Single myosin cross-bridge orientation in cardiac papillary muscle detects lever-arm shear strain in transduction.

Myosin motors transduce ATP free energy into mechanical work. Transduction models allocate specific functions to motor structural domains beginning with ATP hydrolysis in the active site and ending in a lever-arm rotating power-stroke. Myosin light chains, regulatory (RLC) and essential (ELC), bind IQ-domains on the lever-arm and track its movement. Strong evidence exists that light chains stabilize the lever-arm and that light chain mutation undermines stability. Human ventricular RLC tagged with photoactivatable GFP (HCRLC-PAGFP) replaces native RLC in porcine papillary muscle fibers, restores native contractility, and situates PAGFP for single molecule orientation tracking within the crowded fiber lattice. The spatial emission pattern from single photoactivated PAGFP tagged myosins was observed in z-stacks fitted simultaneously to maximize accuracy in estimated dipole orientation. Emitter dipole polar and azimuthal angle pair scatter plots identified an area where steric and molecular crowding constraints depopulated orientations unfavorable for actin interaction. Transitions between pre- and post-power-stroke states represent the lever-arm trajectory sampled by the data and quantify lever-arm shear strain in transduction at three tension levels. These data identify forces acting on myosin in the in situ fiber system due to crowding, steric hindrance, and actomyosin interaction. They induce lever-arm shear strain observed with single molecule orientation detection. A single myosin work histogram reveals discretized power-stroke substates reminiscent of the Huxley-Simmons model for myosin based contraction [Huxley and Simmons ( 1971 ) Nature 233 , 533]. RLC or ELC mutation, should it impact lever-arm shear strain, will be detected as changes in single myosin shear strain or power-stroke substate distribution.

In situ measurements of crossbridge dynamics and lattice spacing in rat hearts by x-ray diffraction: sensitivity to regional ischemia.

BACKGROUND: Synchrotron radiation has been used to analyze crossbridge dynamics in isolated papillary muscle and excised perfused hearts with the use of x-ray diffraction techniques. We showed that these techniques can detect regional changes in rat left ventricle contractility and myosin lattice spacing in in situ ejecting hearts in real time. Furthermore, we examined the sensitivity of these indexes to regional ischemia. METHODS AND RESULTS: The left ventricular free wall of spontaneously beating rat hearts (heart rate, 290 to 404 bpm) was directly exposed to brief high-flux, low-emittance x-ray beams provided at SPring-8. Myosin mass transfer to actin filaments was determined as the decrease in reflection intensity ratio (intensity of 1,0 plane over the 1,1 plane) between end-diastole and end-systole. The distance between 1,0 reflections was converted to a lattice spacing between myosin filaments. We found that mass transfer (mean, 1.71+/-0.09 SEM, n=13 hearts) preceded significant increases in lattice spacing (2 to 5 nm) during systole in nonischemic pericardium. Left coronary occlusion eliminated increases in lattice spacing and severely reduced mass transfer (P<0.01) in the ischemic region. CONCLUSIONS: Our results suggest that x-ray diffraction techniques permit real-time in situ analysis of regional crossbridge dynamics at molecular and fiber levels that might also facilitate investigations of ventricular output regulation by the Frank-Starling mechanism.

Measurement of sarcoplasmic reticulum calcium content in skinned mammalian cardiac muscle.

We developed an optical system for the measurement of the Ca2+ content of sarcoplasmic reticulum (SR) in saponin-treated ventricular muscles of ferrets. After the SR was loaded with Ca2+ by activating the Ca2+ pump of SR, caffeine (50 mM) was applied to release the accumulated Ca2+ from the SR into the bathing solution containing the fluorescent Ca2+ indicator, Fluo-3. As Fluo-3, at high concentrations (approximately 200 microM), predominantly binds most of the Ca2+ released from the SR, the Fluo-3 fluorescence change upon Ca2+ binding gave an estimate of the amount of accumulated Ca2+ in SR before caffeine application. The maximal Ca2+ content of SR, thus estimated, was about 370 mumol/l cytoplasm. The amount of Ca2+ loaded in SR showed bell-shaped dependence on the free Ca2+ concentration ([Ca2+]) of the loading solution, reflecting Ca(2+)-induced Ca2+ release at high [Ca2+] (> or = 1 microM). Mg2+ and H+ decreased the rate of Ca2+ uptake by SR. The present system provides a relatively direct means of measurement of the Ca2+ content of SR, and allows examination of the effects of various interventions on SR Ca2+ uptake, bypassing the large influence of intracellular Ca2+ buffer sites.

