Relation between cardiac dimensions and peak oxygen uptake

BACKGROUND

Long term endurance training is known to increase peak oxygen uptake (VO2peak) and induce morphological changes of the heart such as increased left ventricular mass (LVM). However, the relationship between and the total heart volume (THV), considering both the left and right ventricular dimensions in both males and females, is not completely described. Therefore, the aim of this study was to test the hypothesis that THV is an independent predictor of VO2peak and to determine if the left and right ventricles enlarge in the same order of magnitude in males and females with a presumed wide range of THV.

METHODS AND RESULTS

The study population consisted of 131 subjects of whom 71 were athletes (30 female) and 60 healthy controls (20 female). All subjects underwent cardiovascular MR and maximal incremental exercise test. Total heart volume, LVM and left- and right ventricular end-diastolic volumes (LVEDV, RVEDV) were calculated from short-axis images. was significantly correlated to THV, LVM, LVEDV and RVEDV in both males and females. Multivariable analysis showed that THV was a strong, independent predictor of (R2 = 0.74, p < 0.001). As LVEDV increased, RVEDV increased in the same order of magnitude in both males and females (R2 = 0.87, p < 0.001).

CONCLUSION

Total heart volume is a strong, independent predictor of maximal work capacity for both males and females. Long term endurance training is associated with a physiologically enlarged heart with a balance between the left and right ventricular dimensions in both genders.

Respiratory indices by gas analysis and fat metabolism by indirect calorimetry in normal subjects and triathletes

BACKGROUND

The effects of exercise on different indices reflecting the metabolism have been of interest for a long time, and a relationship between anaerobic indices and maximal oxygen uptake has been established. The inter-relationship between different respiratory indices during an exercise test remains to be studied in order to understand differences between individuals. Therefore, the aim of this study was to determine three respiratory indices and investigate their inter-relationship in individuals with highly variable working capacity. A second aim was to investigate the fat metabolism at the VO(2) corresponding to the respiratory compensation point (Pq) in the different subjects using indirect calorimetry.

METHODS

Sixty control subjects (20 female) and 18 triathletes (six female) performed an exercise test with gas analysis. Three respiratory indices, derivative crossing (Dx), point of crossing (Px) and respiratory compensation point (Pq), were calculated using a computerized method. Fat metabolism at Pq was calculated using indirect calorimetry.

RESULTS

Two different sequences of the respiratory indices were found: Dx<Px<Pq and Dx<Pq<Px. In subjects whom Px occurred before Pq, no fat metabolism was seen at Pq, whilst in subjects with Pq occurring before Px, a combined aerobic fat metabolism and anaerobic carbohydrate metabolism was found at Pq.

CONCLUSION

This study has shown two different sequences of the respiratory indices Dx, Px and Pq in subjects of varying working capacity. The individual differences in the order of occurrence of Px and Pq during the exercise test are most likely caused by different abilities to metabolize fat at high workloads.

Increased left ventricular mass in obese adolescents*1

AIMS

An increase of left ventricular mass (LVM) has been reported in obese adolescents in previous studies using echocardiography. The aim of our study was to determine the extent of the increase in LVM and correlation to other risk factors using cardiac magnetic resonance imaging in obese and lean adolescents.

METHODS AND RESULTS

Nineteen obese and 20 lean adolescents were recruited. Following resting blood pressure measurements and blood sampling for insulin, triglycerides, and cholesterol levels, all subjects underwent cardiac magnetic resonance examination to assess LVM. LVM adjusted for body height was 16% greater in obese compared to lean adolescents (median 66 g/m, p = 0.0042). Obese subjects had higher resting systolic blood pressures than controls (median 115 vs. 110 mmHg, p = 0.0077) and higher fasting triglyceride and insulin levels. HDL-cholesterol levels were lower in the obese group compared with the lean group.

CONCLUSIONS

Obese adolescents had a higher LVM than age-matched lean subjects, which correlated mainly with body mass index and systolic blood pressure. These findings add to the established cardiovascular risk profile of obese adolescents.

