A slow component of intramembranous charge movement during sarcoplasmic reticulum calcium release in frog cut muscle fibers

Cut muscle fibers from Rana temporaria were mounted in a double Vaseline-gap chamber and equilibrated with an end-pool solution that contained 20 mM EGTA and 1.76 mM Ca (sarcomere length, 3.3-3.8 microns; temperature, 14-16 degrees C). Sarcoplasmic reticulum (SR) Ca release, delta[CaT], was estimated from changes in myoplasmic pH (Pape, P.C., D.-S. Jong, and W.K. Chandler. 1995. J. Gen. Physiol. 106:259-336). The maximal value of delta[CaT] obtained during a depleting depolarization was assumed to equal the SR Ca content before stimulation, [CaSR]R (expressed as myoplasmic concentration). After a depolarization to -55 to -40 mV in fibers with [CaSR]R = 1,000-3,000 microM, currents from intramembranous charge movement, Icm, showed an early I beta component. This was followed by an I gamma hump, which decayed within 50 ms to a small current that was maintained for as long as 500 ms. This slow current was probably a component of Icm because the amount of OFF charge, measured after depolarizations of different durations, increased according to the amount of ON charge. Icm was also measured after the SR had been depleted of most of its Ca, either by a depleting conditioning depolarization or by Ca removal from the end pools followed by a series of depleting depolarizations. The early I beta component was essentially unchanged by Ca depletion, the I gamma hump was increased (for [CaSR]R > 200 microM), the slow component was eliminated, and the total amount of OFF charge was essentially unchanged. These results suggest that the slow component of ON Icm is not movement of a new species of charge but is probably movement of Q gamma that is slowed by SR Ca release or some associated event such as the accompanying increase in myoplasmic free [Ca] that is expected to occur near the Ca release sites. The peak value of the apparent rate constant associated with this current, 2-4%/ms at pulse potentials between -48 and -40 mV, is decreased by half when [CaSR]R approximately equal to 500-1,000 microM, which gives a peak rate of SR Ca release of approximately 5-10 microM/ms.

Effects of acidosis on phosphorylation of phospholamban and troponin I in rat cardiac muscle

Acidosis inhibits Ca2+ transport by the sarcoplasmic reticulum of cardiac muscle and decreases the Ca2+ sensitivity of the contractile proteins, although the mechanisms underlying these changes are unclear. We have investigated the hypothesis that changes in the phosphorylation of the regulatory proteins phospholamban and troponin I might play a role in the acidosis-induced changes in the function of the sarcoplasmic reticulum and the myofilaments, respectively. Langendorff-perfused rat hearts were labeled with 32P and then perfused with either control (pH 7.4) or acid (pH 6.8) physiological salt solution, in both the absence and presence of isoproterenol. The incorporation of 32P into phospholamban and troponin I was determined by sodium dodecyl sulfate-polyacrylamide gel electrophoresis of sarcoplasmic reticulum and myofibrillar proteins, followed by autoradiography and liquid scintillation counting. The data show that acidosis has no effect on the phosphorylation of phospholamban in the absence of isoproterenol but that, in the presence of isoproterenol, acidosis increased the phosphorylation of phospholamban. However, acidosis increased the phosphorylation of troponin I, in both the absence and the presence of isoproterenol. Acidosis did not alter the adenosine 3',5'-cyclic monophosphate content of the hearts but did inhibit type 1 phosphatase. These data show that acidosis can alter the phosphorylation of these two proteins and suggest that these changes underlie, in part the changes observed in cardiac muscle during acidosis.

Regulation of cytosolic calcium levels in vascular smooth muscle

Ca2+ plays an important role in the contraction of skeletal, cardiac, and smooth muscle, as well as in a number of important processes, such as secretion and neuronal activity. In this review, I focus on the various mechanisms by which cytosolic Ca2+ concentration is regulated in vascular smooth muscle, in the resting state and during activation. Particular attention is paid to the calcium pumps of the plasmalemma and the sarcoplasmic reticulum, to the inositol 1,4,5-trisphosphate- and ryanodine-sensitive calcium channels of the sarcoplasmic reticulum, and to voltage-dependent and voltage-independent calcium channels of the plasmalemma.

