Calcium-induced calcium release from purified cardiac sarcoplasmic reticulum vesicles. General characteristics

The biophysical properties of TRIC-A and TRIC-B and their interactions with RyR2

Trimeric intracellular cation channels (TRIC-A and TRIC-B) are thought to provide counter-ion currents to enable charge equilibration across the sarco/endoplasmic reticulum (SR) and nuclear membranes. However, there is also evidence that TRIC-A may interact directly with ryanodine receptor type 1 (RyR1) and 2 (RyR2) to alter RyR channel gating. It is therefore possible that the reverse is also true, where the presence of RyR channels is necessary for fully functional TRIC channels. We therefore coexpressed mouse TRIC-A or TRIC-B with mouse RyR2 in HEK293 cells to examine if after incorporating membrane vesicles from these cells into bilayers, the presence of TRIC affects RyR2 function, and to characterize the permeability and gating properties of the TRIC channels. Importantly, we used no purification techniques or detergents to minimize damage to TRIC and RyR2 proteins. We found that both TRIC-A and TRIC-B altered the gating behavior of RyR2 and its response to cytosolic Ca2+ but that TRIC-A exhibited a greater ability to stimulate the opening of RyR2. Fusing membrane vesicles containing TRIC-A or TRIC-B into bilayers caused the appearance of rapidly gating current fluctuations of multiple amplitudes. The reversal potentials of bilayers fused with high numbers of vesicles containing TRIC-A or TRIC-B revealed both Cl- and K+ fluxes, suggesting that TRIC channels are relatively non-selective ion channels. Our results indicate that the physiological roles of TRIC-A and TRIC-B may include direct, complementary regulation of RyR2 gating in addition to the provision of counter-ion currents of both cations and anions.

Diminished inhibition and facilitated activation of RyR2-mediated Ca2+ release is a common defect of arrhythmogenic calmodulin mutations

A number of calmodulin (CaM) mutations cause severe cardiac arrhythmias, but their arrhythmogenic mechanisms are unclear. While some of the arrhythmogenic CaM mutations have been shown to impair CaM-dependent inhibition of intracellular Ca²⁺ release through the ryanodine receptor type 2 (RyR2), the impact of a majority of these mutations on RyR2 function is unknown. Here, we investigated the effect of 14 arrhythmogenic CaM mutations on the CaM-dependent RyR2 inhibition. We found that all the arrhythmogenic CaM mutations tested diminished CaM-dependent inhibition of RyR2-mediated Ca²⁺ release and increased store-overload induced Ca²⁺ release (SOICR) in HEK293 cells. Moreover, all the arrhythmogenic CaM mutations tested either failed to inhibit or even promoted RyR2-mediated Ca²⁺ release in permeabilized HEK293 cells with elevated cytosolic Ca²⁺ , which was markedly different from the inhibitory action of CaM wild-type. The CaM mutations also altered the Ca²⁺ -dependency of CaM binding to the RyR2 CaM-binding domain. These results demonstrate that diminished inhibition, and even facilitated activation, of RyR2-mediated Ca²⁺ release is a common defect of arrhythmogenic CaM mutations.

The Arrhythmogenic Calmodulin p.Phe142Leu Mutation Impairs C-domain Ca2+ Binding but Not Calmodulin-dependent Inhibition of the Cardiac Ryanodine Receptor

A number of point mutations in the intracellular Ca2+-sensing protein calmodulin (CaM) are arrhythmogenic, yet their underlying mechanisms are not clear. These mutations generally decrease Ca2+ binding to CaM and impair inhibition of CaM-regulated Ca2+ channels like the cardiac Ca2+ release channel (ryanodine receptor, RyR2), and it appears that attenuated CaM Ca2+ binding correlates with impaired CaM-dependent RyR2 inhibition. Here, we investigated the RyR2 inhibitory action of the CaM p.Phe142Leu mutation (F142L; numbered including the start-Met), which markedly reduces CaM Ca2+ binding. Surprisingly, CaM-F142L had little to no aberrant effect on RyR2-mediated store overload-induced Ca2+ release in HEK293 cells compared with CaM-WT. Furthermore, CaM-F142L enhanced CaM-dependent RyR2 inhibition at the single channel level compared with CaM-WT. This is in stark contrast to the actions of arrhythmogenic CaM mutations N54I, D96V, N98S, and D130G, which all diminish CaM-dependent RyR2 inhibition. Thermodynamic analysis showed that apoCaM-F142L converts an endothermal interaction between CaM and the CaM-binding domain (CaMBD) of RyR2 into an exothermal one. Moreover, NMR spectra revealed that the CaM-F142L-CaMBD interaction is structurally different from that of CaM-WT at low Ca2+ These data indicate a distinct interaction between CaM-F142L and the RyR2 CaMBD, which may explain the stronger CaM-dependent RyR2 inhibition by CaM-F142L, despite its reduced Ca2+ binding. Collectively, these results add to our understanding of CaM-dependent regulation of RyR2 as well as the mechanistic effects of arrhythmogenic CaM mutations. The unique properties of the CaM-F142L mutation may provide novel clues on how to suppress excessive RyR2 Ca2+ release by manipulating the CaM-RyR2 interaction.

Nebivolol suppresses cardiac ryanodine receptor-mediated spontaneous Ca2+ release and catecholaminergic polymorphic ventricular tachycardia

β-Blockers are a standard treatment for heart failure and cardiac arrhythmias. There are ∼30 commonly used β-blockers, representing a diverse class of drugs with different receptor affinities and pleiotropic properties. We reported that among 14 β-blockers tested previously, only carvedilol effectively suppressed cardiac ryanodine receptor (RyR2)-mediated spontaneous Ca²⁺ waves during store Ca²⁺ overload, also known as store overload-induced Ca²⁺ release (SOICR). Given the critical role of SOICR in arrhythmogenesis, it is of importance to determine whether there are other β-blockers that suppress SOICR. Here, we assessed the effect of other commonly used β-blockers on RyR2-mediated SOICR in HEK293 cells, using single-cell Ca²⁺ imaging. Of the 13 β-blockers tested, only nebivolol, a β-1-selective β-blocker with nitric oxide synthase (NOS)-stimulating action, effectively suppressed SOICR. The NOS inhibitor (N-nitro-l-arginine methyl ester) had no effect on nebivolol's SOICR inhibition, and the NOS activator (histamine or prostaglandin E2) alone did not inhibit SOICR. Hence, nebivolol's SOICR inhibition was independent of NOS stimulation. Like carvedilol, nebivolol reduced the opening of single RyR2 channels and suppressed spontaneous Ca²⁺ waves in intact hearts and catecholaminergic polymorphic ventricular tachycardia (CPVT) in the mice harboring a RyR2 mutation (R4496C). Interestingly, a non-β-blocking nebivolol enantiomer, (l)-nebivolol, also suppressed SOICR and CPVT without lowering heart rate. These data indicate that nebivolol, like carvedilol, possesses a RyR2-targeted action that suppresses SOICR and SOICR-evoked VTs. Thus, nebivolol represents a promising agent for Ca²⁺-triggered arrhythmias.

K201 (JTV519) is a Ca2+-Dependent Blocker of SERCA and a Partial Agonist of Ryanodine Receptors in Striated Muscle

K201 (JTV-519) may prevent abnormal Ca(2+) leak from the sarcoplasmic reticulum (SR) in the ischemic heart and skeletal muscle (SkM) by stabilizing the ryanodine receptors (RyRs; RyR1 and RyR2, respectively). We tested direct modulation of the SR Ca(2+)-stimulated ATPase (SERCA) and RyRs by K201. In isolated cardiac and SkM SR microsomes, K201 slowed the rate of SR Ca(2+) loading, suggesting potential SERCA block and/or RyR agonism. K201 displayed Ca(2+)-dependent inhibition of SERCA-dependent ATPase activity, which was measured in microsomes incubated with 200, 2, and 0.25 µM Ca(2+) and with the half-maximal K201 inhibitory doses (IC50) estimated at 130, 19, and 9 µM (cardiac muscle) and 104, 13, and 5 µM (SkM SR). K201 (≥5 µM) increased RyR1-mediated Ca(2+) release from SkM microsomes. Maximal K201 doses at 80 µM produced ∼37% of the increase in SkM SR Ca(2+) release observed with the RyR agonist caffeine. K201 (≥5 µM) increased the open probability (Po) of very active ("high-activity") RyR1 of SkM reconstituted into bilayers, but it had no effect on "low-activity" channels. Likewise, K201 activated cardiac RyR2 under systolic Ca(2+) conditions (∼5 µM; channels at Po ∼0.3) but not under diastolic Ca(2+) conditions (∼100 nM; Po < 0.01). Thus, K201-induced the inhibition of SR Ca(2+) leak found in cell-system studies may relate to potentially potent SERCA block under resting Ca(2+) conditions. SERCA block likely produces mild SR depletion in normal conditions but could prevent SR Ca(2+) overload under pathologic conditions, thus precluding abnormal RyR-mediated Ca(2+) release.

