Alterations in calcium antagonist receptors and sodium-calcium exchange in cardiomyopathic hamster tissues

An experimental approach to study the function of mitochondria in cardiomyopathy

Cardiomyopathy is an inherited or acquired disease of the myocardium, which can result in severe ventricular dysfunction. Mitochondrial dysfunction is involved in the pathological process of cardiomyopathy. Many dysfunctions in cardiac mitochondria are consequences of mutations in nuclear or mitochondrial DNA followed by alterations in transcriptional regulation, mitochondrial protein function, and mitochondrial dynamics and energetics, presenting with associated multisystem mitochondrial disorders. To ensure correct diagnosis and optimal management of mitochondrial dysfunction in cardiomyopathy caused by multiple pathogenesis, multidisciplinary approaches are required, and to integrate between clinical and basic sciences, ideal translational models are needed. In this review, we will focus on experimental models to provide insights into basic mitochondrial physiology and detailed underlying mechanisms of cardiomyopathy and current mitochondria-targeted therapies for cardiomyopathy. [BMB Reports 2015; 48(10): 541-548]

Implications of protease activation in cardiac dysfunction and development of genetic cardiomyopathy in hamsters

It has become evident that protein degradation by proteolytic enzymes, known as proteases, is partly responsible for cardiovascular dysfunction in various types of heart disease. Both extracellular and intracellular alterations in proteolytic activities are invariably seen in heart failure associated with hypertrophic cardiomyopathy, dilated cardiomyopathy, hypertensive cardiomyopathy, diabetic cardiomyopathy, and ischemic cardiomyopathy. Genetic cardiomyopathy displayed in different strains of hamsters provides a useful model for studying heart failure due to either cardiac hypertrophy or cardiac dilation. Alterations in the function of several myocardial organelles such as sarcolemma, sarcoplasmic reticulum, myofibrils, mitochondria, as well as extracellular matrix have been shown to be due to subcellular remodeling as a consequence of changes in gene expression and protein content in failing hearts from cardiomyopathic hamsters. In view of the increased activities of various proteases, including calpains and matrix metalloproteinases in the hearts of genetically determined hamsters, it is proposed that the activation of different proteases may also represent an important determinant of subcellular remodeling and cardiac dysfunction associated with genetic cardiomyopathy.

Reduced Ca2+ transport across sarcolemma but enhanced spontaneous activity in cardiomyocytes isolated from left atrium-pulmonary veins tissue of myopathic hamster

Background

Several lines of evidence point to a particularly important role of the left atrium (LA) in initiating and maintaining atrial fibrillation (AF). This role may be related to the location of pulmonary veins (PVs) in the LA. The aim of the present study was to investigate the action potential (AP) and ionic currents in LA-PV cardiomyocytes isolated from Bio14.6 myopathic Syrian hamsters (36-57 week-old) versus age-matched F1B healthy control hamsters.

Methods and Results

Whole-cell patch-clamp techniques were used to record AP in current-clamp mode and ionic currents in voltage-clamp mode. The results obtained show that in both healthy and myopathic LA-PV tissue spontaneously discharging cardiomyocytes can be found, but they are more numerous in myopathic (9/29) than in healthy hamsters (4/42, p < 0.05 by χ² analysis). Myopathic myocytes have shorter AP duration (APD) with smaller I_Ca,L and I_NCX than the healthy control. The currents I_TO, I_K, I_K1 and I_Ca,T are not significantly different in myopathic versus healthy cells.

Conclusions

Our results indicate that in myopathic Syrian hamsters LA-PV cardiomyocytes are more prone to automatic rhythms. Also, they show altered electrophysiologic properties, which may be due to abnormal Ca²⁺ channels and may account for contractile dysfunction.

Cardiac contractility modulation electrical signals normalize activity, expression, and phosphorylation of the Na+-Ca2+ exchanger in heart failure

BACKGROUND

Expression and phosphorylation of the cardiac Na(+)-Ca(2+) exchanger-1 (NCX-1) are up-regulated in heart failure (HF). We examined the effects of chronic cardiac contractility modulation (CCM) therapy on the expression and phosphorylation of NCX-1 and its regulators GATA-4 and FOG-2 in HF dogs.

