Maximum Rate Of Contraction Research Articles

Negative functional effects of cGMP mediated by cGMP protein kinase are reduced in T 4 cardiac myocytes

We tested the hypothesis that in isolated rabbit cardiac myocytes, the negative functional effects of cyclic GMP are partly mediated by cyclic GMP-dependent protein kinase activity, and that these effects are altered in thyroxine (T 4, 0.5 mg/kg/day for 16 days)-induced hypertrophic myocytes. Using isolated ventricular myocytes from control ( N=8) and T 4 ( N=8) hypertrophic hearts, data for percent cell shortening (%) and maximum rate of contraction (μm/s) were collected using a video edge detector at baseline, after the addition of 10 −6 M 8-bromo-cyclic GMP (8-Br-cGMP), 10 −5 M 8-Br-cGMP, and 10 −6 M KT5823 (10-methoxy-10-methoxycarbonyl-9, 10, 11, 12-tetrahydro-9, 12-epoxy-(1 H)-diinidolo [1, 2, 3, f–g: 3′, 2′, 1′-k-j]-pyrrolidino-[3,4-i] [1,6]-benzodiazocin-2-methyl-1-one, cyclic GMP protein kinase inhibitor). Protein phosphorylation was determined autoradiographically after gel electrophoresis. In both control and T 4 myocytes, 8-Br-cGMP caused a significant decrease in percent shortening (5.56±0.49% to 3.02±0.47% in control and 4.34±0.33% to 3.13±0.17% in T 4 myocytes) and maximal rate of contraction 57.35±6.05 to 36.82±3.17 μm/s in control and 58.49±3.28 to 42.88±2.29 μm/s in T 4 myocytes). KT5823 significantly increased percent shortening to 3.77±0.28% and rate to 48.68±4.71 μm/s after 8-Br-cGMP only in control myocytes. In T 4 myocytes, the changes in percent shortening and rate after KT5823 were not significant. Protein phosphorylation was increased by 8-Br-cGMP in control and to a lesser extent in T 4 myocytes, but the increment was reduced by KT-5823 in control only. These data demonstrated that cyclic GMP had negative functional effects partially mediated by cyclic GMP protein kinase in control myocytes. Cyclic GMP also exerted negative functional effects in thyroxine-induced hypertrophic myocytes, but cyclic GMP protein kinase activity was not an important regulator of these effects in T 4 ventricular myocytes.

The effects of etomidate on the contractility of failing and nonfailing human heart muscle.

We measured the effects of etomidate on contractility of human cardiac muscle. Muscles were obtained from the left ventricle and right atrium of 12 patients undergoing cardiac transplantation, and from the right atrium of 12 patients undergoing coronary artery bypass surgery. Muscles were studied at 37 degrees C and 1.0 Hz. Variables of isometric contraction were recorded before and after etomidate (0.04-80 microM) or its solvent, propylene glycol. The ability of beta-adrenergic stimulation to cause an inotropic effect after etomidate was also assessed. Etomidate caused a dose-dependent decrease in developed tension, which was statistically significant only at concentrations exceeding clinical doses (> or =20 microM; P < 0. 05). Decreases in maximum rates of contraction and relaxation paralleled changes in developed tension. beta-Adrenergic stimulation reversed the etomidate-induced decreases in developed tension and rates of contraction and relaxation to baseline (P > 0.05 compared with baseline). Thus, in human myocardium, etomidate exerts a dose-dependent negative inotropic effect, which is reversible with beta-adrenergic stimulation. Concentrations required to produce these negative inotropic effects are, however, in excess of those reached during clinical use. Therefore, etomidate-induced negative inotropy is unlikely to be a problem clinically, even in patients with cardiac dysfunction. Etomidate produced a similar dose-dependent negative inotropic effect in both failing and nonfailing human myocardium. This effect was present only at concentrations exceeding those attained clinically and was reversible with beta-adrenergic stimulation.

The effect of heptanol on the electrical and contractile function of the isolated, perfused rabbit heart.