Non-β-blocking R-carvedilol enantiomer suppresses Ca2+ waves and stress-induced ventricular tachyarrhythmia without lowering heart rate or blood pressure

Carvedilol is the current β-blocker of choice for suppressing ventricular tachyarrhythmia (VT). However, carvedilol's benefits are dose-limited, attributable to its potent β-blocking activity that can lead to bradycardia and hypotension. The clinically used carvedilol is a racemic mixture of β-blocking S-carvedilol and non-β-blocking R-carvedilol. We recently reported that novel non-β-blocking carvedilol analogues are effective in suppressing arrhythmogenic Ca(2+) waves and stress-induced VT without causing bradycardia. Thus, the non-β-blocking R-carvedilol enantiomer may also possess this favourable anti-arrhythmic property. To test this possibility, we synthesized R-carvedilol and assessed its effect on Ca(2+) release and VT. Like racemic carvedilol, R-carvedilol directly reduces the open duration of the cardiac ryanodine receptor (RyR2), suppresses spontaneous Ca(2+) oscillations in human embryonic kidney (HEK) 293 cells, Ca(2+) waves in cardiomyocytes in intact hearts and stress-induced VT in mice harbouring a catecholaminergic polymorphic ventricular tachycardia (CPVT)-causing RyR2 mutation. Importantly, R-carvedilol did not significantly alter heart rate or blood pressure. Therefore, the non-β-blocking R-carvedilol enantiomer represents a very promising prophylactic treatment for Ca(2+)- triggered arrhythmia without the bradycardia and hypotension often associated with racemic carvedilol. Systematic clinical assessments of R-carvedilol as a new anti-arrhythmic agent may be warranted.

Eudistomin D and penaresin derivatives as modulators of ryanodine receptor channels and sarcoplasmic reticulum Ca2+ ATPase in striated muscle

Eudistomin D (EuD) and penaresin (Pen) derivatives are bioactive alkaloids from marine sponges found to induce Ca(2+) release from striated muscle sarcoplasmic reticulum (SR). Although these alkaloids are believed to affect ryanodine receptor (RyR) gating in a "caffeine-like" manner, no single-channel study confirmed this assumption. Here, EuD and MBED (9-methyl-7-bromoeudistomin D) were contrasted against caffeine on their ability to modulate the SR Ca(2+) loading/leak from cardiac and skeletal muscle SR microsomes as well as the function of RyRs in planar bilayers. The effects of these alkaloids on [(3)H]ryanodine binding and SR Ca(2+) ATPase (SERCA) activity were also tested. MBED (1-5 μM) fully mimicked maximal activating effects of caffeine (20 mM) on SR Ca(2+) leak. At the single-channel level, MBED mimicked the agonistic action of caffeine on cardiac RyR gating (i.e., stabilized long openings characteristic of "high-open-probability" mode). EuD was a partial agonist at the maximal doses tested. The tested Pen derivatives displayed mild to no agonism on RyRs, SR Ca(2+) leak, or [(3)H]ryanodine binding studies. Unlike caffeine, EuD and some Pen derivatives significantly inhibited SERCA at concentrations required to modulate RyRs. Instead, MBED's affinity for RyRs (EC50 ∼ 0.5 μM) was much larger than for SERCA (IC50 > 285 μM). In conclusion, MBED is a potent RyR agonist and, potentially, a better choice than caffeine for microsomal and cell studies due to its reported lack of effects on adenosine receptors and phosphodiesterases. As a high-affinity caffeine-like probe, MBED could also help identify the caffeine-binding site in RyRs.

Ryanodine receptor current amplitude controls Ca2+ sparks in cardiac muscle

RATIONALE

In cardiac muscle, Ca(2+)-induced Ca(2+) release (CICR) from the sarcoplasmic reticulum (SR) is mediated by ryanodine receptor (RyR) Ca(2+) release channels. The inherent positive feedback of CICR is normally well-controlled. Understanding this control mechanism is a priority because its malfunction has life-threatening consequences.

OBJECTIVE

We show that CICR local control is governed by SR Ca(2+) load, largely because load determines the single RyR current amplitude that drives inter-RyR CICR.

METHODS AND RESULTS

We differentially manipulated single RyR Ca(2+) flux amplitude and SR Ca(2+) load in permeabilized ventricular myocytes as an endogenous cell biology model of the heart. Large RyR-permeable organic cations were used to interfere with Ca(2+) conductance through the open RyR pore. Single-channel studies show this attenuates current amplitude without altering other aspects of RyR function. In cells, the same experimental maneuver increased resting SR Ca(2+) load. Despite the increased load, Ca(2+) spark (inter-RyR CICR events) frequency decreased and sparks terminated earlier.

CONCLUSIONS

Spark local control follows single RyR current amplitude, not simply SR Ca(2+) load. Spark frequency increases with load because spontaneous RyR openings at high loads produce larger currents (ie, a larger CICR trigger signal). Sparks terminate when load falls to the point at which single RyR current amplitude is no longer sufficient to sustain inter-RyR CICR. Thus, RyRs that spontaneously close no longer reopen and local Ca(2+) release ends.

Is ryanodine receptor a calcium or magnesium channel? Roles of K+ and Mg2+ during Ca2+ release

The ryanodine receptor (RyR) is a poorly selective channel that mediates Ca(2+) release from intracellular Ca(2+) stores. How RyR's selectivity between the physiological cations K(+), Mg(2+), and Ca(2+) affects single-channel Ca(2+) current amplitude is examined using a recent model of RyR permeation. It is found that K(+) provides the vast majority of the countercurrent (through RyR itself) that is needed to prevent the sarcoplasmic reticulum (SR) membrane potential from changing and stopping Ca(2+) release. Moreover, intra-pore competition between Ca(2+) and Mg(2+) defines single RyR Ca(2+) current amplitude. Since both [Mg(2+)] and [Ca(2+)](SR) can change during pathophysiological conditions, the RyR unitary Ca(2+) current amplitude during Ca(2+) release may change significantly due to this Ca(2+)/Mg(2+) competition. Compared to the classic action of Mg(2+) on RyR open probability, these Ca(2+) current amplitude changes have as large or larger effects on overall RyR Ca(2+) mobilization. A new aspect of RyR divalent versus monovalent selectivity is also identified where this kind of selectivity decreases as divalent concentration increases.

Single ryanodine receptor channel basis of caffeine's action on Ca2+ sparks

Caffeine (1, 3, 7-trimethylxanthine) is a widely used pharmacological agonist of the cardiac ryanodine receptor (RyR2) Ca(2+) release channel. It is also a well-known stimulant that can produce adverse side effects, including arrhythmias. Here, the action of caffeine on single RyR2 channels in bilayers and Ca(2+) sparks in permeabilized ventricular cardiomyocytes is defined. Single RyR2 caffeine activation depended on the free Ca(2+) level on both sides of the channel. Cytosolic Ca(2+) enhanced RyR2 caffeine affinity, whereas luminal Ca(2+) essentially scaled maximal caffeine activation. Caffeine activated single RyR2 channels in diastolic quasi-cell-like solutions (cytosolic MgATP, pCa 7) with an EC(50) of 9.0 ± 0.4 mM. Low-dose caffeine (0.15 mM) increased Ca(2+) spark frequency ∼75% and single RyR2 opening frequency ∼150%. This implies that not all spontaneous RyR2 openings during diastole are associated with Ca(2+) sparks. Assuming that only the longest openings evoke sparks, our data suggest that a spark may result only when a spontaneous single RyR2 opening lasts >6 ms.