Modulation of left and right ventricular beta-adrenergic receptors from spontaneously hypertensive rats with left ventricular hypertrophy and failure.

Inotropic responsiveness to beta-adrenergic stimulation is generally found to be depressed in cardiac hypertrophy and failure. To investigate whether inotropic responsiveness is associated with alterations in beta-adrenergic receptors in spontaneously hypertensive rats (SHR), we studied left ventricular myocardial contractile responses to isoproterenol and beta-adrenergic receptor density and affinity in age-matched rats (18 to 24 months), including SHR without heart failure, SHR with evidence of heart failure, and normotensive control Wistar-Kyoto rats (WKY). In the baseline state, papillary muscles from failing SHR demonstrated decreased isometric tension development and a reduction in maximal rate of tension development relative to normotensive WKY and compensated SHR. Compared with WKY, beta-adrenergic receptor density of the left ventricle was unchanged in nonfailing SHR and increased in failing SHR (P < .05 versus WKY and nonfailing SHR), and beta-adrenergic receptor affinity did not differ among groups. In the right ventricle, beta-adrenergic receptor density was decreased in failing SHR relative to WKY and nonfailing SHR, and beta-adrenergic receptor affinity was not different among groups. Muscle preparations did not exhibit a positive inotropic response to 10(-8) to 10(-5) mol/L isoproterenol or 6.3 mumol/L forskolin in either failing or nonfailing SHR, whereas a positive inotropic response to both drugs was observed in the normotensive WKY. The lusitropic response to isoproterenol and forskolin was intact and similar in both SHR groups and WKY. The findings suggest that in the SHR model of heart failure, impaired intrinsic left ventricular myocardial function and depressed inotropic responsiveness to beta-adrenergic stimulation are not associated with downregulation of the beta-adrenergic receptor.(ABSTRACT TRUNCATED AT 250 WORDS)

Peroxynitrite irreversibly decreases diastolic and systolic function in cardiac muscle.

Much of the damaging action of nitric oxide in heart may be due to its diffusion-limited reaction with superoxide to form peroxynitrite. Direct infusion of peroxynitrite into isolated perfused hearts fails to model the effects of in situ formation because the bulk of peroxynitrite decomposes before reaching the myocytes. To examine the direct effects of peroxynitrite on the contractile apparatus of the heart, we exposed intact and skinned rat papillary muscles to a steady state concentration of 4-microM peroxynitrite for 5 min, followed by a 30-min recovery period to monitor irreversible effects. In intact muscles developed force fell immediately to 26% of initial force, recovering to 43% by 30 min. Resting tension increased by 600% immediately, and was still elevated 500% by 30 min. Nitrotyrosine immunochemistry showed that peroxynitrite can induce tyrosine nitration at low concentrations and is capable of penetrating 200-380 microm into the papillary muscle after a 5-min infusion. Decomposed peroxynitrite had no effect on either intact or skinned muscle developed force or resting tension. Our results show that peroxynitrite directly damages both developed force and resting tension of isolated heart muscle, which can be extrapolated to systolic and diastolic injury in intact hearts.

Relation between myocardial beta-adrenergic receptor and left ventricular function in patients with left ventricular volume overload due to chronic mitral regurgitation with or without aortic regurgitation.

The relation between myocardial beta-adrenergic receptor and left ventricular (LV) function was studied in 10 patients, aged 41 to 61 years (average 51), with LV volume overload mainly due to chronic mitral regurgitation. Beta-adrenergic receptors were examined using crude membrane in LV papillary muscle obtained at mitral valve replacement. Cardiac function was evaluated at preoperative cardiac catheterization with the interval to surgery of 1 to 33 months (average 7). Beta-adrenergic receptor density in 7 patients with New York Heart Association class II or III congestive heart failure was higher than that in 3 patients with class IV heart failure (59 +/- 19 vs 37 +/- 3 fmol/mg protein, p less than 0.05). Beta-adrenergic receptor density showed a positive correlation with end-systolic stress/end-systolic volume index ratio, the index for contractile function (p less than 0.005). Other parameters such as cardiac index, pulmonary artery wedge pressure and LV ejection fraction had no significant correlation to beta-adrenergic receptor. The results demonstrated that increases in symptom and LV dysfunction, particularly of the contractile state, was associated with decreased myocardial beta-adrenergic receptor density from possible down-regulation in patients with chronic mitral regurgitation with or without aortic regurgitation.