Changes in cardiac gap junction expression, such as those following myocardial infarction and produced in connexin knockout mice, are associated with a predisposition to arrhythmias. The present experiments investigated the effects of heptanol, a reversible gap junction inhibitor, on isolated Langendorff-perfused rabbit hearts. The introduction and withdrawal of heptanol inhibited both pressure generation and electrical conduction. These effects were completely reversible. Possible mechanisms for these findings were investigated through measurement of the concentration dependence of heptanol's effects upon conduction velocity and repolarization duration. Low concentrations of heptanol (less than 0.3 mM) caused small but significant increases in the delay between the stimulus (delivered to the basal septum) artefact and local activation of the left ventricle, as measured from bipolar electrogram (BEG) recordings. There was a steep increase in the latency between stimulus and left-ventricular activation at concentrations of heptanol above 0.3 mM. These findings are explicable by earlier reports of heptanol actions on gap junctions in vitro and modelling studies of the effects of reduced gap junction conductance on conduction velocity. Heptanol decreased repolarization duration, measured from the activation recovery interval (ARI) of BEGs, and monophasic action potential duration at 70% repolarization (MAPD70). Heptanol also reduced left-ventricular developed pressure (LVDP), and the maximum rates of contraction and relaxation of the left ventricle; these effects were concentration dependent and reversible. However, changes in ARIs, LVDP and the maximum rates of change of pressure lacked the steep response to 0.3-1.0 mM heptanol shown by the latency. These other effects are therefore likely to be mediated by cellular targets other than gap junctions. Perfusion of hearts with heptanol was also associated with a high incidence of arrhythmias. During premature stimulation protocols arrhythmias could be induced in hearts perfused with 0.1-0.3 mM heptanol but not at higher concentrations. This suggests that there is a critical range of slowed conduction that permits the development of re-entrant arrhythmias in the normal heart, although the effects of heptanol on repolarization duration may also contribute to its pro-arrhythmic activity.

The role of collagen in the declining mechanical properties of aging human cortical bone

The importance of the mechanical role of collagen in bone is becoming increasingly more clear as evidence mounts on the detrimental effects of altered collagen on the mechanical properties of bone. We previously examined a set of mechanical properties (material stiffness, strength, and toughness) of human femoral bone (ages 35–92) and found that a gradual deterioration in these properties occurs with age. The present study examines the collagen of the same specimens and relates the collagen properties to the mechanical ones. In the collagen we measured the concentration of stable mature crosslinks, the shrinkage temperature, and the rate of contraction during isometric heating. The changes in the concentration of mature (pyridinium and deoxypyridinium) crosslinks showed no clear relationship to age nor did they correlate with the mechanical properties. The shrinkage temperature declined with age and correlated with a bone's toughness. The maximum rate of contraction was strongly correlated with three different measures of tissue toughness, but much less to stiffness and strength. Our results reinforce speculation regarding the toughening role of collagen in bone mechanics and suggest that the fragility of aging bone may be related to collagen changes. © 1999 John Wiley & Sons, Inc. J Biomed Mater Res, 45, 108–116, 1999.

Correlation between myofilament response to Ca2+ and altered dynamics of contraction and relaxation in transgenic cardiac cells that express beta-tropomyosin.

We compared the dynamics of the contraction and relaxation of single myocytes isolated from nontransgenic (NTG) mouse hearts and from transgenic (TG-beta-Tm) mouse hearts that overexpress the skeletal isoform of tropomyosin (Tm). Compared with NTG controls, TG-beta-Tm myocytes showed significantly reduced maximal rates of contraction and relaxation with no change in the extent of shortening. This result indicated that the depression in contraction dynamics determined in TG-beta-Tm isolated hearts is intrinsic to the cells. To further investigate the effect of Tm isoform switching on myofilament activity and regulation, we measured myofilament force and ATPase rate as functions of pCa (-log of [Ca2+]). Compared with controls, force generated by myofilaments from TG-beta-Tm hearts and myofibrillar ATPase activity were both more sensitive to Ca2+. However, the shift in pCa50 (half-maximally activating pCa) caused by changing sarcomere length from 1.8 to 2.4 microm was not significantly different between NTG and TG-beta-Tm fiber preparations. To test directly whether isoform switching affected the economy of contraction, force versus ATPase rate relationships were measured in detergent-extracted fiber bundles. In both NTG and TG-beta-Tm preparations, force and ATPase rate were linear and identically correlated, which indicated that crossbridge turnover was unaffected by Tm isoform switching. However, detergent extracted fibers from TG-beta-Tm demonstrated significantly less maximum tension and ATPase activity than NTG controls. Our results provide the first evidence that the Tm isoform population modulates the dynamics of contraction and relaxation of single myocytes by a mechanism that does not alter the rate-limiting step of crossbridge detachment. Our results also indicate that differences in sarcomere-length dependence of activation between cardiac and skeletal muscle are not likely due to differences in the isoform population of Tm.