Steady-state twitch Ca2+ fluxes and cytosolic Ca2+ buffering in rabbit ventricular myocytes

Intracellular Ca2+ ([Ca2+]i) transients and transsarcolemmal Ca2+ currents were measured in indo 1-loaded isolated rabbit ventricular myocytes during whole cell voltage clamp to quantitate the components of cytosolic Ca2+ influx and to describe the dynamic aspects of cytosolic Ca2+ buffering during steady-state contraction (0.5 Hz, 22 degrees C). Sarcolemmal Ca2+ influx was directly measured from the integrated Ca2+ current (Ica) recorded during the clamp (158 +/- 10 attomoles; amol). Sarcoplasmic reticulum (SR) Ca2+ content was determined from the integrated electrogenic Na+/Ca2+ exchange current (Ix) induced during rapid application and sustained exposure of cells to caffeine to elicit the release of the SR Ca2+ load (1,208 +/- 170 amol). The mean steady-state SR Ca2+ load was calculated to be 87 +/- 13 microM (mumol/l nonmitochondrial cytosolic volume). Ca2+ influx via Ica represented approximately 14% of the stored SR Ca2+ and 23% of the total cytosolic Ca2+ flux during a twitch (47 +/- 6 microM). Comparison of electrophysiologically measured Ca2+ fluxes with Ca2+ transients yields apparent buffering values of 60 for caffeine contractures and 110 for twitches (delta Ca2+ total/delta Ca2+ free). This is consistent with the occurrence of "active" buffering of cytosolic Ca2+ by SR Ca2+ uptake during the twitch.

Ca2+ release from the sarcoplasmic reticulum compared in amphibian and mammalian skeletal muscle

Puzzled by recent reports of differences in specific ligand binding to muscle Ca2+ channels, we quantitatively compared the flux of Ca2+ release from the sarcoplasmic reticulum (SR) in skeletal muscle fibers of an amphibian (frog) and a mammal (rat), voltage clamped in a double Vaseline gap chamber. The determinations of release flux were carried out by the "removal" method and by measuring the rate of Ca2+ binding to dyes in large excess over other Ca2+ buffers. To have a more meaningful comparison, the effects of stretching the fibers, of rapid changes in temperature, and of changes in the Ca2+ content of the SR were studied in both species. In both frogs and rats, the release flux had an early peak followed by fast relaxation to a lower sustained release. The peak and steady values of release flux, Rp and Rs, were influenced little by stretching. Rp in frogs was 31 mM/s (SEM = 4, n = 24) and in rats 7 +/- 2 mM/s (n = 12). Rs was 9 +/- 1 and 3 +/- 0.7 mM/s in frogs and rats, respectively. Transverse (T) tubule area, estimated from capacitance measurements and normalized to fiber volume, was greater in rats (0.61 +/- 0.04 microns-1) than in frogs (0.48 +/- 0.04 micron-1), as expected from the greater density of T tubuli. Total Ca in the SR was estimated as 3.4 +/- 0.6 and 1.9 +/- 0.3 mmol/liter myoplasmic water in frogs and rats. With the above figures, the steady release flux per unit area of T tubule was found to be fourfold greater in the frog, and the steady permeability of the junctional SR was about threefold greater. The ratio Rp/Rs was approximately 2 in rats at all voltages, whereas it was greater and steeply voltage dependent in frogs, going through a maximum of 6 at -40 mV, then decaying to approximately 3.5 at high voltage. Both Rp and Rs depended strongly on the temperature, but their ratio, and its voltage dependence, did not. Assuming that the peak of Ca2+ release is contributed by release channels not in contact with voltage sensors, or not under their direct control, the greater ratio in frogs may correspond to the relative excess of Ca2+ release channels over voltage sensors apparent in binding measurements. From the marked differences in voltage dependence of the ratio, as well as consideration of Ca(2+)-induced release models, we derive indications of fundamental differences in control mechanisms between mammalian and amphibian muscle.

Capsaicin does not inhibit the intracellular calcium handling process in rat ventricular papillary muscle

1. We studied the effects of capsaicin, a pungent agent extracted from red pepper, on rested-state contraction (RSC) of isolated rat ventricular papillary muscles. 2. The RSC was induced by stimulation, after a rest interval of 5 sec to 10 min, after the twitch tension of the muscle preparation stimulated at the regular stimulus frequency of cycle lengths of 5, 1 or 0.2 sec attained the steady state. 3. Drug effects were evaluated on the RSC in the presence of capsaicin 10(-5) M, caffeine 10(-2) M or ryanodine 10(-7) M, respectively. 4. All drugs inhibited the RSC but to different degrees. Ryanodine was the most effective in inhibiting RSC, followed by caffeine and capsaicin, in that order. However, the inhibitory mode varied, depending on the drugs. 5. These findings suggest that capsaicin may not inhibit the function of intracellular Ca2+ store in rat cardiac muscle.