Effects of captopril on contractility after myocardial infarction: experimental observations.

After large myocardial infarction, compromised left ventricular (LV) function and changes in the peripheral circulation result in the syndrome of chronic congestive heart failure. Although treatment with angiotensin-converting enzyme inhibitors improve cardiovascular function, it is difficult to determine whether this benefit is due to changes in organ versus muscle function. The rat model of heart failure, created by ligating the left coronary artery, results in pathophysiology that is similar to that seen in patients, i.e., increased LV end-diastolic pressure and volume, hypertrophy of the noninfarcted myocardium, prolongation of the time constant of LV relaxation, decreased venous compliance, and increased total blood volume. In noninfarcted papillary muscles, isolated from rats with heart failure, maximal developed tension and peak rate of tension rise (+dT/dt) are decreased, time to peak tension is prolonged, and myocardial stiffness is increased. Morphologic changes include an increase in papillary muscle myocyte cross-sectional area and an increase in myocardial hydroxyproline content. Captopril (2 g/liter drinking water) alters LV loading by decreasing arterial pressure, increasing venous compliance, and decreasing blood volume. This results in a decrease in LV end-diastolic pressure and volume. In the noninfarcted myocardium, time to peak tension is shortened, whereas developed tension, +dT/dt, and muscle stiffness remain abnormal. Captopril decreases myocyte cross-sectional area, but collagen content remains elevated. Thus, in the rat infarct model of heart failure, treatment with captopril alters LV remodeling and hypertrophy but produces only modest improvement in muscle function.(ABSTRACT TRUNCATED AT 250 WORDS)

Biochemical abnormalities in the heart of rats fed a sucrose-rich diet: is the low activity of the pyruvate dehydrogenase complex a result of increased fatty acid oxidation?

We have previously shown that normal Wistar rats fed for 3 weeks with an isocaloric sucrose-rich (63%) diet (SRD) develop high levels of plasma free fatty acids and increased triacylglycerol content in the myocardium. We are now reporting that these changes are accompanied by remarkably low levels of the active form of the pyruvate dehydrogenase complex (PDHa; mean +/- SEM, 37.2% +/- 3.7% of the total activity) when compared with levels found in hearts donated by control rats fed the standard chow diet (STD; 71.0% +/- 2.8%; P less than .01). Increased concentrations of both long-chain acyl-CoA (0.21 +/- 0.03 v 0.06 +/- 0.01 mumol.g dry weight-1 found in STD; P less than .01) and acetyl-CoA (0.17 +/- 0.05 v 0.09 +/- 0.01 found in STD; P less than .01), as well as a relative decrease in coenzyme A (CoASH) (0.21 +/- 0.02 v 0.32 +/- 0.05 from STD; P = NS), resulting in an increased acetyl-CoA/CoASH ratio (0.80 +/- 0.13 v 0.29 +/- 0.03 in STD; P less than .01) may have stimulated the PDH kinase, leading in turn to an inactivation of the PDH complex. The above enzymatic and metabolic changes in the in situ heart of SRD-fed rats were still present after perfusing them for 35 minutes with a Krebs-Henseleit buffer containing 11 mmol/L glucose as the only exogenous substrate.(ABSTRACT TRUNCATED AT 250 WORDS)

Increase in cardiac muscle fructose content in streptozotocin-induced diabetic rats.

To evaluate the activation of the sorbitol pathway in cardiac muscle in diabetic rats, we measured sorbitol, fructose, and myo-inositol content in cardiac tissue obtained from control and streptozotocin-diabetic rats, with or without an 8-week insulin treatment, using gas chromatography-mass spectrometry (GC-MS). Cardiac fructose and sorbitol content in 10-week diabetic rats increased by 60-fold and 3.9-fold of those of control rats, respectively (P less than .001). In contrast, cardiac myo-inositol content in 10-week diabetic rats decreased to 56% (P less than .025) of the control value. The abnormalities in cardiac fructose, sorbitol, and myo-inositol content were completely normalized by the 8-week insulin treatment, which was initiated 2 weeks after the induction of diabetes. There was no difference in cardiac aldose reductase activity between control and diabetic rats. However, cardiac sorbitol dehydrogenase activity in diabetic rats was 151% (P less than .005) higher than that of control rats, although hepatic sorbitol dehydrogenase activity was not different between the two groups. These results indicate that the sorbitol pathway is significantly activated in cardiac tissue obtained from streptozotocin-induced diabetic rats, which results in the marked cardiac accumulation of fructose.