The role of collagen in the declining mechanical properties of aging human cortical bone.

The importance of the mechanical role of collagen in bone is becoming increasingly more clear as evidence mounts on the detrimental effects of altered collagen on the mechanical properties of bone. We previously examined a set of mechanical properties (material stiffness, strength, and toughness) of human femoral bone (ages 35-92) and found that a gradual deterioration in these properties occurs with age. The present study examines the collagen of the same specimens and relates the collagen properties to the mechanical ones. In the collagen we measured the concentration of stable mature crosslinks, the shrinkage temperature, and the rate of contraction during isometric heating. The changes in the concentration of mature (pyridinium and deoxypyridinium) crosslinks showed no clear relationship to age nor did they correlate with the mechanical properties. The shrinkage temperature declined with age and correlated with a bone's toughness. The maximum rate of contraction was strongly correlated with three different measures of tissue toughness, but much less to stiffness and strength. Our results reinforce speculation regarding the toughening role of collagen in bone mechanics and suggest that the fragility of aging bone may be related to collagen changes.

Evoked isometric muscle contractions in myopathies: analysis of pathophysiological properties by different stimulus patterns

Isometric twitches of the adductor pollicis muscle following ulnar nerve stimulation were investigated in healthy subjects ( n=35) and patients with different types of myopathies (myotonic dystrophy, n=19; limb girdle muscular dystrophy, n=10; metabolic myopathy, n=6). The changes within the rising part (i.e. within the contraction time, CT) of the isometric twitches after single stimuli were similar in myotonic and limb girdle dystrophies: the first part of CT, which lasts until the maximal contraction rate is achieved, was shortened, whereas the following second part of CT, which lasts until the maximal twitch force is achieved, was normal. In metabolic myopathies the first part was normal, whereas the second part was prolonged. The relaxation was prolonged in all types of myopathies, particularly in metabolic myopathies. Using double stimuli with short interstimulus intervals (ISI), the absolute refractory period of the muscle contraction (healthy subjects: 1.35±0.16 ms) was shortened in patients with myotonic dystrophy (1.02±0.11 ms). In the other types of myopathies, the absolute refractory period was only shortened provided that the single twitch force was clearly reduced. A similar dependence on a reduced single twitch force was also found with regard to the maximal force development with two stimuli and the corresponding ISI: the force contributed by a second stimulus was pathologically enhanced if the single twitch force was clearly reduced. The ISI related to the maximal force with two stimuli was shifted towards very short values (healthy subjects: 10.5 ms, myotonic dystrophy: 4.6 ms, limb girdle dystrophies: 5.0 ms). Our results can be attributed to altered kinetics of calcium release and uptake by the sarcoplasmic reticulum in myopathies.

Contractile properties of human nasal dilator motor units.