Role of CaMKII in RyR leak, EC coupling and action potential duration: a computational model

During heart failure, the ability of the sarcoplasmic reticulum (SR) to store Ca(2+) is severely impaired resulting in abnormal Ca(2+) cycling and excitation-contraction (EC) coupling. Recently, it has been proposed that "leaky" ryanodine receptors (RyRs) contribute to diminished Ca(2+) levels in the SR. Various groups have experimentally investigated the effects of RyR phosphorylation mediated by Ca(2+)/calmodulin-dependent kinase II (CaMKII) on RyR behavior. Some of these results are difficult to interpret since RyR gating is modulated by many external proteins and ions, including Ca(2+). Here, we present a mathematical model representing CaMKII-RyR interaction in the canine ventricular myocyte. This is an extension of our previous model which characterized CaMKII phosphorylation of L-type Ca(2+) channels (LCCs) in the cardiac dyad. In this model, it is assumed that upon phosphorylation, RyR Ca(2+)-sensitivity is increased. Individual RyR phosphorylation is modeled as a function of dyadic CaMKII activity, which is modulated by local Ca(2+) levels. The model is constrained by experimental measurements of Ca(2+) spark frequency and steady state RyR phosphorylation. It replicates steady state RyR (leak) fluxes in the range measured in experiments without the addition of a separate passive leak pathway. Simulation results suggest that under physiological conditions, CaMKII phosphorylation of LCCs ultimately has a greater effect on RyR flux as compared with RyR phosphorylation. We also show that phosphorylation of LCCs decreases EC coupling gain significantly and increases action potential duration. These results suggest that LCC phosphorylation sites may be a more effective target than RyR sites in modulating diastolic RyR flux.

Trifluoperazine: a rynodine receptor agonist

Trifluoperazine (TFP), a phenothiazine, is a commonly used antipsychotic drug whose therapeutic effects are attributed to its central anti-adrenergic and anti-dopaminergic actions. However, TFP is also a calmodulin (CaM) antagonist and alters the Ca(2+) binding properties of calsequestrin (CSQ). The CaM and CSQ proteins are known modulators of sarcoplasmic reticulum (SR) Ca(2+) release in ventricular myocytes. We explored TFP actions on cardiac SR Ca(2+) release in cells and single type-2 ryanodine receptor (RyR2) channel activity in bilayers. In intact and permeabilized ventricular myocytes, TFP produced an initial activation of RyR2-mediated SR Ca(2+) release and over time depleted SR Ca(2+) content. At the single channel level, TFP or nortryptiline (NRT; a tricyclic antidepressant also known to modify CSQ Ca(2+) binding) increased the open probability (Po) of CSQ-free channels with an EC(50) of 5.2 microM or 8.9 microM (respectively). This Po increase was due to elevated open event frequency at low drug concentrations while longer mean open events sustained Po at higher drug concentrations. Activation of RyR2 by TFP occurred in the presence or absence of CaM. TFP may also inhibit SR Ca uptake as well as increase RyR2 opening. Our results suggest TFP and NRT can alter RyR2 function by interacting with the channel protein directly, independent of its actions on CSQ or CaM. This direct action may contribute to the clinical adverse cardiac side effects associated with these drugs.

Melatonin ameliorates calcium homeostasis in myocardial and ischemia-reperfusion injury in chronically hypoxic rats

Chronic hypoxia (CH) leads to the deterioration of myocardial functions with impaired calcium handling in the sarcoplasmic reticulum (SR), which may be mediated by oxidative stress. We hypothesized that administration of antioxidant melatonin would protect against cardiac and ischemia-reperfusion (I/R) injury by ameliorating SR calcium handling. Adult Sprague-Dawley rats that had received a daily injection of melatonin or vehicle were exposed to 10% oxygen for 4 wk. The heart of each rat was then dissected and perfused using a Langendorff apparatus. The ratio of heart-to-body weight, ventricular hypertrophy and hematocrit were increased in the hypoxic rats compared with the normoxic controls. Malondialdehyde levels were also increased in the heart of hypoxic rats and were lowered by the treatment of melatonin. The hearts were subjected to left coronary artery ischemia (30 min) followed by 120-min reperfusion. Lactate dehydrogenase leakage before ischemia, during I/R and infarct size of the isolated perfused hearts were significantly elevated in the vehicle-treated hypoxic rats but not in the melatonin-treated rats. Spectroflurometric studies showed that resting calcium levels and I/R-induced calcium overload in the cardiomyocytes were more significantly altered in the hypoxic rats than the normoxic controls. Also, the hypoxic group had decreased levels of the SR calcium content and reduced amplitude and decay time of electrically induced calcium transients, indicating impaired contractility and SR calcium re-uptake. Moreover, there were reductions in protein expression of calcium handling proteins, markedly shown at the level of SR-Ca(2+) ATPase (SERCA) in the heart of hypoxic rats. Melatonin treatment significantly mitigated the calcium handling in the hypoxic rats by preserving SERCA expression. The results suggest that melatonin is cardioprotective against CH-induced myocardial injury by improving calcium handling in the SR of cardiomyocytes via an antioxidant mechanism.

Luminal Ca2+ regulation of single cardiac ryanodine receptors: insights provided by calsequestrin and its mutants

The luminal Ca2+ regulation of cardiac ryanodine receptor (RyR2) was explored at the single channel level. The luminal Ca2+ and Mg2+ sensitivity of single CSQ2-stripped and CSQ2-associated RyR2 channels was defined. Action of wild-type CSQ2 and of two mutant CSQ2s (R33Q and L167H) was also compared. Two luminal Ca2+ regulatory mechanism(s) were identified. One is a RyR2-resident mechanism that is CSQ2 independent and does not distinguish between luminal Ca2+ and Mg2+. This mechanism modulates the maximal efficacy of cytosolic Ca2+ activation. The second luminal Ca2+ regulatory mechanism is CSQ2 dependent and distinguishes between luminal Ca2+ and Mg2+. It does not depend on CSQ2 oligomerization or CSQ2 monomer Ca2+ binding affinity. The key Ca2+-sensitive step in this mechanism may be the Ca2+-dependent CSQ2 interaction with triadin. The CSQ2-dependent mechanism alters the cytosolic Ca2+ sensitivity of the channel. The R33Q CSQ2 mutant can participate in luminal RyR2 Ca2+ regulation but less effectively than wild-type (WT) CSQ2. CSQ2-L167H does not participate in luminal RyR2 Ca2+ regulation. The disparate actions of these two catecholaminergic polymorphic ventricular tachycardia (CPVT)–linked mutants implies that either alteration or elimination of CSQ2-dependent luminal RyR2 regulation can generate the CPVT phenotype. We propose that the RyR2-resident, CSQ2-independent luminal Ca2+ mechanism may assure that all channels respond robustly to large (>5 μM) local cytosolic Ca2+ stimuli, whereas the CSQ2-dependent mechanism may help close RyR2 channels after luminal Ca2+ falls below ∼0.5 mM.

Chronic intermittent hypoxia alters Ca2+ handling in rat cardiomyocytes by augmented Na+/Ca2+ exchange and ryanodine receptor activities in ischemia-reperfusion

This study examined Ca(2+) handling mechanisms involved in cardioprotection induced by chronic intermittent hypoxia (CIH) against ischemia-reperfusion (I/R) injury. Adult male Sprague-Dawley rats were exposed to 10% inspired O(2) continuously for 6 h daily from 3, 7, and 14 days. In isolated perfused hearts subjected to I/R, CIH-induced cardioprotection was most significant in the 7-day group with less infarct size and lactate dehydrogenase release, compared with the normoxic group. The I/R-induced alterations in diastolic Ca(2+) level, amplitude, time-to-peak, and the decay time of both electrically and caffeine-induced Ca(2+) transients measured by spectrofluorometry in isolated ventricular myocytes of the 7-day CIH group were less than that of the normoxic group, suggesting an involvement of altered Ca(2+) handling of the sarcoplasmic reticulum (SR) and sarcolemma. We further determined the protein expression and activity of (45)Ca(2+) flux of SR-Ca(2+)-ATPase, ryanodine receptor (RyR) and sarcolemmal Na(+)/Ca(2+) exchange (NCX) in ventricular myocytes from the CIH and normoxic groups before and during I/R. There were no changes in expression levels of the Ca(2+)-handling proteins but significant increases in the RyR and NCX activities were remarkable during I/R in the CIH but not the normoxic group. The augmented RyR and NCX activities were abolished, respectively, by PKA inhibitor (0.5 microM KT5720 or 0.5 microM PKI(14-22)) and PKC inhibitor (5 microM chelerythrine chloride or 0.2 microM calphostin C) but not by Ca(2+)/calmodulin-dependent protein kinase II inhibitor KN-93 (1 microM). Thus, CIH confers cardioprotection against I/R injury in rat cardiomyocytes by altered Ca(2+) handling with augmented RyR and NCX activities via protein kinase activation.