METHODS AND RESULTS

Studies were performed in LV tissue from 7 CCM-treated HF dogs, 7 untreated HF dogs, and 6 normal (NL) dogs. mRNA expression of NCX-1, GATA-4, and FOG-2 was measured using reverse transcriptase polymerase chain reaction, and protein level was determined by Western blotting. Phosphorylated NCX-1 (P-NCX) was determined using a phosphoprotein enrichment kit. Compared with NL dogs, NCX-1 mRNA and protein expression and GATA-4 mRNA and protein expression increased in untreated HF dogs, whereas FOG-2 expression decreased. Compared with NL dogs, the level of P-NCX-1 normalized to total NCX-1 increased in untreated HF dogs (0.80+/-0.10 vs 0.37+/-0.04; P < .05). CCM therapy normalized NCX-1 expression, GATA-4, and FOG-2 expression, and the ratio of P-NCX-1 to total NCX-1 (0.62+/-0.10).

CONCLUSION

Chronic monotherapy with CCM restores expression and phosphorylation of NCX-1. These findings are consistent with previous observations of improved LV function and normalized sarcoplasmic reticulum calcium cycling in the left ventricles of HF dogs treated with CCM therapy.

Ca2+- and mitochondrial-dependent cardiomyocyte necrosis as a primary mediator of heart failure

Loss of cardiac myocytes in heart failure is thought to occur largely through an apoptotic process. Here we show that heart failure can also be precipitated through myocyte necrosis associated with Ca2+ overload. Inducible transgenic mice with enhanced sarcolemmal L-type Ca2+ channel (LTCC) activity showed progressive myocyte necrosis that led to pump dysfunction and premature death, effects that were dramatically enhanced by acute stimulation of beta-adrenergic receptors. Enhanced Ca2+ influx-induced cellular necrosis and cardiomyopathy was prevented with either LTCC blockers or beta-adrenergic receptor antagonists, demonstrating a proximal relationship among beta-adrenergic receptor function, Ca2+ handling, and heart failure progression through necrotic cell loss. Mechanistically, loss of cyclophilin D, a regulator of the mitochondrial permeability transition pore that underpins necrosis, blocked Ca2+ influx-induced necrosis of myocytes, heart failure, and isoproterenol-induced premature death. In contrast, overexpression of the antiapoptotic factor Bcl-2 was ineffective in mitigating heart failure and death associated with excess Ca2+ influx and acute beta-adrenergic receptor stimulation. This paradigm of mitochondrial- and necrosis-dependent heart failure was also observed in other mouse models of disease, which supports the concept that heart failure is a pleiotropic disorder that involves not only apoptosis, but also necrotic loss of myocytes in association with dysregulated Ca2+ handling and beta-adrenergic receptor signaling.

Electrical and structural remodeling in left ventricular hypertrophy-a substrate for a decrease in QRS voltage?

Electrical remodeling in advanced stages of cardiovascular diseases creates a substrate for triggering and maintenance of arrhythmias. The electrical remodeling is a continuous process initiated already in the early stages of cardiological pathology. The aim of this opinion article was to discuss the changes in electrical properties of myocardium in left ventricular hypertrophy (LVH), with special focus on its early stage, as well as their possible reflection in the QRS amplitude of the electrocardiogram. It critically appraises the classical hypothesis related to the QRS voltage changes in LVH. The hypothesis of the relative voltage deficit is discussed in the context of supporting evidence from clinical studies, animal experiments, and simulation studies. The underlying determinants of electrical impulse propagation which may explain discrepancies between "normal" ECG findings and increased left ventricular size/mass in LVH are reviewed.

Antagonists of stretch-activated ion channels restore contractile function in hamster dilated cardiomyopathy

BACKGROUND

Stretch-activated ion channels (SACs) mediate abnormal ion currents in dilated cardiomyopathy (DCM), but their role in the contractile defect of DCM is undefined. We hypothesized that SAC antagonists would enhance contractile function in a hamster model of DCM.

METHODS

Left ventricular papillary muscles from Syrian hamsters with a genetic DCM (n = 26), and from non-myopathic controls (n = 26), were superfused and stimulated to contract. Maximum active force (F(max); milli-Newtons per square millimeter) was determined before (baseline) and after subjecting the muscle to a 60-minute period of overstretch (resting length associated with a 20% decay in baseline maximum force [F(max)]). Gadolinium (10 micromol/liter) and streptomycin (40 micromol/liter) were used separately to antagonize SACs.

