, healthy baboons (approximately 30 kg, similar to 5 year old children) were studied to determine whether the requisite 11 views and 52 measurement parameters could be reliably acquired by transesophageal echocardiography (TEE). The obtained measurements were compared to human adult normative literature values and to a large relational database of pediatric "normal heart" echo measurements. Comparisons to humans, when normalized to BSA, revealed a trend in baboons toward larger mitral and aortic valve effective orifice areas and much larger left ventricular muscle mass and wall thickness, but similar pulmonary and tricuspid valves. By modifying probe positioning relative to human techniques, all recommended TEE views except transgastric could be replicated. To supplement, two transthoracic apical views were discovered that in baboons could reliably replace the transgastric TEE view. Thus, all requisite echo views could be obtained for a complete cardiac evaluation in Papio hamadryas anubis to noninvasively quantify cardiac structural anatomy, physiology, and dimensions. Despite similarities between the species, there are subtle and important physiologic and anatomic differences when compared to human.

Bimodal cardiac dysfunction in an animal model of iron overload

Iron-overload cardiomyopathy is the most common cause of death in patients with thalassemia major, yet the associated changes in cardiac function have not been quantified. We studied the effects of iron overload on cardiac function in Mongolian gerbils, a species that responds to iron overload in the same manner as human beings. We injected iron-dextran or dextran alone at low subcutaneous doses (200 mg/kg/wk) for 20 to 60 weeks and at high doses (800 mg/kg/wk) for 6 to 20 weeks. At shorter durations for either dose, the mean values of cardiac work, coronary flow, left ventricular (dP/dt)(max) and left ventricular (dP/dt)(min) in isolated perfused hearts were significantly greater than control values; at longer durations, these values were significantly less than control values. Echocardiography in intact animals showed eccentric cardiac hypertrophy, increased cardiac output, and normal exercise tolerance at shorter durations of dosage. At longer durations, concentric cardiac hypertrophy developed, and cardiac output and exercise capacity were impaired. The response to iron overload in Mongolian gerbils progresses from an initial state of high cardiac output to a subsequent state of low-output failure similar to the course of cardiomyopathy that has been inferred in patients with transfusional iron overload.

Altered excitation-contraction coupling in myocytes from remodeled myocardium after chronic myocardial infarction

Following myocardial infarction (MI), the left ventricle undergoes progressive dilatation and eccentric hypertrophy, i.e., remodeling, which is greater in the adjacent than the remote region. The cellular mechanisms underlying these regional differences were studied. One (n=5) and 8 weeks (n=8) after anteroapical MI in sheep, cardiac myocytes were isolated from the adjacent and remote regions. At 8 weeks after MI, myocyte function in the remote region was not different from values either in sham controls (n=3) or animals 1 week after MI. At 8 weeks after MI, myocyte contractile function (% contraction) was decreased, P<0.01, in the adjacent region (6.4+/-0.4%), as compared with the remote region (8.8+/-0.5%) and was associated with decreased amplitude of Ca(2+)transients (adjacent, 0.69+/-0.09 v remote, 1.08+/-0.20, P<0.05) and L-type Ca(2+)current density (adjacent, 3.6+/-0.2 v remote, 4.8+/-0.2 pA/pF, P<0.05). Relaxation was also impaired significantly in myocytes from the adjacent region, associated with decreased protein levels of SERCA2a. The myocytes were hypertrophied more in the adjacent region than the remote region. Furthermore, focal areas of central myofibrillar lysis and increased glycogen deposition were observed in the adjacent region. These results indicate that impaired excitation-contraction coupling underlies dysfunction of myocytes from the adjacent non-infarcted myocardium after chronic MI, even in the absence of heart failure. Hypertrophy is implicated as the mechanism, since these changes were noted at 8 weeks, but not at 1 week after MI.