Differences in mechanisms of SR dysfunction in ischemic vs. idiopathic dilated cardiomyopathy

We examined 1) contractile properties and the intracellular Ca(2+) concentration ([Ca(2+)](i)) transient in cardiac myocytes and 2) sarcoplasmic reticulum (SR) Ca(2+) uptake and release function in myocardium from patients with end-stage heart failure caused by ischemic (ICM) vs. idiopathic dilated cardiomyopathy (DCM). The amplitude of cell motion was decreased 43 +/- 6% in ICM and 68 +/- 7% in DCM compared with that in normal organ donors (DN). Time to peak of shortening was increased 43 +/- 15% in DCM, but not in ICM. Prolongation of the relaxation time was more predominant in ICM. In DCM the systolic [Ca(2+)](i) was decreased 27 +/- 9% and diastolic [Ca(2+)](i) was increased 36 +/- 11%. In ICM the diastolic [Ca(2+)](i) was increased 59 +/- 12% but the systolic [Ca(2+)](i) was unchanged. A significant decrease of the ATP-dependent SR Ca(2+) uptake rate associated with the reduction of the SR Ca(2+)-ATPase protein level was found in ICM. In contrast, the significant decrease in SR Ca(2+) release rate was distinct in DCM. The large amount of Ca(2+) retained in the SR associated with a significant decrease in the maximum reaction velocity and increase in the Michaelis-Menten constant in the caffeine concentration-response curve suggests a fundamental abnormality in the SR Ca(2+) release channel gating property in DCM. We conclude that potentially important differences exist in the intracellular Ca(2+) homeostasis and excitation-contraction coupling in ICM vs. DCM. The SR Ca(2+) release dysfunction may play an important pathogenetic role in the abnormal Ca(2+) homeostasis in DCM, and the SR Ca(2+) uptake dysfunction may be responsible for the contractile dysfunction in ICM.

Ca2+ sparks and Ca2+ waves in saponin-permeabilized rat ventricular myocytes

1. We carried out confocal Ca2+ imaging in myocytes permeabilized with saponin in 'internal' solutions containing: MgATP, EGTA and fluo-3 potassium salt. 2. Permeabilized myocytes exhibited spontaneous Ca2+ sparks and waves similar to those observed in intact myocytes loaded with fluo-3 AM. 3. In the presence of 'low' [EGTA] (0.05 mM), Ca2+ waves arose regularly, even at relatively low [Ca2+] (50-100 nM, free). Increasing [EGTA] resulted in decreased frequency and propagation velocity of Ca2+ waves. Propagating waves were completely abolished at [EGTA] > 0.3 mM. 4. The frequency of sparks increased as a function of [Ca2+] (50-400 nM range) with no sign of a high affinity Ca2+-dependent inactivation process. 5. The rate of occurrence of Ca2+ sparks was increased by calmodulin and cyclic adenosine diphosphate-ribose (cADPR).

Analysis of calcium-induced calcium release in cardiac sarcoplasmic reticulum vesicles using models derived from single-channel data

The planar lipid bilayer and vesicle release experiments are two alternative approaches used to study the function of the ryanodine receptor (RyR) channel at the subcellular level. In this work, we combine models of gating (Zahradníková and Zahradník, Biophys. J. 71 (1996) 2996-3012) and permeation (Tinker et al., J. Gen. Physiol. 100 (1992) 495-517) of the cardiac RyR channel to simulate calcium release experiments on sarcoplasmic reticulum vesicles. The resulting model and real experimental data agreed well within the experimental scatter, confirming indistinguishable properties of the RyRC in the vesicle preparation and in the planar lipid bilayer. The previously observed differences in calcium dependencies of the release and the gating processes can be explained by binding of calcium within the RyRC conducting pore. A novel method of analysis of calcium dependence of calcium release was developed and tested. Three gating models of the RyRC, showing, respectively, an increase, no change, and a decrease in calcium sensitivity over time, were compared. The described method of analysis enabled determination of temporal changes in calcium sensitivity, giving potential for detection of the adaptation/inactivation phenomena of the RyRC in both vesicle and in situ release experiments.

Regulation of the cardiac ryanodine receptor channel by luminal Ca2+ involves luminal Ca2+ sensing sites

The mechanism of activation of the cardiac calcium release channel/ryanodine receptor (RyR) by luminal Ca2+ was investigated in native canine cardiac RyRs incorporated into lipid bilayers in the presence of 0.01 microM to 2 mM Ca2+ (free) and 3 mM ATP (total) on the cytosolic (cis) side and 20 microM to 20 mM Ca2+ on the luminal (trans) side of the channel and with Cs+ as the charge carrier. Under conditions of low trans Ca2+ (20 microM), increasing cis Ca2+ from 0.1 to 10 microM caused a gradual increase in channel open probability (Po). Elevating cis Ca2+ above 100 microM resulted in a gradual decrease in Po. Elevating trans [Ca2+] enhanced channel activity (EC50 approximately 2.5 mM at 1 microM cis Ca2+) primarily by increasing the frequency of channel openings. The dependency of Po on trans [Ca2+] was similar at negative and positive holding potentials and was not influenced by high cytosolic concentrations of the fast Ca2+ chelator, 1,2-bis(2-aminophenoxy)ethane-N,N,N, N-tetraacetic acid. Elevated luminal Ca2+ enhanced the sensitivity of the channel to activating cytosolic Ca2+, and it essentially reversed the inhibition of the channel by high cytosolic Ca2+. Potentiation of Po by increased luminal Ca2+ occurred irrespective of whether the electrochemical gradient for Ca2+ supported a cytosolic-to-luminal or a luminal-to-cytosolic flow of Ca2+ through the channel. These results rule out the possibility that under our experimental conditions, luminal Ca2+ acts by interacting with the cytosolic activation site of the channel and suggest that the effects of luminal Ca2+ are mediated by distinct Ca2+-sensitive site(s) at the luminal face of the channel or associated protein.

Direct monitoring of the calcium concentration in the sarcoplasmic and endoplasmic reticulum of skeletal muscle myotubes

Direct monitoring of the free Ca2+ concentration in the sarcoplasmic reticulum (SR) was carried out in rat skeletal myotubes transfected with a specifically targeted aequorin chimera (srAEQ). Myotubes were also transfected with a chimeric aequorin (erAEQ) that we have demonstrated previously is retained in the endoplasmic reticulum (ER). Immunolocalization analysis showed that although both recombinant proteins are distributed in an endomembrane network identifiable with immature SR, the erAEQ protein was retained also in the perinuclear membrane. The difficulty of measuring [Ca2+] in 100-1000 microM range was overcome with the use of the synthetic coelenterazine analogue, coelenterazine n. We demonstrate that the steady state levels of [Ca2+] measured with srAEQ is around 300 microM, whereas that measured with erAEQ is significantly lower, i.e. around 200 microM. The effects of caffeine, high KCl, and nicotinic receptor stimulation, in the presence or absence of external calcium or after blockade of the Ca-ATPase, were investigated with both chimeras. The kinetics of [Ca2+] changes revealed by the erAEQ were similar, but not identical, neither quantitatively nor qualitatively, to those monitored with the srAEQ, indicating that at this stage of muscle development, differences exist between SR and ER in their mechanisms of Ca2+ handling. The functional implications of these findings are discussed.

