Oxygen wasting for Ca2+ extrusion activated by partial inhibition of sarcoplasmic reticulum Ca2+ -atpase by cyclopiazonic acid in rat left ventricles

Resolving an inconsistency in the estimation of the energy for excitation of cardiac muscle contraction

In the excitation of muscle contraction, calcium ions interact with transmembrane transporters. This process is accompanied by energy consumption and heat liberation. To quantify this activation energy or heat in the heart or cardiac muscle, two non-pharmacological approaches can be used. In one approach using the "pressure-volume area" concept, the same estimate of activation energy is obtained regardless of the mode of contraction (either isovolumic/isometric or ejecting/shortening). In the other approach, an accurate estimate of activation energy is obtained only when the muscle contracts isometrically. If the contraction involves muscle shortening, then an additional component of heat associated with shortening is liberated, over and above that of activation. The present study thus examines the reconcilability of the two approaches by performing experiments on isolated muscles measuring contractile force and heat output. A framework was devised from the experimental data to allow us to replicate several mechanoenergetics results gleaned from the literature. From these replications, we conclude that the choice of initial muscle length (or ventricular volume) underlies the divergence of the two approaches in the estimation of activation energy when the mode of contraction involves shortening (ejection). At low initial muscle lengths, the heat of shortening is relatively small, which can lead to the misconception that activation energy is contraction mode independent. In fact, because cardiac muscle liberates heat of shortening when allowed to shorten, estimation of activation heat must be performed only under isometric (isovolumic) contractions. We thus recommend caution when estimating activation energy using the "pressure-volume area" concept.

Cardiac mechanoenergetics for understanding isoproterenol-induced rat heart failure

Considering from clinical implication, it is often complained that short-term experimental diseased heart models do not mimic long-term diseased hearts that one often clinically encountered. The left ventricle (LV) function in rat cardiac hypertrophy models treated with isoproterenol (ISO) up to 16 weeks was followed up with a non-invasive echocardiography. Infusion of either ISO (1.2mgkg(-1)day(-1) for 3 days-16 weeks) or vehicle (saline 24μlday(-1) for 3 days-16 weeks; SA group) was performed by subcutaneously implanting osmotic minipump. LV and right ventricle (RV) weight ratios to body weight (mgg(-1)) in SA, ISO3d, ISO7d and ISO4w were: 1.94±0.10 and 0.54±0.04 (n=7), 2.56±0.10 and 0.66±0.05 (n=7), 2.50±0.25 and 0.64±0.07 (n=10) and 2.40±0.08 and 0.59±0.08 (n=9), respectively. From echocardiography, the LV function of the hypertrophy models at 3 days, 1 and 2 weeks was unchanged but the model at the longer-term than 4 weeks resulted in prolonged systolic failure. These results indicated that only 3-day ISO infusion induced the hypertrophy model similar in shape and function to that induced by 2-week ISO infusion; the 3-day model sufficiently represents the effects of 2-week ISO infusion. In this review, left ventricular (LV) function was compared between rat cardiac hypertrophy models treated with ISO for 3 days (ISO3d) and 7 days (ISO7d) by analyzing LV mechanical work and energetics. The LV mechanical work and energetics was unchanged in SA, ISO3d and ISO7d groups. The LV relaxation rate at 240bpm in ISO3d and ISO7d groups was significantly slower than that in SA group with unchanged contraction rate. The amounts of expression of sarcoplasmic reticulum Ca(2+)-ATPase (SERCA2a), phospholamban (PLB), phosphorylated-Ser(16) PLB (p-PLB), phospholemman (PLM) and Na(+)-K(+)-ATPase (NKA) are significantly decreased in ISO3d and ISO7d groups. Furthermore, the marked collagen production (types I and III) was observed in ISO3d and ISO7d groups. These results suggested the possibility that physiological LV function is compensated, although molecular changes have been generated even in the short-term hypertrophy model. Although a novel histone deacetylase (HDAC) inhibitor, has some beneficial effects on hemodynamics, it has no effects of anti-hypertrophic modalities in ISO3d model. However, a selective sodium proton exchanger-1 (NHE-1) inhibitor normalized ISO-induced down-regulation of SERCA2a without changes in pPLB/PLB expression in the ISO7d model and ameliorates cardiac Ca(2+) handling impairment and prevents the development of cardiac dysfunction. This result indicated that SERCA2a is a key molecule in the ISO7d model. Slow LV relaxation rate in ISO7d model may be due to down-regulation of SERCA2a. In conclusion, lowering the heart rate make it possible to rescue the impairment of LV mechanical work and energetics in the ISO-induced compensatory hypertrophied rat hearts, providing basic evidence for clinical therapy for patients with some types of cardiac failure.

