Correlation of changes in cardiac calcium channels with hemodynamics in Syrian hamster cardiomyopathy and 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.

Calcium transport mechanisms in muskrat and rat hearts

Mammalian hearts experience calcium overload during extreme and prolonged hypoxia and the calcium overload may lead to enzyme activation and cell death. Several calcium transport systems were examined in muskrat hearts and compared to those found in rat hearts to determine if there is a species difference that might be related to the muskrats' superior ability to survive hypoxia. Radiolabeled nitredendipine binding was determined in rat and muskrat hearts to estimate the density of voltage gated calcium channels in surface membranes. There were no species differences. Calcium release channel density in the sarcoplasmic reticulum was estimated by the determination of radiolabeled ryanodine binding in muskrat and rat heart SR membranes. No differences were revealed between species. The SR uptake of calcium was measured in SR membranes from the hearts of the two species. No differences were found in the B(max) values, however, the muskrat SR membranes did have a slightly lower K(m) value. There were large species differences in Na(+)/Ca(2+) exchange in SL membranes with the muskrat heart having approximately 3.5 times the transport capacity of rat SL membranes. During hypoxic conditions in which there is extensive ATP depletion leading to [Na(+)](i) accumulation and discharge of cellular membrane potential, the Na(+)/Ca(2+) exchanger may operate in the reverse mode and import calcium into the cell and accelerate hypoxic damage. Prior to reaching this state a robust Na(+)/Ca(2+) exchange would facilitate the maintenance of normal diastolic calcium levels and calcium cycling. Muskrats hearts are hypoxia tolerant by virtue of their ability to reduce metabolic demand and generate ATP anaerobically thus, maintaining a favorable ATP balance. Therefore, the relative overexpression of Na(+)/Ca(2+) exchangers in muskrat hearts may be beneficial in the preservation of contractile function and calcium homeostasis in this freshwater diving mammal.

Intracellular calcium and the relationship to contractility in an avian model of heart failure

Global contractile heart failure was induced in turkey poults by furazolidone feeding (700 ppm). Abnormal calcium regulation appears to be a key factor in the pathophysiology of heart failure, but the cellular mechanisms contributing to changes in calcium fluxes have not been clearly defined. Isolated ventricular myocytes from non-failing and failing hearts were therefore used to determine whether the whole heart and ventricular muscle contractile dysfunctions were realized at the single cell level. Whole cell current- and voltage-clamp techniques were used to evaluate action potential configurations and L-type calcium currents, respectively. Intracellular calcium transients were evaluated in isolated myocytes with fura-2 and in isolated left ventricular muscles using aequorin. Action potential durations were prolonged in failing myocytes, which correspond to slowed cytosolic calcium clearing. Calcium current-voltage relationships were normal in failing myocytes; preliminary evidence suggests that depressed transient outward potassium currents contribute to prolonged action potential durations. The number of calcium channels (as measured by radioligand binding) were also similar in non-failing and failing hearts. Isolated ventricular muscles from failing hearts had enhanced inotropic responses, in a dose-dependent fashion, to a calcium channel agonist (Bay K 8644). These data suggest that changes in intracellular calcium mobilization kinetics and longer calcium-myofilament interaction may be able to compensate for contractile failure. We conclude that the relationship between calcium current density and sarcoplasmic reticulum calcium release is a dynamic process that may be altered in the setting of heart failure at higher contraction rates.

Capillary density and leukocyte adhesion in hamsters with hereditary cardiomyopathy

