Neural regulation of electrical and mechanical activities in the rat tail artery

Stimulation of the perivascular nerves elicited two types of electrical responses in the rat tail artery—excitatory junction potentials (e.j.p.s) and slow depolarization—and two types of mechanical responses—fast and slow contractions. Fast phasic contractions were triggered whenever action potentials were generated from either the e.j.p. or the slow depolarization reaching threshold. Slow tonic contractions and slow depolarizations were sensitive to -adrenergic blockade. However the slow contraction always preceded the slow depolarization. Bolus doses of exogenous noradrenaline also induced slow contraction and slow depolarization and the development of tension also preceded the membrane potential change. Increasing the external KCl also induced membrane depolarization however, contractions were not observed until the membrane was depolarized positive of –49 mV. In contrast, tension developed readily with membrane potential more negative than –49 mV with exogenous noradrenaline and neural stimulation, suggesting that the action of noradrenaline was not mediated by electromechanical coupling. It was concluded that vascular activity in the rat tail artery could be regulated by the e.j.p., the slow depolarization and also by pharmacomechanical coupling.     

Characterization of the prostanoid receptors and of the contractile effects of prostaglandin F2a in human pial arteries

The contractile and relaxant effects of various prostanoids were studied on isolated human pial arteries. Contractions were elicited with the following order of potency: U46619?U44069>PGB₂>PGF_2a>PGE₂?PGD₂>PGF_1a≥TXB₂, indicating that prostanoid-induced contractions probably are mediated by a thromboxane-sensitive receptor. Relaxation of PGF_2a-contracted arteries was induced with the order of potency: PGE₂> PGE₁>PGD₂?PGD₁. Vessels contrated by K⁺ were relaxed only by PGE,. Since PGI₂ was previously found to be more potent than all the prostanoids tested in the present study, relaxant responses are probably mediated via a PGI₂-sensitive receptor. The roles of free extracellular and cellularly bound calcium for the contractile effects of PGF_2a and K⁺ were estimated by incubating the arteries for various times in calcium-free medium containing 10^-5 M EGTA. Incubation for 5–10 min abolished K⁺-induced contractions, whereas after 40 min of incubation PGF_2a still induced contractions that reached 70% of control. The PGF_2a-induced contraction was biphasic in 8 out of 10 preparations. The second phase could be eliminated by increasing the EGTA-concentration to 10^-4 M, as well as by nifedipine pretreatment. In calcium-free, high K⁺ medium calcium-induced contractions were elicited at lower concentrations in the presence of PGF_2a. The results suggest that PGF_2a-induced contractions in human pial arteries are relatively independent of free extracellular calcium. PGF_2a may promote trans-membrane influx of calcium, as well as release calcium from seemingly superficially located cellular stores.     

Effects of 4-aminopyridine on contractile response and action potential of rabbit papillary muscle

The effects of 4-aminopyridine (4-AP) were studied on isolated papillary muscles from the heart of reserpinized rabbits (at 37°C). The preparations were paced to contract at 0.67 Hz under isometric conditions and the muscle length was adjusted to 95 % of the length for optimum force production. Simultaneous recordings of isometric force and membrane potentials were performed. 4-AP (50 μM) increased peak force by approximately 20% of the control and prolonged the action potential by 20%. Higher concentrations of 4-AP (800 μ.M) resulted in further increments of force and action potential duration (60 and 70% of controls, respectively). Prolongation of the action potential and potentiation of the isometric force was still present one hour after withdrawal of the drug from the perfusate. The results are consistent with the view that 4-AP prolongs the action potential by inhibiting the late repolarizing potassium current. It is suggested that the calcium uptake by the ventricular cell during the prolonged action potential is increased and that this leads to the positive inotropic effect.     

Characterization of action potential-triggered [Ca2+]i transients in single smooth muscle cells of guinea-pig ileum

