Regulation of calcium release is gated by calcium current, not gating charge, in cardiac myocytes

In skeletal muscle, intramembrane charge movement initiates the processes that lead to the release of calcium from the sarcoplasmic reticulum. In cardiac muscle, in contrast, the similarity of the voltage dependence of developed tension and intracellular calcium transients to that of calcium current suggests that the calcium current may gate the release of calcium. Nevertheless, a mechanism similar to that of skeletal muscle continues to be postulated for cardiac muscle. By using rapid exchange (20 to 50 milliseconds) of the extracellular solutions in rat ventricular myocytes in which the intracellular calcium transients or cell shortening were measured, it has now been shown that the influx of calcium through the calcium channel is a mandatory link in the processes that couple membrane depolarization to the release of calcium. Thus, intramembrane charge movement does not contribute to the release of calcium in heart muscle.     

Calcium ion release in mechanically disrupted heart cells

In cardiac muscle fibers which have had their sarcolemma disrupted intracellular stores of calcium ions can be released by the same chemical stimuli which cause their release from the sarcoplasmic reticulum of skinned skeletal muscle fibers. These stimuli are increases in calcium or caffeine concentrations and substitution of chloride for propionate or sodium for potassium in solutions bathing the fibers.     

Ryanodine receptor of skeletal muscle is a gap junction-type channel

In the sarcoplasmic reticulum membrane of skeletal muscle, the ryanodine receptor forms an aqueous pore identified as the calcium-release pathway that operates during excitation-contraction coupling. The purified ryanodine receptor channel has now been shown to have four properties usually associated with gap junction channels: (i) a large nonspecific voltage-dependent conductance consisting of several open states; (ii) an inhibition of open probability by low pH; (iii) an inhibition of open probability by calcium; and (iv) a sensitivity to blockade by heptanol and octanol but not other alcohols. This functional homology may provide an insight into the mechanism of how muscle cells transduce depolarization into an intracellular release of calcium.     

Effect of membrane potential changes on the calcium transient in single rat cardiac muscle cells

The mechanism that links membrane potential changes to the release of calcium from internal stores to cause contraction of cardiac cells is unclear. By using the calcium indicator fura-2 under voltage-clamp conditions, changes in intracellular calcium could be monitored in single rat ventricular cells while controlling membrane potential. The voltage dependence of the depolarization-induced increase in intracellular calcium was not the same as that of the calcium current (Isi), which suggests that only a small fraction of Isi is required to trigger calcium release from the sarcoplasmic reticulum. In addition, sarcoplasmic reticulum calcium release may be partly regulated by membrane potential, since repolarization could terminate the rise in intracellular calcium. Thus, changes in the action potential will have immediate effects on the time course of the calcium transient beyond those associated with its effects on Isi.     

骨骼肌兴奋收缩偶联潜伏期肌浆网内Ca~(2 )变化的研究——细胞化学法及X-射线能量色散谱微区分析

报道了采用红外探测－计算机控制的超低温快速冷冻固定技术、Ｘ－射线能量色散谱微区定量分析及Ｃａ２＋细胞化学技术对骨骼肌收缩潜伏期时肌浆网内的Ｃａ２＋进行了分析,从超微结构形态学的角度对骨骼肌兴奋－收缩偶联发生时,肌浆网内Ｃａ２＋的作用进行了研究,这对揭示骨骼肌兴奋－收缩偶联时,肌浆网内Ｃａ２＋的释放机理研究有重要意义。     

Sodium current-induced release of calcium from cardiac sarcoplasmic reticulum

The role of sodium-calcium exchange at the sarcolemma in the release of calcium from cardiac sarcoplasmic reticulum was investigated in voltage-clamped, isolated cardiac myocytes. In the absence of calcium entry through voltage-dependent calcium channels, membrane depolarization elicited release of calcium from ryanodine-sensitive internal stores. This process was dependent on sodium entry through tetrodotoxin-sensitive sodium channels. Calcium release under these conditions was also dependent on extracellular calcium concentration, suggesting a calcium-induced trigger release mechanism that involves calcium entry into the cell by sodium-calcium exchange. This sodium current-induced calcium release mechanism may explain, in part, the positive inotropic effects of cardiac glycosides and the negative inotropic effects of a variety of antiarrhythmic drugs that interact with cardiac sodium channels. In response to a transient rise of intracellular sodium, sodium-calcium exchange may promote calcium entry into cardiac cells and trigger sarcoplasmic calcium release during physiologic action potentials.     