The technique of intramuscular microstimulation was used to activate facial nerve fibers while acquiring simultaneous twitch force measurements to measure the contractile properties and force-frequency responses of human nasal dilator (ND) motor units. Twitch force amplitude (TF), contraction time (CT), half-relaxation time (HRT), and the maximal rate of rise of force normalized to the peak force (maximum contraction rate, MCR) were recorded from 98 ND motor units in 37 subjects. The average CT, HRT, MCR, and TF were 47.9 +/- 1.8 ms, 42.6 +/- 2.1 ms, 28.6 +/- 1.8 s-1, and 1.06 +/- 0.1 mN, respectively. Neither CT nor HRT were significantly correlated with TF. The average CT and HRT were similar to values recorded for small muscles of the hand but were faster than the values recorded from human toe extensor motor units. However the lack of an association between twitch force and CT or HRT was similar to the findings obtained for both human hand and foot muscles. Force-frequency curves were recorded from eight ND motor units. The force produced by the eight motor units was recorded in response to stimuli delivered at 1, 5, 10, 15, 20, 25, 30, 35, and 40 Hz to assess force-frequency relationships. The mean twitch force of the eight motor units was 0.91 +/- 0.3 mN and the average tetanic force was 8.1 +/- 1.8 mN. Therefore the average twitch force was equal to 12.7% of the tetanic force. Fifty percent of the unit tetanic force was achieved at an average frequency of 16. 4 +/- 1.7 Hz, which is greater than the value recorded for human toe extensor motor units (9.6 Hz). Thus the force produced by the ND motor units was more sensitive to changes in discharge frequency over the range of approximately 10-30 Hz and less sensitive to changes in the range of 0-10 Hz because of their fast contractile properties. The mean slope of the regression lines that were fit to the steep portion of each force-frequency curve was 5.15 +/- 0.5% change in force/Hz. This value was greater than the slope measured for human toe extensor muscles (4.2% change in force/Hz). These observations suggest that force gradation by ND motor units is more sensitive to changes in stimulation frequency than human toe extensor motor units. We conclude that most ND motor units have fast contractile properties and that rate coding may play a significant role in the gradation of force produced by the ND muscle. Furthermore, the findings of this investigation have demonstrated that contractile speed and TF in a human facial muscle are not correlated. This supports previous findings obtained from human hand and foot muscles and suggests that there may be a fundamental difference in the contractile speed-twitch force relationship between many human muscles and most muscles of other mammals.

Regulatory effects of phospholamban on cardiac function in intact mice.

Phospholamban (PLB) regulates Ca(2+)-adenosinetriphosphatase activity in cardiac sarcoplasmic reticulum and participates in the regulation of myocardial performance. Animal models with altered levels of PLB permit in vivo evaluation of the physiological role of PLB. This study examined left ventricular (LV) performance in intact PLB heterozygous and homozygous mice under basal and stimulated conditions. A Millar Mikro-Tip transducer was inserted into the right carotid artery and advanced into the LV for direct measurement of ventricular pressure and the first derivative of intraventricular pressure (dP/dt). Baseline blood pressures were increased in PLB heterozygotes and even more so in PLB homozygotes compared with wild types (WT), and there were no differences in heart rate or LV end-diastolic pressure. The increase in pressure was primarily caused by an increase in systolic pressure. Baseline values for positive and negative dP/dt were linearly correlated with PLB levels. In PLB heterozygotes, contractile response to isoproterenol (Iso) was blunted compared with WT, but maximum rates of contraction were similar between the two groups. Contractile performance in PLB homozygous mice, which under baseline conditions was similar to maximum levels seen in WT, showed a blunted response to Iso, and maximum rates of contraction were significantly greater than in either of the other groups, indicating an essential but perhaps not exclusive role for PLB in mediating the inotropic effects of beta-adrenergic agonists. The effects of Iso on negative dP/dt were also blunted in both PLB heterozygous and PLB homozygous animals. Our results demonstrate that myocardial function in highly dependent on PLB level and suggest that the cardiovascular effects of PLB perturbations are largely uncompensated for in the intact mouse.

Alterations in cardiac contractility and gene expression during low-T3 syndrome: prevention with T3.

