GPR55 Deletion in Mice Leads to Age-Related Ventricular Dysfunction and Impaired Adrenoceptor-Mediated Inotropic Responses

G protein coupled receptor 55 (GPR55) is expressed throughout the body, and although its exact physiological function is unknown, studies have suggested a role in the cardiovascular system. In particular, GPR55 has been proposed as mediating the haemodynamic effects of a number of atypical cannabinoid ligands; however this data is conflicting. Thus, given the incongruous nature of our understanding of the GPR55 receptor and the relative paucity of literature regarding its role in cardiovascular physiology, this study was carried out to examine the influence of GPR55 on cardiac function. Cardiac function was assessed via pressure volume loop analysis, and cardiac morphology/composition assessed via histological staining, in both wild-type (WT) and GPR55 knockout (GPR55^−/−) mice. Pressure volume loop analysis revealed that basal cardiac function was similar in young WT and GPR55^−/− mice. In contrast, mature GPR55^−/− mice were characterised by both significant ventricular remodelling (reduced left ventricular wall thickness and increased collagen deposition) and systolic dysfunction when compared to age-matched WT mice. In particular, the load-dependent parameter, ejection fraction, and the load-independent indices, end-systolic pressure-volume relationship (ESPVR) and E _max, were all significantly (P<0.05) attenuated in mature GPR55^−/− mice. Furthermore, GPR55^−/− mice at all ages were characterised by a reduced contractile reserve. Our findings demonstrate that mice deficient in GPR55 exhibit maladaptive adrenergic signalling, as evidenced by the reduced contractile reserve. Furthermore, with age these mice are characterised by both significant adverse ventricular remodelling and systolic dysfunction. Taken together, this may suggest a role for GPR55 in the control of adrenergic signalling in the heart and potentially a role for this receptor in the pathogenesis of heart failure.     

Beta2-microglobulin knockout mice treated with anti-asialoGM1 exhibit improved hemodynamics and cardiac contractile function during acute intra-abdominal sepsis

We previously showed that beta2-microglobulin knockout mice treated with anti-asialoGM1 (beta2M/alphaAsGM1 mice) exhibit less hypothermia, reduced production of proinflammatory cytokines, less metabolic acidosis, and improved survival after cecal ligation and puncture (CLP) compared with wild-type mice. The present study was designed to assess hemodynamics and left ventricular contractility at 18 h after CLP. Arterial pressure was measured by carotid artery cannulation, and left ventricular pressure-volume loops were obtained by insertion of a 1.4-F conductance catheter into the left ventricle. Heart rate, stroke volume, and cardiac output were not significantly different between wild-type and beta2M/alphaAsGM1 mice after CLP. However, beta2M/alphaAsGM1 mice exhibited improved mean arterial pressure and systemic vascular resistance compared with wild-type mice. Myocardial function was also better preserved in beta2M/alphaAsGM1 mice as indicated by improved left ventricular pressure development over time, time-varying maximum elastance, endsystolic pressure-volume relationship, and preload recruitable stroke work. Overall, this study shows that cardiovascular collapse characterized by hypotension, myocardial depression, and low systemic vascular resistance occurs after CLP in wild-type mice. However, beta2M/alphaAsGM1 mice exhibit improved hemodynamics and cardiac contractile function after CLP that may account, in part, for our previously observed survival benefit.     

Rat Heterotopic Abdominal Heart/Single-lung Transplantation in a Volume-loaded Configuration

Herein, we describe a novel technique for heterotopic abdominal heart-lung transplantation (HAHLT) in rats. The configuration of the transplant graft involves anastomosis of donor inferior vena cava (IVC) to recipient IVC, and donor ascending aorta (Ao) to recipient abdominal Ao. The right upper and middle lung lobes are preserved and function as conduits for blood flow from right heart to left heart.There are several advantages to using this technique, and it lends itself to a broad range of applications. Because the graft is transplanted in a configuration that allows for dyamic volume-loading, cardiac function may be directly assessed in vivo. The use of pressure-volume conductance catheters permits characterization of load-dependent and load-independent hemodynamic parameters. The graft may be converted to a loaded configuration by applying a clamp to the recipient’s infra-hepatic IVC. We describe modified surgical techniques for both donor and recipient operations, and an ideal myocardial protection strategy. Depending on the experimental aim, this model may be adapted for use in both acute and chronic studies of graft function, immunologic status, and variable ventricular loading conditions. The conducting airways to the transplanted lung are preserved, and allow for acute lung re-ventilation. This facilitates analysis of the effects of the mixed venous and arterial blood providing coronary perfusion to the graft.A limitation of this model is its technical complexity. There is a significant learning curve for new operators, who should ideally be mentored in the technique. A surgical training background is advantageous for those wishing to apply this model. Despite its complexity, we aim to present the model in a clear and easily applicable format. Because of the physiologic similarity of this model to orthotopic transplantation, and its broad range of study applications, the effort invested in learning the technique is likely to be worthwhile.     