Functional Co-expression of the canine cardiac Ca2+ pump and phospholamban in Spodoptera frugiperda (Sf21) cells reveals new insights on ATPase regulation

The utility of the baculovirus cell expression system for investigating Ca2+-ATPase and phospholamban regulatory interactions was examined. cDNA encoding the canine cardiac sarco(endo)plasmic Ca2+-ATPase pump (SERCA2a) was cloned for the first time and expressed in the presence and absence of phospholamban in Spodoptera frugiperda (Sf21) insect cells. The recombinant Ca2+ pump was produced in high yield, contributing 20% of the total membrane protein in Sf21 microsomes. At least 70% of the expressed pumps were active. Co-expression of wild-type, pentameric phospholamban with the Ca2+-ATPase decreased the apparent affinity of the ATPase for Ca2+, but had no effect on the maximum velocity of the enzyme, similar to phospholamban's action in cardiac sarcoplasmic reticulum vesicles. To investigate the importance of the oligomeric structure of phospholamban in ATPase regulation, SERCA2a was co-expressed with a monomeric mutant of phospholamban, in which leucine residue 37 was changed to alanine. Surprisingly, monomeric phospholamban suppressed SERCA2a Ca2+ affinity more strongly than did wild-type phospholamban, demonstrating that the pentamer is not essential for Ca2+ pump inhibition and that the monomer is the more active species. To test if phospholamban functions as a Ca2+ channel, Sf21 microsomes expressing either SERCA2a or SERCA2a plus phospholamban were actively loaded with Ca2+ and then assayed for unidirectional 45Ca2+ efflux. No evidence for a Ca2+ channel activity of phospholamban was obtained. We conclude that the phospholamban monomer is an important regulatory component inhibiting SERCA2a in cardiac sarcoplasmic reticulum membranes, and that the channel activity of phospholamban previously observed in planar bilayers is not involved in the mechanism of ATPase regulation.

A dynamic model of the cardiac ventricular action potential. I. Simulations of ionic currents and concentration changes

A mathematical model of the cardiac ventricular action potential is presented. In our previous work, the membrane Na+ current and K+ currents were formulated. The present article focuses on processes that regulate intracellular Ca2+ and depend on its concentration. The model presented here for the mammalian ventricular action potential is based mostly on the guinea pig ventricular cell. However, it provides the framework for modeling other types of ventricular cells with appropriate modifications made to account for species differences. The following processes are formulated: Ca2+ current through the L-type channel (ICa), the Na(+)-Ca2+ exchanger, Ca2+ release and uptake by the sarcoplasmic reticulum (SR), buffering of Ca2+ in the SR and in the myoplasm, a Ca2+ pump in the sarcolemma, the Na(+)-K+ pump, and a nonspecific Ca(2+)-activated membrane current. Activation of ICa is an order of magnitude faster than in previous models. Inactivation of ICa depends on both the membrane voltage and [Ca2+]i. SR is divided into two subcompartments, a network SR (NSR) and a junctional SR (JSR). Functionally, Ca2+ enters the NSR and translocates to the JSR following a monoexponential function. Release of Ca2+ occurs at JSR and can be triggered by two different mechanisms, Ca(2+)-induced Ca2+ release and spontaneous release. The model provides the basis for the study of arrhythmogenic activity of the single myocyte including afterdepolarizations and triggered activity. It can simulate cellular responses under different degrees of Ca2+ overload. Such simulations are presented in our accompanying article in this issue of Circulation Research.

Structure and function of ryanodine receptors

Membrane depolarization, neurotransmitters, and hormones evoke a release of Ca2+ from intracellular Ca(2+)-storing organelles like the endoplasmic reticulum and, in muscle, the sarcoplasmic reticulum (SR). In turn, the released Ca2+ serves to trigger a variety of cellular responses. The presence of Ca2+ pumps to replenish intracellular stores was described more than 20 years ago. The presence of Ca2+ channels, like the ryanodine receptor, which suddenly release the organelle-stored Ca2+, is a more recent finding. This review describes the progress made in the last five years on the structure, function, and regulation of the ryanodine receptor. Numerous reports have described the response of ryanodine receptors to cellular ions and metabolites, kinases and other proteins, and pharmacological agents. In many cases, comparative measurements have been made using Ca2+ fluxes in SR vesicles, single-channel recordings in planar bilayers, and radioligand binding assays using [3H]ryanodine. These techniques have helped to relate the activity of single ryanodine receptors to global changes in the SR Ca2+ permeability. Molecular information on functional domains within the primary structure of the ryanodine receptor is also available. There are at least three ryanodine receptor isoforms in various tissues. Some cells, such as amphibian muscle cells, express more than a single isoform. The diversity of ligands known to modulate gating and the diversity of tissues known to express the protein suggest that the ryanodine receptor has the potential to participate in many types of cell stimulus-Ca(2+)-release coupling mechanisms.

Transport and release of calcium by sarcoplasmic reticulum in chemically skinned ventricular muscle of the rat

Strips of rat ventricle were treated with EGTA (5 mM) for 24 h at 4 degrees C and used to perform isotopical measurements of transport and release of Ca2+ in the SR in situ. 45Ca accumulated by these preparations showed dependence on time of incubation until it reached saturation after 30 min. Rate and capacity of Ca2+ accumulation were calculated in 0.075 nmol mg ww-1 min-1 and 0.402 nmol mg ww-1, respectively. These values were increased by a factor of 2.6 and 8.6 when K oxalate was present in the incubation media as could be expected for Ca2+ transported by SR. Ca2+ release was assayed on 45Ca desaturation curves at three free Ca2+ concentrations: 0.3, 1 and 10 microM. Significant increases in the velocity of Ca2+ efflux and net release of Ca2+ were induced only by 1 microM free Ca2+, and the Ca2+ release could be inhibited by 75% when 50 microM of ruthenium red was included in the washout solution. These results are in agreement with those obtained in assessing the SR function by mechanical measurements in skinned cardiac cells or by biochemical determinations in isolated cardiac SR vesicles. In spite of the fact that the resolution time is not as high as that required for the physiological handling of Ca2+ by SR, this methodology looks promising for approaching the SR function in cardiac pathologies as well as the effects of drugs on transport and release of Ca2+ by cardiac SR.

Reactive disulfide compounds induce Ca2+ release from cardiac sarcoplasmic reticulum

Reactive disulfide compounds (RDSs) with a pyridyl ring adjacent to a disulfide bond, 2,2'dithiodipyridine (2,2' DTDP) and 4,4' dithiodipyridine (4,4' DTDP), induce Ca2+ release from isolated canine cardiac sarcoplasmic reticulum (SR) vesicles. RDSs are absolutely specific to free sulfhydryl (SH) groups and oxidize SH sites of low pKa via a thiol-disulfide exchange reaction, with the stoichiometric production of thiopyridone in the medium. As in skeletal SR, this reaction caused large increases in the Ca2+ permeability of cardiac SR and the number of SH sites oxidized by RDSs was kinetically and quantitatively measured through the absorption of thiopyridone. RDS-induced Ca2+ release from cardiac SR was characterized and compared to the action of RDSs on skeletal SR and to Ca2(+)-induced Ca2+ release. (i) RDS-induced Ca2+ release from cardiac SR was dependent on ionized Mg2+, with maximum rates of release occurring at 0.5 and 1 mM Mg2+free for 2,2' DTDP and 4,4' DTDP, respectively. (ii) In the presence of adenine nucleotides (0.1-1 mM), the oxidation of SH sites in cardiac SR by exogenously added RDS was inhibited, which, in turn, inhibited Ca2+ release induced by RDSs. (iii) Conversely, when the oxidation reaction between RDSs and cardiac SR was completed and Ca2+ release pathways were opened, subsequent additions of adenine nucleotides stimulated Ca2+ efflux induced by RDSs. (iv) Sulfhydryl reducing agents (e.g., dithiothreitol, DTT, 1-5 mM) inhibited RDS-induced Ca2+ efflux in a concentration-dependent manner. (v) RDSs elicited Ca2+ efflux from passively loaded cardiac SR vesicles (i.e., with nonfunctional Ca2+ pumps in the absence of Mg-ATP) and stimulated Ca2(+)-dependent ATPase activity, which indicated that RDS uncoupled Ca2+ uptake and did not act at the Ca2+, Mg2(+)-ATPase. These results indicate that RDSs selectively oxidize critical sulfhydryl site(s) on or adjacent to a Ca2+ release channel protein channel and thereby trigger Ca2+ release. Conversely, reduction of these sites reverses the effects of RDSs by closing Ca2+ release channels, which results in active Ca2+ reuptake by Ca2+, Mg2(+)-ATPase. These compounds can thus provide a method to covalently label and identify the protein involved in Ca2+ release from cardiac SR.