Left ventricular function of isoproterenol-induced hypertrophied rat hearts perfused with blood: mechanical work and energetics

We investigated left ventricular (LV) mechanical work and energetics in the cross-circulated (blood-perfused) isoproterenol [Iso 1.2 mg x kg(-1).day(-1) for 3 days (Iso3) or 7 days (Iso7)]-induced hypertrophied rat heart preparation under isovolumic contraction-relaxation. We evaluated pressure-time curves per beat, end-systolic pressure-volume and end-diastolic pressure-volume relations, and myocardial O(2) consumption per beat (Vo(2))-systolic pressure-volume area (PVA; a total mechanical energy per beat) linear relations at 240 beats/min, because Iso-induced hypertrophied hearts failed to completely relax at 300 beats/min. The LV relaxation rate at 240 beats/min in Iso-induced hypertrophied hearts was significantly slower than that in control hearts [saline 24 microl/day for 3 and 7 days (Sa)] with unchanged contraction rate. The Vo(2)-intercepts (composed of basal metabolism and Ca(2+) cycling energy consumption in excitation-contraction coupling) of Vo(2)-PVA linear relations were unchanged associated with their unchanged slopes in Sa, Iso3, and Iso7 groups. The oxygen costs of LV contractility were also unchanged in all three groups. The amounts of expression of sarcoplasmic reticulum Ca(2+)-ATPase, phospholamban (PLB), phosphorylated-Ser(16) PLB, phospholemman, and Na(+)-K(+)-ATPase are significantly decreased in Iso3 and Iso7 groups, although the amount of expression of NCX1 is unchanged in all three groups. Furthermore, the marked collagen production (types I and III) was observed in Iso3 and Iso7 groups. These results suggested the possibility that lowering the heart rate was beneficial to improve mechanical work and energetics in isoproterenol-induced hypertrophied rat hearts, although LV relaxation rate was slower than in normal hearts.

The proinflammatory cytokines TNF-alpha and IL-1 beta impair economy of contraction in human myocardium

Considerable experimental evidence has accumulated over the past years that proinflammatory cytokines, especially TNF-alpha and IL-1beta, impair myocardial function in different animal species. On the other hand, several prospective clinical trials studying TNF-alpha antagonist in patients with chronic heart failure were not able to demonstrate a benefit. As there might be a relevant species-related discrepancy, we intended to prove our previous results demonstrating impaired myocardial economy after exogenous administration of recombinant TNF-alpha in rat myocardium. In the present study, both TNF-alpha and IL-1beta not only revealed an immediate negative inotropic effect but also increased specific oxygen demand in human right-atrial myocardium. Enhanced oxygen consumption was not caused by an elevated basal metabolism but an impaired economy of contraction. Our results suggest that proinflammatory cytokines have a considerable effect on myocardial mechano-energetic parameters in human myocardium as well.

Na+/Ca2+ exchange inhibition protects the rat heart from ischemia-reperfusion injury by blocking energy-wasting processes

We have recently reported that exposure of rat hearts to high Ca2+ produces a Ca2+ overload-induced contractile failure in rat hearts, which was associated with proteolysis of α-fodrin. We hypothesized that contractile failure after ischemia-reperfusion (I/R) is similar to that after high Ca2+ infusion. To test this hypothesis, we investigated left ventricular (LV) mechanical work and energetics in the cross-circulated rat hearts, which were subjected to 15 min global ischemia and 60 min reperfusion. Sixty minutes after I/R, mean systolic pressure-volume area (PVA; a total mechanical energy per beat) at midrange LV volume (mLVV) (PVAmLVV) was significantly decreased from 5.89 ± 1.55 to 3.83 ± 1.16 mmHg·ml·beat−1·g−1 ( n = 6). Mean myocardial oxygen consumption per beat (Vo2) intercept of (Vo2-PVA linear relation was significantly decreased from 0.21 ± 0.05 to 0.15 ± 0.03 μl O2·beat−1·g−1 without change in its slope. Initial 30-min reperfusion with a Na+/Ca2+ exchanger (NCX) inhibitor KB-R7943 (KBR; 10 μmol/l) significantly reduced the decrease in mean PVAmLVV and Vo2 intercept ( n = 6). Although Vo2 for the Ca2+ handling was finally decreased, it transiently but significantly increased from the control for 10–15 min after I/R. This increase in Vo2 for the Ca2+ handling was completely blocked by KBR, suggesting an inhibition of reverse-mode NCX by KBR. α-Fodrin proteolysis, which was significantly increased after I/R, was also significantly reduced by KBR. Our study shows that the contractile failure after I/R is similar to that after high Ca2+ infusion, although the contribution of reverse-mode NCX to the contractile failure is different. An inhibition of reverse-mode NCX during initial reperfusion protects the heart against reperfusion injury.

