GLUT2 expression and function in beta-cells of GK rats with NIDDM. Dissociation between reductions in glucose transport and glucose-stimulated insulin secretion

GLUT2 underexpression has been reported in the beta-cells of Zucker diabetic fatty rats and db/db mice, models of spontaneously occurring NIDDM with antecedent obesity. To determine whether the beta-cells of a nonobese rodent model of NIDDM exhibit the same abnormalities in GLUT2, we studied Goto-Kakizaki rats. In these mildly diabetic animals glucose-stimulated insulin secretion was reduced at all ages examined from 8 to 48 wk. In normal control Wistar rats, immunostainable GLUT2 was present on all insulin-positive cells in the pancreatic islets. Only 85% of beta-cells were GLUT2-positive in GK rats at 12 wk of age, and only 34% were positive at 48 wk of age. GLUT2 mRNA was 50% of normal in 12-wk-old GK rats. In the latter age-group, glucose-stimulated insulin secretion was only 28% of normal at a time when 85% of beta-cells were GLUT2-positive and initial 3-O-methyl-D-glucose transport rate was 77% of the control value. We conclude that although GLUT2 is underexpressed, neither the magnitude of the underexpression of GLUT2 nor of the reduction in GLUT2 transport function in islets of GK rats is sufficient by itself to explain the profound reduction in glucose-stimulated insulin secretion.

Multiple coronary artery dissections diagnosed in vivo in a pregnant woman

A 27-year-old, 37-weeks' primigravida survived an acute myocardial infarction. Coronary angiography 25 days after infarction demonstrated multiple dissections in the left coronary artery. She had no vascular risk factors or connective tissue disease. This is a rare case of spontaneous, multiple coronary artery dissections that were diagnosed by coronary angiography.

Characterization of the stable, acid-induced, molten globule-like state of staphylococcal nuclease

Titration of a salt-free solution of native staphylococcal nuclease by HCl leads to an unfolding transition in the vicinity of pH 4, as determined by near- and far-UV circular dichroism. At pH 2-3, the protein is substantially unfolded. The addition of further HCl results in a second transition, this one to a more structured species (the A state) with the properties of an expanded molten globule, namely substantial secondary structure, little or no tertiary structure, relatively compact size as determined by hydrodynamic radius, and the ability to bind the hydrophobic dye 1-anilino-8-naphthalene sulfonic acid. The addition of anions, in the form of neutral salts, to the acid-unfolded state at pH 2 also causes a transition leading to the A state. Fourier transform infrared analysis of the amide I band was used to compare the amount and type of secondary structure in the native and A states. A significant decrease in alpha-helix structure, with a corresponding increase in beta or extended structure, was observed in the A state, compared to the native state. A model to account for such compact denatured states is proposed.

Increased generation of reactive oxygen species in embryos cultured in vitro

Previous studies have suggested that oxygen toxicity is closely related to the developmental blockage of embryos cultured in vitro. In this study, to obtain an actual proof of the increase in production of reactive oxygen species within embryos, we have measured the level of H2O2 in individual embryos using a fluorimetric method. Mouse (ICR) pronuclear stage embryos from the oviducts were cultured for a specified time under various conditions in a medium to which 2',7'-dichlorodihydrofluorescin diacetate was added. After washing the embryos, the fluorescence emissions of the H2O2-dependent oxidative product in embryos were measured. The fluorescent emissions were lowest in embryos cultured under 5% O2 and highest under 40% O2 (5% < 20% < 40%), just the inverse of the culture efficacy. The fluorescence emmissions of embryos cultured in Ham's F-10, which contains hypoxanthine and transition metals such as Cu and Fe, were higher than those cultured in BWW and alpha MEM, which do not contain these components (alpha MEM < BWW < Ham's F-10; again this is the inverse of the culture efficacy). The fluorescence emissions of embryos increased with the time of the exposure to visible light. L-cysteine and thioredoxin, both of which have been shown to promote embryo development, decreased the fluorescence emissions of embryos. All of these results would provide direct evidence for the hypothesis that oxygen radicals are involved in the developmental blockage.