A method for assembling a collaborative research team from multiple disciplines and academic centers to study the relationships between ECG estimation and MRI measurement of myocardial infarct size

A method has been developed for establishing a "University Without Walls" for the purpose of studying the relationship between electrocardiographic estimation and magnetic resonance imaging measurements of myocardial infarct size. The research team includes faculty and students from 4 medical centers, with expertise extending from clinical to technical. Weekly interactive videoconferences provide the key research communication method. Study patients are recruited from 2 of the sites, and the correlations between their electrocardiographic and magnetic resonance imaging data are considered by the research team in conference. Outcomes of this program are both scientific publications in international peer-review journals and formal postdoctoral degree attainment by the research trainees.

Accuracy of segmented MR velocity mapping to measure small vessel pulsatile flow in a phantom simulating cardiac motion

The purpose of this study was to investigate the accuracy of conventional, segmented, and echo-shared MR velocity mapping sequences to measure pulsatile flow in small moving vessels using a phantom with simulated cardiac motion. The phantom moved either cyclically in-plane, through-plane, in- and through-plane, or was stationary. The mean error in average flow was -2% +/- 3% (mean +/- SD) for all sequences under all conditions, with or without background correction, as long as the region of interest (ROI) size was equal to the vessel cross-sectional size. Overestimation of flow as a result of an oversized ROI was less than 20%, and independent of field of view (FOV) and matrix, as long as the offset in angle between the imaging plane and flow direction was less than 10 degrees. Segmented velocity mapping sequences are surprisingly accurate in measuring average flow and render flow profiles in small moving vessels despite the blurring in the images due to vessel motion. J. Magn. Reson. Imaging 2001;13:722-728.

Coronary sinus flow measurement by means of velocity-encoded cine MR imaging: validation by using flow probes in dogs

PURPOSE

To validate coronary sinus flow measurements for quantification of global left ventricular (LV) perfusion by means of velocity-encoded cine (VEC) magnetic resonance (MR) imaging and flow probes.

MATERIALS AND METHODS

Measurements of coronary sinus flow were performed in seven dogs by using VEC MR imaging at baseline, single coronary arterial stenosis, dipyridamole stress, and reactive hyperemia. These measurements were compared with flow probe measurements of coronary blood flow (CBF) in the left anterior descending coronary (LAD) and circumflex (CFX) arteries (CBF(LAD+CFX)) and coronary sinus. LV blood perfusion was calculated in milliliters per minute per gram from coronary sinus flow, and LV mass was obtained by using VEC and cine MR imaging. LV mass was validated at autopsy.

RESULTS

CBF(LAD+CFX) and coronary sinus flow at VEC MR imaging showed close correlation (r = 0.98, P: <.001). The difference between CBF(LAD+CFX) and MR coronary sinus flow was 3.1 mL/min +/- 8.5 (SD). LV mass at cine MR imaging was not significantly different from that at autopsy (73.2 g +/- 12.8 vs 69. 4 g +/- 12.8). At baseline, myocardial perfusion was 0.40 mL/min/g +/- 0.09 at VEC MR imaging, and CBF(LAD+CFX) was 0.44 mL/min/g +/- 0. 08 (not significant). Reactive hyperemia resulted in 2.7- and 2. 3-fold increases in coronary sinus flow at VEC MR imaging and flow probe CBF(LAD+CFX), respectively.

CONCLUSION

VEC MR imaging has the potential to measure coronary sinus flow during different physiologic conditions and can serve as a noninvasive modality to quantify global LV perfusion in patients.

Normal and infarcted myocardium: differentiation with cellular uptake of manganese at MR imaging in a rat model

PURPOSE

To assess whether normal myocardium can be distinguished from infarction at magnetic resonance (MR) imaging with low doses of manganese dipyridoxyl diphosphate (Mn-DPDP).