Myoglobin content and citrate synthase activity in different parts of the normal human heart.

Myoglobin (Mb) content and citrate synthase (CS) activity were determined in myocardial samples from nine human brain-dead organ donors with normal hearts. Six regions of each heart were analyzed: right and left atria, right ventricle, left ventricular subepicardium, subendocardium, and anterior papillary muscle. The Mb content was similar, whereas the CS activity was higher in the left than in the right heart at both atrial and ventricular levels. Mb content and CS activity were higher in ventricles than in atria. The subendocardial layer and papillary muscle of the left ventricle had a higher Mb content than the subepicardial layer, whereas CS activity was similar in these three locations. The results suggested a closer relationship between CS activity (oxidative potential) and work load than between Mb content and work load. Mb content may, instead, be related to intramuscular oxygen tension (PO2) on the basis of a comparison between our Mb data and those of others on regional variations in myocardial PO2.

Contractile properties of thin (actin) filament-reconstituted muscle fibers.

Selective removal and reconstitution of the components of muscle fibers (fibrils) is a useful means of examining the molecular mechanism underlying the formation of the contractile apparatus. In addition, this approach is powerful for examining the structure-function relationship of a specific component of the contractile system. In previous studies, we have achieved the partial structural and functional reconstitution of thin filaments in the skeletal contractile apparatus and full reconstitution in the cardiac contractile apparatus. First, all thin filaments other than short fragments at the Z line were removed by treatment with plasma gelsolin, an actin filament-severing protein. Under these conditions, no active tension could be generated. By incorporating exogenous actin into these thin filament-free fibers, actin filaments were reconstituted by polymerization on the short actin fragments remaining at the Z line, and active tension, which was insensitive to Ca2+, was restored. The active tension after the reconstitution of thin filaments reached as high as 30% of the original level in skeletal muscle, while it reached 140% in cardiac muscle. The augmentation of tension in cardiac muscle is mainly attributable to the elongation of reconstituted filaments, longer than the average length of thin filaments in an intact muscle. These results indicate that a muscle contractile apparatus with a high order structure and function can be constructed by the self-assembly of constituent proteins. Recently, we applied this reconstitution system to the study of the mechanism of spontaneous oscillatory contraction (SPOC) in thin (actin) filament-reconstituted cardiac muscle fibers. As a result, we found that SPOC occurs even in regulatory protein-free actin filament-reconstituted fibers (Fujita & Ishiwata, manuscript submitted), although the SPOC conditions were slightly different from the standard SPOC conditions. This result strongly suggests that spontaneous oscillation is intrinsic to actomyosin motors. We here summarize the contractile properties of the reconstitution system.

Cyclic AMP-receptor responses to hypergravity.

BACKGROUND: Altered gravity (G) encountered during spaceflight causes physiologic changes in humans and in experimental animals. In addition to weightlessness (0G) in space, sharply increased G forces are exerted on the spacecraft during the lift-off and reentry phases. Previous studies showed major changes in cAMP-associated activity of rat heart muscle after spaceflight, indicating that (hormone) signaling pathways may have been affected. HYPOTHESIS: The present study was designed to test the hypothesis that cAMP-related cellular responses of exocrine glands after simulated hypergravity (centrifugation at 1.7G) differ from the effects of 0G. METHODS: A portion of the parotid and lachrymal gland tissue was fixed for morphologic and immunocytochemical study, and another was used for biochemical determinations. A short-term tissue culture was established from each gland to determine the effects of stimulation by norepinephrine. Heart muscle (ventricle) was also studied. Soluble and particulate fraction extracts of tissue homogenates were prepared, photoaffinity labeled with the [32P]8-N3-analog of cAMP, proteins separated by electrophoresis and the cAMP-reactive proteins (cARP) identified by autoradiography. RESULTS: Differences were seen in protein banding patterns of the gland extracts and in altered cARP distribution in the 1.7G samples of heart ventricle and exocrine gland tissues, when compared with 1G controls. In the heart, cARP increased in the soluble fraction, while the particulate fraction extract showed no change. In acinar cells of the parotid, labeled cARP had accumulated, but decreased after stimulation to the level of the 1G controls. Immunogold labeling showed an increased content of amylase in the secretory granules of the 1.7G animals, while morphologic observation revealed few changes in the structure of parotid acinar cells. The response in the lachrymal gland was translocation of an isoform of cARP from the particulate to the cytoplasmic compartment. CONCLUSIONS: Changes distinct from those due to 0G, but specific for hyper-G were found in cARP activity, protein synthesis, as well as in an apparent inhibition of regulated secretion.