Calpain inhibitor-1 protects the rat heart from ischemia-reperfusion injury: analysis by mechanical work and energetics

We hypothesized that calpain inhibitor-1 protected left ventricular (LV) function from ischemia-reperfusion injury by inhibiting the proteolysis of α-fodrin. To test this hypothesis, we investigated the effect of calpain inhibitor-1 on LV mechanical work and energetics in the cross-circulated rat hearts that underwent 15-min global ischemia and 60-min reperfusion ( n = 9). After ischemia-reperfusion with calpain inhibitor-1, mean end-systolic pressure at midrange LV volume and systolic pressure-volume area (PVA) at midrange LV volume (total mechanical energy per beat) were hardly changed, although they were significantly ( P < 0.01) decreased after ischemia-reperfusion without calpain inhibitor-1. Mean myocardial oxygen consumption per beat (Vo2) intercepts (PVA-independent Vo2; Vo2 for the total Ca2+ handling in excitation-contraction coupling and basal metabolism) of Vo2-PVA linear relations were also unchanged after ischemia-reperfusion with calpain inhibitor-1, although they were significantly ( P < 0.01) decreased after ischemia-reperfusion without calpain inhibitor-1. There were no significant differences in O2 costs of LV PVA and contractility among the hearts in control (or normal) postischemia-reperfusion and postischemia-reperfusion with calpain inhibitor-1. Western blot analysis of α-fodrin and the immunostaining of 150-kDa products of α-fodrin confirmed that calpain inhibitor-1 almost completely protected the proteolysis of α-fodrin. Our results indicate that calpain inhibitor-1 prevents the heart from ischemia-reperfusion injury associated with the impairment of total Ca2+ handling by directly inhibiting the proteolysis of α-fodrin.

Myocardial contractility of the isovolumetrically beating isolated rat heart

Several indexes of myocardial contractility have been proposed to assess ventricular function in the isovolumetrically beating isolated heart. However, the conclusions reached on the basis of these indexes may be influenced by ventricular geometry rather than contractility itself. The objective of the present study was to assess the performance of widely used contractility indexes in the isovolumetrically beating isolated heart in two experimental models of hypertrophy, the spontaneously hypertensive rat (SHR) and infrarenal aortocava fistula. Compared to normotensive controls (N = 8), SHRs with concentric hypertrophy (N = 10) presented increased maximum rate of ventricular pressure rise (3875 +/- 526 vs 2555 +/- 359 mmHg/s, P < 0.05) and peak of isovolumetric pressure (187 +/- 11 vs 152 +/- 11 mmHg, P < 0.05), and decreased developed stress (123 +/- 20 vs 152 +/- 26 g/cm(2), P < 0.05) and slope of stress-strain relationship (4.9 +/- 0.42 vs 6.6 +/- 0.77 g/cm(2)/%). Compared with controls (N = 11), rats with volume overload-induced eccentric hypertrophy (N = 16) presented increased developed stress (157 +/- 38 vs 124 +/- 22 g/cm(2), P < 0.05) and slope of stress-strain relationship (9 +/- 2 vs 7 +/- 1 g/cm(2)/%, P < 0.05), and decreased maximum rate of ventricular pressure rise(2746 +/- 382 vs 3319 +/- 352 mmHg, P < 0.05) and peak of isovolumetric pressure (115 +/- 14 vs 165 +/- 13 mmHg/s, P < 0.05). The results suggested that indexes of myocardial contractility used in experimental studies may present opposite results in the same heart and may be influenced by ventricular geometry. We concluded that several indexes should be taken into account for proper evaluation of contractile state, in the isovolumetrically beating isolated heart.

Left Ventricular Mechanoenergetics in Small Animals

Studies on left ventricular mechanical work and energetics in rat and mouse hearts are reviewed. First, left ventricular linear end-systolic pressure-volume relation (ESPVR) and curved end-diastolic pressure-volume relation (EDPVR) in canine hearts and left ventricular curved ESPVR and curved EDPVR in rat hearts are reviewed. Second, as an index for total mechanical energy per beat in rat hearts as in canine hearts, a systolic pressure-volume area (PVA) is proposed. By the use of our original system for measuring continuous oxygen consumption for rat left ventricular mechanical work, the linear left ventricular myocardial oxygen consumption per beat (VO2)-PVA relation is obtained as in canine hearts. The slope of VO2-PVA relation (oxygen cost of PVA) indicates a ratio of chemomechanical energy transduction. VO2 intercept (PVA-independent VO2) indicates the summation of oxygen consumption for Ca2+ handling in excitation-contraction coupling and for basal metabolism. An equivalent maximal elastance (eEmax) is proposed as a new left ventricular contractility index based on PVA at the midrange left ventricular volume. The slope of the linear relation between PVA-independent VO2 and eEmax (oxygen cost of eEmax) indicates changes in oxygen consumption for Ca2+ handling in excitation-contraction coupling per unit changes in left ventricular contractility. The key framework of VO2-PVA-eEmax can give us a better understanding for the biology and mechanisms of physiological and various failing rat heart models in terms of mechanical work and energetics.