MATERIALS AND METHODS

After 1-hour coronary arterial occlusion and 2-hour reperfusion, three groups of eight rats each were injected with 25, 50, or 100 micromol of Mn-DPDP per kilogram of body weight. The longitudinal relaxation rate (R1) in normal myocardium, reperfused infarction, and blood was repeatedly measured at inversion-recovery echo-planar imaging before and for 1 hour after the administration of contrast material. Afterward, several animals from each group were examined at high-spatial-resolution inversion-recovery spin-echo (SE) MR imaging.

RESULTS

Manganese accumulated in normal myocardium but was cleared from reperfused infarction and blood. One hour after the administration of Mn-DPDP, R1 in normal myocardium (1.53 sec(-1) +/- 0.03, 1.73 sec(-1) +/- 0.03, and 1.94 sec(-1) +/- 0.02, respectively, for 25, 50, and 100 micromol/kg) was significantly (P <.05) faster than that of reperfused infarction (0.99 sec(-1) +/- 0.03, 1.11 sec(-1) +/- 0.03, and 1.48 sec(-1) +/- 0.06). Normal myocardium appeared hyperintense on T1-weighted inversion-recovery SE MR images and was clearly distinguishable from reperfused infarction.

CONCLUSION

Mn-DPDP-enhanced inversion-recovery echo-planar and SE MR images demonstrated retention of manganese in normal myocardium and clearance of manganese from infarction. Mn-DPDP has characteristics similar to those of widely used thallium and may be useful in the assessment of myocardial viability at MR imaging.

Transcatheter closure of post-infarction ventricular septal defect with the Amplatzer Septal Occluder device

There is an 80-90% mortality rate within the first 2 months of the occurrence of a post-infarction ventricular septal defect (VSD) with medical treatment alone. The muscular VSD presents a technical problem for the surgeon. Surgical treatment was unsuccessful in two patients. They were treated successfully using the Amplatzer Septal Occluder, with improvement in their condition.

Magnetic resonance-based assessment of global coronary flow and flow reserve and its relation to left ventricular functional parameters: a comparison with positron emission tomography

BACKGROUND

Measurement of coronary sinus blood flow (CSF) by phase-contrast magnetic resonance (PC-MR) imaging at rest and during hyperemia may allow noninvasive assessment of global coronary hemodynamics.

METHODS AND RESULTS

Sixteen healthy volunteers (age, 22 to 32 years) were examined with MR and PET in random order within 1 to 2 days. At rest and during hyperemia (dipyridamole 0.56 mg/kg), CSF was measured by a cine PC-MR technique (temporal resolution, 40 ms; spatial resolution, 1.25x0.8 mm(2)), and myocardial blood flow (MBF) was measured by [(13)N]NH(3) PET. PET and MR agreed closely for coronary flow reserve (CFR; mean difference, 2.2+/-14.7%; Bland-Altman method). CSF divided by either total left ventricular mass or an estimate of drained myocardium (LVM(drain)) correlated highly with PET flow data (r=0.93 and 0.95, respectively) and with measures of oxygen demand, ie, heart rate, afterload-corrected fiber shortening, and peak systolic stress determined by MR (overall correlation coefficients, 0.81 and 0.87, respectively, multivariate analysis). CSF/LVM(drain) did not differ significantly from PET-derived MBF (difference, 3.6+/-16.6%). In orthotopic heart transplant recipients (n=9), CFR was reduced and blood supply-demand relationships at rest were shifted toward higher flows (P<0.0001).

CONCLUSIONS

This integrated MR approach allows comprehensive assessment of autoregulated and hyperemic coronary flow and is suitable for serial measurements in patients. In transplanted hearts, elevated resting flow is the major cause of reduced CFR.