Regulation of calcium release channel in sarcoplasmic reticulum

In this review, we summarized the results obtained mainly by flux measurements through Ca2+ channel in HSR vesicles. The Ca2+ channel has a large pore which passes not only divalent cations such as Ca2+, Mg2+, and Ba2+ and monovalent cations such as Na+, K+, and Cs+, but also large ions such as choline and tris. The permeation rates of choline and glucose through the Ca2+ channel were measured quantitatively by the light scattering method. The slow permeation of such molecules may reflect the structure of pores since the permeation process is the rate-limiting step for such large molecules. Neutral molecules such as glucose became permeable in the presence of submolar KCl, which suggests that pore size of the channel becomes larger in KCl. The apparent permeation rates of Ca2+ and Mg2+ obtained from the flux measurement were the same, although their single-channel conductances were different. This discrepancy was explained by the fact that flux measurements reflects the open rate of the channel. Thus, complementarity between the flux measurement and single-channel recording was demonstrated. From the effects of K+ on the action of regulators on Ca2+ channel, it was suggested that the Ca2+ channel has many binding sites for activators and inhibitors. There are two kinds of Ca2+ binding sites for activation and inhibition. Activation sites for Ca2+, caffeine, and ATP are different and inhibition sites for Ca2+ and procaine are different. The binding sites for ruthenium red and Mg2+ are the same as the activation and/or inhibition sites for Ca2+. Ryanodine-treated Ca2+ channel became permeable to glucose even in the absence of KCl. The conformational state of the channel opened by ryanodine is different from that opened by Ca2+, caffeine, and ATP. The maximal flux rates of choline and glucose induced by ryanodine were smaller than those attained by caffeine and ATP. This result is consistent with the observation obtained by single-channel recording; the maximal value of single-channel conductance after ryanodine treatment becomes 40-50% of the value before the treatment. It is likely that the radius of the pore opened by ryanodine is smaller than that opened by Ca2+, caffeine, or ATP. The flexibility of the channel may be decreased in the open locked state induced by ryanodine. The Ca2+ response to open the channel by micromolar Ca2+ was lost when calsequestrin was released from the vesicles. It is possible that calsequestrin acts as an endogenous regulator of Ca2+ channel through triadin in excitation-contraction coupling.

Synergistic actions of glucagon and miniglucagon on Ca2+ mobilization in cardiac cells

It has been recently shown that the physiological processing of glucagon into its C-terminal (19-29) fragment, miniglucagon, by cardiac cells was essential for the contractile positive inotropic effect of the hormone. However, the mechanisms underlying the effects of miniglucagon remained undetermined. In the present study, we assessed the effects of miniglucagon on Ca2+ homeostasis in embryonic chick ventricular myocytes. In quiescent cells, short-term applications of 0.1 nmol/L miniglucagon markedly increased the accumulation of 45Ca into intracellular compartments resistant to digitonin lysis and sensitive to caffeine. Ca2+ accumulation into the sarcoplasmic reticular (SR) store was further attested by fura 2 imaging studies on quiescent or prestimulated cells: miniglucagon potentiated Ca2+ release from the SR compartment triggered by caffeine and evoked a rise in cytosolic Ca2+ when applied on cells pretreated with 1 mumol/L thapsigargin, a specific inhibitor of the SR Ca2+ pump. Glucagon alone produced a small cytosolic Ca2+ signal that was considerably amplified by miniglucagon. The action of glucagon was mimicked by 8-bromo-cAMP and was blocked by isradipine, suggesting that it relied on the activation of L-type Ca2+ channels, via phosphorylation. We conclude that the combined actions of miniglucagon and glucagon on Ca2+ accumulation into SR stores and Ca2+ release from the same stores are likely to support the positive inotropic effect elicited in vivo by glucagon on heart contraction.