The aim of this study was to characterize microvascular networks in cheek pouch of cardiomyopathic Syrian hamster (CM) (Bio 14.6), which is an interesting model of idiopathic cardiomyopathy and congestive heart failure. Microcirculation was visualized by fluorescence microscopy. Diameter and length of arterioles, classified according to centrifugal ordering scheme, were measured. A computational method was arranged to determine the density of arterioles and capillaries (total vessel length per unit area, cm-1), fractal dimension of capillaries, and the associated Voronoi tesselation. Furthermore, leukocyte adhesion to venules and arteriolar reactivity to drugs were studied. Increase in the number of terminal arterioles and capillary rarefication characterized CM microvasculature compared with that of age-matched controls (58 +/- 7 versus 25 +/- 5 cm-1, and 128 +/- 15 versus 240 +/- 10 cm-1, respectively). Fractal dimension of capillaries was reduced in CM compared with controls (1. 40 +/- 0.10 versus 1.85 +/- 0.09) and associated with increased avascular spaces, as shown by Voronoi tesselation results. Leukocyte adhesion to venules increased significantly in CM. In CM responsiveness of arterioles to nitric oxide inhibition and propranolol was slighter but more marked to norepinephrine and angiotensin II compared with that of control hamsters. In conclusion, the different geometry, increased leukocyte adhesion, and altered arterial responsiveness may contribute to flow disturbances in the microcirculation of CM hamsters.

The dystrophinopathies: an alternative to the structural hypothesis

Abnormal expression of the cytoskeletal protein dystrophin has deleterious consequences for skeletal muscle, cardiac muscle, and the central nervous system. A complete failure to express the protein produces Duchenne muscular dystrophy (DMD), in which there is extensive and progressive skeletal muscle necrosis, the development of a life-threatening dilated cardiomyopathy, and mild mental retardation. Dystrophin binds the F-actin cytoskeleton and is normally expressed in a complex of transmembrane proteins (the "dystrophin protein complex") that interact with external components of the basal lamina. One pathogenic model for DMD (the "structural hypothesis") suggests that this complex forms a structural bridge between the external basal lamina and the internal cytoskeleton and that the absence of dystrophin produces a defect in membrane structural support that renders skeletal muscle susceptible to plasmalemmal ruptures (or "tears") during the course of contractile activity. This review attempts to critically evaluate the structural hypothesis for DMD and presents an opposing model (the "channel aggregation model") that highlights the role of dystrophin in organizing the membrane cytoskeleton and the role of the cytoskeleton in aggregating ion channels and neurotransmitter receptors. Since ion channel aggregation is a process that is common across organ systems, the idea that channel function can be altered when aggregated ion channels interact with a dystrophic cytoskeleton has immediate implications for the expression of the dystrophinopathies in skeletal muscle, cardiac muscle, and the central nervous system.

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)

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.

Regulation of cardiac sarcolemmal Ca2+ channels and Ca2+ transporters by thyroid hormone

In order to examine the regulatory role of thyroid hormone on sarcolemmal Ca(2+)-channels, Na(+)-Ca2+ exchange and Ca(2+)-pump as well as heart function, the effects of hypothyroidism and hyperthyroidism on rat heart performance and sarcolemmal Ca(2+)-handling were studied. Hyperthyroid rats showed higher values for heart rate (HR), maximal rates of ventricular pressure development +(dP/dt)max and pressure fall -(dP/dt)max, but shorter time to peak ventricular pressure (TPVP) and contraction time (CT) when compared with euthyroid rats. The left ventricular systolic pressure (LVSP) and left ventricular end-diastolic pressure (LVEDP), as well as aortic systolic and diastolic pressures (ASP and ADP, respectively) were not significantly altered. Hypothyroid rats exhibited decreased values of LVSP, HR, ASP, ADP, +(dP/dt)max and -(dP/dt)max but higher CT when compared with euthyroid rats; the values of LVEDP and TPVP were not changed. Studies with isolated-perfused hearts showed that while hypothyroidism did not modulate the inotropic response to extracellular Ca2+ and Ca2+ channel blocker verapamil, hyperthyroidism increased sensitivity to Ca2+ and decreased sensitivity to verapamil in comparison to euthyroid hearts. Studies of [3H]-nitrendipine binding with purified cardiac sarcolemmal membrane revealed decreased number of high affinity binding sites (Bmax) without any change in the dissociation constant for receptor-ligand complex (Kd) in the hyperthyroid group when compared with euthyroid sarcolemma; hypothyroidism had no effect on these parameters. The activities of sarcolemmal Ca(2+)-stimulated ATPase, ATP-dependent Ca2+ uptake and ouabain-sensitive Na(+)-K+ ATPase were decreased whereas the Mg(2+)-ATPase activity was increased in hypothyroid hearts. On the other hand, sarcolemmal membranes from hyperthyroid samples exhibited increased ouabain-sensitive Na(+)-K+ ATPase activity, whereas Ca(2+)-stimulated ATPase, ATP-dependent Ca2+ uptake, and Mg(2+)-ATPase activities were unchanged. The Vmax and Ka for Ca2+ of cardiac sarcolemmal Na(+)-Ca2+ exchange were not altered in both hyperthyroid and hypothyroid states. These results indicate that the status of sarcolemmal Ca(2+)-transport processes is regulated by thyroid hormones and the modification of Ca(2+)-fluxes across the sarcolemmal membrane may play a crucial role in the development of thyroid state-dependent contractile changes in the heart.