1. To characterize increases in cytosolic free Ca²⁺ concentration ([Ca²⁺]_i) associated with discharge of action potentials, membrane potential and [Ca²⁺]_i were simultaneously recorded from single smooth muscle cells of guinea-pig ileum by use of a combination of nystatin-perforated patch clamp and fura-2 fluorimetry techniques.
2. A single action potential in response to a depolarizing current pulse elicited a transient rise in [Ca²⁺]_i. When the duration of the current pulse was prolonged, action potentials were repeatedly discharged during the early period of the pulse duration with a progressive decrease in overshoot potential, upstroke rate and repolarization rate. However, such action potentials could each trigger [Ca²⁺]_i transients with an almost constant amplitude.
3. Nicardipine (1 μM) and La³⁺ (10 μM), blockers of voltage-dependent Ca²⁺ channels (VDCCs), abolished both the action potential discharge and the [Ca²⁺]_i transient.
4. Charybdotoxin (ChTX, 300 nM) and tetraethylammonium (TEA, 2 mM), blockers of large conductance Ca²⁺-activated K⁺ channels, decreased the rate of repolarization of action potentials but increased the amplitude of [Ca²⁺]_i transients.
5. Thapsigargin (1 μM), an inhibitor of SR Ca²⁺-ATPase, slowed the falling phase and somewhat increased the amplitude, of action potential-triggered [Ca²⁺]_i transients without affecting action potentials. In addition, in voltage-clamped cells, the drug had little effect on the voltage step-evoked Ca²⁺ current but exerted a similar effect on its concomitant rise in [Ca²⁺]_i to that on the action potential-triggered [Ca²⁺]_i transient.
6. Similar action potential-triggered [Ca²⁺]_i transients were induced by brief exposures to high-K⁺ solution. They were not decreased, but rather increased, after depletion of intracellular Ca²⁺ stores by a combination of ryanodine (30 μM) and caffeine (10 mM) through an open-lock of Ca²⁺-induced Ca²⁺ release (CICR)-related channels.
7. The results show that action potentials, discharged repeatedly during the early period of a long membrane depolarization, undergo a progressive change in configuration but can each trigger a constant rise in [Ca²⁺]_i. Intracellular Ca²⁺ stores have a role, especially in accelerating the falling phase of the action potential-triggered [Ca²⁺]_i transients by replenishing cytosolic Ca²⁺. No evidence was provided for the involvement of CICR in the action potential-triggered [Ca²⁺]_i transient.

     

当药苷调节兴奋-收缩耦联改善心肌缺血再灌注损伤的机制

目的 探索当药苷通过兴奋-收缩耦联信号通路对缺血再灌注损伤后心脏收缩/舒张功能的影响。方法 24只健康雄性SD大鼠随机分为正常组、模型组、10 μmol·L^-1当药苷给药组与1 μmol·L^-1地高辛给药组，并采用Langendorff系统结合左前降支冠状动脉结扎建立缺血再灌注损伤模型（I/R），2，3，5-氯化三苯基四氮唑（TTC）染色检测各组心脏梗死面积，Powerlab生理记录仪检测血流动力学参数，如左心室舒张压（LVDP）、左心室终末舒张压（LVEDP）、左心室终末收缩压（LVESP）、左心室内压最大上升速率（+dp/dt_max）和左心室内压最大下降速率（-dp/dt_max）；提取分离乳鼠原代心肌细胞（NRCMs），建立缺氧/复氧（H/R）损伤模型，随机分为正常组、模型组、1 μmol·L^-1当药苷给药组和10 μmol·L^-1当药苷给药组，多功能成像细胞分析仪和激光共聚焦显微镜测定各组心肌细胞活力、心率、收缩幅度、收缩时程、达峰时程和舒张时程及钙瞬变峰值。根据前期转录组学测序结果及文献调研，利用实时荧光定量聚合酶链式反应（Real-time PCR）检测L型钙通道相关基因（Cacnb2），细胞色素C氧化酶相关基因（Cox6a2），肌钙蛋白（Tnnc1、Tnni3、Tnnt2）、肌动蛋白（Actc1）、肌球蛋白（Myh6、Myl2、Myl4）等兴奋-收缩耦联通路基因的mRNA表达并对差异基因进行聚类分析。结果 与正常组比较，模型组心肌梗死面积显著增加（P<0.01）、LVDP显著降低（P<0.01）、LVEDP显著上升（P<0.01）、LVESP明显下降（P<0.05）、+dp/dt_max有下降趋势和-dp/dt_max上升趋势及心肌细胞活力降低、心率降低、收缩幅度降低、收缩时程升高、达峰时程升高和舒张时程升高（P<0.01），而当药苷可以逆转上述指标（P<0.05）。此外，心肌细胞经H/R损伤后，Cacnb2、Cox6a2、Tnnc1、Tnni3、Tnnt2、Actc1、Myh6、Myl2、Myl4等兴奋-收缩耦联通路的基因表达下降（P<0.05、P<0.01）。而使用当药苷预处理后，可以增强上述基因的表达（P<0.05）。结论 当药苷通过调节兴奋-收缩耦联信号通路，从而调节原代乳鼠心肌细胞内钙离子水平，增强心肌收缩功能，对抗I/R损伤。     