The low-T3 syndrome is a metabolic response resulting in a decreased serum triiodothyronine (T3) concentration that has uncertain effects on thyroid hormone-responsive gene expression and function. We measured cardiac myocyte gene expression and cardiac contractility in young adult female rats using chronic calorie deprivation as a model of the low-T3 syndrome. Sarcoplasmic reticulum calcium adenosinetriphosphatase (SERCA2) and myosin heavy chain (MHC) isoform mRNA content were measured after 28 days on a 50% calorie-restricted diet (low T3) with or without T3 treatment (6 micrograms.kg body wt-1.day-1). The low-T3 animals had decreased maximal rates of contraction (-13%; P < 0.05) and relaxation (-18%; P < 0.05) compared with the control and the T3-treated groups. There was a 21% (P < 0.05) increase in left ventricular (LV) relaxation time in the low-T3 animals vs. both control and T3-treated groups. The LV content of the SERCA2 mRNA was decreased significantly (37%) in the low-T3 rats and was increased (P < 0.05) with T3 treatment vs. controls. The alpha-MHC mRNA isoform decreased in the low-T3 animals but was unchanged in the T3-treated animals. T3 supplementation normalized both cardiac function and phenotype of calorie-restricted animals, suggesting a role for the low-T3 syndrome in the pathophysiological response to calorie restriction.

Cardiac adaptations to low temperature in non-polar teleost fish

Acclimation of teleost fish to low temperature results in a number of well-characterized adaptive responses in skeletal muscle. Our understanding of the response of cardiac muscle to such a challenge though is still poorly understood. It is considered of interest to learn more about the impact of chronic low temperature exposure on the cardiac response since the heart is required to support other tissues through the delivery of oxygen and metabolic fuels and this system may be a useful model in which to identify loci of cardiac plasticity. An overview of the available data reveals a number of fundamental responses. These include an increase in heart mass, an increase in maximal contraction frequency, an increase in complex lipid biosynthesis, and an increase in aerobic-based fatty acid metabolism. Two different strategies seem to emerge and lead to working hypotheses. As one example, rainbow trout develop larger hearts which may lead to enhanced stroke volume. The net accretion of protein is associated with either a more effective use of the available protein synthetic machinery and/or a decrease in protein degradation. The net accretion of lipid is associated with an increased rate of biosynthesis. Energy metabolism at low temperature appears to be much more dependent upon the oxidation of fatty acids. As another example, perch acclimated to low temperature show enhanced maximal rates of contraction which may be associated with increase in the volume of sarcoplasmic reticulum. The metabolic processes necessary to supply ATP appear quite adequate and require no further expansion as evidenced by constant mitochondrial volume density and activity of mitochondrial marker enzymes. The data however, in both cases are fragmentary and call for comprehensive studies on selected species. © 1996 Wiley-Liss, Inc.

ADAPTATIONS IN MOTOR UNIT FIRING RATES AND CONTRACTILE PROPERTIES AFTER RESISTANCE TRAINING 1220

The purpose of this study was to determine if motor unit firing rates change in parallel with the contractile properties of vastus lateralis following eight weeks of isometric resistance training. The firing rates from more than 400 motor units of vastus lateralis were obtained during voluntary isometric contractions of 50% MVC, before and after training in male subjects(N=10) and their untrained controls (N=10). Single motor unit spike trains were recorded with tungsten microelectrodes. Training resulted in a 36%(p<0.05) increase in MVC. We also found significant increases (p<0.05) in maximal twitch amplitude (+17%), time to peak tension (+9%) and the maximal rate of contraction (+20%) in the trained leg of the experimental group. Neither the maximal integrated EMG nor the rate of increase of integrated EMG was different after training. There were no significant changes in any of these measures from the untrained leg or the control group. Average firing rates were not different after training despite the increase in twitch contractile speed. These findings suggest that the control properties of muscular contraction are not altered despite sizable changes in the contractile properties following eight weeks of resistance training.