RESULTS

In the absence of SAC antagonist, baseline F(max) was greater in controls (1.79 +/- 0.26) vs DCM (0.69 +/- 0.12; p < 0.05). Overstretch caused further decrease in F(max) in DCM (to 0.50 +/- 0.08; p = 0.03 vs baseline), but not in controls. The SAC antagonists increased baseline F(max) in DCM to equal that of untreated controls (gadolinium 1.64 +/- 0.34, streptomycin 2.13 +/- 0.33), but neither agent increased baseline F(max) in controls (gadolinium 1.91 +/- 0.20, streptomycin 2.25 +/- 0.49). Both agents abolished the stretch-induced decrease in contractile function in DCM.

CONCLUSIONS

Antagonists of SACs enhance contractile function in DCM to equal that of normal controls, and abolish sensitivity to further stretch. They do not alter contractile function in normal muscle. These data suggest an important role of SACs in the contractile dysfunction of DCM and further suggest that SAC antagonists may represent novel therapy in heart failure.

Electrical and structural remodeling of the failing ventricle

Heart failure (HF) is a complex disease that presents a major public health challenge to Western society. The prevalence of HF increases with age in the elderly population, and the societal disease burden will increase with prolongation of life expectancy. HF is initially characterized by an adaptive increase of neurohumoral activation to compensate for reduction of cardiac output. This leads to a combination of neurohumoral activation and mechanical stress in the failing heart that trigger a cascade of maladaptive electrical and structural events that impair both the systolic and diastolic function of the heart.

BK(Ca) channels compensate for loss of NOS-dependent coronary artery relaxation in cardiomyopathy

Previously, we showed that development of myocardial necrotic lesions is associated with impaired endothelium-dependent coronary artery relaxation in young cardiomyopathic hamsters. Since active necrosis declines with aging, this study was designed to determine whether coronary artery endothelium-dependent relaxation to ACh is restored and to identify the mechanisms mediating this effect. Intraluminal diameter was recorded in coronary arteries (150-250 micrometer) from control (C, 297 +/- 5 days old) and cardiomyopathic (M, 296 +/- 4 days old) hamsters. Relaxation to ACh (10(-9)-3 x 10(-5) M) was similar in vessels from C and M hamsters. However, mechanisms mediating relaxation to ACh were altered. Inhibition of nitric oxide synthase (NOS) activity with N-nitro-L-arginine (1 mM) had a greater inhibitory effect in vessels from C hamsters, indicating a reduction in NOS-dependent relaxation in vessels from M hamsters. Conversely, inhibition of large Ca(2+)-dependent K(+) (BK(Ca)) channels with charybdotoxin (CTX, 0.1 microM) had a greater inhibitory effect in vessels from M hamsters. In the presence of both N-nitro-L-arginine and CTX, relaxation to ACh was abolished in both groups. CTX (0.1 micrometer) produced a 50 +/- 4 and 30 +/- 3% contraction of vessels from M and C hamsters, respectively, indicating an enhanced role for BK(Ca) channels in regulation of coronary artery tone in M hamsters. Finally, vasodilatory cyclooxygenase products contributed to ACh-induced relaxation in vessels from M, but not C, hamsters. In conclusion, NOS-dependent relaxation of coronary small arteries is reduced in the late stage of cardiomyopathy. An increase in relaxation mediated by BK(Ca) channels and vasodilatory cyclooxygenase products compensates for this effect.

Reduced myocardial sarcoplasmic reticulum Ca(2+)-ATPase protein expression in compensated primary and secondary human cardiac hypertrophy