Electrical membrane events underlying contraction of guinea pig bladder muscle

The relationship of electrical to mechanical activity of guinea pig bladder muscle has been studied in vitro by intracellular microelectrode and sucrose gap techniques. A discrete relationship between individual electrical action potentials and muscle contractions of small amplitude can be seen in muscle which is spontaneously active or stimulated by muscarinic activation or electrical current. An increase in the frequency of action potentials is associated with an increase in the frequency of small-amplitude contractions and the generation of muscle tension. The frequency of action potential firing is sensitive to changes in the resting membrane potential of the muscle. The findings suggest that an increase in the frequency of action potential firing may be a major mechanism in the development of muscular tension in the wall of the bladder.     

Restitution of Ca2+ Release and Vulnerability to Arrhythmias

New information has recently been obtained along two essentially parallel lines of research: investigations into the fundamental mechanisms of Ca²⁺-induced Ca²⁺ release (CICR) in heart cells, and analyses of the factors that control the development of unstable rhythms such as repolarization alternans. These lines of research are starting to converge such that we can begin to understand unstable and potentially arrhythmogenic cardiac dynamics in terms of the underlying mechanisms governing not only membrane depolarization and repolarization but also the complex bidirectional interactions between electrical and Ca²⁺ signaling in heart cells. In this brief review, we discuss the progress that has recently been made in understanding the factors that control the beat-to-beat regulation of cardiac Ca²⁺ release and attempt to place these results within a larger context. In particular, we discuss factors that may contribute to unstable Ca²⁺ release and speculate about how instability in CICR may contribute to the development of arrhythmias under pathological conditions.     

Effects of prostanoids on isolated feline cerebral arteries

The roles of extra-and intracellular calcium for the contractile effects of PGF_2α in the feline basilar artery (BA) were investigated. Comparisons were made with contractions induced by K⁺ and noradrenaline (NA). Addition of nifedipine to PGF_2α-or K⁺ (124 mM)-contracted arteries resulted in an incomplete relaxation, whereas NA-contracted vessels were completely relaxed. Incubation of the preparations in a calcium-free medium containing 10^-5 M EGTA for 5–10 min almost abolished contractions induced by K⁺ and NA. In contrast, 63 % of the response to PGF_2α remained after pretreatment of the arteries in a calcium-free solution for 40 min; PGF_2α produced a biphasic contraction in 17 out of 20 preparations consisting of a rapidly developing initial phase followed by a second increase in tension after 1–6 min. The second phase was absent if the EGTA-concentration was increased to 10^-4 M, or if the arteries were pre-treated with nifedipine. After incubation of the arteries in a calcium-free medium for 40–120 min and K⁺-depolarization, re-addition of calcium elicited contractions at lower concentrations in the presence of PGF_2α than in controls. The results suggest that PGF_2α-induced contractions in the feline BA are considerably less dependent on extracellular calcium than contractions evoked by K⁺ or NA. PGF_2α appears to be able to release calcium from two cellular stores, and may also promote calcium influx through the cell membrane.     

Connexins and junctional channels. Roles in the spreading of cardiac electrical excitation and heart development

The electrical activity in heart is generated in the sinoatrial node and then propagates to the atrial and ventricular tissues. The junctional channels that couple the cardiomyocytes are responsible for this propagation process. These channels are dodecamers of transmembrane proteins of the connexin (Cx) family. Four Cxs - Cx30.2, -40, -43 and -45--have been demonstrated to be synthesized in the cardiomyocytes. In addition, each of these Cxs has a unique expression pattern in the myocardium. A fruitful approach of the role of these Cxs in the cardiac functions came with the development of transgenic mouse models. It has been shown that Cx43 was mainly involved in influx propagation in the ventricles and that inactivation in the cardiomyocytes of the gene of this Cx predisposed to development of cardiac abnormalities. Cx40 very significantly contributes to the propagation of electrical activity in the atria and the conduction system. Cx45 is essential to coordinate the synchronization of contractile activities of embryonic cardiomyocytes and for the normal progress of cardiogenesis. Finally, Cx30.2 contributes to the slowing of propagation of excitation in the atrioventricular node. These observations enable to better understand the relationships between alteration in Cx expression or gap junction remodelling and arrhythmias in the human heart.