[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.

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+.

Regulation of [3H]nitrendipine binding by phospholipases A2 and C through direct and GTP-sensitive mechanisms

We compared the effects of Phospholipases A2, C, B and D on [3H]nitrendipine binding to hamster cardiac membranes, in the absence and presence of ATP or GTP. Phospholipase A2, competitively inhibited [3H]nitrendipine binding to hamster cardiac membranes unchanged by ATP or GTP (Ki = 5 ng/ml); as evidenced by complete and reversible displacement of [3H]nitrendipine binding and increase in KD on Scatchard analyses. Phospholipase C also completely inhibited [3H]nitrendipine binding to hamster cardiac membranes (Ki = 5 micrograms/ml) with a decrease in Bmax and no change in KD on Scatchard analyses. The addition of GTP alone inhibited the PLC effect in EGTA-treated membranes. The addition of GTP with either CaCl2 or ATP or both resulted in an equal and opposite enhancement of the PLC effect. Phospholipases B and D had no effect on [3H]nitrendipine binding. These data support: (1) Direct effect of PLA2 on dihydropyridine binding. (2) Indirect regulation of dihydropyridine binding by Phospholipase C through a GTP and ATP-sensitive mechanism.

L-type cardiac calcium channels in doxorubicin cardiomyopathy in rats morphological, biochemical, and functional correlations

Doxorubicin (DXR) is an effective antitumor agent in a wide spectrum of neoplasms. Chronic treatment is associated with cardiomyopathy and characteristic myocardial ultrastructural changes, which include swelling of the t tubules. Accordingly, we investigated excitation-contraction coupling in cardiomyopathic rat heart resulting from chronic DXR treatment. Using the whole-cell patch clamp technique, we studied the L-type calcium channel in single cells enzymatically isolated from normal (CTRL) and DXR rat hearts. Despite similar cell dimensions, the total membrane capacitance was significantly smaller in the DXR cells (138 +/- 9 pF) than in the CTRL cells (169 +/- 11 pF) (mean +/- SEM, n = 9, P less than 0.05). The mean current and the current density-voltage relationships of the CTRL and the DXR cells were significantly different (n = 9, P less than 0.001) with the maximal peak L-type calcium current (ICa) density increased from 6.4 +/- 0.9 in CTRL cells to 10.5 +/- 2.4 microA/cm2 in the DXR cells (P less than 0.05). There was no shift either in the current-voltage relationship or the steady-state inactivation curve in the two cell groups. However, the fast time constant of inactivation was increased at a membrane voltage of -10 to 10 mV. Calcium channel antagonist equilibrium binding assays using [3H]-PN200-110 revealed no difference in the maximal receptor binding capacity (CTRL, 194 +/- 27 and DXR 211 +/- 24 fmol/mg protein; P greater than 0.05, n = 6) and in receptor affinity (CTRL, 0.15 +/- 0.05 and DXR 0.13 +/- 0.03 nM; P less than 0.05). These data suggest that a decrease in effective capacitance might be associated with t-tubular damage. Despite this decrease, ICa was increased in the DXR cells. Such an increase may result from an alteration in the properties of the calcium channels and/or recruitment of "hibernating" channels in the remaining surface and t-tubular membranes.