Enhanced postischemic myocardial recovery following exercise induction of HSP 72

The inducible isoform of the 70-kDa heat shock protein (HSP) family, HSP 72, has been shown to protect cells from protein-damaging stressors and has been associated with myocardial protection. Because exercise is capable of increasing HSP 72 content, we determined whether exercise induction of HSP 72 also provided myocardial protection. Twenty-eight rats (n = 7 per group) were divided into control, heat-shocked (15 min at 42 degrees C), and two exercised groups. Exercise consisted of either one or three bouts (on 3 consecutive days) of treadmill running for 60 min at 30 m/min. Twenty-four hours after heat shock or exercise, hearts were placed on a Langendorff apparatus and subjected to 30 min of global ischemia followed by 30 min of reperfusion. Left ventricular developed pressure (LVDP), maximal rate of contraction and relaxation (+/- dP/dt, respectively), coronary flow, catalase activity, and HSP 72 content were determined. During reperfusion, hearts from heat-shocked animals and animals subjected to three bouts of exercise recovered a greater percentage of preischemic LVDP and +/- dP/dt compared with controls or animals that exercised only once. Compared with hearts from controls, HSP 72 content was significantly elevated in the hearts of heat-shocked animals and in animals subjected to three bouts of exercise, but not in animals that exercised only once. These results suggest that exercise induction of HSP 72 can confer an enhanced postischemic recovery and may explain, at least in part, the myocardial protection associated with exercise.

A PROSPECTIVE STUDY OF FUNCTIONAL ASSESSMENT OF POSTOPERATIVE CHANGES OF THE GALLBLADDER IN THE EARLY PERIODS OF FOLLOWING GASTRECTOMY FOR GASTRIC CANCER BY ULTRASONOGRAPHY AND HEPATOBILIALLY SCINTIGRAPHY

In 30 patients following curative gastrectomy for gastric cancer in our institution, functional assess-ment (about (1) maximal fasting area (2) maximal contraction rate (3) presence of gallstones and debris echoes) at the gallbladder was made serially before, and 2, 4 weeks and 6 months after the operation by ultrasonography, and bile flow assessment was done 4 weeks after the operation by 99mTc-N-pyridoxyl-5-methyltryptophan (99mTc-PMT) hepatobiliary scintigraphy, to clarify the etiology of postoperative cholecystolithiasis. By ultrasonography, significant enlargement and contractile dysfunction of the gallbladder was found in 29 patients except one patient who had undergone pylorus preserving gastrectomy (PPG), compared with preoperative values. However, these findings recovered gradually thereafter. Debris echoes were visualized in 17 (58.6%) in 2 weeks, and in 14 (48.3%) in 4 weeks out of 29 patients. Transition from debris echoes to gallstones was comfirmed in only 2 (6.9%) of these 29 patients within 4 weeks. However, these findings disappeared gradually thereafter. By hepatobiliary scintigraphy, we found a delayed tendency of bile flow of gallbladder, peak time of the common bile duct and time of RI appearance in the duodenum, and regurgitation of bile flow to intrahepatic duct after caerulein injection (0.2μg/kg i.m.) in 7 (24.1%) out of 29 patients except one patient with PPG in addition to dysfunction of gallbladder in 4 weeks. It is concluded that denervation due to gastrectomy with lymph node dissection might cause dysfunc-tion of the sphincter of Oddi, especially failure of coordinated movement between the sphincter of Oddi and gallbladder, in addition to dysfunction of the gallbladder in an early period after gastrectomy for gastric cancer.

Lusitropic effects of alpha- and beta-adrenergic stimulation in amphibian heart.

The effects of beta and alpha-adrenergic stimulation in amphibian superfused hearts and ventricular strips were studied. Superfusion with 3 x 10(-8) M isoproterenol produced a positive inotropic effect, as detected by a 92 +/- 24% increase in the maximal rate of contraction (+T) and a positive lusitropic effect characterized by a decrease in both the ratio +T/-T (23 +/- 5%) and the half relaxation time (t1/2) (19 +/- 4%). The mechanical behavior induced by the beta-agonist was associated with an increase in the intracellular cAMP levels from control values of 173 +/- 19 to 329 +/- 28 nmol/mg wet tissue. Hearts superfused with 32P in the presence of isoproterenol showed a significant increase in Tn 1 phosphorylation (from 151 +/- 13 to 240 +/- 44 pmol 32P/mg MF protein) without consistent changes in phosphorylation of C-protein. In sarcoplasmic reticulum membrane vesicles, no phospholamban phosphorylation was detected either by beta-adrenergic stimulation of superfused hearts or when phosphorylation conditions were optimized by direct treatment of the vesicles with cAMP-dependent protein kinase (PKA) and [gamma 32P] ATP. The effect of alpha-adrenergic stimulation on ventricular strips was studied at 30 and 22 degrees C. At 30 degrees C, the effects of 10(-5) to 10(-4) M phenylephrine on myocardial contraction and relaxation were diminished to non significant levels by addition of propranolol. At 22 degrees C, blockage with propranolol left a remanent positive inotropic effect (10% of the total effect of phenylephrine) and changed the phenylephrine-induced positive lusitropic effect into a negative lusitropic action.(ABSTRACT TRUNCATED AT 250 WORDS)