Pathological intracellular calcium handling has been proposed to underlie the alterations of contractile behavior in hypertrophied myocardium. However, the myocardial protein expression of intracellular calcium transport proteins in compensated human left ventricular hypertrophy has not yet been studied. We investigated septal myocardial specimens of patients suffering from hypertrophic obstructive cardiomyopathy (n=14) or from acquired aortic valve stenosis (n=11) undergoing myectomy or aortic valve replacement, respectively. For comparison, we studied non-hypertrophied myocardium of six non-failing hearts which could not be transplanted for technical reasons. The myocardial density of the calcium release channel of the sarcoplasmic reticulum (SR) was determined by(3)H-ryanodine binding. Myocardial contents of SR Ca(2+)-ATPase, phospholamban, calsequestrin and Na(+)/Ca(2+)-exchanger were analysed by Western blot analysis. The myocardial SR calcium release channel density was not significantly different in hypertrophied and non-failing human myocardium. In both hypertrophic obstructive cardiomyopathy and in aortic valve stenosis, SR Ca(2+)-ATPase expression was reduced by about 30% compared to non-failing myocardium (P<0.05), whereas the expression of phospholamban, calsequestrin, and the Na(+)/Ca(2+)-exchanger was unchanged. The decrease of SR Ca(2+)-ATPase expression was still observable when related to its regulatory protein phospholamban or to the myosin content of the homogenates (P<0.05). Furthermore, the SR Ca(2+)-ATPase expression was inversely correlated to the septum thickness assessed by echocardiography, but not to age, cardiac index or outflow tract gradient. In primary as well as in secondary hypertrophied human myocardium, the expression of SR Ca(2+)-ATPase is reduced and inversely related to the degree of the hypertrophy. The diminished SR Ca(2+)-ATPase expression might result in reduced Ca(2+)reuptake into the SR and might contribute to altered contractile behavior in hypertrophied human myocardium.

Minimum alveolar anesthetic concentration of volatile anesthetics in normal and cardiomyopathic hamsters

Minimum alveolar anesthetic concentrations (MAC) values of volatile anesthetics in cardiovascular diseases remain unknown. We determined MAC values of volatile anesthetics in spontaneously breathing normal and cardiomyopathic hamsters exposed to increasing (0.1%-0.3% steps) concentrations of halothane, isoflurane, sevoflurane, or desflurane (n = 30 in each group) using the tail-clamp technique. MAC values and their 95% confidence interval were calculated using logistic regression. In normal hamsters, inspired MAC values were: halothane 1.15% (1.10%-1.20%), isoflurane 1.62% (1.54%-1.69%), sevoflurane 2.31% (2.22%-2.40%), and desflurane 7.48% (7.30%-7.67%). In cardiomyopathic hamsters, they were: halothane 0.89% (0.83%-0.95%), isoflurane 1.39% (1.30%-1.47%), sevoflurane 2.00% (1.85%-2.15%), and desflurane 6.97% (6.77%-7.17%). Thus, MAC values of halothane, isoflurane, sevoflurane, and desflurane were reduced by 23% (P < 0.05), 14% (P < 0.05), 13% (P < 0.05), and 7% (P < 0.05), respectively in cardiomyopathic hamsters.

IMPLICATIONS

Minimum alveolar anesthetic concentrations of volatile anesthetics were significantly lower in cardiomyopathic hamsters than in normal hamsters.

The molecular and cellular pathophysiology of heart failure

In the United States, it is estimated that heart failure develops in 465,000 people each year. Heart failure occurs in both men and women and is associated with a high morbidity and mortality rate in both sexes and in all races. Our knowledge of the pathophysiology of heart failure has advanced beyond the cardiorenal-neurohumoral model and now includes changes in myocyte structure and function. Cellular changes in heart failure include myocyte hypertrophy, abnormalities in calcium homeostasis, excitation-contraction coupling, cross-bridge cycling, and changes in the cytoskeletal architecture. Data also indicate that some of these changes are found during the compensated stage of heart failure; whereas other changes are found during overt decompensation and are associated with changes in systolic and diastolic function. The transition from compensated to decompensated heart failure is more than likely related to the overexpression of neurohormones and peptides such as norepinephrine, angiotensin II, and proinflammatory cytokines. The purpose of this article is to review the epidemiology and cellular pathophysiology of heart failure.

Effects of neuropeptide Y on L-type calcium current in guinea-pig ventricular myocytes

1. Neuropeptide Y (NPY) reduces cell shortening at high concentrations in guinea-pig ventricular myocytes. We have studied the effects of the peptide on calcium current in cardiac myocytes. 2. We have recorded L-type calcium current in guinea-pig ventricular myocytes under conditions in which the effects of other overlapping currents have been minimised by using Na(+)-free, K(+)-free external solution and patch-clamp electrodes containing Cs+. 3. Peak inward calcium current is reduced by NPY at concentrations in excess of 1 nM, and maximal inhibition (31%) was found at and above concentrations of 100 nM. The IC50 value for NPY inhibition of peak calcium current was 1.72 nM. 4. NPY had no effect on the voltage-dependence of calcium current amplitude, on the time course of current inactivation, or on the voltage-dependence of the steady-state gating variables. 5. NPY did not reduce the calcium current in the presence of 8-Br-cyclic AMP, and it was also without effect when GTP-gamma-S or GDP-beta-S were included in the patch pipette. 6. We conclude that in guinea-pig ventricular myocytes NPY acts at low concentration to reduce L-type calcium current, via a G-protein-mediated pathway and reduction in intracellular cyclic AMP.