Three-dimensional MR imaging of pulmonary vessels and parenchyma with NC100150 injection (Clariscan)

The influence of increasing doses of NC100150 Injection (Clariscantrade mark) and echo times on visualization of pulmonary vessels and parenchyma was evaluated. The effects of 0.5, 1, 2, 4, and 8 mg Fe/kg NC100150 Injection and echo times (TE) of 1.1, 1.8, 2. 2, and 4.3 msec were determined in six dogs using breath-hold three-dimensional (3D) spoiled gradient-echo magnetic resonance (MR) sequence. At 2 mg Fe/kg and TE of 1.1 msec, the signal-to-noise ratio of the central pulmonary arteries and parenchyma was significantly increased (5.3 +/- 2.2 to 50.3 +/- 2.4) and (2.2 +/- 0. 9 to 6.4 +/- 1.1), respectively. Using the TE of 1.1 msec, signal intensity in the main arteries continued to increase with increasing dose. Moreover, the enhancement of pulmonary parenchyma and microvasculature had a positive dose response. 3D MR imaging with ultrashort echo time and 2 mg Fe/kg NC100150 Injection produces angiograms with strong vascular contrast and allows qualitative assessment of pulmonary parenchyma and microvasculature.

Reperfused rat myocardium subjected to various durations of ischemia: estimation of the distribution volume of contrast material with echo-planar MR imaging

PURPOSE

To estimate and compare the fractional distribution volume (fDV) of gadodiamide injection and technetium 99m-diethylenetriaminepentaacetic acid (DTPA) in the reperfused myocardium of rat hearts subjected to various durations of ischemia.

MATERIALS AND METHODS

Magnetic resonance (MR) imaging and autoradiography were performed in rats subjected to 20, 30, 40, or 60 minutes of regional ischemia followed by 1 hour of reperfusion. The fDVs of gadodiamide injection and (99m)Tc-DTPA were measured and compared by using inversion-recovery echo-planar imaging and autoradiographic phosphor imaging, respectively.

RESULTS

The mean fDV of both tracers (gadodiamide and (99m)Tc-DTPA) in normal myocardium was 18% +/- 1, whereas that in the entire area at risk increased significantly (P <.05) with 20, 30, 40, and 60 minutes of ischemia to 32% +/- 1, 57% +/- 4, 66% +/- 2, and 68% +/- 2, respectively. The fDV was significantly (P <.05) greater in the core of infarction-78% +/- 4, 89% +/- 5, and 88% +/- 5 with 30, 40, and 60 minutes of ischemia, respectively-than in the normal myocardium or in the area at risk.

CONCLUSION

The fDV of MR contrast material in the periinfarcted rim was significantly (P <. 05) greater than that in the normal myocardium, but significantly less than that in the core of infarcted myocardium.

Clinical implication of down-scatter in attenuation-corrected myocardial SPECT

BACKGROUND

Interpretation of myocardial perfusion single photon emission computed tomography (SPECT) studies is hampered by attenuation artifacts. Attenuation correction methods with simultaneous emission and transmission are now commercially available. However, it has been observed in clinical practice that attenuation correction without down-scatter correction in a 1-day rest/stress myocardial perfusion protocol may lead to serious interpretation errors. Therefore the aim of this study was to study errors resulting from down-scatter under realistic conditions, thus providing a background for the assessment of further corrections.

METHODS AND RESULTS

Forty-six patients underwent myocardial perfusion scintigraphy in a 1-day technetium 99m-tetrofosmin rest-stress SPECT protocol, with a moving 153Gd line-source device for attenuation correction without down-scatter correction. Short-axis slices were quantified as inferior/anterior, septal/lateral, and apical/remainder count ratios. The changes at rest (350 MBq) and exercise (900 MBq) induced by attenuation correction were studied. Attenuation correction gave differences in apparent perfusion between rest and exercise not seen before correction. The gender differences in inferior-anterior ratio were greatly reduced after correction at rest but remained at exercise. A torso phantom study indicated that these results were due to under-correction at exercise because of down-scatter.

CONCLUSIONS

Down-scatter results in an underestimation of attenuation in simultaneous emission and transmission, if not accurately accounted for. Particularly, a high-dose study compared with a low-dose study, as in the 1-day protocol, might cause serious interpretation errors.