Biphasic response of the rabbit detrusor muscle to changing the extracellular concentrations of potassium and calcium

1. In the presence of 1.8 mM calcium (normal Tyrode's solution) increasing the potassium concentration significantly enhanced the phasic response of the rabbit detrusor muscle to 1 Hz field stimulation but did not affect either the tonic tension or the maximal rate of contraction. 2. At low calcium (0.6 mM) increasing the potassium concentration enhanced the phasic response to 1 and 4 Hz and significantly increased the tonic tension and the rate of tension generation. 3. Increasing the extracellular calcium concentration enhanced all parameters in a dose-dependent manner at all frequencies. The effect of increasing calcium was significantly greater at 1 and 4 Hz than at 32 Hz. Surface spectroflurometry demonstrated that the increase in intracellular calcium stimulated by field stimulation paralleled the phasic contractile responses. 4. These results can be explained by potassium-induced increases in the intracellular bound calcium.

Alpha-skeletal actin is associated with increased contractility in the mouse heart.

BALB/c mice express abnormally high levels of alpha-skeletal actin in the heart, which may be related to a duplication in the promoter of the alpha-cardiac actin gene. To evaluate the effects of overexpression of the alpha-skeletal actin isoform on cardiac contractile function, we studied these mice using the isolated perfused work-performing murine heart model and measured actin isoform expression in the same hearts. We quantified myocardial contractility from the maximum rate of contraction (+dP/dt) and time to peak pressure and relaxation from -dP/dt and time to half relaxation of left intraventricular pressure. Dot blots of total RNA hybridized against oligonucleotide sequences specific for either alpha-skeletal or alpha-cardiac actin mRNA showed that increased levels of alpha-skeletal actin RNA correlated significantly with increased contractility of hearts from the BALB/c mice (r = .80, n = 15, P < .001). The present study demonstrates a significant functional correlation between alpha-actin isoform content and cardiac contractile function and also that alpha-skeletal actin may promote an increased contractile function in the heart compared with alpha-cardiac actin.

Direct Contractile Effect of Motilin on Isolated Smooth Muscle Cells from Human Gallbladder

To test the hypothesis that motilin receptors are present on smooth muscle cells of the human gallbladder, smooth muscle cells were isolated from the gallbladder and their direct contractile responses to motilin were examined. Motilin elicited a dose-dependent contraction of human gallbladder smooth muscle cells. Maximal contraction rate as defined by the percentage decrease in cell length was 23.7 ± 0.7% at 10 -7 M for motilin and was similar to that for cholecystokinin octapeptide (CCK-OP) (24.2 ± 1.5%) observed at 10 -10 M. The contractile sensitivity of gallbladder muscle cells to motilin (ED 50 = 6 p M) was two orders of magnitude less than that to CCK-OP (ED 50 = 0.07 p M). The motilin-induced contraction was inhibited in a dose-dependent manner by addition of 8-( N,N-diethylamino)octy1-3,4,5-trimethoxybenzoate hydrochloride, an inhibitor of intracellular Ca 2+ release. These results indicate that distinct motilin receptors are likely to be present on human gallbladder smooth muscle cells, as evidenced by their direct contractile responses, and suggest that Ca 2+ release from intracellular calcium stores is important in the contractile response to motilin.