Myofibrillar Ca2+ sensitivity of cardiomyopathic hamster hearts

We studied the Ca2+ responsiveness of skinned muscle fibre preparations from the right and left ventricles of normal (FIB) and genetically cardiomyopathic (Bio-To-2) Syrian hamsters. Thus, we compared the Ca2+/force relationships of preparations from myopathic hamsters to those of age-matched (11-16 months old) normal animals. The pCa (i.e. -log10 [Ca2+]) required for 50% force activation (Ca2+ sensitivity) was higher in the myopathic hamsters than in controls (pCa50 values of 5.3 +/- 0.03 and 5.17 +/- 0.04, respectively); this difference might be due to an alteration in regulatory proteins. Indeed, after extraction (with vanadate) and replacement of troponin I with bovine cardiac troponin the pCa50 values were similar (pCa 5.35) to those of bovine ventricular fibres. The Ca2+ sensitizer EMD 53998 (10 microM) increased Ca2+ sensitivity in preparations from normal and cardiomyopathic hamsters equally, by 0.4 pCa units. Incubation of fibre bundles with the catalytic subunit of cyclic-adenosine-monophosphate-dependent protein kinase decreased Ca2+ sensitivity, thereby "normalizing" the enhanced Ca2+ responsiveness of fibres from cardiomyopathic hamsters. It is not clear, however, whether the pathologically increased Ca2+ sensitivity of the hearts of aged myopathic hamsters reflects a maladaptation, or a compensatory mechanism of the failing heart.

Intracellular calcium handling in isolated ventricular myocytes from cardiomyopathic hamsters (strain BIO 14.6) with congestive heart failure

Intracellular [Ca2+]i handling has been shown to be altered in isolated ventricular myocytes from patients with terminal heart failure. The aim of this study was to evaluate if alterations of intracellular [Ca2+]i handling and triggering Ca2+ currents in cardiomyopathic hamsters (strain BIO 14.6) with congestive heart failure might be similar to changes found in myocytes of patients with terminal heart failure and, therefore if the hamster might serve as a model for heart failure in man. Cells were isolated from hearts of hamsters developing hereditary cardiomyopathy (CMP) (strain BIO 14.6) at 12-14 months of age with overt signs of congestive heart failure. Results were compared with age-matched, undiseased control animals (CTRL). [Ca2+]i transients and Ca2+ currents were recorded simultaneously from isolated cells under voltage clamp perfused internally with the Ca2+ indicator, Fura-2. Ca2+ current densities in myocytes from CMP hamsters were -6.6 +/- 0.6 versus -8.3 +/- 0.5 microA/cm2 (P < 0.05) in CTRL. Resting [Ca2+]i levels were not significantly different. Peak [Ca2+]i transients were significantly decreased in CMP cells (450 +/- 52 nM versus 1031 +/- 98 nM in CTRL, P < 0.05). The rate of diastolic [Ca2+]i decay was slower in cells from CMP animals (t1/2: 167 +/- 19 versus 109 +/- 16 ms; P < 0.05). A moderate negative correlation was found between cell surface area and [Ca2+]i transients (r = 0.42; P < 0.05). It is concluded that changes of intracellular [Ca2+]i handling may play an important role in altered contractility of the myocardium of hamsters with hereditary cardiomyopathy in the late stage of congestive heart failure.(ABSTRACT TRUNCATED AT 250 WORDS)

Morphometric analysis of cultured normal and cardiomyopathic hamster heart cells after immunofluorescent staining for tubulin and alpha-actinin