Relation between steady-state force and intracellular [Ca2+] in intact human myocardium. Index of myofibrillar responsiveness to Ca2+

A novel approach was developed allowing the measurement of steady-state force and intracellular calcium concentration ([Ca2+]i) in tetanized human ventricular trabeculae carneae without pharmacological intervention. We compared and contrasted three methods of assessing calcium sensitivity of the myofilaments: 1) force-[Ca2+] relations in skinned fiber preparations, 2) peak twitch force-peak [Ca2+]i relations, and 3) steady-state force-[Ca2+]i relations in intact muscles. Steady-state contractile activation was achieved rapidly by tetanizing intact human trabeculae, loaded with aequorin, a Ca2(+)-sensitive bioluminescent protein, at a stimulation frequency of 15-20 Hz. Steady-state force and [Ca2+]i were measured during tetani, and the force versus [Ca2+]i relation was obtained by varying the extracellular calcium concentration ([Ca2+]o). Force-[Ca2+]i relations obtained from control and myopathic hearts were fitted to the Hill equation: %Force = [Ca2+]inh/([Ca2+]inh50% + [Ca2+]inh), where nh is the Hill coefficient, and [Ca2+]50% is the [Ca2+] required for 50% activation. The curves of tetani had Hill coefficients of 5.21 +/- 0.20 (n = 6) and 5.61 +/- 0.60 (n = 10) and [Ca2+]50% of 0.56 +/- 0.05 microM (n = 6) and 0.54 +/- 0.09 microM (n = 10) in control and myopathic muscles, respectively. We also constructed peak force-peak [Ca2+]i relations using isometric twitches from the same muscles. These curves were shifted toward higher [Ca2+]i compared with the steady-state force-[Ca2+]i curve derived from tetani. Ryanodine (1 microM), which increased the time course of the Ca2+ and force transients, shifted the peak force-peak [Ca2+]i relation to the left, without affecting the steady-state force-[Ca2+]i relation. Exposure to 10 mM caffeine shifted the steady-state force-[Ca2+]i relation to the left, whereas exposure to 3 microM isoproterenol shifted this relation to the right. Experiments using skinned fiber preparations were performed in parallel with experiments on intact muscles from the same hearts. The force-pCa (-log[Ca2+]) relations in saponin-skinned trabeculae from control and myopathic tissue were superimposable. Ryanodine (1 microM) had no effect on the force-pCa relation in skinned fibers. Maximal tension was evoked by the posttetanic twitch, which was larger than the tetanus. This potentiation was abolished in the presence of ryanodine, a sarcoplasmic reticulum inhibitor. We propose that the changes in the steady-state force-[Ca2+]i relations are correlated with alterations in the sensitivity of the myofilaments to Ca2+, whereas changes in the peak force-peak [Ca2+]i relations represent temporal changes in the twitch transient.(ABSTRACT TRUNCATED AT 400 WORDS)

The heavy metal ions Ag+ and Hg2+ trigger calcium release from cardiac sarcoplasmic reticulum

Heavy metal ions have been shown to induce Ca2+ release from skeletal sarcoplasmic reticulum (SR) by binding to free sulfhydryl groups on a Ca2+ channel protein and are now examined in cardiac SR. Ag+ and Hg2+ (at 10-25 microM) induced Ca2+ release from isolated canine cardiac SR vesicles whereas Ni2+, Cd2+, and Cu2+ had no effect at up to 200 microM. Ag(+)-induced Ca2+ release was measured in the presence of modulators of SR Ca2+ release was compared to Ca2(+)-induced Ca2+ release and was found to have the following characteristics. (i) Ag(+)-induced Ca2+ release was dependent on free [Mg2+], such that rates of efflux from actively loaded SR vesicles increased by 40% in 0.2 to 1.0 mM Mg2+ and decreased by 50% from 1.0 to 10.0 mM Mg2+. (ii) Ruthenium red (2-20 microM) and tetracaine (0.2-1.0 mM), known inhibitors of SR Ca2+ release, inhibited Ag(+)-induced Ca2+ release. (iii) Adenine nucleotides such as cAMP (0.25-2.0 mM) enhanced Ca2(+)-induced Ca2+ release, and stimulated Ag(+)-induced Ca2+ release. (iv) Low Ag+ to SR protein ratios (5-50 nmol Ag+/mg protein) stimulated Ca2(+)-dependent ATPase activity in Triton X-100-uncoupled SR vesicles. (v) At higher ratios of Ag+ to SR proteins (50-250 nmol Ag+/mg protein), the rate of Ca2+ efflux declined and Ca2(+)-dependent ATPase activity decreased gradually, up to a maximum of 50% inhibition. (vi) Ag+ stimulated Ca2+ efflux from passively loaded SR vesicles (i.e., in the absence of ATP and functional Ca2+ pumps), indicating a site of action distinct from the SR Ca2+ pump. Thus, at low Ag+ to SR protein ratios, Ag+ is very selective for the Ca2+ release channel. At higher ratios, this selectivity declines as Ag+ also inhibits the activity of Ca2+,Mg2(+)-ATPase pumps. Ag+ most likely binds to one or more sulfhydryl sites "on" or "adjacent" to the physiological Ca2+ release channel in cardiac SR to induce Ca2+ release.

Calcium and pH in anoxic and toxic injury

The critical events that lead to the transition from reversible to irreversible injury remain unclear. Studies are reviewed that have suggested that a rise in cytosolic free Ca2+ initiates plasma membrane bleb formation and a sequence of events that leads ultimately to cell death. In recent studies, we have measured changes in cytosolic free Ca2+, mitochondrial membrane potential, cytosolic pH, and cell surface blebbing in relation to the onset of irreversible injury and cell death following anoxic and toxic injury to single hepatocytes utilizing multiparameter digitized video microscopy (MDVM). MDVM is an emerging new technology that permits single living cells to be labeled with multiple probes whose fluorescence is responsive to specific cellular parameters of interest. Fluorescence images specific for each probe are collected over time, and then digitized and stored. Image analysis and processing then permits quantitation of the spatial distribution of the various parameters within the single living cells. Our results indicate the following: (1) formation of plasma membrane blebs accompanies all types of injury in hepatocytes; (2) cell death is a rapid event, initiated by rupture of a plasma membrane bleb, and is coincident with the onset of irreversible injury; (3) an increase of cytosolic free Ca2+ is not the stimulus for bleb formation or the final common pathway leading to cell death; (4) a decrease of mitochondrial membrane potential precedes loss of cell viability; (5) cytosolic pH falls by more than 1 pH unit during chemical hypoxia. This acidosis protects against the onset of cell death.

ATP receptor-induced Ca2+ transients in cardiac myocytes: sources of mobilized Ca2+

Addition of micromolar concentrations of extracellular ATP to adult rat cardiac ventricular myocytes increases cytosolic Ca2+ concentration ([Ca2+]). Experiments were performed on fura-2-loaded myocytes to determine whether the [Ca2+] rise was due to Ca2+ influx, release of Ca2+ from the sarcoplasmic reticulum (SR), or a combination of both. BAY K 8644 and nifedipine affected ATP-induced [Ca2+] transients, indicating involvement of voltage-sensitive Ca2+ channels. Addition of ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA) or Ca2+ channel blockers significantly reduced cytosolic [Ca2+] changes due to addition of ATP or KCl without depleting Ca2+ stores (shown by ionomycin treatment in a Ca2+-free medium), demonstrating that these responses require Ca2+ influx. Depletion of intracellular Ca2+ stores by caffeine or ryanodine also diminished cytosolic [Ca2+] responses, indicating that a portion of the increased cytosolic [Ca2+] is due to Ca2+ release from SR. Norepinephrine potentiates the ATP-Ca2+ response, and this effect was not inhibited by depletion of intracellular stores. Although the data show that there are two Ca2+ sources in the cytosolic Ca2+ response to ATP, the pattern is also consistent with the hypothesis of Ca2+-induced Ca2+ release from cardiac SR.

