早期胚胎心肌细胞兴奋收缩耦联模式

目的　研究胚胎心肌细胞兴奋收缩耦联的机理；方法　使用膜片钳与钙离子浓度分析系统测量酶消化法得到的小鼠胚胎心肌细胞的膜电位与细胞内钙离子浓度；结果　细胚胎心肌细胞存在两种兴奋收缩耦联模式，一种与正常成熟心肌细胞的兴奋收缩耦联模式类似，与细胞膜上的钠通道、L-钙离子通道相关；另一种由细胞内钙振荡诱发，这种钙振荡通过细胞膜上的钠钙交换蛋白引起了细胞膜电位的小幅度变化，该模式是一种更基本的兴奋收缩模式。近似熵分析表明，与后一种模式相比较，前一种模式的规律性更强。结论　胚胎心肌细胞存在两种兴奋收缩耦联模式。     

Regulation of isometric force and isotonic shortening velocity by phosphorylation of the 20,000 dalton myosin light chain of rat uterine smooth muscle

The regulation of isometric force maintenance and isotonic shortening velocity by phosphorylation of the 20,000 dalton light chain of myosin has been examined for potassium-depolarized rat uterine smooth muscle. Following a transient peak in myosin light chain (LC₂₀) phosphorylation at 20 s of contraction (0.46 mol PO₄/mol LC₂₀), phosphorylation declined to a steady-state by 2 min (0.28 mol PO₄/mol LC₂₀) with no significant change from 2–90 min of contraction. Isometric force developed more slowly, reaching a maximum at 2 min with no further change out to 90 min. Lightly-loaded (0.1 F₀) shortening velocity, like LC₂₀ phosphorylation, increased initially to a peak of 0.034 L₀/s at 20 s of contraction and then declined to 0.023 L₀/s by 2 min. However, unlike LC₂₀ phosphorylation and isometric force, shortening velocity decreased approximately 4-fold from 0.023 L₀/s at 2 min to 0.006 L₀/s at 90 min of contraction. Graded activation with reduced extracellular calcium was associated with proportional changes in steady-state isometric force and LC₂₀ phosphorylation. Shortening velocity was also decreased with reduced calcium, however, unlike LC₂₀ phosphorylation, the greatest changes in velocity occurred at low levels of developed force. Moreover, in contrast to the large reductions in shortening velocity observed during 90 min contractions where force and LC₂₀ phosphorylation were unchanged, similar reductions in shortening velocity did not occur with graded activation in spite of significant (>3-fold) decreases in both force and LC₂₀ phosphorylation. These results suggest that factors other than light chain phosphorylation are involved in the regulation of isotonic shortening velocity during extended isometric contractions of uterine smooth muscle.     

Effect of hind-limb immobilization on contractile and histochemical properties of skeletal muscle

Summary Both hind limbs of male rats were immobilized in casts. After 4 weeks, serial sections of hind limb muscles were stained for myosin ATPase and NADH-diaphorase. The soleus from immobilized limbs had significantly fewer muscle fibers than the control soleus. Moreover, the soleus from immobilized limbs had a significantly lower percentage and lower number of fibers with low myosin ATPase activity than the soleus from control rats. Immobilization also resulted in the speed of contraction for the soleus being significantly faster than the soleus from control rats. There were no significant differences in the contractile properties or in the percentages of fibers with low myosin ATPase between rectus femoris muscles from immobilized and control limbs. The deep portion of the rectus femoris from immobilized limbs had a significantly smaller percentage of muscle fibers with high NADH-diaphorase activity than did the rectus femoris from control rats.The animals involved in this study were maintained in accordance with the Guide for Laboratory Animal Facilities and Care as published by the National Academy of Sciences-National Research Council.The research reported in this paper was conducted by personnel of the Environmental Sciences Division, USAF School of Aerospace Medicine, Aerospace Medical Division, AFSC, Brooks AFB, Texas. Further reproduction is authorized to satisfy the needs of the U.S. Government.     

The time course of changes in contraction kinetics during the tonic activation of the rat tracheal smooth muscle

The tension increase after onset of electrical stimulation (30 Hz square wave; 1.65 ms pulse duration) and after the cessation of inhibiting length vibration (frequency 100 Hz sinus; amplitude 6% of the muscle length) was analysed in the isolated rat tracheal smooth muscle. In the first experimental series, tonic contraction was interrupted by a 2 s vibration applied 4–256 s after a preceding stimulation. In the second series, the onset of force development was delayed by a long-term vibration stopping 6–258 s after the commencement of a simultaneously performed electrical stimulation. In both the experimental series tested, the time course of post-vibration tension recovery showed an initial fast and a subsequent slow component. The former reflects the kinetics of cross-bridge reattachment and the latter those of the normal actin-myosin interaction cycle. The time constants of both these components reached a minimum of 0.58±0.04 s (fast) and 3.49±0.28 s (slow component) when the vibration stopped 18 s after the start of stimulation. Both values increased to 1.29±0.15 s and 9.98±1.17 s after a preceding stimulation of 256 s. These changes in the time constants may reflect the slowing of cross-bridge action under prolonged contraction. Such variations in the time constants of post-vibration tension recovery occurred without any corresponding changes in the steady state tension developed after cessation of vibration. These results lend further support to the supposition that different mechanisms might control the rate and extent of smooth muscle contraction.     