Comparison of exercise effects on the hemodynamics of the Bio 14.6 cardiomyopathic hamster

1. Comparisons of the effects of 4 and 16 weeks of exercise were made on; cardiac output, stroke volume, heart rate, left intraventricular systolic and diastolic pressures, dP/dt, and heart calcium in the Bio 14.6 cardiomyopathic and F1 B hamsters. 2. In the cardiomyopathic hamster the cardiac output, stroke volume, left intraventricular systolic pressure and dP/dt, which were all depressed in the age related sedentary animals, were increased by both periods of exercise. The left intraventricular diastolic pressure which was elevated was likewise decreased by both exercise periods. Only the 16 week exercise period decreased the resting heart rate. 3. In the normal F1 B hamster, both periods of exercise increased the cardiac output and stroke volume while the left intraventricular systolic pressure was decreased. Only the 16 week exercise decreased the resting heart rate and left intraventricular diastolic pressure and increased the left ventricular dP/dt. 4. Both periods of exercise increased the total heart calcium in the Bio 14.6 hamster while the heart calcium in the F1 B was increased only by the 16 week exercise period.

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.

Nitrendipine binding in congestive heart failure due to myocardial infarction

Depressed cardiac pump function is the hallmark of congestive heart failure, and it is suspected that decreased influx of Ca2+ into the cardiac cell is responsible for depressed contractile function. Since Ca2+ channels in the sarcolemmal membrane are considered to be an important route for the entry of Ca2+, we examined the status of Ca2+ receptors/channels in failing rat hearts after myocardial infarction of the left ventricular free wall. For this purpose, the left coronary artery was ligated and hearts were examined 4, 8, and 16 weeks later; sham-operated animals served as controls. Hemodynamic assessment revealed decreased total mechanical energy (left ventricular systolic pressure x heart rate), increased left ventricular diastolic pressure, and decreased positive and negative dP/dt in experimental animals at 4, 8, and 16 weeks. Although accumulation of ascites in the abdominal cavity was evident at 4 weeks, other clinical signs of congestive heart failure in experimental rats were evident from the presence of lung congestion and cardiac dilatation at 8 and 16 weeks after induction of myocardial infarction. The density of Ca2+ receptors/channels in crude membranes, as assessed by [3H]nitrendipine binding assay, was found to be decreased in the uninfarcted experimental left ventricle at 8 and 16 weeks; however, no change in the affinity of nitrendipine was evident. A similar depression in the specific binding of another dihydropyridine compound, [3H]PN200-110, was also evident in failing hearts. Brain and skeletal muscle crude membrane preparations, unlike those of the right ventricle and liver, revealed a decrease in Ca2+ receptors/channels density in experimental animals at 16 weeks.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparisons of hemodynamics throughout the life span of the Bio 14.6 cardiomyopathic with the F1B normal hamster

1. Comparisons of left intraventricular end diastolic and systolic pressures, cardiac output, dP/dt, stroke volume and heart rate were made between the Bio 14.6 cardiomyopathic and F1B normal hamster at 45, 80, 150 and 240 days of age. 2. Comparisons of the ventricular calcium and taurine contents were made between the two strains of hamsters at similar ages. 3. Interstrain comparisons of the 240 day Bio 14.6 with age matched F1B hamsters and intrastrain comparisons with 45 day Bio 14.6 hamsters showed a decreased stroke volume, cardiac output and dP/dt with an increased left intraventricular end diastolic pressure, ventricular weight, ventricular weight/body weight ratio, heart calcium and taurine. 4. Despite the decreased left ventricular systolic pressure and cardiac output in the 80 day and older groups of Bio 14.6 hamsters, no compensatory increase in heart rate was observed.