Impact of physiological variables and genetic background on myocardial frequency-resistivity relations in the intact beating murine heart

Conductance measurements for generation of an instantaneous left ventricular (LV) volume signal in the mouse are limited, because the volume signal is a combination of blood and LV muscle, and only the blood signal is desired. We have developed a conductance system that operates at two simultaneous frequencies to identify and remove the myocardial contribution to the instantaneous volume signal. This system is based on the observation that myocardial resistivity varies with frequency, whereas blood resistivity does not. For calculation of LV blood volume with the dual-frequency conductance system in mice, in vivo murine myocardial resistivity was measured and combined with an analytic approach. The goals of the present study were to identify and minimize the sources of error in the measurement of myocardial resistivity to enhance the accuracy of the dual-frequency conductance system. We extended these findings to a gene-altered mouse model to determine the impact of measured myocardial resistivity on the calculation of LV pressure-volume relations. We examined the impact of temperature, timing of the measurement during the cardiac cycle, breeding strain, anisotropy, and intrameasurement and interanimal variability on the measurement of intact murine myocardial resistivity. Applying this knowledge to diabetic and nondiabetic 11- and 20- to 24-wk-old mice, we demonstrated differences in myocardial resistivity at low frequencies, enhancement of LV systolic function at 11 wk and LV dilation at 20-24 wk, and histological and electron-microscopic studies demonstrating greater glycogen deposition in the diabetic mice. This study demonstrated the accurate technique of measuring myocardial resistivity and its impact on the determination of LV pressure-volume relations in gene-altered mice.     

Assessment of cardiac function with the pressure-volume conductance system following myocardial infarction in mice

Myocardial infarction (MI) is a major cause of heart failure (HF) with the progressive worsening of cardiac performance due to structural and functional alterations. Therefore, we studied cardiac function in adult mice following MI using the Millar pressure-volume (P-V) conductance catheter system in vivo during the later phase of compensatory remodeling and decompensation to HF. We evaluated load-dependent and -independent parameters in control and 2-, 4-, 6-, and 10-wk post-MI mice and integrated changes in function with changes in gene expression. Our results indicated a significant deterioration of cardiac function in post-MI mice over time, reflected first by systolic dysfunction, followed by a transient improvement before further decline in both systolic and diastolic function. Associated with the function and adaptive remodeling were transient changes in fetal gene and extracellular matrix gene expression. However, undermining the compensatory remodeling response was a continual decline in cardiac contractility, which promoted the transition into failure. Our study provided a scheme of integrated cardiac function and gene expression changes occurring during the adaptive and maladaptive response of the heart independent of systemic vascular properties during the transition to HF following MI in mice. P-V loop analysis was used to quantitatively evaluate the gradual deterioration in cardiac function post-MI. P-V loop analysis was found to be an appropriate method for assessment of global cardiac function under varying load-dependent and -independent conditions in the murine model with many similarities to data obtained from larger animals and humans.     

Validation of a mouse conductance system to determine LV volume: comparison to echocardiography and crystals

The application of left ventricular pressure-volume analysis to transgenic mice to characterize the cardiac phenotype has been problematic due to the small size of the mouse heart and the rapid heartbeat. Conductance technology has been miniaturized for the mouse and can solve this problem. However, there has been no validation of this technique. Accordingly, we performed echocardiography followed by simultaneous ultrasonic crystals, flow probe, and conductance studies in 18 CD-1 mice. Raw conductance volumes were corrected for an inhomogenous electrical field (alpha) and parallel conductance (G(pi)) yielding a stroke volume of 14.1 +/- 3.7 microliter/beat, end-diastolic volume of 20.8 +/- 6.5 microliter, and end-systolic volume of 9.0 +/- 5.8 microliter. The mean conductance volumes were no different from those derived by flow probe and echocardiography but did differ from ultrasonic crystals. G(pi) was determined to be 14.9 +/- 8.7 microliter. However, hypertonic saline altered dimension and pressure in the mouse left ventricle. Although G(pi) can be determined by the hypertonic saline method, saline altered hemodynamics, questioning its validity in the mouse. Although mean measures of absolute volume may be similar among different techniques, individual values did not correlate.     