Selective activation of Ca2+ influx by extracellular ATP in a pancreatic beta-cell line (HIT)

The action of exogenous ATP on cytoplasmic free Ca2+ ([Ca2+]i) was studied in insulin secreting cells using fura-2. Stimulation of clonal pancreatic beta-cells (HIT) with ATP (range 2-20 microM) evoked a sustained elevation in [Ca2+]i. ATP selectively promoted Ca2+ influx and not Ca2+ mobilization since (1) the effect required external Ca1+ and (2) was observed in cells in which internal stores were depleted with ionomycin (3) the rate of Mn2+ influx, measured as the quenching of the fura-2 signal, was accelerated by ATP. The action of ATP was unaffected by the voltage-sensitive Ca2+ channel blockers nifedipine and verapamil as well as by a depolarizing concentration of K+. The effect on [Ca2+]i was highly specific for ATP since AMP, ADP, adenosine 5'-[gamma-thio]triphosphate, adenosine 5'-[beta, gamma-methylene]triphosphate, GTP and adenosine were ineffective. In normal pancreatic islet cells, both exogenous ATP (range 0.2-2 microM) and ADP induced a transient Ca2+ elevation that did not require external Ca2+. The nucleotide specificity of the effect on [Ca2+]i suggests that ATP activates P2 gamma purinergic receptors in normal beta-cells. Thus, ATP evokes a Ca2+ signal in clonal HIT cells and normal islet cells by different transducing systems involving distinct purinoreceptors. A novel mechanism for increasing [Ca2+]i by extracellular ATP is reported in HIT cells, since the nucleotide specificity and the selective activation of Ca2+ influx without mobilization of internal Ca2+ stores cannot be explained by mechanisms already described in other cell systems.

Spontaneous sarcoplasmic reticulum calcium release in rat and rabbit cardiac muscle: relation to transient and rested-state twitch tension

Scattered light intensity fluctuations (SLIF) (which monitor spontaneous Ca2+ release from the sarcoplasmic reticulum [SR]) and resting and twitch tension were measured during intervals after stimulation in rat and rabbit papillary muscles. For 1 to 5 seconds after stimulation of rat muscle bathed in 1.5 mM [Ca2+] (Cao), twitch and resting tension are depressed and SLIF are transiently abolished. SLIF and resting tension then recover simultaneously and monotonically but lag behind the restitution of twitch tension. In the absence of further stimulation, SLIF persist and the rested state twitch amplitude remains potentiated. When Cao is increased above 2.5 mM, the restitution of all parameters following stimulation is accelerated and becomes oscillatory; the lag of SLIF and resting tension restitution behind that of the twitch increases such that twitch amplitude increases, overshoots, and decreases to a nadir as SLIF and resting tension reach their initial maximum. In a given muscle, the maximum twitch amplitude occurs at approximately the same level of SLIF; when this level is exceeded, either transiently during monotonic or oscillatory recovery after stimulation in a given Cao or in the steady rested state by an increase in Cao, twitch tension decreases. Ryanodine (1 microM), caffeine (10 mM), or replacement of Cao with strontium abolishes SLIF and causes twitch amplitude to decay with rest. In contrast to rat, twitch amplitude in rabbit muscle bathed in physiological Cao decays with rest and SLIF are nonmeasurable at any interval following stimulation. When Cao is increased to 20 mM during rest, SLIF occur and the rest decay of the twitch is abolished. We interpret the parallel behavior of SLIF and rest potentiation to indicate that in the presence of SLIF, the average SR Ca2+ load within the tissue is high. Depolarization of a tissue exhibiting SLIF causes a large twitch but also a transient depletion of the average SR Ca2+ load. That the restitution of SLIF lags behind recovery of twitch amplitude suggests that the onset of spontaneous SR Ca2+ release requires a delay following SR Ca2+ replenishment. The simultaneous occurrence of spontaneous Ca2+ release in a sufficient number of cells places an upper limit on twitch amplitude either during recovery following stimulation or at rest.

Cardiac sarcoplasmic reticulum characteristics in hypertrophic hearts from spontaneously hypertensive rats

Sarcoplasmic reticulum (SR) from left ventricles of rats that developed spontaneous hypertension was studied in vitro. Similar increases of left ventricular mass were found when grouping the animals into mild and severe hypertensives (average systolic arterial pressure of 168 +/- 4 and 202 +/- 6 mmHg, respectively). The amount of SR protein (mg/g of left ventricle) was higher when obtained from hypertrophic ventricles of both hypertensive groups than from ventricles of the control group. The result agreed with the enhanced Ca2+ uptake exhibited by left ventricular homogenates of the hypertensive groups. Consequently, Ca2+ uptake in SR microsomes isolated per gram of left ventricle (nmol Ca2+/g muscle) was 51.62 +/- 10.06 and 64.99 +/- 12.84 in mildly and severely hypertensive groups vs. 17.37 +/- 5.79 in the control group (P less than 0.05). The SR microsomes obtained from ventricles of hypertensive rats showed an enhanced Ca2+ activated ATPase activity that was not accompanied by increased Ca2+ uptake at saturating calcium concentrations, but by increased affinity for calcium (K'app. of 1.09 +/- 0.28 and 2.67 +/- 0.16 microM in SR microsomes of hypertrophic and control ventricles respectively; P less than 0.05). The rates of calcium loss, measured in SR vesicles passively loaded with 45Ca, were similar when assayed in SR obtained from ventricles of both hypertensive and normal rats. These results enable us to suggest that in hearts of rats presenting spontaneous hypertension, the function of the SR system could account for a normal handling of cytosolic calcium. They might support the absence of mechanical alterations described in hearts of young rats of the SHR strain.

Lack of effects of calcium X calmodulin-dependent phosphorylation on Ca2+ release from cardiac sarcoplasmic reticulum

Canine cardiac sarcoplasmic reticulum is phosphorylated by an endogenous calcium X calmodulin-dependent protein kinase and phosphorylation occurs mainly on a 27 kDa proteolipid, called phospholamban. To determine whether this phosphorylation has any effect on Ca2+ release, sarcoplasmic reticulum vesicles were phosphorylated by the calcium X calmodulin-dependent protein kinase, while non-phosphorylated vesicles were preincubated under identical conditions but in the absence of ATP to avoid phosphorylation. Both non-phosphorylated and phosphorylated vesicles were centrifuged to remove calmodulin, and subsequently used for Ca2+ release studies. Calcium loading was carried out either by the active calcium pump or by incubation with high (5 mM) calcium for longer periods. Phosphorylation of sarcoplasmic reticulum by calcium X calmodulin-dependent protein kinase had no appreciable effect on the initial rates of Ca2+ released from cardiac sarcoplasmic reticulum vesicles loaded under passive conditions and on the apparent 45Ca2+-40Ca2+ exchange from cardiac sarcoplasmic reticulum vesicles loaded under active conditions. Thus, it appears that calcium X calmodulin-dependent protein kinase mediated phosphorylation of cardiac sarcoplasmic reticulum is not involved in the regulation of Ca2+ release and 45Ca2+-40Ca2+ exchange.

Failure of inositol 1,4,5-trisphosphate to elicit or potentiate Ca2+ release from isolated skeletal muscle sarcoplasmic reticulum

This study was conducted to resolve the conflicting reports regarding the ability of inositol 1,4,5-trisphosphate (IP3) to elicit the release of Ca2+ from isolated sarcoplasmic reticulum derived from skeletal muscle. Three different conditions were employed, one of which has been reported to produce an IP3 induced release of Ca2+. Sarcoplasmic reticulum vesicles with and without intact feet structures failed to respond to added IP3. In addition, IP3 had no effect on the Ca2+-induced release of Ca2+. These results suggest that, unlike other tissue, IP3 does not mobilize the release of Ca2+ from skeletal muscle sarcoplasmic reticulum. IP3 is therefore unlikely to be the physiological signal linking transverse-tubule depolarization with Ca2+ release from the sarcoplasmic reticulum.