Physiological correlates of abnormal behaviors in magnesium-deficient rats

In order to elucidate the mechanism of behavioral alterations in magnesium-deficient rats, changes in the electroencephalogram (EEG) and electrocardiogram (ECG) were studied during auditory stimulation and correlated with the behavioral alterations. Weanling rats were fed either a Mg-deficient diet or a control synthetic diet for 2-3 weeks before the experiment. EEGs were recorded from the hippocampus and the sensorimotor and auditory cortices, and ECGs with a telemetry system. White noise with an intensity of 100 dB was given continuously to induce behavioral changes. The Mg-deficient rats developed consistent and graded behavioral changes in response to the stimulation, showing running-jumping behavior (stage 1), followed by tonic limb convulsion (stage 2) and finally by falling down on the floor (stage 3). The EEGs also showed consistent changes with spike activity, initiating in the hippocampus (stage 2) and then spreading to the neocortices bilaterally (stage 3). These findings indicate that the behavioral changes induced by auditory stimulation in the Mg-deficient rats are due to seizures arising in deeper brain structures, particularly in the limbic system, and projecting secondarily to the neocortices. The ECG changes, mainly consisting of marked bradyarrhythmia, occurred as early as the appearance of the EEG spikes, indicating that they were also related to the seizure. We conclude therefore that Mg deficiency in rats causes increased excitability of the central nervous system, resulting in seizures possibly originated in the limbic system, later developing secondary generalization, and also causing cardiac dysfunctions.

Effects of intraischemic hypothermia on cerebral damage in a model of reversible focal ischemia

Considerable evidence indicates that brain temperature during ischemia affects the extent and distribution of ischemic injury. However, only limited data have been presented concerning the influence of temperature on ischemic damage after reversible focal cerebral ischemia. Because focal ischemic events of this type resemble conditions observed in the clinic, studies were undertaken to examine the effects of mild and moderate hypothermia on the extent of cerebral infarction after focal neocortical ischemia. Under halothane anesthesia, the left middle cerebral artery and both carotid arteries were occluded reversibly for a period of 3 hours in adult Sprague-Dawley rats. The animals were killed 3 days later. Brain sections were stained with triphenyltetrazolium chloride and analyzed for infarction using a computerized image analysis system. Temporal muscle temperature and rectal temperature were monitored continuously. The following groups with different intraischemic temporal muscle temperatures were analyzed: 1) control, 35.8 to 36.2 degrees C; 2) mild hypothermia, 33.0 to 33.5 degrees C; and 3) moderate hypothermia, 27.5 to 29.2 degrees C. The volumes of infarction were 214.5 +/- 17.9, 166.5 +/- 6.8, and 108.2 +/- 5.9 mm3 (mean +/- SEM) for the control, mild hypothermia, and moderate hypothermia groups, respectively. These findings demonstrate that both mild and moderate hypothermia reduce the impact of temporary focal ischemia in Sprague-Dawley rats.

[Interaction of cardiovascular effect of calcium channel blocking drugs and those of inhalation anesthetics in humans]

The hemodynamics in the patients undergoing craniotomy was studied to evaluate the interaction of cardiovascular effects of calcium blocking drugs (nicardipine NCR or diltiazem DL) and those of inhalation anesthetics (0.6% isoflurane Iso or 0.9% sevoflurane Sevo). (Group I = NCR alone, Group II = NCR+Iso, Group III = NCR+Sevo, Group IV = DL alone, Group V = DL+Iso, Group VI = DL+Sevo) Anesthesia was maintained with neuroleptanesthesia (N2O 60%). A 30% reduction in systolic blood pressure was achieved and this was maintained for 60 minutes in each groups. The following effects were observed during induced hypotension. 1) Heart rate increased in the group I and II, but decreased in the group IV, V and VI. 2) Cardiac output increased in the group I and II, but significantly decreased in the group VI. 3) Systemic vascular resistance and left ventricular stroke work decreased in all the groups. 4) Pulmonary vascular resistance decreased slightly in all the groups. 5) Cardiac arrhythmias were observed in 4 cases of the group IV and VI. These results suggest that the presence of Iso or Sevo at low concentrations does not have a marked effect on the pharmacologic action of NCR or DL in humans.

Changing chemosusceptibility in the second-stage larvae of Toxocara canis by long-term incubation

Second-stage larvae of Toxocara canis were maintained in vitro for one year. Susceptibility of the larvae to drugs was evaluated by means of minimal larvicidal concentration (MLC) and larval bursting percentage. MLCs of citral and decanoic acid were almost constant throughout all stages of incubation. However, bursting percentage markedly varied within the first 20 weeks of incubation. Therefore, while larvae are available for use in the MLC assay at any stage of incubation, those beyond the first 20 weeks after incubation should be used for the bursting assay to obtain reproducible results.