The cardiomyopathic (CM) hamster (Strain UM X7.1) develops a progressive cardiomyopathy characterized by cellular necrosis, hypertrophy and congestive heart failure. To better understand these abnormalities, this study was undertaken to investigate possible abnormalities in the morphology and distributions of cytoskeletal proteins in normal and cardiomyopathic hamster heart cells in vitro. Primary cultures of cardiac myocytes from normal and CM newborn hamsters were analyzed and compared by indirect immunofluorescent microscopy after 3, 5, 7 and 9 days in culture. The distributions of the cytoskeletal proteins, alpha-actinin and tubulin, were examined in cultured hamster cardiac myocytes. After the cells attach to coverslips, both normal and CM myocytes appear rounded in shape. After 5 days in culture, CM myocytes show fewer cytoplasmic projections than normal. To assess this phenomenon, the area and perimeter dimensions of normal and CM myocytes were analyzed by morphometric methods. It was determined that cardiomyopathic cells in culture become progressively larger in area but smaller than normal in their perimeter dimensions. A statistically significant difference was noted from day 3 onward. This result confirms that cardiomyopathic cells have abnormal shapes in vitro. It is conceivable that a reduction of the perimeter dimension in CM cells may be related to the reported calcium overload or to other biochemical or physiological lesions. In addition, the greatest density of tubulin staining is present immediately around the nucleus, with fluorescent "rays" radiating out to the cell periphery. Most of the myofibrils labelled by anti-alpha-actinin antibody showed parallel arrangements with respect to each other in normal myocytes whereas in CM heart cells the myofibrils were disarrayed. There were no differences in the distributions of tubulin and alpha-actinin in normal and cardiomyopathic myocytes in culture.

Uniformity of calcium channel number and isometric contraction in human right and left ventricular myocardium

We compared contractile performance in trabeculae carneae (n = 25) from non-failing right and left ventricles (n = 25) of brain dead organ donors without known cardiovascular disease and measured connective tissue content in trabeculae carneae from both non-failing and failing human hearts. Peak twitch force and time-course of contraction were not different between muscles taken from right or left ventricles. Peak twitch force was 13.9 +/- 3 vs. 13.7 +/- 2.7 mN/mm2 for right and left ventricular trabeculae carneae, respectively in 2.5 mM [Ca2+]0 at a 0.33 Hz stimulation frequency. Time to peak tension (405 +/- 21 vs. 405 +/- 12 ms), time to 50% relaxation from peak contractile response (277 +/- 21 vs. 278 +/- 14.6 ms) and time to 80% relaxation (428 +/- 29 vs. 433 +/- 22) were not different between right and left ventricular trabeculae carneae. Calcium channel number determined by [3H]PN200-100 dihydropyridine-radioligand binding assay was also not different (56.2 +/- 6.5 fmol/mg protein vs. 58.6 +/- 8.4 fmol/mg protein for right and left heart preparations, respectively). However, in myocardium obtained from ischemic hearts the left ventricle showed a reduced number of calcium channels compared to the right ventricle (55.3 +/- 3.8 vs. 36.6 +/- 3.9 fmol/mg protein for right and left ventricle, respectively p = 0.027). No differences were noted in the number of DHP receptor binding sites between right and left ventricular myocardium from patients with idiopathic dilated cardiomyopathy (51.4 +/- 7.6 fmol/mg protein vs. 61.8 +/- 6.5 fmol/mg protein respectively). Our data indicate that calcium channel number is similar for non-failing left and right human ventricle. Contractile response to changes in [Ca2+]0 and frequency were similar for trabeculae carneae from the left and right ventricles of non-failing human hearts. Studies involving calcium channel activation or inhibition in ischemic human myocardium, where there may be differences in calcium channel number and/or function are warranted. Whether changes in calcium channel number have biological consequences on contractile function remains to be determined. Importantly, careful studies of calcium channel function under in vivo conditions are warranted.

Changes in cell length consequent on depolarization in single left ventricular myocytes from guinea-pigs with pressure-overload left ventricular hypertrophy

Cell length was measured in single guinea-pig left ventricular myocytes by using a high-resolution photodiode array. Step depolarizations from a holding potential of -45 mV were applied using a switch-clamp technique with 2 M KCl microelectrodes, which were devoid of Ca2+ buffering. Comparison was made between myocytes from sham-operated guinea-pigs and guinea-pigs with mild pressure-overload left ventricular hypertrophy induced by infra-renal aortic constriction. The relation between cell shortening and membrane voltage was bell shaped, and a phasic component of shortening was evident at the range of potentials over which the L-type calcium current was activated. Mean cell shortening was increased in the hypertrophy group, and was maximal at +15 mV in both groups (control, 7.6 +/- 0.9 microns, n = 11, hypertrophy 11.0 +/- 1.2 microns, n = 20, p < 0.05). The latency to the onset of contraction was significantly shorter in the hypertrophy myocytes at -25 mV and at potentials positive to +50 mV. The relation between time-to-peak shortening and voltage showed a trend to shorter times in the hypertrophy group. At very positive potentials a slow component of contraction was identified which was relatively larger in the hypertrophy myocytes. This finding is consistent with increased calcium entry via sarcolemmal sodium-calcium exchange in the myocytes from the hypertrophy group.

