糖尿病大鼠心肌电学的改变及环维黄杨星D对其电生理的影响

目的：观察糖尿病大鼠心肌电学变化特点，探讨环维黄杨星D(CVB-D)对糖尿病心肌电生理的影响。 方法： 以SD雄性大鼠尾静脉注射四氧嘧啶(alloxan,50 mg·kg^-1)复制糖尿病模型，选择以年龄相匹配的SD雄性大鼠作为对照组。2周后，记录大鼠体表心电图和右心室乳头肌跨膜电位，观察CVB-D对跨膜电位的影响。 结果： 糖尿病造模后第2周时，心率明显减慢于对照组，体表心电图QT间期和右室乳头肌动作电位时程（APD）各水平均明显长于对照组大鼠（P<0.01）,而静息膜电位（RP）、动作电位幅度(APA)和 超射值（OS）以及0期去极化最大速率（Vmax）均无明显变化。CVB-D具有剂量依赖效应，在13.3-63.3 μmol·L^-1范围内呈剂量依赖性地延长糖尿病大鼠和对照组大鼠APD30、APD50、APD70和APD90，对糖尿病大鼠APD延长作用更大。在33.3-63.3 μmol·L^-1浓度范围内，CVB-D呈剂量依赖性抑制糖尿病大鼠和对照组大鼠的静息电位(RP)、动作电位幅值（APA）和0期最大去极化速度（Vmax），但对糖尿病组抑制更大。研究结果还显示，CVB-D还具有时间依赖效应，当20 μmol·L^-1灌流心室肌10 min后开始出现作用，对照组到40 min左右作用达高峰，而糖尿病组40 min后仍继续延长。 结论： 糖尿病大鼠右室乳头肌动作电位时程和QT间期明显延长。CVB-D可进一步延长糖尿病大鼠的APD，抑制其RP、APA、OS以及Vmax，作用较对照组明显。     

Paradox response of frog muscle membrane to changes in external potassium

Applying conventional microelectrode technique the anomalous behaviour of membrane potential in response to changes in [K⁺]_o was demonstrated in normal and cevadine-treated muscles bathed in Cl^–-free medium. Partial repolarization of the cevadine-depolarized membrane and reappearance of the slow membrane potential oscillation (SMPO) were induced by elevating [K⁺]_o from 2.5 mM to 10–20 mM. Both effects were reversed by return to 2.5 mM [K⁺]_o. The K-induced repolarization was markedly reduced by 20 mM Cs⁺, but not by 0.1 mM ouabain, 1 mM 4-aminopyridine, or 1 mM diethyl-pyrocarbonate. The elevation of [K⁺]_o failed to repolarize muscle fibers that had been depolarized only to a small extent. No K-induced repolarization has been observed in Cl^–-containing fluid. In cevadine-free experiments the omission of potassium from the extracellular space in Cl^–-free solution hyperpolarized some of the fibers, while depolarized others. Strong electrical stimuli applied in zero K-zero Cl solution turned all the fibers into depolarized state; on returning to 2.5 mM [K⁺]_o complete repolarization was achieved in most of the fibers. It has been concluded that the paradox response of the muscle membrane to changes in [K⁺]_o can be attributed to the K-dependent conductance changes of the inward rectifier K channel providing an explanation for the plateau-formation of SMPO and for the existence of two stable levels of membrane potential of the skeletal muscle bathed in Cl^–-free medium.     

In skeletal muscle the relaxation of the resting membrane potential induced by K+ permeability changes depends on Cl− transport

In resting skeletal muscle the potassium permeability is determined by the permeability of the inwardly potassium rectifier. Continuous resting membrane potential measurements are done to follow the relaxation of the membrane potential upon changes in potassium permeability. Inhibition of the inwardly potassium rectifier, by extracellular application of 80 µM Ba²⁺, causes the cell to depolarize with mean time constants as follows: in control 127±7 s (n=23), in the presence of bumetanide, as an inhibitor of the Na⁺/K⁺/2Cl^– cotransporter, 182±23 s (n=7), in hypertonic media (340 mosmol/kg) 90.4±5 s (n=7) and in reduced chloride medium 64±8 s (n=5). The depolarizing relaxation of the membrane potential induced by reduction of extracellular potassium produces similar results. These time constants are at least three orders of magnitude slower than the time constants reported in the literature for the inhibition of the inwardly potassium rectifier. Chloride transport affects the relaxation of the membrane potential. A further characterization of chloride transport is done by following the relaxation of the membrane potential upon application of chloride transport modulators. It is argued that the electroneutral cotransporter, for which a flux was preliminarily estimated of 13.4 pmol cm^–2 s^–1, has a considerable role in the processes related to the resting membrane potential.     