The Role of Extracellular Ca2+Influx, Intracellular Ca2+ Release and Calmodulin in Mouse Egg Fertilization

The effects of various Ca^2 -modifying drugs on moue egg fertilization were studied.Ca^2 chelator,ethylen glycol-bis-(2-aminoethyl)-tetracetic acid(EGTA),and calmodulin(CaM) antagonist,trifluoperzaine (TFP),inhibited fertilization in a dose-dependent manner,whild Ca^2 channel bolcker,verspamil,did not have any effect.When intracellular Ca^2 release was blocked by 8-(N,N-diethylamino) octy 1-3,4,5-trimethoxy-benzonate(TME-8) or the Ca^2 oscillations were inhibited by an inhibitor of endoplasmic reticulum Ca^2 -At-Pase,thapsigargin,the second polar body emission and pronuclear formation were significantly decreased.In contrast,inhibition of intracellular Ca^2 release via bolckage of inositol 1,4,5-triphosphate (IP3) production by neomycin or lithium did not affect fertilization.The results sugest that both extracellular influx,intracellular Ca^2 release and CaM activation are required for mormal fertilization.However,extracellular influx through voltage-gated Ca^2 channel and intracellular release induced by IP3 and not the only pathways for producing Ca^2 transients in moue eggs.     

Sodium-calcium exchange in heart: membrane currents and changes in [Ca2+]i

Recordings have been made of changes in intracellular calcium ion concentration ([Ca2+]i) that can be attributed to the operation of an electrogenic, voltage-dependent sodium-calcium (Na-Ca) exchanger in mammalian heart cells. Guinea pig ventricular myocytes under voltage clamp were perfused internally with fura-2, a fluorescent Ca2+-indicator, and changes in [Ca2+]i and membrane current that resulted from Na-Ca exchange were identified through the use of various organic channel blockers and impermeant ions. Depolarization of cells elicited slow increases in [Ca2+]i, with the maximum increase depending on internal [Na+], external [Ca2+], and membrane voltage. Repolarization was associated with net Ca2+ efflux and a decline in the inward current that developed instantaneously upon repolarization. The relation between [Ca2+]i and current was linear, and the slope was made steeper by hyperpolarization.     

Diazacholesterol myotonia: accumulation of desmosterol and increased adenosine triphosphatase activity of sarcolemma

Myotonia induced in rats by 20,25-diazacholesterol is accompanied by accumulation of desmosterol in serum, fragmented sarcoplasmic reticulum, and sarcolemma. Activities of (Na(+),K(+))- and Ca(2+)-stimulated adenosine triphosphatases of the sarcolemma are increased, but not the Mg(2+)-stimulated adenosine triphosphatase. The altered sterol composition of the sarcolemma may cause this type of myotonia by decreasing the chloride conductance of the membrane.     

Intracellular impulse conduction in muscle cells

A hypothesis, suggested previously by morphological studies, for impulse conduction from the sarcolemma to the contractile material via the sarcoplasmic reticulum is discussed. The relation of reticulum morphology and cell size to speed of contraction in smooth and striated muscle agrees with the hypothesis and thus supports it. Additional support comes from evidence concerning an unusual morphological relationship between the sarcolemma and contractile fibrils in striated muscle of amphioxus.     

Regenerative calcium release within muscle cells

Free calcium appears to trigger the release of stored calcium from the sarcoplasmic reticulum of skinned skeletal muscle fibers immersed in solutions with a low concentration of magnesium ion.     