Calcium cycling proteins and force-frequency relationship in heart failure

Myocardial function, intracellular calcium and levels of calcium cycling proteins were analyzed in failing and nonfailing human myocardium. Myocardial function was evaluated by the isometric force-frequency relation, and intracellular calcium was studied by aequorin light emission. When stimulation frequency was increased above 30 min-1, there was a continuous increase in isometric tension development in the nonfailing myocardium. In contrast, in failing myocardium, frequency potentiation of contractile force was blunted or inverse. As a consequence, at higher rates of stimulation, twitch tension was reduced significantly in failing compared to nonfailing human myocardium. Aequorin measurements indicated that the contractile deficit in the failing myocardium at higher rates of stimulation is associated with decreased free intracellular calcium concentration. Western blot analysis indicated that in the failing myocardium protein levels of SR-Ca(2+)- ATPase are significantly reduced and protein levels of Na(+)-Ca(2+)- exchanger are significantly increased. Levels of phospholamban are slightly reduced in the failing myocardium, and ryanodine receptor and calsequestrin protein levels are unchanged. There was a close positive correlation between the protein levels of SR-Ca(2+)-ATPase and frequency potentiation of contractile force. From these data, we conclude that in failing compared to nonfailing human myocardium 1) force-frequency relation is blunted or inverse. 2) Frequency-dependence of contractile force is closely correlated with frequency-dependence of intracellular calcium cycling. 3) Protein levels of SR-Ca(2+)-ATPase may determine frequency-dependence of sarcoplasmic reticulum calcium release. 4) Calcium elimination by an increased number of Na(+)-Ca2-exchanger molecules may be a compensatory mechanism to prevent diastolic calcium accumulation in failing myocardium with a reduced number of SR calcium pumps.

Possible functional linkage between the cardiac dihydropyridine and ryanodine receptor: acceleration of rest decay by Bay K 8644

The effect of the dihydropyridine L-Type Ca chanel agonist Bay K 8644 on post-rest contractions in ferret ventricular muscle and isolated myocytes was investigated. Bay K 8644 was shown to abolish rest potentiation and greatly accelerate rest decay. The post-rest contraction suppressed by Bay K 8644 was accompanied by action potentials of large amplitude and longer duration, but voltage-clamp measurements showed that this suppression was not due to a supra-optimal ICa trigger. Caffeine-induced contractures and rapid cooling contractures demonstrated an accelerated rest-dependent decline in sarcoplasmic reticulum (SR) Ca content in the presence of Bay K 8644, which was present even with Ca-free superfusion during rest. Thus, the Bay K 8644-induced decline of SR Ca during rest was independent of extracellular Ca or ICa. To explore whether the binding of Bay K 8644 to the dihydropyridine receptor could alter the SR Ca release channel/ryanodine receptor in a more direct way, ryanodine binding was measured in the absence and presence of Bay K 8644. Ryanodine binding to isolated ferret ventricular myocytes was increased by Bay K 8644 under conditions where sarcolemmal-SR junctions might be expected to be intact, but not after physical disruption. These results are consistent with a working hypothesis where Bay K 8644 may bind to the dihydropyridine receptor and this may lead to physical changes in the linkage between the dihydropridine receptor and a subset of ryanodine receptors, thereby increasing the opening of the SR Ca release channel during rest (and accelerating resting Ca loss).

Functional alterations in sarcoplasmic reticulum membranes of magnesium-deficient rat skeletal muscle as consequences of free radical-mediated process

Free radical-induced physiopathologies are generally thought to be mediated by membrane injuries. Using a pro-oxidant model induced by dietary magnesium deficiency, we have recently shown that skeletal muscle lesions occurred with a rise in the calcium level and enhanced free radical production. In this study, we investigated the physicochemical and biochemical properties of sarcoplasmic reticulum membranes isolated from hind limb muscles of weanling male rats pair fed magnesium-deficient or control diets for 12 d. The calcium-induced calcium efflux from preloaded vesicles was increased in membranes isolated from Mg-deficient rat muscle. In agreement with this latter observation, we demonstrated increased ryanodine binding affinity of the calcium channel. The Ca2(+)-ATPase activity of the pump was shown to be reduced. The viscosity state of the membranes, assessed by 1,6-diphenyl-1,3,5-hexatriene fluorescence anisotropy, was significantly increased in Mg-deficient membranes. Moreover, these membranes demonstrated an increased content of protein carbonyls as compared with controls. These functional as well as structural changes are closed to those described in sarcoplasmic reticulum membranes oxidatively modified in vitro. Together, these data fitted well with the concept that free radical-induced membrane damages resulting in calcium overload may be at the origin of skeletal muscle lesion during Mg-deficiency.