Guanidine hydrochloride-induced folding of proteins

Guanidine hydrochloride (Gdn-HCl) is the most commonly used denaturant for proteins. Contrary to expectation, we found that Gdn-HCl at low concentrations refolds acid-unfolded apomyoglobin and cytochrome c, stabilizing the molten globule state, i.e. a compact denatured state with a significant amount of secondary structure, but substantially disordered tertiary structure. A further increase in Gdn-HCl concentration, above 1 M, caused co-operative unfolding of the molten globule state. Similar sequential folding and unfolding transitions were also observed at neutral pH with a synthetic amphiphilic peptide consisting of Lys and Leu residues, indicating the generality of the phenomenon. Although the Gdn-HCl-induced refolding and unfolding transitions were puzzling at first glance, we show that they are readily interpreted in terms of the differential action of Gdn-HCl. We also show that the comparison of the unfolding curves for the molten globule and native states provides a measure of the buried surface area upon formation of the molten globule state.

Cavernous hemangioma in the cavernous sinus

Cavernous hemangioma, a rare vascular hamartoma, in the cavernous sinus is described in this report. This case showed a slowly progressive course, which first involved the right optic nerve, followed by the left optic nerve, and then the right oculomotor, trochlear, abducens, and trigeminal nerves. Cavernous hemangioma should be suspected in the case of mass in the cavernous sinus with a very slowly progressive course. MRI is very useful to help diagnose this hemangioma.

Biochemical remodeling of collagen in the heart of spontaneously hypertensive rats--prominent increase in type V collagen

To determine how collagen is remodeled in the heart during the development of cardiac hypertrophy, the collagen concentration and the proportions of types I, III, and V collagen were analyzed in the hearts of 10-, 20-, 32-, and 40-week-old spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats. The collagen concentration was calculated after the hydroxyproline content was measured and the proportions of types I, III, and V collagen were determined by non-interrupted SDS-polyacrylamide gel electrophoresis (SDS-PAGE). There was no significant difference between the collagen concentrations in 10-week-old SHR and WKY. At 20 weeks, the collagen concentration in the hearts of the SHR had decreased significantly (p < 0.01) when compared to that in the WKY, at 40 weeks, this concentration in SHR had increased significantly (p < 0.05) when compared to that in the WKY. Type V collagen in WKY increased with age and type I collagen in the 20- and 40-week-old WKY decreased significantly (p < 0.05) when compared to that in the 10-week-old WKY. However, the proportion of type I collagen in the 20-week-old WKY did not differ from that of the 40-week-old WKY. There was no significant difference between the proportions of the various types of collagen in 10-week-old SHR and those in age- and sex-matched WKY. However, the proportion of type V collagen in the 20- and 40-week-old SHR was significantly (p < 0.01) higher than that in age- and sex-matched WKY.(ABSTRACT TRUNCATED AT 250 WORDS)

Coupling between regional myocardial oxygen consumption and contraction under altered preload and afterload

OBJECTIVES

This study was designed to assess the relation between left ventricular regional myocardial oxygen consumption (VO2) and variables of regional myocardial contractile function under various loading conditions.

BACKGROUND

Although the relation between global VO2 and global ventricular function has been extensively studied, the relation between regional VO2 and regional myocardial contraction is not fully understood.

METHODS

Myocardial shortening (regional area shrinkage), regional work, regional total mechanical energy index and regional VO2 were measured under variously altered loading conditions in the isolated, blood-perfused dog left ventricle. Regional total mechanical energy per beat was quantified by wall tension-regional area area (TAA) by the analogy of left ventricular pressure-volume area. Left ventricular loading conditions were altered by changing end-diastolic volume and stroke volume with a servo pump as follows: 1) increased preload (increased end-diastolic volume and stroke volume at a constant ejection fraction), 2) decreased afterload (increased stroke volume at a constant end-diastolic volume), 3) increased preload and afterload (increased end-diastolic volume at a constant stroke volume), and 4) altered mode of contraction (ejecting vs. isovolumetric contractions).

RESULTS

During increased preload, all three variables correlated positively with regional VO2 (r = 0.78 to 1.00). During decreased afterload, the correlation was negative for area shrinkage (r = -0.65 to -0.91) and variable for regional work (r = -0.55 to 0.98) but positive and highly linear for TAA (r = 0.80 to 0.99). During increased preload and afterload, the correlation was again negative for area shrinkage (r = -0.77 to -0.97) but positive for regional work (r = 0.83 to 0.93) and TAA (r = 0.95 to 0.99). During altered mode of contraction, the correlation was insignificant for area shrinkage (r = 0.24 to 0.57) and moderate for regional work (r = 0.50 to 0.79), whereas again highly linear for TAA (r = 0.95 to 0.98). Thus, only TAA correlated closely with regional VO2 under any loading conditions. Furthermore, the slope and regional VO2 intercept of the regional VO2-TAA relation was remarkably consistent among the different hearts and loading conditions.