Prerequisites and limitations to isokinetic measurements in humans

Physical and technical limitations have to be considered when measuring small torques over a wide range of angles during joint movements in humans using isokinetic methods, i.e. at constant angular velocities. In the concentric mode, during the initial phase, the subject must actively accelerate the lever of the isokinetic dynamometer to a preset velocity. To guarantee an adequate duration of the isokinetic phase at high preset velocities, greater torques are necessary to produce appropriate accelerations, otherwise the isokinetic phase will be increasingly shortened until the preset velocity can no longer be attained. The servomotor-controlled dynamometer used (LIDO-Active 2.1) continuously records torque during the whole movement, irrespective of differences between the actual and the preset velocity, but the user is not informed about which torques have been recorded outside the isokinetic phase and at which actual velocity. Thus, in the evaluation of torques in the nonisokinetic range, errors occur due to assigning torque measurements to incorrect velocities and due to the non-negligible accelerations involved. In exploratory tests on the dynamometer using physical methods to produce constant torques, an attempt has been made to ascertain the range over which isokinetic conditions were satisfied, hence providing a reliable basis for isokinetic studies in sports medicine and sports sciences.     

Effects of enflurane on excitation-contraction coupling in frog skeletal muscle fibers

The effect of enflurane on the excitation-contraction (E-C) coupling of frog skeletal muscle fiber was studied.Low (19.13±0.70 mg%) and high (108.52±4.52 mg%) concentrations of enflurane did not cause substantial changes of the resting membrane potential but a high concentration of enflurane reduced the peak of action potentials and prolonged the duration measured at 50% peak amplitude.During the twitch response, enflurane reduced the peak response of light signal measured with calcium-sensitive photoprotein, aequorin, butpeak tension was not diminished by a low concentration of enflurane. A high concentration of enflurane remarkably inhibited both light signal and tension.During tetanus, a low concentration of enflurane partially abolished light signal and tension. At the high concentration, this inhibitory effect was pronuunced and action potentials were only observed in the initial phase of tetanic stimulation.Enflurane enhanced the increase of light signal observed in rapid cooling contracture.In glycerinated muscle fiber, enflurane shifted the pCa-tension relation to the left in low calcium and suppressed maximally activated tension at high calcium concentration.Inhibitory effect of enflurane on light signal in twitch response without abolition of action potential suggests that enflurane might inhibit E-C coupling in frog skeletal muscle fiber (but it enhances direct calcium release from SR induced by caffeine) and increase calcium sensitivity of contractile element.     

Skeletal muscle excitation-contraction coupling II

Bundles of cells, intact from tendon to tendon, were dissected from muscles of normal and malignant hyperthermia susceptible (MHS) pigs. Intact bundles were stimulated either (1) electrically, to elicit twitches and tetani, or (2) ionically with elevated extracellular K⁺ (K ₀ ⁺ ), to elicit K-contractures. Maximal tetanic force was the same for MHS and normal intact bundles. In MHS muscles, when responses were elicited from control resting plasmalemma polarization (4 mM K ₀ ⁺ ), twitches and K-contractures were significantly larger and the K-contracture activation curve was shifted towards lower [K⁺]₀ with respect to normal bundles. Resting hyperpolarization (2 mM K ₀ ⁺ ) selectively reduced MHS twitch force to normal and K-contracture force toward normal. For K-contracture and twitches, there was a range of K ₀ ⁺ concentrations (7–10 mM), representing resting depolarization, which enhanced subsequent twitch and K-contracture magnitude in both MHS and normal intact bundles as compared to responses elicited from control (4 mM K ₀ ⁺ ). These results are consistent with the hypothesis that resting plasmalemmal voltage sets the gain of sarcoplasmic reticulum calcium release in MHS and normal intact bundles independent of type of stimulation and suggest that a defect in this mechanism may be responsible for the enhanced twitches and K-contractures of MHS muscles.     