Pressure-volume-based single-beat estimations cannot predict left ventricular contractility in vivo

The end-systolic pressure-volume relationship is regarded as a useful index for assessing the contractile state of the heart. However, the need for preload alterations has been a serious limitation to its clinical applications, and there have been numerous attempts to develop a method for calculating contractility based on one single pressure-volume loop. We have evaluated four of these methods. Pressure-volume data were obtained by combined pressure and conductance catheters in 37 pigs. All four methods were applied to 88 steady-state pressure-volume files, including eight files sampled during dopamine infusions. Estimates of single-beat contractility (elastance) were compared with preload-varied multiple-beat elastance [E(es(MB))]. All methods had a low average bias (-0.3 to 0.5 mmHg/ml) but limits of agreement (+/-2 SD) were unacceptably high (+/-2.6 to +/-3.8 mmHg/ml). In the dopamine group, E(es(MB)) showed an increase of 1.7 +/- 0.8 mmHg/ml (mean +/- SD) compared with baseline (P < 0.001). None of the single-beat methods predicted this increase in contractility. It is therefore doubtful whether any of the methods allow for single-beat assessment of contractility.     

Rosiglitazone treatment improves cardiac efficiency in hearts from diabetic mice

Isolated perfused hearts from type 2 diabetic (db/db) mice show impaired ventricular function, as well as altered cardiac metabolism. Assessment of the relationship between myocardial oxygen consumption (MVO(2)) and ventricular pressure-volume area (PVA) has also demonstrated reduced cardiac efficiency in db/db hearts. We hypothesized that lowering the plasma fatty acid supply and subsequent normalization of altered cardiac metabolism by chronic treatment with a peroxisome proliferator-activated receptor-gamma (PPARgamma) agonist will improve cardiac efficiency in db/db hearts. Rosiglitazone (23 mg/kg body weight/day) was administered as a food admixture to db/db mice for five weeks. Ventricular function and PVA were assessed using a miniaturized (1.4 Fr) pressure-volume catheter; MVO(2) was measured using a fibre-optic oxygen sensor. Chronic rosiglitazone treatment of db/db mice normalized plasma glucose and lipid concentrations, restored rates of cardiac glucose and fatty acid oxidation, and improved cardiac efficiency. The improved cardiac efficiency was due to a significant decrease in unloaded MVO(2), while contractile efficiency was unchanged. Rosiglitazone treatment also improved functional recovery after low-flow ischemia. In conclusion, the present study demonstrates that in vivo PPARgamma-treatment restores cardiac efficiency and improves ventricular function in perfused hearts from type 2 diabetic mice.     

Enhanced systolic function of the right ventricle during respiratory distress syndrome in newborn lambs

Respiratory distress syndrome (RDS) causes pulmonary hypertension. It is often suggested that this increased afterload for the right ventricle (RV) might lead to cardiac dysfunction. To examine this, we studied biventricular function in an experimental model. RDS was induced by lung lavages in seven newborn lambs. Five additional lambs served as controls. Cardiac function was quantified by indexes derived from end-systolic pressure-volume relations obtained by pressure-conductance catheters. After lung lavages, a twofold increase of mean pulmonary arterial pressure (from 15 to 34 mmHg) was obtained and lasted for the full 4-h study period. Stroke volume was maintained (5.2 +/- 0.6 ml at baseline and 6.1 +/- 1.4 ml at 4 h of RDS), while RV end-diastolic volume showed only a slight increase (from 6.5 +/- 2.3 ml at baseline to 7.7 +/- 1.3 ml at 4 h RDS). RV systolic function improved significantly, as indicated by a leftward shift and increased slope of the end-systolic pressure-volume relation. Left ventricular systolic function showed no changes. In control animals, pulmonary arterial pressure did not increase and right and left ventricular systolic function remained unaffected. In the face of increased RV afterload, the newborn heart is able to maintain cardiac output, primarily by improving systolic RV function through homeometric autoregulation.     