CONCLUSIONS

We conclude that there is a tight coupling between regional VO2 and regional total mechanical energy represented by TAA regardless of left ventricular afterload and preload conditions.

Isolated fracture-dislocation of the sacrum: case report

An unusual case of isolated fracture-dislocation of the sacrum is reported. A 19-year-old woman had a fracture-dislocation of S-1-2 combined with two longitudinal fractures of the sacrum, but no other pelvic fractures were identified. The possible mechanisms of fracture-dislocation of the sacrum are discussed. The fracture site was identified accurately on a CT scan. Pain relief was obtained by application of an external fixator. A mild neurologic deficit in the S-1 root distribution disappeared uneventfully.

Inhibitory effect of amiloride on glucose transport in isolated rat adipocytes

The effect of amiloride on 3-O-methylglucose (3-O-MG) uptake was studied in isolated rat adipocytes to define to what extent amiloride inhibited the process of insulin action or glucose transport. Amiloride (1 mM), which did not change the intracellular water space of adipocytes, inhibited by 43.3% the insulin-stimulated uptake of 3-O-MG, while it did not appear to inhibit the basal uptake. To distinguish the inhibitory effect on glucose transport activity from that on the process of insulin action, the effect of amiloride was evaluated in the transport system using adipocytes deprived of ATP, in which glucose transporters were considered immobile. Amiloride (1 mM) inhibited this transport by 32.8% in an insulin-stimulated state, which was obtained using adipocytes that had been treated with 20 nM insulin and exposed to 2 mM KCN, whereas it did not inhibit the transport system at the basal state. In the inhibitory effect, 76% was thus attributable to the inhibition of glucose transport activity recruited by insulin and 24% to the inhibition of the action of 20 nM insulin itself. These results indicate that amiloride can not be used as a specific inhibitor of the insulin action itself.

Mitochondrial DNA mutations at nucleotide positions 3243 and 3271 in mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes: a comparative study

Of 50 patients with the clinical characteristics of mitochondrial myopathy, encephalopathy, lactic acidosis, and stroke-like episodes (MELAS), 38 had a point mutation at nucleotide position (nt) 3243 in the tRNA(Leu(UUR)) region in mitochondrial DNA and 6 at nt 3271 in the same tRNA(Leu(UUR)) gene. Except for the later onset of the disease in the patients with the 3271 mutation, there were no clinical, biochemical and pathological differences between the two groups. Since the nt 3271 region is not located in the binding site for mitochondrial transcription termination (mTERM) factor, which has been proposed to be defective in the 3243 mutation, a functional defect in tRNA itself might be responsible for the enzyme defects in MELAS patients; however the mechanism by which the defective tRNA(Leu(UUR)) induces the stroke-like episodes remains to be clarified.

The fate of embryos transferred into the uterus

PURPOSE

The purpose of this study is to elucidate the fate of embryos transferred into the uterus.

METHODS

Implantation rates were compared between synchronous and asynchronous intrauterine embryo transfers in mice. Then transferred embryos were recovered from the uterus and examined morphologically 24 hr after transfer. Moreover, the same transfer-recovery experiments were performed after ligation of the uterus two-thirds from the uterotubal junction immediately before transfer.

RESULTS

The implantation rate was high (58.4% per embryo, 87.5% per recipient) when blastocysts were transferred into the uteri of recipients of pseudopregnant Day 4, but it was very low (0-2.4% per embryo, 0-8.3% per recipient) when blastocysts were transferred to recipients of pseudopregnant Day 2, or two-cell embryos into recipients of pseudopregnant Days 2 and 4. When transferred embryos were recovered from the uterus 24 hr after transfer, few embryos (1.4%) were recovered in asynchronous transfers, while 66.3% of the embryos were recovered in synchronous transfers. However, from recipients in which the lower portions of uteri were ligated, embryos at more advanced stages were recovered even in asynchronous transfers.

CONCLUSIONS

These results suggest that discharge from the uterus is one of the major causes of the low implantation rate for asynchronous transfer.