Zinc in rheumatic heart valves.

This study included 48 patients with chronic rheumatic heart disease, 60 control subjects for plasma zinc comparison and 20 control specimens of heart valves from postmortem cases of accident deaths. Plasma and cardiac tissue levels of zinc in patients with rheumatic heart disease were significantly lowered compared to controls. Since zinc is important in the synthesis of nucleic acids and proteins it may influence tissue growth, reparative process and structure and function of biomembrane. Low zinc levels may also influence cell mediated immunity and may increase susceptibility of patients to infection and increased rheumatic activity which needs further study.

[Potassium deficiency in rat cardiomyocytes after whole-body hyperthermia]. / Issledovanie kalievogo defitsita v kardiomiotsite krysy posle gipertermii vsego organizma.

Potassium distribution and content were studied in different compartments of rat heart papillary muscle by X-ray microanalysis. A higher concentration of potassium was measured in normal rat as compared to that in animals treated with high physiological temperature (45 degrees C), to be 120 and 80 mM, respectively.

[A comparative analysis of the intracellular potassium content for the cardiomyocyte in papillary muscle tissue and in a primary heart culture]. / Sravnitel'nyi analiz vnutrikletochnogo soderzhaniia kaliia dlia kardiomiotsita v tkani papilliarnoi myshtsy i v pervichnoi kul'ture serdtsa.

This work was aimed to examine potassium contents in heart myocytes. The intracellular concentration of this element was measured with electron probe microanalysis (EPMA). The cardiomyocyte was studied in samples of the papillary muscle tissue and in heart primary culture. The low temperature fixation was applied to the sample preparation. As a result, cells of the primary heart culture were shown to be separated into three kinds of myocytes, defined by their different shape, size and cellular potassium content. The rod cardial cell, to be used for electrophysiological assays, has potassium and sodium concentrations close to those of the papillary muscle. The difference in potassium concentrations is to be explained presumably by the injury of cell membrane during the primary culture preparation. Healing over the membrane is likely to parallel with the membrane damage resulting from a continuous destruction of the cellular assembly in primary culture.

[The effect of elevated environmental temperature on the development of a potassium deficiency in the body]. / Vliianie povyshennoi temperatury okruzhaiushchei sredy na razvitie defitsita kaliia v organizme.

Electron microscopy and roentgen microanalysis were used to study the ultrastructure and electrolytic balance of K+, Na+ and Cl- of the animal myocardial tissue during general body hyperthermia. It was established that long-term effect of heating microclimate causes a distinct reduction of K+ in the cardiomyocytes accompanied by essential changes of the ultrastructure of ischemic character.

Down regulation of immuno-detectable cardiac connexin-43 in BALB/c mice following acute fasting.

Acute starvation effects for connexin-43 protein expression, in the heart, had not been previously explored. Hence we examined acute fasting on the myocardial immuno-histochemical expression of connexin-43 in 3 groups of 8-week old female BALB/c mice. Groups consisted of control mice (n=5), fasting for 24 h (N=5) and 48 h (N=3). Under light microscopy all control fed cases revealed the presence of some immuno-detectable staining for connexin-43 that is either present or weakly observed in some or all of the regions of interest, that include the cross-sectional left ventricular sub-endocardium, mid-myocardium and papillary muscle. Whereas mice that underwent 24 or 48 h of acute starvation, connexin-43 expression was either difficult to detect visually (N=3) or was completely absent (N=5) at 40x magnification using a light microscope. In starved mice with no membrane staining for connexin-43 we observed an increase in the intracellular accumulation of cytoplasmic connexin-43 expression.