Chronic intermittent hypoxia exposure improves left ventricular contractility in transgenic mice with heart failure

We previously reported the unexpected finding that 4 wk of exposure to intermittent hypoxia (IH), which simulates the hypoxic stress of obstructive sleep apnea, improved LV cardiac function in healthy, lean C57BL/6J mice. The purpose of the present study was to assess the impact of 4 wk of IH on cardiac function in a transgenic murine model that exhibits a natural history of heart failure. We hypothesized that IH exposure would exacerbate cardiac decompensation in heart failure. Adult male FVB (wild type) and transgenic mice with cardiac overexpression of tumor necrosis factor α (TNF-αTG) at 10-12 wk of age were exposed to 4 wk of IH (nadir inspired oxygen 5-6% at 60 cycles/h for 12 h during light period) or intermittent air (IA) as control. Cardiac function was assessed by echocardiography and pressure-volume loop analyses, and mRNA and protein expression were performed on ventricular homogenates. TNF-αTG mice exposed to IA exhibited impaired LV contractility and increased LV dilation associated with markedly elevated cardiac expression of atrial natriuretic peptide and brain natriuretic peptide compared with wild-type mice. When wild-type FVB mice were exposed to IH, they exhibited increases in arterial pressure and dP/dt(max), consistent with our previous report in C57BL/6J mice. Surprisingly, we found that TNF-αTG mice exposed to IH showed a reduction in end-diastolic volume (38.7 ± 3.8 to 22.2 ± 2.1 ul; P < 0.01) and an increase in ejection fraction (29.4 ± 2.5 to 41.9 ± 3.1%; P < 0.05). In contrast to our previous study in C56Bl/6J mice, neither FVB nor TNF-αTG mice exhibited an upregulation in β-adrenergic expression or cAMP in response to IH exposure. We conclude that 4 wk of exposure to IH in mice induces adaptive responses that improve cardiac function in not only healthy animals but also in animals with underlying heart failure.     

Partial Endocardial Cushion Defects

Seven partial endocardial cushion defects have been diagnosed and treated surgically without mortality—six of the ostium primum type and one with a defect from left ventricle to right atrium. The last patient likely has an associated cardiomyopathy. The most serious complication of repair is damage to the bundle of His.The electrocardiogram is helpful in diagnosis, showing left axis deviation in standard leads and right ventricular hypertrophy in chest leads (92%). The vector-cardiogram in the frontal plane shows counterclockwise rotation with the loop usually above the isoelectric line (97%). This is due to congenital aberration of the left bundle branch rather than to left ventricular hypertrophy from mitral incompetence. Radiography demonstrates moderate cardiac enlargement with right-sided hypertrophy, a prominent pulmonary artery, and pleonemic lung fields. Cardiac catheterization reveals a moderate rise in oxygen saturation at atrial level and a further minimal increase at ventricular level. The left atrial trace may show a minimal “c-v” configuration, with the “v” wave equal to the “a”.     

Left ventricular volume measurement in mice by conductance catheter: evaluation and optimization of calibration

The conductance catheter (CC) allows thorough evaluation of cardiac function because it simultaneously provides measurements of pressure and volume. Calibration of the volume signal remains challenging. With different calibration techniques, in vivo left ventricular volumes (V(CC)) were measured in mice (n = 52) with a Millar CC (SPR-839) and compared with MRI-derived volumes (V(MRI)). Significant correlations between V(CC) and V(MRI) [end-diastolic volume (EDV): R(2) = 0.85, P < 0.01; end-systolic volume (ESV): R(2) = 0.88, P < 0.01] were found when injection of hypertonic saline in the pulmonary artery was used to calibrate for parallel conductance and volume conversion was done by individual cylinder calibration. However, a significant underestimation was observed [EDV = -17.3 microl (-22.7 to -11.9 microl); ESV = -8.8 microl (-12.5 to -5.1 microl)]. Intravenous injection of the hypertonic saline bolus was inferior to injection into the pulmonary artery as a calibration method. Calibration with an independent measurement of stroke volume decreased the agreement with V(MRI). Correction for an increase in blood conductivity during the in vivo experiments improved estimation of EDV. The dual-frequency method for estimation of parallel conductance failed to produce V(CC) that correlated with V(MRI). We conclude that selection of the calibration procedure for the CC has significant implications for the accuracy and precision of volume estimation and pressure-volume loop-derived variables like myocardial contractility. Although V(CC) may be underestimated compared with MRI, optimized calibration techniques enable reliable volume estimation with the CC in mice.     