[Diabetic macroangiopathy in the elderly]

Autopsy cases from Annual Report of Autopsy Cases published by the Japan Society of Pathology showed an increase of myocardial infarction from 13.2% in the years 1958-65 to 26.6% in the years 1983-85. However, cerebral infarction and hemorrhage showed no definite increase during the period. Frequency of diabetes mellitus was extremely increased among the cases of myocardial infarction who were admitted to our hospital. Arteriographic characteristics of coronary atherosclerosis in diabetics consist of multiple tight stenoses in one major artery and two or three arterial obstructions. Carotid arterial blood flow and plaque formation and calcification of the arteries were examined by doppler imaging technology and B-mode (5 MHz) real time ultrasound using ultrasonographic equipment in diabetic patients. Both blood flow volume and blood flow velocity in the elderly patients with diabetes mellitus (over 65 years old) were significantly reduced compared with those in the younger patients with diabetes mellitus (7.4 +/- 0.4 vs. 8.5 +/- 0.2 in blood flow volumes. p < 0.01; 12.4 +/- 0.8 in blood flow velocities. p < 0.01). Plaque formation and calcification of carotid arteries were significantly more frequent in the elderly patients with diabetes mellitus than in the younger patients with diabetes mellitus (p < 0.05). Asymptomatic cerebral infarction was studied in 37 diabetic patients by brain magnetic resonance imaging (MRI) in the absence of prior stroke. T2 weighted MRI imaging showed 27 patients among 37 patients (73%) to suffer from lacunar infarction. Hyperintensities were seen in the brain stem (28.6%), white matter (62.9%), basal ganglia (60.0%), and paraventricular areas (PVH) (20.0%).(ABSTRACT TRUNCATED AT 250 WORDS)

Criteria for screening diabetes mellitus using serum fructosamine level and fasting plasma glucose level. The Japanese Society of Multiphasic Health Testing and Services (JMHT), Fructosamine Working Committee

A screening method using serum fructosamine level and the fasting plasma glucose level was used for screening patients with diabetes mellitus. The criteria for positive tests recommended by the Japanese Society of Multiphasic Health Testing and Services were evaluated. It was found that levels for the serum fructosamine of 290 mumol/l or higher (or, for the fasting plasma glucose of 110 mg/dl or higher) agreed with the standard oral glucose tolerance test in identifying patients with diabetes mellitus in 96.7% of cases, and the serum fructosamine test was simpler and less expensive.

Pathological mineralization in a serially passaged cell line from rat pulp

The ultrastructure of crystal formation in association with dental pulp cells isolated from rat incisor was studied in vitro. A clone, RPC-K, was obtained and incubated with Na-beta-glycerophosphate (BGP). Growing pulp cells showed low alkaline phosphatase (ALP) activity, which began to increase with cell proliferation. Pulp cells formed cell multilayers after day 14 of culture. Mineralized tissues were observed within cell multilayers on day 28 of culture. Vesicular structures were found around degenerate and necrotic cells. Some of these vesicles contained needle-like crystals. Organic structures appeared at the periphery of mineralized tissues with a post-embedding demineralization and staining method. Electron diffraction patterns of the newly formed crystals revealed a pattern consistent with hydroxyapatite (HAP). These findings suggest that the RPC-K cell line might be useful for a model system to investigate pathological mineralization.

Determinants of myocardial oxygen consumption in fibrillating dog hearts. Comparison between normothermia and hypothermia

The purpose of the present study was to elucidate the mechanism of the difference in myocardial oxygen consumption between heating and fibrillating states during normothermia and hypothermia. In five isolated cross-circulated dog hearts, we measured left ventricular pressure at several ventricular volumes and myocardial oxygen consumption at V0 and V100, at which peak isovolumic pressures were zero and approximately 100 mm Hg, respectively, in beating and fibrillating states during normothermia and hypothermia (29 degrees C). As a measure of the total mechanical energy at V100, we obtained pressure-volume area in the beating state and equivalent pressure-volume area for fibrillation. We calculated equivalent heart rate as an estimate of the contraction frequency of individual myocytes in a fibrillating ventricle from myocardial oxygen consumption at V0 in the beating and fibrillating states. During normothermia, myocardial oxygen consumption per minute at V0 and V100 and myocardial oxygen consumption for mechanical purposes at V100 (myocardial oxygen consumption at V100-myocardial oxygen consumption at V0) were significantly higher during fibrillation than in the beating state. Equivalent pressure-volume area during fibrillation and pressure-volume area in the beating state at V100 were comparable, whereas equivalent heart rate during fibrillation was significantly higher than heart rate in the beating state. During hypothermia, myocardial oxygen consumption was comparable between beating and fibrillating states at V0, although myocardial oxygen consumption at V100 was slightly lower during fibrillation than in the beating state. Myocardial oxygen consumption for mechanical purposes during fibrillation was half of that in the beating state. Equivalent pressure-volume area was significantly smaller than pressure-volume area, whereas equivalent heart rate and heart rate were comparable. We conclude that during normothermia, higher myocardial oxygen consumption during fibrillation than in the beating state at V0 and V100 is attributable to the higher contraction frequency. During hypothermia the comparable myocardial oxygen consumption values at V0 are attributable to the comparable contraction frequencies, whereas slightly lower myocardial oxygen consumption during fibrillation at V100 is ascribed to the lower total mechanical energy.