Fiber differentiation of the human laryngeal muscles using the inhibition reactivation myofibrillar ATPase technique

Summary The aim of the present study was to further subdivide the type II fibers of the human thyroarytenoid and posterior cricoarytenoid muscles by means of a modified myosin ATPase reaction. In order to understand the functioning of these highly strained muscles better, it is important to know the respective percentage of fatigue-resistant type IIA fibers and fatigable type IIB fibers. The material comprised the larynges of seven laryngectomized males aged between 45 and 70 years and four laryngectomized females aged between 39 and 72 years. After having been frozen in nitrogen, 10-m-thick sections were cut from the laryngeal muscles in a cryostat. The pH-lability of the enzyme that can be utilized in a classical myosin ATPase reaction permits a differentiation between fiber types I, IIA and IIB. Evidently, this is not possible with every human muscle. The fiber types IIA and IIB of the thyroarytenoid and the posterior cricoarytenoid muscles could be clearly distinguished by means of the inhibition reactivation myofibrillar ATPase technique. Using this method, the myosin ATPase enzyme was initially inhibited by hydroxymer curibenzoate and subsequently reactivated by cysteine. Regarding the incidence of type I and IIA fibers, there was a statistically significant difference between the thyroarytenoid and the posterior cricoarytenoid muscles. The type IIA fiber content was statistically significantly higher in the arytenoid muscle than in the posterior cricoarytenoid muscle. The percentage of type IIB fibers was low, not only in the thyroarytenoid muscle and the posterior cricoarytenoid muscle but also in the other laryngeal muscles. The share of fiber types I, IIA and IIB in the thyroarytenoid muscles varied greatly from one patient to another. This was also true for the other laryngeal muscles. This aspect may be especially significant with regard to an individual's vocal character and vocal fatigability under stress.     

Alterations of molecular architectures in skeletal muscle plasma membranes of dehydrated and water-loaded mice

It is likely that orthogonal arrays (OAs) and caveolae seen on the replicas of freeze-fractured muscle plasma membranes are involved in maintaining osmotic homeostasis. Therefore, using the freeze-fracture technique, we examined the ultrastructural changes in OAs and caveolae of the skeletal muscle plasma membrane of dehydrated and water-loaded mice. In the muscle plasma membranes of 6 dehydrated and 6 water-loaded mice, caveolar distribution was not altered, and the densities of caveolae and OAs did not show statistically significant differences when compared with those in 6 control mice, although the skeletal muscles of water-loaded mice sometimes had muscle plasma membranes with extremely numerous OAs. In contrast, the muscle plasma membranes of dehydrated mice often revealed changes in the distribution of OAs, which existed in a group at the confined area of the muscle plasma membranes and were frequently accompanied by the aggregations of intramembranous particles (IMPs) around OAs. Thus, on the basis of the present study, we suggest that OAs in skeletal muscles as well as those in brain may play an important role in maintaining osmotic homeostasis of these organs under abnormal water balance.     

Patterns of neck muscle activation in cats during reflex and voluntary head movements

Summary When the head rotates, vestibulocollic reflexes counteract the rotation by causing contraction of the neck muscles that pull against the imposed motion. With voluntary head rotations, these same muscles contract and assist the movement of the head. The purpose of this study was to determine if an infinite variety of muscle activation patterns are available to generate a particular head movement, or if the CNS selects a consistent and unique muscle pattern for the same head movement whether performed in a voluntary or reflex mode. The relationship of neck muscle activity to reflex and voluntary head movements was examined by recording intramuscular EMG activity from six neck muscles in three alert cats during sinusoidal head rotations about 24 vertical and horizontal axes. The cats were trained to voluntarily follow a water spout with their heads. Vestibulocollic reflex (VCR) responses were recorded in the same cats by rotating them in an equivalent set of planes with the head stabilized to the trunk so that only the vestibular labyrinths were stimulated. Gain and phase of the EMG responses were calculated, and data analyzed to determine the directions of rotation for which specific muscles produced their greatest EMG output. Each muscle exhibited preferential activation for a unique direction of rotation, and weak responses during rotations orthogonal to that preferred direction. The direction of maximal activation could differ for reflex and voluntary responses. Also, the best excitation of the muscle was not always in the direction that would produce a maximum mechanical advantage for the muscle based on its line of pull. The results of this study suggest that a unique pattern of activity is selected for VCR and tracking responses in any one animal. Patterns for the two behaviors differ, indicating that the CNS can generate movements in the same direction using different muscle patterns.