Pressure-volume relation analysis of mouse ventricular function

Nearly 40 years ago, the Sagawa laboratory spawned a renaissance in the use of instantaneous ventricular pressure-volume (P-V) relations to assess cardiac function. Since then, this analysis has taken hold as the most comprehensive way to quantify ventricular chamber function and energetics and cardiovascular interactions. First studied in large mammalian hearts and later in humans employing a catheter-based method, P-V analysis was translated to small rodents in the late 1990s by the Kass laboratory. Over the past decade, this approach has become a gold standard for comprehensive examination of in vivo cardiac function in mice, facilitating a new era of molecular cardiac physiology. The catheter-based method remains the most widely used approach in mice. In this brief review, we discuss this instrumentation, the theory behind its use, and how volume signals are calibrated and discuss elements of P-V analysis. The goal is to provide a convenient summary of earlier investigations and insights for users whose primary interests lie in genetic/molecular studies rather than in biomedical engineering.     

Assessment of systolic and diastolic ventricular properties via pressure-volume analysis: a guide for clinical, translational, and basic researchers

Assessment of left ventricular systolic and diastolic pump properties is fundamental to advancing the understanding of cardiovascular pathophysiology and therapeutics, especially for heart failure. The use of end-systolic and end-diastolic pressure-volume relationships derived from measurements of instantaneous left ventricular pressure-volume loops emerged in the 1970s as a comprehensive approach for this purpose. As invasive and noninvasive techniques for measuring ventricular volume improved over the past decades, these relations have become commonly used by basic, translational, and clinical researchers. This review summarizes 1) the basic concepts underlying pressure-volume analysis of ventricular and myocardial systolic and diastolic properties, 2) deviations from ideal conditions typically encountered in real-life applications, 3) how these relationships are appropriately analyzed, including statistical analyses, and 4) the most common problems encountered by investigators and the appropriate remedies. The goal is to provide practical information and simple guidelines for accurate application and interpretation of pressure-volume data as they pertain to characterization of ventricular and myocardial properties in health and disease.     

Catecholamines help snakes have a change of heart

JGP study on python snakes reveals that the regulation of ventricular repolarization by the sympathetic nervous system is evolutionarily conserved.

The T-wave of an electrocardiogram (ECG) arises from local differences in ventricular repolarization and represents a vulnerable period for the generation of arrythmias when some, but not all, of the myocardium is still refractory and unable to generate a new action potential. In mammals, ventricular repolarization is regulated by catecholamines released by the autonomic nervous system. In this issue of JGP, Boukens et al. show that this mode of regulation is conserved in the ball python, Python regius (1).Bas Boukens (left), Bjarke Jensen (center), and colleagues reveal that, similar to mammals, catecholamines released by the autonomic nervous system regulate ventricular repolarization in ball pythons. An ECG (right) shows that, by altering the pattern of ventricular repolarization, adrenaline treatment causes an inversion of the T-wave (red arrowhead). A similar phenomenon is observed in snakes undergoing a rise in body temperature, when autonomic tone increases.Working together at Amsterdam UMC, Bas Boukens and Bjarke Jensen are interested in the electrophysiological adaptations that have occurred during cardiac evolution. Cold-blooded reptiles have a much longer ventricular repolarization phase than warm-blooded mammals, even at 37°C (2). Moreover, the T-wave is typically negative in reptiles, whereas in mammals it is usually positive. Anecdotal observations, however, suggest that, in some reptiles, the T-wave can invert and become positive at higher body temperatures (3, 4). “We were curious about what might underlie these observations,” Jensen says.The researchers therefore recorded ECGs in living ball pythons as their body temperatures were increased (1). The ball python’s heart is unique in having functionally distinct ventricles, with a high-pressure left side and a low-pressure right side, even though, as in other snakes, the two sides are not anatomically separated. Though results varied across individual pythons and ECG leads, raising body temperature from 25 to 35°C caused an inversion of the snakes’ T-wave, reflecting temperature-dependent changes in the pattern of ventricular repolarization.In 1880, Burdon-Sanderson and Page (5) showed in their classic experiments that local differences in temperature change T-wave polarity in the ectothermic heart, presumably due to a direct effect of temperature on the activity of cardiac ion channels. However, when Boukens et al. recorded an ECG from a decapitated python, they found that raising temperature did not cause T-wave inversion (1). “So, we realized that it might not be a direct effect of temperature but might involve another factor, namely catecholamines released by the autonomic nervous system,” says Boukens. Autonomic activity increases at higher temperatures, but the ability of catecholamines to modulate ventricular repolarization would be blunted in decapitated snakes lacking a functional nervous system.Sure enough, the researchers found that stimulating the β-adrenergic receptor induced T-wave inversion in pythons maintained at a stable temperature. In contrast, the β blocker propranolol largely prevented higher temperatures from inducing T-wave inversion.Thus, similar to mammals, catecholamines regulate ventricular repolarization in ball pythons, and the increase in autonomic tone at higher temperatures alters the pattern of repolarization and changes T-wave shape. “The T-wave inversion suggests that certain regions of the python heart respond more strongly to adrenergic stimulation than other regions,” Boukens says.To test this idea, the researchers performed RNA sequencing of tissue samples taken from different regions of the python heart. “Catecholamine-associated genes exhibited differential expression between the left and right sides of the ventricle, consistent with the repolarization of these regions being differentially modulated by adrenergic signaling,” says Jensen.This may provide some sort of advantage to pythons as their body temperature rises, though the resulting changes in repolarization pattern could also leave them vulnerable to developing arrythmias. Boukens and Jensen are now extending their studies to a different branch of the evolutionary tree, examining repolarization and arrhythmogenesis in zebra finches (6).     