Intra-operative blood pressure control by prostaglandin E1 in patients with hypertension and ischemic heart disease--a multi-center study

The purpose of this multi-center study was to evaluate the efficacy and safety of prostaglandin E1 (PGE1) administration in achieving deliberate hypotension and in treating intraoperative hypertension for patients with a history of hypertension and ischemic heart disease. PGE1 (0.08 microg.kg(-1).min(-1)) decreased systolic blood pressure from 125 +/- 29 to 106 +/- 22 mmHg (mean +/- SD) in the deliberate hypotension group (n = 158) and from 155 +/- 34 to 125 +/- 32 mmHg in the antihypertension group (n = 55). The heart rate significantly increased from 80 +/- 15 to 85 +/- 18 beats.min(-1) in the deliberate hypotension group, but was not significantly altered in the antihypertension group. The time required to obtain the desired level of blood pressure was approximately 20 min in the deliberate hypotension group. When the infusion was stopped, blood pressure returned approximately to the preinfusion level within about 20 min. No rebound hypertension was observed. PGE1 significantly increased the urine flow in patients who had a low urine flow before PGE1 infusion. Thirteen out of 213 patients (5.6%) had side effects such as excessive hypotension (1%), phlebitis (3%), and unexpected tachycardia (1%), which were alleviated gradually after discontinuation of PGE1 infusion. No dysarrhythmia and further ST segment changes in the electrocardiograms were observed. These findings suggest that PGE1 can be safely used to control arterial blood pressure during surgery in patients having preoperative hypertension and ischemic heart disease.

[The microcirculatory dynamics of prostaglandin E1 and/or nicardipine and their different reactions in the hyper- and hypodynamic state of septic shock in a rat model]

The reactions of the mesenteric microcirculation dynamics to prostaglandin E1(PGE1) and nicardipine were investigated under continuous biomicroscopy with a high speed video analyzer in septic shock models using rats. Both of the drugs induced similar reactions. In the hyperdynamic state, the vasodilation and increasing velocity of red blood cells (RBC) stimulated by both of these vasodilators were similar to those in normal control state. However, in the hypodynamic state the vasodilators especially PGE1 had little effect on increasing the velocity of RBC in spite of their vasodilating action. Therefore these vasodilators are useful in order to improve the microcirculatory blood flow in states such as early septic-shock state.

Variable cross-bridge cycling-ATP coupling accounts for cardiac mechanoenergetics

Cardiac twitch contractions were simulated by Huxley's sliding filament cross-bridge muscle model coupled with parallel and series elastic components. The energetics of the contraction were based on the ATP hydrolysis for the cross-bridge cycling. Force-length area (FLA), as a measure of the total mechanical energy, was computed for both isometric and isotonic contractions in a manner similar to the pressure-volume area (PVA) (Suga, H. Physiol. Rev. 70: 247-277, 1990). PVA correlates linearly with cardiac oxygen consumption, and since FLA is analogous to PVA, FLA should correlate with the ATP expended. Simulations comparing FLA with the cross-bridge cycling ATP usage showed that at lower muscle fiber activation levels (shorter initial fiber lengths and lower preload levels) FLA decreased more rapidly than the number of muscle fiber cross-bridge cycles in both isometric and isotonic contraction cases. This suggests that one ATP can cause more than one cross-bridge cycle at lower activation levels as was proposed by Yanagida, Arata, and Oosawa (Nature 316: 366-369, 1985). If the number of cross-bridge cycles to ATP ratio is allowed to increase at lower activation levels as suggested by Yanagida et al., Huxley's model is compatible with the experimental findings on FLA and PVA.