Calcium sparks and [Ca2+]i waves in cardiac myocytes

Local elevations in intracellular calcium ("Ca2+ sparks") in heart muscle are elementary sarcoplasmic reticulum (SR) Ca(2+)-release events. Ca2+ sparks occur at a low rate in quiescent cells but can also be evoked by electrical stimulation of the cell to produce the cell-wide Ca2+ transient. In this study we investigate how Ca2+ sparks are related to propagating waves of elevated cytosolic Ca2+ induced by "Ca2+ overload." Single ventricular myocytes from rat were loaded with the Ca(2+)-sensitive indicator fluo 3 and imaged with a confocal microscope. After extracellular Ca2+ concentration was increased from 1 to 10 mM to produce Ca2+ overload, the frequency of spontaneous Ca2+ sparks, which occur at the t tubule/SR junction, increased approximately 4-fold, whereas the spark amplitude and spatial size increased 4.1-and 1.7-fold, respectively. In addition, a spectrum of larger subcellular events, including propagating Ca2+ waves, was observed. Ca2+ sparks were seen to occur at the majority (65%) of the sites of wave initiation. For slowly propagating Ca2+ waves, discrete Ca(2+)-release events, similar to Ca2+ sparks, were detected in the wave front. These Ca2+ sparks appeared to recruit other sparks along the wave front so that the wave progressed in a saltatory manner. We conclude that Ca2+ sparks are elementary events that can explain both the initiation and propagation of Ca2+ waves. In addition, we show that Ca2+ waves and electrically evoked Ca2+ transients have the same time course and interact with each other in a manner that is consistent with both phenomena having the same underlying mechanism(s). These results suggest that SR Ca2+ release during Ca2+ waves, like that during normal excitation-contraction coupling, results from the spatial and temporal summation of Ca2+ sparks.

Low concentrations of ouabain increase cytosolic free calcium concentration in rat vascular smooth muscle cells

1. Low ouabain concentrations in the nanomolar range significantly increased cytosolic free calcium concentration. 2. The ouabain-induced cytosolic free calcium concentration increase was due to transplasmamembrane calcium influx, which could be prevented in the absence of extracellular calcium or by addition of the calcium channel blocker nifedipine. 3. The amount of stored cellular Ca2+, as determined by the thapsigargin-induced cytosolic free calcium concentration increase, was also enhanced by 1 nmol/l ouabain. 4. It is concluded that low ouabain concentrations affect intracellular cytosolic free calcium concentration homoeostasis.

The role of Ca2+ release from sarcoplasmic reticulum in the regulation of sinoatrial node automaticity

The role of Ca2+ release channels in the sarcoplasmic reticulum in modulating physiological automaticity of the sinoatrial (SA) node was studied by recording transmembrane action potentials and membrane ionic currents in small preparations of the rabbit SA node. Ryanodine, which modifies the conductance and gating behavior of the Ca2+ release channels, was used to block Ca2+ release from the sarcoplasmic reticulum. Superfusion of 1-mM ryanodine decreased the spontaneous firing frequency as well as the maximal rate of depolarization of the SA, and these reductions reached a steady state within approximately 5 min. The action potential recordings revealed that the latter part of diastolic depolarization was depressed and that the take-off potential became less negative. This suggested that the negative chronotropic effect of ryanodine resulted from the blockade of physiological Ca2+ release from the sarcoplasmic reticulum. In voltage clamp experiments, using double-microelectrode techniques, ryanodine did not markedly reduce the Ca2+ current (ICa) but decreased the delayed rectifying K+ current (IK), the steady-state inward current (Iss), and the hyperpolarization-activated inward current (Ih). These observations suggest that, even when the function of C2+ channels in the cell membrane is normally maintained, depression of Ca2+ release channels in the sarcoplasmic reticulum would prevent sufficient elevation of the Ca2+ concentration in SA node cells for the activation of various ionic currents, and, thus adversely affect the physiological automaticity of this primary cardiac pacemaker.

Effect of ryanodine on cardiac calcium current and calcium channel gating current

The effects of 100 microM ryanodine on the L-type calcium channel were studied using the pacth-clamp technique in isolated guinea pig ventricular myocytes. The inactivation kinetics of the calcium current were slowed down in the presence of ryanodine in agreement with the blockade of the release of calcium from the sarcoplasmic reticulum by the drug. The I-V and steady-state inactivation curves of the calcium current were shifted to negative values by ryanodine. A similar shift was observed in the activation and inactivation curves of the intramembrane charge movement associated with the calcium channel. Due to this shift, ryanodine slightly reduced the maximal amount of displaced charge although it did not modify the transition from the inactivated to the activated state (i.e., charge movement repriming). This result is in notable contrast with that obtained in skeletal muscle, where it has been found that ryanodine interferes with charge movement repriming. These results provide additional evidence of the postulated differences between the architecture of the excitation-contraction coupling system in cardiac and skeletal muscle.