Cardiac-specific overexpression of calsequestrin results in left ventricular hypertrophy, depressed force-frequency relation and pulsus alternans in vivo

Cardiac-specific overexpression of calsequestrin has been shown to result in significant decreases in contractile parameters and intracellular Ca(2+)transients in vitro. Therefore, the purpose of the present study was to determine the effects of calsequestrin overexpression on basal cardiac function and the force-frequency relation in vivo. Calsequestrin overexpression mice (CSQ-OE, n=20) and their isogenic controls (WT) were studied with an integrative approach using transthoracic echocardiography, stress-shortening relations, and invasive hemodynamics in intact closed-chest mice. M-mode echocardiography indicated that calsequestrin overexpression resulted in concentric hypertrophy (+52%) and an increase in LV ejection phase indices. However, mean end-systolic stress-shortening coordinates revealed that at matched end-systolic wall-stress, fractional shortening was depressed in CSQ-OE mice. This was confirmed by depressed indices of LV isovolumic contraction and relaxation in CSQ-OE v. WT mice. Furthermore, overexpression of calsequestrin resulted in a downward and leftward shift of the biphasic force-frequency relation; thus, the critical heart (HR(crit)) was significantly lower in calsequestrin-overexpression mice (264+/-15 bpm) than in wild-type controls (365+/-21 bpm). Surprisingly, calsequestrin overexpression was associated with the induction of pulsus alternans in every animal (at an average heart rate of 428+/-26 bpm), whereas none of the wild-type controls displayed this phenomenon. We conclude that: (i) although increased levels of calsequestrin result in decreased myocardial contractility and a depressed force-frequency relation, LV wall stress is reduced and chamber function is normal, and (ii) an increase in SR Ca(2+)storage capacity induces pulsus alternans in the intact anesthetized mouse.

Tachycardia-induced primate model of heart failure in cardiovascular drug discovery

Congestive heart failure (CHF) is a complex, multifactoral disease involving genetic and environmental factors that represents a large unmet medical need. There are currently many animal models of CHF that have provided some insight into the etiology of this disease. However, due to the complex interactions of environmental and genetic components of this disease most animal models are somewhat limited. Nonhuman primates offer a unique opportunity to investigate the genetic aspects of this complex disease due to their close genetic and phenotypic similarity to humans. Here we describe a novel tachycardia-induced primate model of CHF characterized by depressed global function that progresses to a symptomatic stage consistent with clinical data. No animal model, including this one, can exactly mimic the clinical pathophysiology of CHF. However, this tachycardia-induced primate model of CHF has similarities to the dynamic state of CHF in humans and affords the opportunity to evaluate changes in gene expression using genomic and proteomic technologies throughout the progression of the disease.

Noninvasive evaluation of cardiac dysfunction by echocardiography in streptozotocin-induced diabetic rats

BACKGROUND

There has not been a noninvasive in vivo longitudinal evaluation of cardiac function in diabetic rats. The objective of this study is to examine the time course of development of cardiac dysfunction in streptozotocin (STZ)-induced diabetic rats.

METHODS AND RESULTS

Cardiac function was evaluated by M-mode and Doppler echocardiography in anesthetized Wistar rats at 2, 4, 5, 6, and 8 weeks after injection with 65 mg of STZ/kg and in age-matched control rats before and after the administration of isoproterenol. Body weight (BW) was significantly less and blood glucose level significantly greater in diabetic rats compared with controls at 2 weeks and remained at these levels at all time points. The calculated left ventricular (LV) mass appeared slightly decreased in diabetic rats. However, LV mass-BW ratios were similar in controls and diabetic rats at 2, 4, and 5 weeks, but were significantly greater in diabetic rats at 6 and 8 weeks. Basal heart rate (HR) was significantly lower in diabetic rats at all time points studied. Basal LV systolic and diastolic dimensions, fractional shortening (FS), velocity of circumferential shortening (Vcf), peak emptying rate (PER), peak filling rate (PFR), and aortic peak velocity (APV) were not significantly different between controls and diabetic rats at 2 and 4 weeks. PER and PFR were significantly less in 5-week diabetic rats. However, Vcf, PER, and PFR were significantly less and FS and APV were similar at 6 and 8 weeks. Administration of isoproterenol increased HR, Vcf, FS, PFR, and PER in controls at all time points, but the increases in diabetic rats at 5, 6, and 8 weeks were less compared with those in controls. The increase in APV was significantly less in diabetic rats at all time points studied.

CONCLUSION

STZ-induced diabetic rats showed bradycardia before contractile dysfunction. Overt and covert contractile dysfunction unmasked by isoproterenol begins at 5 weeks of diabetes. The overt LV systolic and diastolic dysfunction are fully manifested after 6 weeks of diabetes.