Cardiac dysfunction in terms of left ventricular mechanical work and energetics in hypothyroid rats

We hypothesized that cardiac dysfunction in hypothyroidism is mainly caused by the impairment of Ca(2+) handling in excitation-contraction coupling. To prove this hypothesis, we investigated left ventricular (LV) mechanical work and energetics without interference of preload and afterload in an excised, blood-perfused whole heart preparation from hypothyroid rats. We found that LV inotropism and lusitropism were significantly depressed, and these depressions were causally related to decreased myocardial oxygen consumption for Ca(2+) handling and for basal metabolism. The oxygen costs of LV contractility for Ca(2+) and for dobutamine in the hypothyroid rats did not differ from those in age-matched normal rats. The expression of sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA2) significantly decreased and that of phospholamban significantly increased. The present results revealed that changes in LV energetics associated with decreased mechanical work in hypothyroid rats are mainly caused by the impairment of Ca(2+) uptake via SERCA2. We conclude that the impairment of Ca(2+) uptake plays an important role in the pathogenesis of cardiac dysfunction in hypothyroidism.

Oxygen costs of left ventricular contractility for dobutamine and Ca(2+) in normal rat hearts and the cost for dobutamine in Ca(2+) overload-induced failing hearts

The aim of the present study was to compare the oxygen (O(2)) cost of left ventricular (LV) contractility (equivalent maximal elastance; eEmax, an index for contractility) for dobutamine (a beta-receptor stimulant) with that for calcium (Ca(2+)) in normal rat hearts and to assess the O(2) cost of LV eEmax for dobutamine in Ca(2+) overload-induced failing rat hearts. The mean O(2) cost of LV eEmax (x10(-4) microl O(2) x beat(-1) x mmHg(-1) x ml x g(-2)) for Ca(2+) was 1.30+/-0.37 in 12 normal hearts, and for dobutamine it was 1.26+/-0.30 in eight different normal hearts. In the same five normal hearts, the mean O(2) cost of LV eEmax for dobutamine was 1.15+/-0.27, and for Ca(2+) it was 0.81+/-0.36. These mean values showed no significant differences between Ca(2+) and dobutamine. In five Ca(2+) overload-induced failing hearts, the mean O(2) cost of LV eEmax for Ca(2+) could not be assessed, but the mean O(2) cost of LV eEmax for dobutamine was 1.04+/-0.40. This mean value for dobutamine did not differ significantly from those (see above 1.26+/-0.30 or 1.15+/-0.27) in the normal hearts. The present results indicate, in terms of the coupling of mechanical work and energetics of the heart, that the total Ca(2+) handling VO(2) in excitation-contraction coupling against unit LV contractility change for dobutamine in the contractile failing hearts does not differ from that in the normal hearts. This suggests that in the Ca(2+) overload-induced contractile failing hearts, there were no changes in the sensitivity of the contractile machinery for Ca(2+), in the Ca(2+)/ATP in the total Ca(2+) handling, and in the ATP/VO(2) in the mitochondria.

Left ventricular diastolic dysfunction in type 2 diabetes mellitus model rats

To gain insight into the pathogenesis of diabetic cardiomyopathy, we investigated cardiac function in terms of the coupling of left ventricular mechanical work and the energetics in Otsuka Long-Evans Tokushima Fatty rats, which are well known as a model of type 2 diabetes mellitus (DM). Neither left ventricular systolic function and mean coronary flow nor coronary flow reserve differed even in late DM rats. The amount of oxygen required for mechanical work and contraction was unaltered, although myosin isozyme was finally transformed from V(1) to V(3). The maximum pacing rate was decreased from 300 to 240 beats/min, and the left ventricular relaxation rate was significantly (P < 0.05) slower only in late DM rats, resulting in decreased oxygen consumption per minute for total Ca(2+) handling in excitation-contraction coupling mainly consumed by sarco(endo)plasmic reticulum Ca(2+)-ATPase (SERCA2) without significant changes in basal metabolism or in mitochondrial oxidative phosphorylation. The protein level of SERCA2 in membranes was significantly (P < 0.001) lower in severe DM rats. We conclude that the only lusitropic dysfunction due to the depressed expression of SERCA2 is related to generating diabetic cardiomyopathy even in the present type 2 diabetic rats.