Purification, primary structure, and immunological characterization of the 26-kDa calsequestrin binding protein (junctin) from cardiac junctional sarcoplasmic reticulum

Previously we identified a protein of apparent M(r) = 26,000 as the major calsequestrin binding protein in junctional sarcoplasmic reticulum vesicles isolated from cardiac and skeletal muscle (Mitchell, R. D., Simmerman, H. K. B., and Jones, L. R. (1988) J. Biol. Chem. 263, 1376-1381). Here we describe the purification and primary structure of the 26-kDa calsequestrin binding protein. The protein was purified 164-fold from cardiac microsomes and shown by immunoblotting to be highly enriched in junctional membrane subfractions. It ran as a closely spaced doublet on SDS-polyacrylamide gel electrophoresis and bound 125I-calsequestrin intensely. Cloning of the cDNA predicted a protein of 210 amino acids containing a single transmembrane domain. The protein has a short N-terminal region located in the cytoplasm, and the bulk of the molecule, which is highly charged and basic, projects into the sarcoplasmic reticulum lumen. Significant homologies were found with triadin and aspartyl beta-hydroxylase, suggesting that all three proteins are members of a family of single membrane-spanning endoplasmic reticulum proteins. Immunocytochemical labeling localized the 26-kDa protein to junctional sarcoplasmic reticulum in cardiac and skeletal muscle. The same gene product was expressed in these two tissues. The calsequestrin binding activity of the 26-kDa protein combined with its codistribution with calsequestrin and ryanodine receptors strongly suggests that the protein plays an important role in the organization and/or function of the Ca2+ release complex. Because the 26-kDa calsequestrin binding protein is an integral component of the junctional sarcoplasmic reticulum membrane in cardiac and skeletal muscle, we have named it Junctin.

Increased calcium sequestration by sarcoplasmic reticulum in small muscular arteries in young spontaneously hypertensive rats

1. Sarcoplasmic reticulum (SR) in small resistance arteries plays a role in the regulation of the cytosolic free calcium concentration by sequestration of calcium from cytoplasm. 2. To examine the contribution of calcium (Ca2+) sequestration by the SHR to both contraction and relaxation in young spontaneously hypertensive rats (SHR), we measured evoked tension before and after depletion of SR Ca2+ stores in the rings of the first branch of superior mesenteric artery in 5 week old SHR and age-matched Wistar-Kyoto (WKY) rats. Contractile responses were induced by 40 mmol/L potassium and 10 mmol/L caffeine before and after the treatment with ryanodine or thapsigargin, which depletes SR calcium stores. 3. The magnitude of potassium-induced contraction was almost the same in both strains. 4. Ryanodine and thapsigargin did not affect the resting tension and almost decreased caffeine-induced contraction in both strains. 5. After the treatment with ryanodine or thapsigargin, the magnitude and the rate of potassium-induced contraction were augmented greatly in SHR, but not in WKY. 6. The relative relaxation rate after exposure of potassium was significantly slowed in SHR by ryanodine or thapsigargin, but only slightly in WKY. 7. These results suggest that Ca2+ sequestration by the SR in SHR was greater than in WKY. Therefore, it is concluded that SR plays an important role in preventing the development of hypertension in SHR via a buffering effect on the elevation of cytosolic free Ca2+.

Ca2+ activation and Ca2+ inactivation of canine reconstituted cardiac sarcoplasmic reticulum Ca(2+)-release channels

1. Calcium-release channels (ryanodine receptors) of canine cardiac sarcoplasmic reticulum (SR) were incorporated into lipid bilayer membranes at the tip of a patch pipette. Using symmetrical 150 mM KCl solutions, [Ca2+] > 0.3 microM activated single channels of 627 pS conductance. The kinetics of Ca(2+)-mediated channel activation, deactivation and inactivation were studied by stepwise changes in pCa (-log[Ca2+]) and analysis of current means. 2. Steps of [Ca2+] activated the channel open probability (Po) along a time course which could be fitted by a single exponential. The activation time constant was dependent on [Ca2+], which decreased from 4.9 ms at pCa 6.5 to 0.2 ms at pCa 3. Subsequent rapid reduction in [Ca2+] decreased Po along a mono-exponential deactivation time course, the time constant of which was independent of the [Ca2+] during the preceding activation period. Further analysis yielded the rate constants kon of 2 x 10(8) (M s)-1 and koff of 2 x 10(2) s-1, an apparent dissociation constant (KD) of 1 microM, and a Hill coefficient of 1.05. 3. The open probability increased with [Ca2+], reaching a peak at about pCa 5.5. At pCa < or = 5.5, Po decreased time dependently, the time constants decreasing along with [Ca2+] from 1 s at 3 microM to 0.2 s at 1 mM. During the 0.5 s period at 3 microM Ca2+, Po fell by 13% due to an extension of the closed times. At 1 mM Ca2+, Po 'inactivated' by 72%, which was due mostly to long closures. These differences suggest that the Ca(2+)-mediated decay of Po was dependent on Ca2+ binding to an intermediate (KD, 3 microM) and a low affinity site (KD, 360 microM). On the return of pCa from 3 to > 8, the channels briefly re-opened. 4. A 'refractory' behaviour of the channel was not observed for 20 ms steps between < 10 nM and < 10 microM [Ca2+] (25 Hz). For steps between 10 nM and 1 mM, however, such behaviour was marked by infrequent and irregular channel openings. 5. The results are described by a three Ca2+ binding site model and compared with the literature.