Usefulness of cardiac calcification on two-dimensional echocardiography for distinguishing ischaemic from nonischaemic dilated cardiomyopathy: a preliminary report.

BACKGROUND: Aortic valve calcification (AVC) and/or mitral annulus calcification (MAC) is considered to be a marker of atherosclerosis and has been demonstrated to predict cardiovascular morbidity and mortality. AIM: We hypothesized that the presence of cardiac calcification by echocardiography can be used in the differential diagnosis between ischaemic (DCMI+) and nonischaemic dilated cardiomyopathy (DCMI-). METHODS: We evaluated 62 patients with DCM (38 males, mean age 66 +/- 10 years, LVEF < 40%), without any prior history of myocardial infarction or coronary intervention, who were undergoing coronary angiography for aetiological diagnosis. DCMI+ was considered present when a > or = 70% stenosis of at least one coronary artery was found. AVC, MAC, aortic wall and papillary muscle calcifications were semiquantitatively assessed by two-dimensional echocardiographic examination with a calcium score ranging from 0 (no calcifications) to 8 (calcium in all four sites). RESULTS: DCMI+ was found in 20 out of 62 patients. As expected, there were no differences in LVEF and LV end-diastolic diameters between DCMI+ and DCMI--patients (29 +/- 8% versus 31 +/- 10% and 66 +/- 6 versus 68 +/- 8 mm, respectively; not significant). Regional wall motion abnormalities and conventional risk factors for atherosclerosis, such as hypertension and hypercholesterolaemia, were significantly more frequent in the DCMI+ compared to the DCMI- group. On the other hand, the calcium echo score was 4.6 +/- 2 (range 1.7-7.3) in DCMI+ patients and 0.8 +/- 0.95 (range 0-4) in DCMI--patients (P < 0.05). A calcium score > or = 3 was observed in 18 out of 20 (90%) DCMI+ patients and only in three of 42 (8%) DCMI--patients. CONCLUSIONS: The assessment of cardiac calcification by two-dimensional echocardiography could represent a simple, noninvasive and inexpensive approach to assess the aetiology (ischaemic versus nonischaemic) of dilated cardiomyopathy.

Dynamics of crossbridge-mediated activation in the heart.

Both intracellular calcium and strongly bound crossbridges contribute to thin filament activation in the heart, but the magnitude and the duration of the effects due to crossbridges are not well characterized. In this study, crossbridge attachment was altered in tetanized ferret papillary muscles and changes in the rate constant for the recovery of force (k (TR)) and unloaded shortening velocity (V (U)) were measured to track thin filament activation. k (TR) decreased as the time the muscles spent at low levels of crossbridge attachment (shortening deactivation) increased (0.02 s=17.9+/-2.3 s(-1), 0.32 s=3.3+/-0.4 s(-1); half-time=0.052 s; P<0.05). Furthermore, the deactivation was reversible and k (TR) recovered when muscles were allowed to regenerate force isometrically during the same tetanus. V (U) also decreased when the preceding load was lower (isometric load, V (U)=1.93+/-0.26 muscle lengths/s (ML/s); zero load, V (U)=0.93+/-0.14 ML/s, P<0.05) and as the length of time the muscle spent unloaded increased (>60% decline after 0.3 s). In addition, V (U) recovered when the muscle was allowed to regenerate force isometrically. These results indicate that crossbridge attachment increases thin filament activation as reflected in measurements of V (U) and k (TR). This 'extra' activation by crossbridges appears to be a dynamic process that decays during unloaded shortening and redevelops during isometric contraction.

Study of potassium deficiency in cardiac muscle: quantitative X-ray microanalysis and cryotechniques.

An imbalance of potassium in cardiac muscle causes an alteration of heart function. The distribution and concentration of potassium in rat papillary heart muscle was studied using cryofixation and X-ray microanalysis. Freeze-dried cryosections and sections of freeze-dried, embedded tissue were analysed. Bulk frozen specimens were freeze-dried either in a vacuum or by a new technique using liquid propane as a cryodehydration medium. These two methods of freeze-drying were tested for elemental retention in other specimens, with comparable results. A potassium concentration of 120 mmol/l was measured in normal myocytes of cardiac papillary muscle compared to 80 mmol/l in myocytes of animals stressed by a temperature of 45 degrees C for 1 h. The presumed physiological significance of the findings is discussed.