Two types of spontaneous contractions in rat myocardium and their possible relation to different Ca release processes

The potency of putative blockers (lidocaine, dantrolene Na, DAN, SKF 525A) for various forms of Ca2+ release from sarcoplasmic reticulum (SR) against two types of spontaneous contractions induced by 0.5 mM caffeine (caffeine response) or myocardium Ca2+ overload (after-contractions) was tested in rat papillary muscle. There were differences in the sensitivity of spontaneous contractions to the agents employed. The caffeine response was depressed by DAN but not affected by lidocaine. Conversely, conventional diastolic after-contractions were eliminated by lidocaine and SKF 525A but DAN failed to affect them. The two types of spontaneous contractions were suggested to be triggered by different CA2+ release processes supposedly originating from different SR compartments.

Direct and indirect effects of ciguatoxin on guinea-pig atria and papillary muscles

The mode of action of ciguatoxin (CTX) on the isolated atrial and papillary muscle of the guinea-pig heart was investigated using conventional methods for the measurement of mechanical and electrophysiological parameters. CTX induced positive inotropic and positive klinotropic responses in atrial and papillary muscles. Each response consisted of two phases. The initial positive inotropic response developed rapidly and resulted from the previously reported indirect action of CTX. The second phase of positive inotropy developed more slowly and was well maintained at doses of CTX up to 0.15 mouse units/ml in atria and up to 0.8 M.U./ml in papillary muscles. This phase was found to result from a direct action of CTX on the myocardium which was not reversed by washing. Tetrodotoxin (TTX) reversed the positive inotropic effects stemming from the direct action of CTX. The (-) and (+) enantiomers of propranolol were equally effective in inhibiting the direct effect of CTX. These antagonists did not displace CTX from the myocardium. CTX induced a TTX-sensitive depolarization of stimulated or quiescent atrial cells. All the effects of CTX on the atrial action potential were reversed by TTX. It was therefore concluded that CTX opens voltage dependent Na+ channels. CTX bound equally to resting and K+-depolarized Na+ channels but there were indications that electrical stimulation enhanced the rate of CTX binding. CTX overrides the positive staircase effect of increasing stimulation frequency. Na+ channels found in the atria which were particularly sensitive to TTX did not play a prominent role in mediating the CTX effect. CTX appeared to have little effect on the normal Na+ channel inactivation process. CTX did not restore contractions in the K+-depolarized cardiac muscles examined. The sensitivity of CTX action to TTX distinguished it from cardiac glycoside activity. Established mechanisms of Na+/Ca2+ exchange and Ca2+-induced release of Ca2+ can explain the link between CTX-induced increase of intracellular [Na+] and the positive inotropic response.

Solubilization and separation of Ca2+-ATPase from the Ca2+-ryanodine receptor complex

Heavy sarcoplasmic reticulum (SR) preparations of rabbit skeletal muscle, which are enriched in Ca2+-release vesicles from the terminal cisternae (TC) and [3H]ryanodine receptor density, exhibit 60% of the Ca2+-ATPase activity, 58% of the EP level, and 30% of the steady state Ca2+ loading compared to membrane vesicles from the longitudinal SR. The Ca2+-ATPase of TC SR is solubilized and separated from the Ca2+-ryanodine receptor complex in the insoluble fraction on treatment with the detergent C12E9. However, a 50% decrease in receptor density is observed upon removal of the Ca2+-ATPase, suggesting a significant contribution of this protein to maintaining optimal receptor complex density.

Interaction of myotoxin a with the Ca2+-ATPase of skeletal muscle sarcoplasmic reticulum

Myotoxin a is a muscle-damaging toxin isolated from the venom of Crotalus viridis viridis. Its interaction with the Ca2+-ATPase of sarcoplasmic reticulum (SR) vesicles purified from rabbit skeletal muscle was investigated. Myotoxin a inhibited Ca2+ loading and stimulated Ca2+-dependent ATPase without affecting unidirectional Ca2+ efflux. Its action was dose, time, and temperature dependent. Myotoxin a partially blocked the binding of specific anti-(rabbit SR Ca2+-ATPase) antibodies. It is concluded that myotoxin a attaches to the SR Ca2+-ATPase and uncouples Ca2+ uptake from Ca2+-dependent ATP hydrolysis. Myotoxin a also prevented the formation of decavanadate-induced two-dimensional crystalline arrays of the SR Ca2+-ATPase.

Calcium oxalate and calcium phosphate capacities of cardiac sarcoplasmic reticulum

Both oxalate-supported and phosphate-supported calcium uptake by canine cardiac sarcoplasmic reticulum initially increase linearly with time but fall to a steady-state level within 20 min. The departure from linearity could be due to a decrease in influx or to an increase in efflux of calcium. Because Ca2+-ATPase activity is linear, a decrease in the influx of calcium is an unlikely cause of the non-linear calcium uptake curves. A possible cause of an increase in calcium efflux is rupture of the vesicles. This hypothesis was tested by investigating the amount of calcium which could be released upon addition of 5 mM EGTA. The amount of rapidly releasable calcium was zero until a threshold calcium uptake of about 4-6 mumol calcium oxalate or calcium phosphate per mg was reached. After that point the rapidly releasable calcium continued to increase with calcium oxalate to reach more than 23 mumol/mg, but stayed constant at about 0.7 mumol/mg for calcium phosphate. The rapidly releasable calcium was attributed to calcium oxalate or calcium phosphate crystals externalized by vesicle rupture. The differences in the amounts of rapidly releasable calcium were attributed to different kinetics of calcium phosphate and calcium oxalate dissolution. Addition of ryanodine caused a marked increase in the threshold for rapidly releasable calcium oxalate. Transmission electron micrographs showed that vesicles can become filled with calcium oxalate crystals, but the vesicles were heterogeneous with respect to their size and their sensitivity to ryanodine. These observations support the hypothesis that calcium oxalate and calcium phosphate capacities are limited by vesicle rupture and that ryanodine increases the capacity by closing a calcium channel in a subpopulation of vesicles that otherwise would not accumulate calcium.

Ca2+ transport studied with arsenazo III in Tetrahymena microsomes. Effects of calcium ionophore A23187 and trifluoperazine

Transport of Ca2+ in microsomal membrane vesicles of the Tetrahymena has been investigated using arsenazo III as a Ca2+ indicator. The microsomes previously shown to carry a Mg2+-dependent, Ca2+-stimulated ATPase (Muto, Y. and Nozawa, Y. (1984) Biochim. Biophys. Acta 777, 67-74) accumulated calcium upon addition of ATP and Ca2+ sequestered into microsomal vesicles was rapidly discharged by the Ca2+ ionophore A23187. Kinetic studies indicated that the apparent Km for free Ca2+ and ATP are 0.4 and 59 microM, respectively. The Vmax was about 40 nmol/mg protein per min at 37 degrees C. The calcium accumulated during ATP-dependent uptake was released after depletion of ATP in the incubation medium. Furthermore, addition of trifluoperazine which inhibited both (Ca2+ + Mg2+)-ATPase and ATP-dependent Ca2+ uptake rapidly released the calcium accumulated in the microsomal vesicles. These observations suggest that Tetrahymena microsome contains both abilities to take up and to release calcium and may act as a Ca2+-regulating site in this organism.

The calcium-ryanodine receptor complex of skeletal and cardiac muscle

[3H]Ryanodine binds with high affinity to saturable and Ca2+-dependent sites in heavy sarcoplasmic reticulum (SR) preparations from rabbit skeletal and cardiac muscle. Ruthenium red, known to interfere with Ca2+-induced Ca2+ release from SR vesicles, inhibits [3H]ryanodine specific binding in both skeletal and cardiac preparations whereas Mg2+, Ba2+, Cd2+ and La3+ selectively inhibit the skeletal preparation. The toxicological relevance of the [3H]ryanodine binding site is established by the correlation of binding inhibition with toxicity for seven ryanoids including two botanical insecticides. These findings provide direct evidence for Ca2+-ryanodine receptor complexes that may play a role in excitation-contraction coupling.