X-ROS signaling: rapid mechano-chemo transduction in heart

We report that in heart cells, physiologic stretch rapidly activates reduced-form nicotinamide adenine dinucleotide phosphate (NADPH) oxidase 2 (NOX2) to produce reactive oxygen species (ROS) in a process dependent on microtubules (X-ROS signaling). ROS production occurs in the sarcolemmal and t-tubule membranes where NOX2 is located and sensitizes nearby ryanodine receptors (RyRs) in the sarcoplasmic reticulum (SR). This triggers a burst of Ca(2+) sparks, the elementary Ca(2+) release events in heart. Although this stretch-dependent "tuning" of RyRs increases Ca(2+) signaling sensitivity in healthy cardiomyocytes, in disease it enables Ca(2+) sparks to trigger arrhythmogenic Ca(2+) waves. In the mouse model of Duchenne muscular dystrophy, hyperactive X-ROS signaling contributes to cardiomyopathy through aberrant Ca(2+) release from the SR. X-ROS signaling thus provides a mechanistic explanation for the mechanotransduction of Ca(2+) release in the heart and offers fresh therapeutic possibilities.     

Increased activity of calcium leak channels in myotubes of Duchenne human and mdx mouse origin

Elevated free Ca2+ concentrations found in adult dystrophic muscle fibers result in enhanced protein degradation. Since the difference in concentrations may reflect differences in entry, Ca2+ leak channels in cultures of normal and Duchenne human myotubes, and normal and mdx murine myotubes, have been identified and characterized. The open probability of leak channels is markedly increased in dystrophic myotubes. Other channel properties, such as mean open times, single channel conductance, ion selectivity, and behavior in the presence of pharmacological agents, were similar among myotube types. Compared to the Ca2+ concentrations in normal human and normal mouse myotubes, intracellular resting free Ca2+ concentrations ([Ca2+]i) in myotubes of Duchenne and mdx origin were significantly higher at a time when dystrophin is first expressed in normal tissue. Taken together, these findings suggest that the increased open probability of Ca2+ leak channels contributes to the elevated free intracellular Ca2+ concentration in Duchenne human and mdx mouse myotubes.     

BAX and BAK regulation of endoplasmic reticulum Ca2+: a control point for apoptosis

BAX and BAK are "multidomain" proapoptotic proteins that initiate mitochondrial dysfunction but also localize to the endoplasmic reticulum (ER). Mouse embryonic fibroblasts deficient for BAX and BAK (DKO cells) were found to have a reduced resting concentration of calcium in the ER ([Ca2+]er) that results in decreased uptake of Ca2+ by mitochondria after Ca2+ release from the ER. Expression of SERCA (sarcoplasmic-endoplasmic reticulum Ca2+ adenosine triphosphatase) corrected [Ca2+]er and mitochondrial Ca2+ uptake in DKO cells, restoring apoptotic death in response to agents that release Ca2+ from intracellular stores (such as arachidonic acid, C2-ceramide, and oxidative stress). In contrast, targeting of BAX to mitochondria selectively restored apoptosis to "BH3-only" signals. A third set of stimuli, including many intrinsic signals, required both ER-released Ca2+ and the presence of mitochondrial BAX or BAK to fully restore apoptosis. Thus, BAX and BAK operate in both the ER and mitochondria as an essential gateway for selected apoptotic signals.     

Ca(2+)-induced Ca2+ release in sea urchin egg homogenates: modulation by cyclic ADP-ribose

Calcium-induced calcium release (CICR) may function widely in calcium-mediated cell signaling, but has been most thoroughly characterized in muscle cells. In a homogenate of sea urchin eggs, which display transients in the intracellular free calcium concentration ([Ca2+]i) during fertilization and anaphase, addition of Ca2+ triggered CICR. Ca2+ release was also induced by the CICR modulators ryanodine and caffeine. Responses to both Ca2+ and CICR modulators (but not Ca2+ release mediated by inositol 1,4,5-trisphosphate) were inhibited by procaine and ruthenium red, inhibitors of CICR. Intact eggs also displayed transients of [Ca2+]i when microinjected with ryanodine. Cyclic ADP-ribose, a metabolite with potent Ca(2+)-releasing properties, appears to act by way of the CICR mechanism and may thus be an endogenous modulator of CICR. A CICR mechanism is present in these nonmuscle cells as is assumed in various models of intracellular Ca2+ wave propagation.     

Localization of calcium pump activity in smooth muscle

A microsomal fraction isolated from longitudinal smooth muscle of guinea pig ileum actively sequesters calcium ion in the presence of magnesium and adenosine triphosphate in a fashion previously described for microsomes of the rabbit aorta. This activity in guinea pig ileum appears to be associated primarily with the plasma membrane as is found in the red cell. By contrast the uptake of calcium in aortic smooth muscle appears to be associated to an appreciable extent with intracellular membranes, possibly analogous to the sarcoplasmic reticulum of skeletal muscle.     