Measurement of the distribution volume of gadopentetate dimeglumine at echo-planar MR imaging to quantify myocardial infarction: comparison with 99mTc-DTPA autoradiography in rats

PURPOSE

To measure the fractional distribution volume of gadopentetate dimeglumine in normal and reperfused infarcted myocardium at magnetic resonance (MR) imaging by using the fractional distribution volume of technetium 99m-diethylenetriaminepentaacetic acid (DTPA) as an independent reference.

MATERIALS AND METHODS

Rats were subjected to 1 hour of coronary artery occlusion and 1 hour of reperfusion before inversion-recovery echo-planar imaging or autoradiography. Regional change in relaxation rate (delta R1) ratios for myocardium over blood were compared with radioactivity ratios for myocardium over blood after the injection of 99mTc-DTPA.

RESULTS

Both delta R1 and radioactivity ratios demonstrated equilibrium distribution and hence represent partition coefficients (lambda). The fractional distribution volumes were greater in infarcted myocardium (0.90 +/- 0.05 for gadopentetate dimeglumine and 0.89 +/- 0.04 for 99mTc-DTPA) than in normal myocardium (0.23 +/- 0.02 for gadopentetate dimeglumine and 0.16 +/- 0.01 for 99mTc-DTPA). Area at risk at autoradiography was not significantly different from that at histomorphometry. The infarction size defined by using triphenyltetrazolium chloride was 13% +/- 4 smaller than that defined by using autoradiography.

CONCLUSION

The fractional distribution volumes of gadopentetate dimeglumine and 99mTc-DTPA are similar and indicate extracellular distribution in normal myocardium and intracellular as well as extracellular distribution in reperfused infarction. Because the failure of cells to exclude these agents is indicative of necrosis, contrast medium-enhanced MR imaging may be useful to quantify myocardial infarction.

Left-to-right cardiac shunts: comparison of measurements obtained with MR velocity mapping and with radionuclide angiography

PURPOSE

To investigate the agreement between two noninvasive methods, magnetic resonance (MR) velocity mapping and first-pass radionuclide angiography, to quantify the pulmonary-to-systemic blood flow ratio (QP/QS) in adults, adolescents, and children with left-to-right cardiac shunts.

MATERIALS AND METHODS

The accuracy and precision of MR velocity mapping were studied in 12 control subjects (six men, six women) and in a phantom. MR velocity mapping and radionuclide angiography were performed on the same day in 24 patients (16 adults, two adolescents, six children; five male patients, 19 female patients).

RESULTS

The mean error in QP/QS at MR velocity mapping in phantom experiments was -1% +/- 1 (mean +/- SD). In control subjects, QP/QS at MR velocity mapping was 1.03 +/- 0.03, and the cardiac index was 3.1 L/min/m2 +/- 0.2 and 3.2 L/min/m2 +/- 0.3 for women and men, respectively. In patients, QP/QS at radionuclide angiography was 14% +/- 13, higher than at MR velocity mapping. Interobserver variability was four times higher for radionuclide angiography compared with MR velocity mapping, 0% + 16 versus 0% +/- 4 (n = 12). The difference between repeated MR flow measurements in the same vessel was -1% +/- 5 (n = 36).

CONCLUSION

The data suggest that MR velocity mapping is accurate and precise for measurements of shunt size over the whole range of possible QP/QS values.

Detection of acute myocardial ischemia using first-pass dynamics of MnDPDP on inversion recovery echoplanar imaging

Previous studies used manganese N,N'-bis-(pyridoxal 5-phosphate)ethylenediamine-N,N'-diacetic acid (MnDPDP) to detect myocardial ischemia at a dose of 0.4 mmol/kg with spin echo imaging. The purpose of this study was to detect acute myocardial ischemia using MnDPDP at a dose range near that approved for hepatobiliary imaging (0.005 mmol/kg) in conjunction with inversion recovery echoplanar imaging (IR EPI). Regional ischemia was produced in 26 rats by occluding the left coronary artery for 20-30 minutes before imaging. Consecutive 32 IR EP images (inversion time [TI]/TR/TE 700/2000/10 msec) were obtained to monitor the first pass of MnDPDP at four incremental doses (0.005, 0.01, 0.02, or 0.04 mmol/kg, n = 6-8). MnDPDP produced dose-dependent enhancement of left ventricular blood and normal myocardium, but not ischemic myocardium. Quantitative analysis revealed a difference in signal intensities (P<0.05) between normal and ischemic myocardium at the time of peak enhancement in all groups. However, differential enhancement between normal and ischemic myocardium produced clear visual delineation of the ischemic region only at doses > or =0.01 mmol/kg. In conclusion, acute myocardial ischemia can be detected with IR EPI using doses close to the clinically approved dose of MnDPDP.