Evaluation of extent of shortening versus velocity of shortening at the endocardium and midwall in hypertensive heart disease

To assess the incremental value of velocity of shortening velocity parameters compared with simpler, more widely used, extent of shortening parameters in compensated left ventricular hypertrophy, we studied 52 patients with left ventricular hypertrophy and 63 age-matched controls. Velocity parameters did not provide incremental information beyond that obtained by extent of shortening parameters.

Reversal of hypocalcemia and decreased afterload in sepsis. Effect on myocardial systolic and diastolic function

Sepsis is a major cause of death in intensive care units. Clinically, sepsis induces a number of physiologic and metabolic abnormalities, including decreased myocardial contractility and decreased plasma ionized calcium. There is debate about the proper therapy of hypocalcemia in sepsis because calcium administration may worsen cell function by causing intracellular Ca2+ overload. We investigated the effect of Ca2+ administration on myocardial systolic and diastolic function in an extensively utilized rat model of sepsis, i.e., the cecal ligation and puncture model (CLP). Approximately 24 h after CLP or sham surgery, rats were anesthetized and myocardial function assessed in vivo by a left ventricular Millar catheter and simultaneous two-dimensional guided M-mode echocardiography. Septic rats had a 28% decrease in peak left ventricular developed pressure, a 30% decrease in +dP/ dt, and a 23% decrease in -dP/dt (p < 0.05). Plasma ionized Ca2+ was decreased in septic compared with that in sham rats: 4.9 +/- 0.9 and 5.6 +/- 0.01 mg/dl, respectively (p < 0.05). CaCl2 improved both systolic and diastolic function and there was no evidence of adverse effects of Ca2+ even at supraphysiologic levels. Surprisingly, correction of decreased afterload in septic rats, using the pure alpha-agonist phenylephrine, caused normalization of all indices of cardiac contractility, indicating that the presumed decrease in cardiac function was due entirely to an effect of the decreased afterload to "unload" the left ventricle. We conclude that Ca2+ administration is not detrimental to cardiac function in the rat CLP model. Although the rat CLP model is widely utilized and reproduces many of the clinical hallmarks of sepsis, it does not cause intrinsic myocardial depression and, therefore, it may not be an appropriate model to investigate the clinical cardiac dysfunction that occurs in patients with sepsis.

Effect of myocardial hypertrophy on systolic and diastolic function in children: insights from the force-frequency and relaxation-frequency relationships

OBJECTIVE

The objective of this study was to evaluate the effect of myocardial hypertrophy on systolic and diastolic properties of the left ventricle in children.

BACKGROUND

In children with myocardial hypertrophy, ejection phase indices are invariably increased. However, indices of force-generation, e.g., end-systolic elastance and invasive indices of diastolic properties, have been studied infrequently in children with myocardial hypertrophy.

METHODS

We studied 10 children with congenital aortic stenosis or coarctation of aorta and nine control patients. Systolic properties were assessed from shortening fraction, end-systolic fiber elastance (Ef(es)) measured at resting heart rates, and force-frequency relationship measured at heart rates increasing from 110 to 160 beats per minute. Diastolic properties were assessed from time constant of relaxation (tau) at matched heart rates, chamber stiffness constant, myocardial stiffness constant, and relaxation-frequency relationship measured at gradually increasing heart rates.

RESULTS

Ef(es) remained unchanged by myocardial hypertrophy, however, tau was prolonged (tauL: 27.3+/-2.3 vs. 21.8+/-2.2 ms, p < 0.001; and tauD: 43.2+/-3.1 vs. 34.3+/-3.3 ms, p < 0.001). Both chamber and myocardial stiffness constants remained unchanged. Incremental increases in heart rate produced incremental improvement in both contraction and relaxation. Slopes of force-frequency and relaxation-frequency relationships remained unchanged in the experimental group. However, the relaxation-frequency relationship manifested a parallel shift upward.

CONCLUSIONS

In conscious, sedated children with myocardial hypertrophy, systolic function assessed by an index of force generation remains unchanged. However, relaxation is prolonged but passive diastolic properties remain unaffected. The combined effect of hypertrophy and heart rate does not alter the force-frequency and relaxation-frequency relationships.