Effects of chronic methamphetamine exposure on heart function in uninfected and retrovirus-infected mice

Methamphetamine (MA) increases catecholamine levels, which have detrimental effects on heart function through vasoconstriction, myocardial hypertrophy, and fibrosis. Murine retrovirus infection induces dilated cardiomyopathy (DCM). The present study investigated the cardiovascular effects of chronic MA treatment on uninfected and retrovirus-infected mice. C57BL/6 mice were studied after 12 weeks treatment. The four study groups were (group I) uninfected, MA placebo; (group II) infected, MA placebo; (group III) uninfected, MA treatment; and (group IV) infected and MA treatment. MA injections were given i.p. once a day for 5 days/week with a increasing dose from 15 mg/kg to 40 mg/kg. Left ventricular mechanics were measured in situ a using Millar conductance catheter system for pressure-volume loop analysis. Cardiac pathology was determined with histological analysis. In the uninfected mice, the load independent contractile parameters, pre-load recruitable stroke work (PRSW) and dP/dt(max) vs. Ved, significantly decreased by 32% and 35% in MA treated mice when compared to the saline injected mice. In retrovirus-infected mice, although there were no significant difference in Ees, PRSW, and dP/dt(max) vs. Ved due to MA treatment, they were increased 45%, 15% and 42% respectively when compared to saline treated mice. No further lowered heart function during murine AIDS may be due to the counteraction of the retroviral DCM and the MA induced myocardial fibrosis and hypertrophy (thickening of the ventricular walls). This is supported by increases in the End-diastolic volume (Ved, 38%) and End-systolic volume (Ves, 84%) in the retrovirus-infected saline injected mice, the decreases of 33% and 17% in the uninfected MA-treated mice, but no significant changes in the retrovirus-infected MA treated mice when compared to uninfected saline injected mice. These data suggest that MA induced myocardial cellular changes compensate for retrovirus induced DCM.     

Effects of changes in left ventricular contractility on indexes of contractility in mice

Measurement of left ventricular (LV) function is often overlooked in murine studies, which have been used to analyze the effects of genetic manipulation on cardiac phenotype. The goal of this study was to address the effects of changes in LV contractility on indexes of contractility in mice. LV function was assessed in vivo in closed-chest mice by echocardiography and by LV catheterization using a conductance pressure-volume (P-V) catheter with three different interventions that alter contractility by 1) atrial pacing to increase inotropy by augmentation of the force-frequency relation (modest increment of inotropy), 2) dobutamine to maximize inotropy, and 3) esmolol infusion to decrease contractility. Load-independent parameters derived from P-V relations, such as slope of end-systolic P-V relations (ESPVR) and slope of the first maximal pressure derivative over time (dP/dt(max))-end-diastolic volume relation (dP/dt-EDV), and standard echocardiographic parameters were measured. The dP/dt-EDV changed the most among parameters after atrial pacing and dobutamine infusion (percent change, 162.8 +/- 95.9% and 271.0 +/- 44.0%, respectively). ESPVR was the most affected by a decrease in LV contractility during esmolol infusion (percent change, -49.8 +/- 8.3%). However, fractional shortening failed to detect changes in contractility during atrial pacing and esmolol infusion and its percent change was <20%. This study demonstrated that contractile parameters derived from P-V relations change the most during a change in LV contractility and should therefore best detect a small change in contractility in mice. Heart rate has a modest but significant effect on P-V relationship-derived indexes and must be considered in the evaluation of murine cardiac physiology.