Dissociation between force and [Ca2+]i during extra systoles in guinea-pig ventricular muscle microinjected with fura-2

Thin trabeculae were dissected from the right ventricle of guinea-pig heart and stimulated to contract isometrically at 0.5 Hz (26 degrees C). Rapid and transient changes of force were obtained by inducing three extra systoles (ES1-3) at 450-ms intervals. The two regular contractions (P1-2) following (ES1-3) were potentiated. Fura-2 salt was microinjected into the preparation to monitor intracellular calcium ([Ca2+]i). Three distinct phases of [Ca2+]i were seen: (1) a rapid rising phase to about 200 nmol L(-1), (2) a slower rising phase to a peak at 400 nmol L(-1), and (3) a slow decline to about 50 nmol L(-1). During ES1, there was a discrepancy between force, which decreased, and peak [Ca2+]i, which increased to 600 nmol L(-1). It is likely that the increased [Ca2+]i during the extra systoles reflects increased sarcolemmal calcium inflow, causing post-extra-systolic potentiation. Ryanodine (1-2 microM) was added to inhibit the intracellular calcium release and thus reduce the intracellular [Ca2+]i gradients following excitation. Ryanodine inhibited phase 1 of [Ca2+]i and abolished post-extra-systolic potentiation. There was a close relationship between dF/dt and [Ca2+]i with ryanodine during control and ES1-3. It is likely that fura-2 reports a spatially averaged [Ca2+]i and that phase 1 of the signal therefore apparently underestimates activator calcium in the close vicinity of the contractile elements.

Microvascular injury in reperfused infarcted myocardium: noninvasive assessment with contrast-enhanced echoplanar magnetic resonance imaging

OBJECTIVES

The purpose of this study was to measure the accumulation of labeled albumin and to visualize its distribution pattern in reperfused infarcted myocardium as a function of time between onset of reperfusion and administration of the tracer.

BACKGROUND

Myocardial microvascular injury leads to leakage of albumin from the intravascular space. Quantitative measurements of GdDTPA-albumin with inversion recovery echoplanar imaging (IR-EPI) may allow noninvasive monitoring of microvascular injury.

METHODS

After 1 h of coronary artery occlusion, 56 rats were injected with GdDTPA-albumin or 123I-GdDTPA-albumin either immediately before reperfusion or 1/2, 1 or 24 h after reperfusion. GdDTPA-albumin in blood, normal myocardium and reperfused infarction was dynamically measured with IR-EPI during 1 h postinjection (PI). Autoradiograms were obtained at 15 min PI. Accumulation of labeled albumin in myocardium was expressed as the ratio of myocardial to blood content.

RESULTS

In normal myocardium, the ratio of changes of relaxation rate-ratio (deltaR1-ratio) was 0.12+/-0.01 and did not change over 1 h. In reperfused infarction, however, the deltaR1-ratio increased after administration. Animals given GdDTPA-albumin before reperfusion exhibited fastest accumulation (deltaR1-ratio 15 min PI: 0.56+/-0.03) and essentially homogeneous distribution. The accumulation was slower when administered at 1/2, 1 and 24 h after reperfusion (deltaR1-ratios 15 min PI: 0.39+/-0.03; 0.31+/-0.04; 0.16+/-0.01; p < 0.001 compared to administration before reperfusion). Moreover, the tracer accumulated predominantly in the periphery of the injury zone.