Depressed sliding velocity of isolated cardiac myosin from cardiomyopathic hamsters: evidence for an alteration in mechanical interaction of actomyosin

We measured the relative sliding velocity of cardiomyopathic hamster cardiac myosin on actin cables by using an in vitro motility assay system. We also investigated the relationship between the velocity and both myosin isozyme content and ATPase activity. Cardiac myosin was obtained from cardiomyopathic hamsters (BIO 14.6; B) aged 3, 6, 9, and 18 months and age-matched controls (F1B; F). Long well-organized actin cables of an alga, Nitellopsis, were used for the motility assay. Small latex beads (2 microns in diameter) were coated with purified cardiac myosin. When myosin-coated beads were introduced into an algal cell in the presence of Mg-ATP, myosin interacted with actin and dragged the beads. Active movement of the beads along the actin cables was observed under a photomicroscope and the velocity was measured. The velocity was significantly lower in B than in F for each age group (0.47 vs. 0.71 microns/s at the age of 3 months, p < 0.05; 0.44 vs. 0.88 microns/s at 6 months, p < 0.01; 0.44 vs. 0.67 microns/s at 9 months, p < 0.01; 0.35 vs. 0.52 microns/s at 18 months, p < 0.05). Both Ca(2+)-activated ATPase activity and the percentage of alpha-myosin heavy chain were also lower in B than in F for each age group. When examined for individual specimens, there was a positive correlation between the velocity and both myosin Ca(2+)-activated ATPase activity (r = 0.84) and percentage of alpha-myosin heavy chain (r = 0.83). These data points of both control and cardiomyopathic hamsters were distributed near the regression line obtained from control and thyroxine-treated rabbits reported previously. The present results indicate that the difference in mechanical properties between control and cardiomyopathic cardiac myosin is attributed to isozyme redistribution and not to a qualitative change in each myosin molecule.

[3H]PN200-110 and [3H]glibenclamide binding in normal and cardiomyopathic hamsters

1. We examined the binding of the Ca2+ channel ligand [3H]PN200-110 and the ATP-sensitive K+ channel ligand [3H]glibenclamide to brain and heart from cardiomyopathic hamsters and compared them to controls. 2. We found that [3H]PN200-110 binding site density was elevated in the heart, but not in the brain, of 30- and 180-day old cardiomyopathic hamsters when compared to controls. 3. [3H]Glibenclamide binding site density was greatly reduced in the heart of 180-day old cardiomyopathic animals compared with all other groups. 4. Quantitative autoradiography revealed that [3H]glibenclamide binding was elevated in several brain areas of 30-day old cardiomyopathic hamsters relative to controls. 5. It is concluded that alterations in both Ca2+ and K+ channels exist in the cardiomyopathic hamster.

Expression of dihydropyridine receptor (Ca2+ channel) and calsequestrin genes in the myocardium of patients with end-stage heart failure

Cytoplasmic free calcium ions (Ca2+) play a central role in excitation-contraction coupling of cardiac muscle. Abnormal Ca2+ handling has been implicated in systolic and diastolic dysfunction in patients with end-stage heart failure. The current study tests the hypothesis that expression of genes encoding proteins regulating myocardial Ca2+ homeostasis is altered in human heart failure. We analyzed RNA isolated from the left ventricular (LV) myocardium of 30 cardiac transplant recipients with end-stage heart failure (HF) and five organ donors (normal control), using cDNA probes specific for the cardiac dihydropyridine (DHP) receptor (the alpha 1 subunit of the DHP-sensitive Ca2+ channel) and cardiac calsequestrin of sarcoplasmic reticulum (SR). In addition, abundance of DHP binding sites was assessed by ligand binding techniques (n = 6 each for the patients and normal controls). There was no difference in the level of cardiac calsequestrin mRNA between the HF patients and normal controls. In contrast, the level of mRNA encoding the DHP receptor was decreased by 47% (P less than 0.001) in the LV myocardium from the patients with HF compared to the normal controls. The number of DHP binding sites was decreased by 35-48%. As reported previously, expression of the SR Ca(2+)-ATPase mRNA was also diminished by 50% (P less than 0.001) in the HF group. These data suggest that expression of the genes encoding the cardiac DHP receptor and SR Ca(2+)-ATPase is reduced in the LV myocardium from patients with HF. Altered expression of these genes may be related to abnormal Ca2+ handling in the failing myocardium, contributing to LV systolic and diastolic dysfunction in patients with end-stage heart failure.