Bone disorder in cardiomyopathic hamsters

Bones of cardiomyopathic hamsters (UM-X7.1 Syrian hamsters), at 5, 10 and 20 weeks of age, were compared chemically and histomorphologically with those of normal Syrian hamsters. Femurs of UM-X7.1 hamsters were significantly shorter than those of normal hamsters, and the mean dry weight, mean volume, mean ash weight per unit bone volume and mean ash as a percentage of dry weight of femurs were all significantly less in UM-X7.1 hamsters. The bone disorder preceded the myocardial calcium precipitation and myocardial hypertrophy in the cardiomyopathic hamsters. In addition, the percentage of cortical area measured on the cross-section of tibia and the appositional rate of bone minerals, determined by a tetracycline labelling technique, were also lower in the UM-X7.1 hamsters. These findings suggest that the bone disorder was associated with decreased bone formation in the UM-X7.1 Syrian hamsters.

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

The Syrian cardiomyopathic (CM) hamster (BIO 14.6) develops a progressive cardiomyopathy characterized by cellular necrosis, hypertrophy, and, eventually, cardiac dilatation and congestive heart failure. Several lines of evidence implicate cellular calcium overload as an important etiologic factor. We previously reported an increased number of receptors for calcium antagonist drugs, which block voltage-dependent calcium channels, in heart, skeletal muscle, and brain tissue of these hamsters in the early necrotic stage of the disease. To better characterize the pathophysiological significance of this abnormality we evaluated calcium antagonist receptor binding and Na+-Ca2+ exchange in CM and control hamsters at different stages of disease as documented by quantitative histopathologic assessment. In CM hamsters as young as 10 days, an age previously thought to be before the onset of disease, we identified cardiac myocyte hypertrophy, a twofold increase in calcium antagonist receptor binding in heart and brain, and a 50% increase in skeletal muscle. Overt histological lesions were present in skeletal muscle at 25 days and in heart between 28-30 days. The size of cardiac lesions increased over time and changed from necrotic foci with cellular infiltration to fibrotic or calcified lesions by 360 days. Myocardial cellular hypertrophy persisted through the late stages of the disease (360 days), but increased calcium antagonist binding was present in heart only to 6 months of age, in skeletal muscle to 90 days, and in brain to 30 days. Na+-Ca2+ exchange in heart was normal until 15 days and then increased by 400% at 30 days suggesting that this augmentation might be a secondary response to the earlier increase in calcium antagonist receptors. At 360 days cardiac Na+-Ca2+ exchange was decreased by 50%, likely reflecting progressive cardiac damage. The increase in calcium antagonist receptors in CM animals as young as 10 days supports the hypothesis that abnormalities in voltage-dependent calcium channels play a role in the pathophysiology of CM hamsters.

Calcium channel binding characteristics in the human heart

The number, affinity, pharmacologic specificity and regional distribution of calcium channel binding sites in human hearts obtained at autopsy and open heart surgery were characterized using the radioligand [3H]nitrendipine. Scatchard analyses of saturation data from 6 autopsy hearts revealed a homogeneous distribution of high affinity binding sites (affinity-1 [KD] = 0.44 +/- 0.06, 0.52 +/- 0.07, 0.32 +/- 0.02, 0.30 +/- 0.03, and 0.45 +/- 0.01 nM; binding capacity [Bmax] = 30 +/- 4, 27 +/- 6, 25 +/- 7, 33 +/- 3, and 28 +/- 4 fmol/mg protein in right atrium, right ventricle, left atrium, left ventricle and ventricular septum, respectively). In ligand competition experiments, nifedipine and nitrendipine completely displaced binding with partial displacement by verapamil and 35% enhancement of binding by 10(-5) M diltiazem at 37 degrees. Analyses of right atrial appendages obtained at open heart surgery from 5 coronary artery bypass patients provided similar results (KD = 0.2 +/- 0.03 nM, Bmax = 42 +/- 2 fmol/mg protein). In addition, no significant differences in KD or Bmax were found in 3 hamster hearts assayed at the time of death or up to 18 hours postmortem at either 4 or 25 degrees. In contrast, there was a significant increase in Bmax (110 fmol/mg protein) with no change in KD (0.3 nM) in a myomectomy specimen from a patient with obstructive hypertrophic cardiomyopathy compared with either autopsy or surgical specimens. These studies illustrate the feasibility and potential advantages of studying calcium channels directly in human hearts.