Comparative Myocardial Depressive Effects of Propofol and Thiopental

To compare the direct myocardial effects of thiopental and propofol, concentration-contractility curves were determined by using an isolated guinea pig left atrial preparation. Contractility was measured as peak twitch tension, maximum rate of contraction (dT/dt), and maximum rate of relaxation (-dT/dt). Drug concentrations ranged from 56 μM (10 μg/mL) to 2800 μM (500 μg/mL) propofol and 3.8 μM (1 μg/mL) to 190 μM (50 μg/mL) thiopental. Data were fitted to a sigmoidal inhibitory pharmacodynamic model in which an increasing incremental drug concentration is needed to cause a consistent incremental effect. Comparisons were then made of the drug concentrations at which a 50% inhibition in contractility was noted (IC50). To compare drugs on an equi-anesthetic basis, comparisons also were made between IC50 values normalized to anesthetic concentrations needed to produce anesthesia (67.8 μM and 28.1 μM for thiopental and propofol, respectively). Data from seven atrial preparations for thiopental and seven atrial preparations for propofol were analyzed. The IC50 (μM) for peak twitch tension were 20.9 ± 6.3 and 96.6 ± 10.1 (P < 0.05), for dT/dt were 21.1 ± 5.9 and 86.9 ± 8.3 (P < 0.05), and for -dT/dt were 21.0 ± 6.1 and 84.3 ± 9.2 (P < 0.05) for thiopental and propofol, respectively. When the normalized IC50 were compared, the differences between anesthetics became greater. For guinea pig left atria in vitro, thiopental is a more potent myocardial depressant than propofol, especially when compared relative to anesthetic potencies; neither anesthetic preferentially affects peak twitch tension, maximum rate of contraction, or maximum rate of relaxation. (Anesth Analg 1993;77:324-9)

Effect of Tetrodotoxin on the Phasic and Tonic Responses of Isolated Rabbit Urinary Bladder Smooth Muscle to Field Stimulation

The response of the rabbit urinary bladder to field stimulation (80 volts, 2–32Hz, 1 msec duration) is biphasic, consisting of an initial phasic contraction mediated by cholinergic and purinergic neurotransmitters, followed by a prolonged tonic contraction which is solely cholinergic. Obstructive hypertrophy of the bladder induces a variety of contractile alterations including a significantly greater reduction in the tonic component of the contractile response as compared to the phasic component. This results in a severe dysfunction in the ability of the bladder to empty. One possibility is that the inability of the bladder to maintain tension and empty efficiently may be related to a degeneration of nerves innervating the bladder smooth muscle.In addition to the well documented neuropathy, the bladder undergoes hypertrophy ± hyperplasia of both smooth muscle and interstitial cellular elements, alterations in the metabolism of substrates, alterations in the synthesis of structural and smooth muscle proteins, and alterations in the deposition of collagen. The purpose of this study was to 1) to create a specific neuropathy in the absence of the additional structural, smooth muscle, and metabolic changes that are induced by partial outlet obstruction; and 2) determine if the contractile dysfunctions induced by the neuropathy have properties similar to the contractile dysfunctions induced by outlet obstruction.In the present study, a progressive “smooth muscle neuropathy” was induced in isolated strips of male rabbit urinary bladder smooth muscle by incubating isolated strips of urinary bladder body in the presence of increasing concentrations of tetrodotoxin (15–1500nM). In these studies, we determined the effect of increasing concentrations of tetrodotoxin (TTX) on the response to field stimulation utilizing 2Hz and 32Hz, at 80V and 1ms duration. The effects of TTX on maximum rate of contraction, peak contraction and tonic contraction were monitored. In addition, the effects of atropine (cholinergic muscarinic blockage) and ATP-desensitization (purinergic inhibition) on the effects of TTX were also determined. The results can be summarized as follows: 1) Both atropine and ATP desensitization individually inhibited significantly the peak response to field stimulation. 2) Atropine abolished the tonic response. 3) TTX inhibited the tonic contraction at significantly lower concentrations than it inhibited peak contraction. Thus, at low concentrations of TTX, a condition similar to that seen in obstructive hypertrophy was created. 4) The ED50 in the presence of atropine was significantly greater than the ED50 following ATP desensitization. This may indicate that there are separate synaptic elements for cholinergic and purinergic transmission. TTX blockade offers an interesting in vitro model to study the effects of neuropathy on the smooth muscle function.