Expression of sarcoplasmic reticulum Ca2+ -ATPase mRNA in the hypertrophied heart of young spontaneously hypertensive rats

1. Recent evidence indicates that cardiac hypertrophy induced by pressure overload is associated with a decrease in sarcoplasmic reticulum (SR) Ca2+ -ATPase of myocytes, which may contribute to a diastolic dysfunction of the heart by causing intracellular Ca2+ overload. To elucidate whether or not this is also the case in genetic hypertension, we examined cardiac mRNA levels of SR Ca2+ ATPase in 11 week old spontaneously hypertensive rats (SHR) by northern blot analysis. 2. Furthermore, to test the effects of short-term inhibition of the renin-angiotensin system on its expression, we treated 10 week old SHR with angiotensin-converting enzyme inhibitors (alacepril and imidapril) or an AT1 receptor antagonist (SC-52458) for 7 days. 3. Though the left ventricular weight of SHR was significantly higher than that of Wistar-Kyoto (WKY) rats (277 +/- 6 vs 237 +/- 4 mg/100 g bodyweight, respectively, P < 0.05), the level of SR Ca2+ -ATPase mRNA showed no difference between SHR and WKY at this age. 4. Moreover, the aforementioned three drugs did not at all affect the SR Ca2+ -ATPase expression of SHR. 5. Thus, the expression of SR Ca2+ -ATPase was not down-regulated in the heart of 11 week old SHR, and seemed not to be mediated by angiotensin AT1 receptor at this age. Since some evidence on pressure-overloaded cardiac hypertrophy indicate that the decrease in SR Ca2+ -ATPase expression occur in prominent hypertrophy and in the failured heart, further studies on cardiac SR Ca2+ -ATPase expression in more aged SHR will be required.

The ryanodine binding sarcoplasmic reticulum calcium release channel in nonfailing and in failing human myocardium

The ryanodine-sensitive Ca2+ release channel (RyaCRC) of the sarcoplasmic reticulum plays a key role in the intracellular Ca2+ handling in cardiomyocytes. Altered expression of the RyaCRC has been supposed to contribute to abnormal cellular Ca2+ handling and to myocardial dysfunction in dilated and ischemic cardiomyopathy. In the present study the 3H-ryanodine binding site in human myocardial homogenates was characterized and the density of the RyaCRC (which corresponds to the cardiac ryanodine receptor) was determined in nonfailing and in failing human myocardium. Homogenates were prepared from nonfailing left ventricular myocardium from the hearts of 5 organ donors (NF) and from failing myocardium from 14 explanted hearts of transplant recipients with end-stage heart failure resulting from dilated (DCM, n = 5) or ischemic (ICM, n = 9) cardiomyopathy. Radioligand saturation binding experiments revealed a specific, high-affinity 3H-ryanodine binding site (Kd-values: NF: 0.65 +/- 0.11 nmol/l, DCM: 0.66 +/- 0.09 nmol/l, ICM: 0.88 +/- 0.18 nmol/l; n.s.) in all preparations. Specific 3H-ryanodine binding depended on the free Ca2+ concentration in the assay. It was maximal at 3-100 micro mol/l Ca2+. The binding was inhibited by the RyaCRC antagonists ruthenium red (Ki-value: 0.32 [0.18-0.56] micromol/l, n = 5) and Mg2+ (Ki-value: 2.95 [1.23-7.11] mmol/l, n = 5). The RyaCRC density was 103.5 +/- 11.9 fmol/mg protein in nonfailing myocardium. There was no significant change in the RyaCRC density in dilated or ischemic cardiomyopathy (112.4 +/- 17.1 and 122.7 +/- 13.9 fmol/mg protein) compared to nonfailing control myocardium. In summary, 3H-ryanodine binds specifically and with high-affinity to the RyaCRC in human myocardium. There is no change in the RyaCRC density in failing myocardium of patients with DCM or ICM in comparison to non-failing controls.