Protection from cardiac arrhythmia through ryanodine receptor-stabilizing protein calstabin2

Ventricular arrhythmias can cause sudden cardiac death (SCD) in patients with normal hearts and in those with underlying disease such as heart failure. In animals with heart failure and in patients with inherited forms of exercise-induced SCD, depletion of the channel-stabilizing protein calstabin2 (FKBP12.6) from the ryanodine receptor-calcium release channel (RyR2) complex causes an intracellular Ca2+ leak that can trigger fatal cardiac arrhythmias. A derivative of 1,4-benzothiazepine (JTV519) increased the affinity of calstabin2 for RyR2, which stabilized the closed state of RyR2 and prevented the Ca2+ leak that triggers arrhythmias. Thus, enhancing the binding of calstabin2 to RyR2 may be a therapeutic strategy for common ventricular arrhythmias.     

Mitogens and oncogenes can block the induction of specific voltage-gated ion channels

The mechanisms underlying the ontogeny of voltage-gated ion channels in muscle are unknown. Whether expression of voltage-gated channels is dependent on mitogen withdrawal and growth arrest, as is generally true for the induction of muscle-specific gene products, was investigated in the BC3H1 muscle cell line by patch-clamp techniques. Differentiated BC3H1 myocytes expressed functional Ca2+ and Na+ channels that correspond to those found in T tubules of skeletal muscle. However, Ca2+ and Na+ channels were first detected after about 5 days of mitogen withdrawal. In order to test whether cellular oncogenes, as surrogates for exogenous growth factors, could prevent the expression of ion channels whose induction was contingent on mitogen withdrawal, BC3H1 cells were modified by stable transfection with oncogene expression vectors. Expression vectors containing v-erbB, or c-myc under the control of the SV40 promoter, delayed but did not prevent the appearance of functional Ca2+ and Na+ channels. In contrast, transfection with a Val12 c-H-ras vector, or cotransfection of c-myc together with v-erbB, suppressed the formation of functional Ca2+ and Na+ channels for greater than or equal to 4 weeks. Potassium channels were affected neither by mitogenic medium nor by transfected oncogenes. Thus, the selective effects of certain oncogenes on ion channel induction corresponded to the suppressive effects of mitogenic medium.     

钙离子在植物生理调节中的作用

钙是植物必须的营养元素,同时也是植物体内转导多种生理过程的胞内胞外信号物质之一。胞外Ca2+通过Ca2+通道内流进入胞质,并通过Ca2+-ATPase和Ca2+/H+反向转运蛋白外流,以保持胞质内低Ca2+浓度。同时为了应对植物发育和环境胁迫信号,Ca2+由质膜、液泡膜和内质网膜的Ca2+通道内流进入胞质,导致胞质Ca2+浓度迅速增加,产生钙瞬变和钙振荡,传递到钙信号靶蛋白(如钙调素、钙依赖型蛋白激酶及钙调磷酸酶B类蛋白,引起特异的生理生化反应),这一系列钙信号调节、应答机制构成了植物的钙信号系统。对钙转运系统、钙信号调节和放大及应答方式进行了综述。     

Calcium-induced movement of troponin-I relative to actin in skeletal muscle thin filaments

The role of troponin-I (the inhibitory subunit of troponin) in the regulation by Ca2+ of skeletal muscle contraction was investigated with resonance energy transfer and photo cross-linking techniques. The effect of Ca2+ on the proximity of troponin-I to actin in reconstituted rabbit skeletal thin filaments was determined. The distance between the cysteine residue at position 133 (Cys133) of troponin-I and Cys374 of actin increases by approximately 15 angstroms on binding of Ca2+ to troponin-C. Also, troponin-I labeled at Cys133 with benzophenone-4-maleimide could be photo cross-linked to actin in the absence of Ca2+, but not in its presence. These results suggest that troponin-I is attached to actin in the Ca2(+)-free or relaxed state of muscle, and that it detaches from actin on Ca2+ activation of contraction. Thus, troponin-I may function as a Ca2(+)-dependent molecular switch in regulation of skeletal muscle contraction.