CONCLUSIONS

Amount and distribution pattern of labeled albumin in reperfused infarction are modulated by duration of reperfusion. The accumulation of GdDTPA-albumin can be quantified by IR-EPI. Thus, IR-EPI may be useful to noninvasively monitor myocardial microvascular injury in reperfused infarction.

Effects of magnesium pyrophosphate on mechanical properties of skinned smooth muscle from the guinea pig taenia coli

Effects of the non-hydrolyzable nucleotide analogue magnesium pyrophosphate (MgPPi) on cross-bridge properties were investigated in skinned smooth muscle of the guinea pig Taenia coli. A "high" rigor state was obtained by removing MgATP at the plateau of an active contraction. Rigor force decayed slowly towards an apparent plateau of approximately 25-35% of maximal active force. MgPPi markedly increased the rate of force decay. The initial rate of the force decay depended on [MgPPi] and could be described by the Michaelis-Menten equation with a dissociation constant of 1.6 mM. The decay was irreversible amounting to approximately 50% of the rigor force. Stiffness decreased by 20%, suggesting that the major part of the cross-bridges were still attached. The results can be interpreted as "slippage" of PPi-cross-bridges to positions of lower strain. The initial rate of MgPPi-induced force decay decreased with decreasing ionic strength in the range 45-150 mM and was approximately 25% lower in thiophosphorylated fibers. MgADP inhibited the MgPPi-induced force decay with an apparent Ki of 2 microM. The apparent Km of MgATP for the maximal shortening velocity in thiophosphorylated fibers was 32 microM. This low Km of MgATP suggests that steps other than MgATP-induced detachment are responsible for the low shortening velocity in smooth muscle. No effects were observed of 4 mM MgPPi on the force-velocity relation, suggesting that cross-bridges with bound MgPPi do not constitute an internal load or that binding of MgPPi is weaker in negatively strained cross-bridges during shortening.

Force response to rapid length change during contraction and rigor in skinned smooth muscle of guinea-pig taenia coli

1. Mechanical transients in fibre bundles of skinned smooth muscle of guinea-pig taenia coli at 21-22 degrees C were investigated by recording tension responses to length changes of up to 9%, complete within 0.3 ms. 2. The length-force relationship, recorded continuously during rapid stretch of a Ca(2+)-activated contracted muscle, was linear up to at least 2.5 times the isometric force, corresponding to a stretch of about 1%. The slope of the relationship (stiffness) increased with the velocity of stretch. 3. During rapid release (about 120 muscle lengths s-1) the length-force relationship was linear down to about 50% of the initial isometric force, reached at about 80 microseconds after the beginning of the release. At lower force the length-force relationship was concave upwards. The linear portion extrapolated to zero force at about -0.008 muscle lengths. In large releases the length-force plot approached the force baseline under an acute angle, and negative force was transiently exerted. 4. When the muscle was stretched back to the initial length after a shortening step, force transiently rose above the isometric force, but decayed back within a few milliseconds. Stiffness at the time of restretch was compared with that in the initial shortening step by plotting force vs. length, and was found to be decreased to 63% within 0.3 ms of a step to zero force. Stiffness decreased further with time at zero force, and after 256 ms was about 29% of the isometric value. 5. In rigor, caused by the introduction of ATP-free solution during the plateau of isometric contraction, fibre tension decreased to about 30% of the active tension, whereas stiffness relative to force increased; 82% of the initial stiffness in rigor was detected in a restretch immediately after a shortening step, decreasing to 59% at 256 ms. When the fibre was activated at suboptimal [Ca2+] to cause the same force as in rigor, stiffness was lower than in rigor and decreased more after a release. 6. After completion of a release-stretch cycle, stiffness was rapidly restored to the same value as in isometric contraction. Test stretches at different points in time after completion of the cycle revealed that most of the stiffness had been restored within 1 ms of the restretch, occurring concomitantly with a decay in force.(ABSTRACT TRUNCATED AT 400 WORDS)