Simultaneous reduction of the sarcolemmal and SR calcium ATPase activities and gene expression in cardiomyopathic hamster

Altered calcium regulation is a prominent feature in the hereditary cardiomyopathy of the Syrian hamster. However, the activity of the two systems necessary for intracellular calcium homeostasis in the heart, the sarcolemmal and sarcoplasmic reticulum calcium ATPase pumps, have not been correlated. Using age- and pair-matched myopathic and control hamsters, a simultaneous reduction in gene expression and enzyme activity for these two pumps has been demonstrated. The concomitant alteration in gene expression as early as 1 month of age, preceding noticeable myocytolysis suggests that the depressed activity in these two calcium ATPase systems is not due to cell necrosis but at least in part due to reduction in their mRNA levels. Reduced capacity of the calcium pumps would result in calcium overload as well as impaired contractility that leads to the eventual heart failure in this animal model.

Alterations in Ca(2+)-channels during the development of diabetic cardiomyopathy

In order to examine the status of Ca2+ channels in heart sarcolemma during the development of diabetes, rats were injected intravenously with 65 mg/kg streptozotocin and hearts were removed 1, 3 and 8 weeks later. Crude membranes from the ventricular muscle were prepared and the specific binding of 3H-nitrendipine was studied by employing different concentrations of this Ca(2+)-antagonist. A significant decrease in both dissociation constant and maximal number of 3H-nitrendipine binding was observed in 3 and 8 weeks diabetic preparations. No such alterations were evident in diabetic brain membranes. Treatment of diabetic animals with insulin prevented the occurrence of these changes in the myocardium. The altered 3H-nitrendipine binding characteristics in diabetic heart membranes may not be due to the high levels of circulating catecholamines in this experimental model because no such changes were seen upon injecting a high dose (40 mg/kg) of isoproterenol in rats for 24 hr. The reduced number of 3H-nitrendipine binding sites may decrease Ca(2+)-influx through voltage sensitive Ca2+ channels and partly explain the depressed cardiac contractile force development in chronic diabetes whereas the increased affinity of Ca2+ channels may partly explain the increased sensitivity of diabetic heart to Ca2+.

Vitamin E deficiency has a pathological role in myocytolysis in cardiomyopathic Syrian hamster (BIO14.6)

This study revealed the occurrence of vitamin E deficiency in the myocardium of 60-day-old Syrian cardiomyopathic hamsters (BIO14.6), and that this deficiency might be related to the increase in lipid peroxide. Vitamin E administration for ten days effectively restored creatininekinase activity and decreased the lipid peroxide content in the myocardium, returning these to normal control levels (F1b). These results indicate that vitamin E deficiency, possibly combined with oxidative stress in the early cardiomyopathic stage plays an important role in initiating the pathogenesis of myocardial lesions.

Structure and function of the respiratory system of the dystrophic hamster

The BIO 14.6 dystrophic hamster has been used extensively over the past 30 years as an animal model in which to study the mechanisms responsible for the development of cardiomyopathy and skeletal muscle dysfunction associated with muscular dystrophy. More recently, structural and functional aspects of the respiratory system of this animal model have been investigated. This review summarizes our current knowledge of ventilation, lung morphometry and mechanics, the structure and function of the diaphragm, tracheal and pulmonary vascular smooth muscle, and pulmonary macrophages in the BIO 14.6 dystrophic hamster. We conclude that many aspects of the structure and function of the respiratory system of this hamster warrant further investigation, including the development of alveolar hypoventilation, the causes of pulmonary vascular hyporeactivity, and the potential contribution of abnormal pulmonary macrophages to the pathogenesis of life-threatening respiratory disease in muscular dystrophy.