Arachidonic acid disrupts calcium dynamics in neonatal rat cardiac myocytes

OBJECTIVES

The purpose of this study was to investigate the effects of prolonged arachidonic acid (AA) exposure on electrically induced fluctuations of cytosolic free Ca2+ concentration ([Ca2+]i) in cardiac myocytes and to identify intracellular biochemical events that may play a role in the actions of AA on [Ca2+]i dynamics.

METHODS

Electrically induced [Ca2+]i transients were investigated in cultured single neonatal rat ventricular myocytes using spectrofluorometric analysis of fura-2-[Ca2+]i binding. KCl-induced depolarization, caffeine and ryanodine were used to assess the effects of AA on Ca2+ handling by the sarcolemma and the sarcoplasmic reticulum. Prostanoid formation was measured with an ELISA technique. alpha-Tocopherol was used to determine if free radical formation was a factor in the AA effects on [Ca2+]i.

RESULTS

Exposure to 10-30 microM AA produced a concentration-dependent and reversible configuration change and eventually a cessation of [Ca2+]i transients. Continued exposure resulted in a Ca2+ overload (tonic [Ca2+]i greater than peak systolic [Ca2+]i). AA did not influence KCl-induced [Ca2+]i increase but did eliminate caffeine-induced [Ca2+]i transients. AA exposure stimulated the formation of 6-oxo-prostaglandin F1 alpha in a concentration-dependent manner, but thromboxane B2 formation was not influenced. alpha-Tocopherol pretreatment significantly delayed times till cessation of [Ca2+]i transients and Ca2+ overload, whereas ryanodine and cyclo-oxygenase inhibitors were without effect.

CONCLUSIONS

The present data provide evidence that the initial action of AA on [Ca2+]i transients during excitation-contraction coupling involves an effect of AA on sarcolemmal Ca2+ influx and sarcoplasmic reticulum Ca2+ handling. AA-induced cessation of electrically induced [Ca2+]i transients and Ca2+ overload may involve the formation of free radicals.

Unchanged protein levels of SERCA II and phospholamban but reduced Ca2+ uptake and Ca(2+)-ATPase activity of cardiac sarcoplasmic reticulum from dilated cardiomyopathy patients compared with patients with nonfailing hearts

BACKGROUND

The aim of the present study was to investigate whether Ca2+ uptake into the sarcoplasmic reticulum (SR) is altered in failing human myocardium resulting from dilated cardiomyopathy.

METHODS AND RESULTS

Ca(2+)-ATPase (SERCA II) activity and Ca(2+)-dependent 45Ca2+ uptake (oxalate supported, steady state) in isolated vesicles from the SR (VSR) and in crude membrane preparations (CSR) (free Ca2+, 0.01 to 100 mumol/L) from nonfailing (donor hearts, n = 13) and terminally failing (heart transplants, dilated cardiomyopathy, n = 17) human myocardium were studied. In the same hearts, protein levels (Western blot analysis) and mRNA levels (Northern blot analysis) of SERCA II and phospholamban were measured. Increasing concentrations of Ca2+ were followed by an increased Ca(2+)-ATPase activity and Ca2+ uptake. Ca2+ uptake activity and Ca(2+)-ATPase activity in CSR preparations from failing myocardium were significantly reduced compared with nonfailing hearts (Ca(2+)-ATPase, 163 +/- 8 and 125 +/- 7 nmol ATP/mg protein per minute for nonfailing tissue and failing tissue in New York Heart Association [NYHA] class IV, respectively; Ca2+ uptake, 7.1 +/- 0.8 and 3.5 +/- 0.3 nmol/mg protein per minute in CSR from nonfailing and NYHA class IV hearts, respectively P < .05). In contrast, no significant difference was measured in VSR. In the same preparations (CSR and VSR), both SERCA II and phospholamban levels (Western blot technique with monoclonal antibodies) were unchanged in failing compared with nonfailing tissue. mRNA expression relative to GAPDH mRNA for SERCA IIa and for phospholamban was significantly reduced in failing human myocardium (P < .05).

CONCLUSIONS

These findings provide evidence that in failing human myocardium caused by dilated cardiomyopathy, protein levels of SERCA II and phospholamban are unchanged even though mRNA levels for SERCA II and phospholamban and the SERCA II function are reduced compared with nonfailing myocardium.