Effect of low extracellular calcium on shortening velocity in isolated single smooth muscle cells

To understand how smooth muscle modulates its shortening velocity within the time course of a single contraction, the factors that govern crossbridge cycling in smooth muscle must be characterized. Since calcium plays an important role in regulation of the contractile apparatus, we studied the effect of lowering extracellular calcium on shortening velocity in single smooth muscle cells isolated enzymatically from the toad,Bufo marinus, stomach muscularis. Shortening velocity was estimated by three independent methods: (1) isotonic releases; (2) slack test; (3) video images of freely shortening cells. To determine the shortening velocity from isotonic releases and the slack test method, a single cell was attached at one end to an ultrasensitive force transducer and piezoelectric displacement device at its other end. At peak isometric force, in response to electrical stimulation, a series of isotonic releases to varying force levels were imposed and the resultant shortening responses used to construct force: velocity relationships in physiological saline containing low (0.18 mM) extracellular calcium. From force: velocity relationships and slack test data, an estimate of maximum shortening velocity (V _max) was 0.19 cell lengths/s which was 3 times lower than in normal calcium (Warshaw 1987). AlthoughV _max was reduced in low extracellular calcium, the curvature of the force: velocity relationship estimated from the hyperbolic constant,a/F _max, was unchanged (a/F _max=0.17). Additional evidence for an effect of extracellular calcium on shortening velocity was obtained from shortening responses of freely shortening cells attached at one end only. Data from cells exposed first to low (0.18 mM) and then normal (1.8 mM) calcium suggest at least a twofold decrease in free shortening velocity with lower extracellular calcium. These results suggest that calcium exerts its effects by altering the rate constants for specific steps in the crossbridge cycle. For example, the lowerV _max would correlate with a reduction in the rate of crossbridge detachment.This work was supported by grants from the National Institutes of Health: AR34872 and HL35684, D. M. Warshaw is a fellow of the Culpepper Foundation     

Action of a serum protein on muscular contraction

Summary The mechanism of action of a serum protein isolated from human serum was assessed in several experimental preparations including glycerol-treated muscle fibers, rat heart papillary muscle and isolatedin vitro perfused rat heart. The action of the serum protein was studied also on canine and human heart papillary muscles which were made to respond to electrical stimulation with ultrasonication modified epinephrine. In addition the action of the protein on adenosine 5 triphosphate generated precipitation of purified human actomyosin was investigated.The serum protein enhanced and intensified the generation of ATP induced tension in glycerol-extracted muscle fibers. It intensified the developed tension (DT) and increased the rate of development of tension (dT/dt) without influencing the time peak tension (TPT) of capillary muscles from rat, canine and human hearts in response to electrical stimulation. The serum protein increased the force of contraction of the isolatedin vitro perfused rat heart, and accelerated the adenosine 5 triphosphate generated precipitation of purified human heart actomyosin.     

Contraction band necrosis at the lateral borders of the area at risk in reperfused infarcts

The aim of this study was to test the hypothesis that increased mechanical stress at the lateral borders of the area at risk may render this area more susceptible to ischaemia/reperfusion injury in the absence of collateral flow. The spatial distribution of myocardial necrosis within the territory of a transiently occluded left anterior descending coronary artery was investigated in 31 porcine hearts submitted to 48 min of coronary occlusion and 6 h of reperfusion. Immediately before excising the heart, the left anterior descending coronary artery was re-occluded and 10% fluorescein was injected in the left atrium. The area at risk was imaged by ultraviolet illumination of the myocardial slices, and the area of necrosis by incubation in triphenyltetrazolium chloride. The area at risk was divided in four sectors and an index of eccentricity was calculated as the percent of the area of necrosis located in the two lateral sectors of the area at risk. The area of contraction band necrosis was measured in whole heart histological sections. Infarcts were generally small, and were composed almost exclusively of contraction band necrosis. There was a good correlation between the extent of the area of contraction band necrosis and infarct size (r=0.831, P<0.0005). The area of necrosis had a patchy appearance and was predominantly distributed along the lateral borders of the area at risk. This eccentric distribution was more prominent in smaller infarcts, and the eccentricity index was inversely correlated with infarct size (r=–0.471, P=0.007), suggesting that contraction band necrosis occurres first at the interface between control and reperfused myocardium in this model. These results are in agreement with a prominet role of mechanical factors in the genesis of myocardial necrosis during transient coronary occlusion.Partially supported by Grants 92/0443 from the Fondo de Investigaciones Sanitarias de la Seguridad Social, CRHG 07-92/79 from the Institut Català de la Salut, and Concerted Action BMH1-CT92/1501, BIOMED I Program from the European Union.     

Characterisation of some pharmacological effects of the venom from Vipera lebetina.

Vipera lebetina is one of the most venomous snakes on the Iran plateau. Serious clinical problems such as edema, hemorrhage and tissue necrosis are observed in humans following V. lebetina envenomating. However, little information on the pharmacological properties of the venom is available. To determine haemodynamic actions of the venom of V. lebetina, the changes in the mean arterial blood pressure of anaesthetised rats following the administration of the venom were recorded. Venom (1 mg/kg, i.v.) produced rapid cardiovascular collapse, while 0.3 mg/kg (i.v.) caused only a small transient decrease in mean arterial blood pressure. Effects of the venom on perfusion pressure in the isolated rat mesenteric bed, and on contractions of the isolated rat right atrium and the isolated guinea-pig ileum, were also studied. Exposure of the isolated rat right atrium to venom (0.1-1 mg/ml) caused a transient increase followed by a sustained reduction in the amplitude and frequency of spontaneous contractions. The transient positive inotropic and chronotropic effects were abolished when the preparation was preincubated with propranolol, but not with tolazoline. N(G)-nitro-l-arginine methyl ester pretreatment attenuated the vascular hyporeactivity to phenylephrine induced by the venom in the isolated rat mesenteric vascular bed. This suggests that nitric oxide (NO) or NO-like compounds may be present in the venom and involved in its hypotensive effect. The venom (0.3-1 mg/ml) caused concentration-dependant blockade of isolated guinea-pig ileum contractions induced by electrical field stimulation, acetylcholine or KCl. This inhibitory effect of the venom was significantly reduced by prior incubation of the venom with manoalide (1 microM) indicating involvement of a phospholipase A(2) component. Further, investigation is required to identify specific toxins responsible for the above pharmacological effects.     

硬膜外镇痛对剖宫产术后子宫收缩的影响

目的：观察硬膜外镇痛对剖宫产术后子宫收缩的影响。方法：将90例ASA（Ⅰ～Ⅱ）级剖宫产病例随机分成两组,A组：对照组,B组：硬膜外镇痛（PCEA）组。A组术后拔除硬膜外导管,常规以哌替啶止痛;B组术后用韩国产镇痛泵止痛,其方法：0．75％布比卡因30ml、吗啡8mg、氟哌啶5mg用0．9％氯化钠稀释至100ml,每小时2ml镇痛液持续硬膜外注入。回病房后由麻醉医师和主管医师共同观察止痛效果、不良反应、哺乳时间、术后24h内阴道出血量及术后48h内宫底高度变化并记录。结果：B组效果显著优于A组,恶心、呕吐等不良反应也较少;B组哺乳时间稍有提前（P〈0．05）;阴道出血量及宫底高度变化两组差别无显著性（P〉0．05）。结论：硬膜外镇痛用于剖宫产术后临床效果良好,对子宫收缩无不良影响,值得推广。     

The influence of changes in the stimulation pattern on force and fusion in motor units of the rat medial gastrocnemius muscle

The effects of irregularity in the pattern of stimulation on the tension produced by motor units in the rat medial gastrocnemius muscle were investigated. The effects of decreasing as well as increasing the interpulse intervals were observed for each motor unit in tetani fused to different degrees. For each motor-unit type, it was found that the effects of these changes depended on the extent of tetanic fusion. Decreasing the interpulse interval produced an increase in tension during the tetanus: the more fused the profile of tetanus, the smaller the tension increase. Increasing the interpulse interval resulted in a decrease in tetanic tension. This effect was most prominent when the tetanic fusion index was approximately 0.75. This phenomenon resulted from the prolongation in relaxation when tetanic fusion increased, thereby preventing a decrease in tension when the interpulse interval increased. We also investigated the effects of introducing a short interpulse interval (”doublet”) at the beginning of the stimulation. The doublets produced increased tetanic tension with a more fused profile. However, the doublet enhanced the sensitivity of the tetanus to increases in interpulse interval and decreased its sensitivity to decreases in interpulse intervals. Slow-twitch motor units appeared to be significantly less sensitive to both increases and decreases in interpulse interval than fast-twitch units. This suggests that slow-twitch units are better suited for producing long-lasting contractions with a constant tension level. Conversely, the high sensibility of fast-twitch units to changes in stimulation frequency enhances their participation in regulation of tension of the muscular contraction. Received: 25 March 1998 / Accepted: 24 March 1999