Excitation-contraction coupling in cardiac Purkinje fibers. Effects of caffeine on the intracellular [Ca2+] transient, membrane currents, and contraction

Blinded, Multicenter Evaluation of Drug-induced Changes in Contractility Using Human-induced Pluripotent Stem Cell-derived Cardiomyocytes

Animal models are 78% accurate in determining whether drugs will alter contractility of the human heart. To evaluate the suitability of human-induced pluripotent stem cell-derived cardiomyocytes (hiPSC-CMs) for predictive safety pharmacology, we quantified changes in contractility, voltage, and/or Ca²⁺ handling in 2D monolayers or 3D engineered heart tissues (EHTs). Protocols were unified via a drug training set, allowing subsequent blinded multicenter evaluation of drugs with known positive, negative, or neutral inotropic effects. Accuracy ranged from 44% to 85% across the platform-cell configurations, indicating the need to refine test conditions. This was achieved by adopting approaches to reduce signal-to-noise ratio, reduce spontaneous beat rate to ≤ 1 Hz or enable chronic testing, improving accuracy to 85% for monolayers and 93% for EHTs. Contraction amplitude was a good predictor of negative inotropes across all the platform-cell configurations and of positive inotropes in the 3D EHTs. Although contraction- and relaxation-time provided confirmatory readouts forpositive inotropes in 3D EHTs, these parameters typically served as the primary source of predictivity in 2D. The reliance of these “secondary” parameters to inotropy in the 2D systems was not automatically intuitive and may be a quirk of hiPSC-CMs, hence require adaptations in interpreting the data from this model system. Of the platform-cell configurations, responses in EHTs aligned most closely to the free therapeutic plasma concentration. This study adds to the notion that hiPSC-CMs could add value to drug safety evaluation.

Dipeptidyl peptidase-4 independent cardiac dysfunction links saxagliptin to heart failure

Saxagliptin treatment has been associated with increased rate of hospitalization for heart failure in type 2 diabetic patients, though the underlying mechanism(s) remain elusive. To address this, we assessed the effects of saxagliptin on human atrial trabeculae, guinea pig hearts and cardiomyocytes. We found that the primary target of saxagliptin, dipeptidyl peptidase-4, is absent in cardiomyocytes, yet saxagliptin internalized into cardiomyocytes and impaired cardiac contractility via inhibition of the Ca²⁺/calmodulin-dependent protein kinase II-phospholamban-sarcoplasmic reticulum Ca²⁺-ATPase 2a axis and Na⁺-Ca²⁺ exchanger function in Ca²⁺ extrusion. This resulted in reduced sarcoplasmic reticulum Ca²⁺ content, diastolic Ca²⁺ overload, systolic dysfunction and impaired contractile force. Furthermore, saxagliptin reduced protein kinase C-mediated delayed rectifier K⁺ current that prolonged action potential duration and consequently QTc interval. Importantly, saxagliptin aggravated pre-existing cardiac dysfunction induced by ischemia/reperfusion injury. In conclusion, our novel results provide mechanisms for the off-target deleterious effects of saxagliptin on cardiac function and support the outcome of SAVOR-TIMI 53 trial that linked saxagliptin with the risk of heart failure.

In-vitro examination of the positive inotropic effect of caffeine and taurine, the two most frequent active ingredients of energy drinks

Background

Our study aimed to evaluate changes in the contractile behavior of human myocardium after exposure to caffeine and taurine, the main active ingredients of energy drinks (EDs), and to evaluate whether taurine exhibits any inotropic effect at all in the dosages commonly used in EDs.

Methods

Myocardial tissue was removed from the right atrial appendages of patients undergoing cardiac surgery and prepared to obtain specimens measuring 4 mm in length. A total of 92 specimens were exposed to electrical impulses at a frequency of 75 bpm for at least 40 min to elicit their maximum contractile force before measuring the isometric contractile force (ICF) and duration of contraction (CD). Following this, each specimen was treated with either taurine (group 1, n = 29), or caffeine (group 2, n = 31) or both (group 3, n = 32). After exposure, ICF and CD measuring were repeated. Post-treatment values were compared with pre-treatments values and indicated as percentages.

Results

Exposure to taurine did not alter the contraction behavior of the specimens. Exposure to caffeine, in contrast, led to a significant increase in ICF (118 ± 03%, p < 0.01) und a marginal decrease in CD (95 ± 1.6%, p < 0.01). Exposure to a combination of caffeine and taurine also induced a statistically significant increase in ICF (124 ± 4%, p < 0.01) and a subtle reduction in CD (92 ± 1.4%, p < 0.01). The increase in ICF achieved by administration of caffeine was similar to that achieved by a combination of both caffeine and taurine (p = 0.2).

The relative ICF levels achieved by administration of caffeine and a combination of taurine and caffeine, respectively, were both significantly higher (p < 0.01) than the ICF resulting from exposure to taurine only.

Conclusion

While caffeine altered the contraction behavior of the specimen significantly in our in-vitro model, taurine did not exhibit a significant effect. Adding taurine to caffeine did not significantly enhance or reduce the effect of caffeine.

Cardiotoxicodynamics: Toxicity of Cardiovascular Xenobiotics

Maintaining adequate tissue perfusion depends on a variety of factors, all of which can be influenced by xenobiotics (substances foreign to the body, including pharmaceuticals, chemicals, and natural compounds). Volume status, systemic vascular resistance, myocardial contractility, and cardiac rhythm all play a significant role in ensuring hemodynamic stability and proper cardiovascular function. Direct effects on the nervous system, the vasculature, or the heart itself as well as indirect metabolic effects may play a significant role in the development of cardiotoxicity. This article is dedicated to discussion of the disruption of cardiovascular physiology by xenobiotics.

A model of canine purkinje cell electrophysiology and Ca(2+) cycling: rate dependence, triggered activity, and comparison to ventricular myocytes

Purkinje cells (Pcell) are characterized by different electrophysiological properties and Ca(2+) cycling processes than ventricular myocytes (Vcell) and are frequently involved in ventricular arrhythmias. Yet, the mechanistic basis for their arrhythmic vulnerability is not completely understood. The objectives were to: (1) characterize Pcell electrophysiology, Ca(2+) cycling, and their rate dependence; (2) investigate mechanisms underlying Pcell arrhythmogenicity; and compare Pcell and Vcell electrophysiology, Ca(2+) cycling, and arrhythmic properties. We developed a new mathematical model of Pcell. The Ca(2+) subsystem includes spatial organization and receptors distribution unique to Pcell. Results were: (1) in Pcell and Vcell, Na(+) accumulation via its augmentation of repolarizing I(NaK) dominates action potential duration adaptation and, in Pcell, I(NaL) contributes additional action potential duration shortening at short cycle length; (2) steep Pcell restitution is attributable to slow recovery of I(NaL); (3) biphasic Ca(2+) transients of Pcell reflect the delay between Ca(2+) release from junctional sarcoplasmic reticulum and corbular sarcoplasmic reticulum; (4) Pcell Ca(2+) alternans, unlike Vcell, can develop without inducing action potential alternans; (5) Pcell action potential alternans develops at a shorter cycle length than Vcell, with increased subcellular heterogeneity of Ca(2+) cycling attributable to refractoriness of Ca(2+) release from corbular sarcoplasmic reticulum and junctional sarcoplasmic reticulum; (6) greater Pcell vulnerability to delayed afterdepolarizations is attributable to higher sarcoplasmic reticulum Ca(2+) content and ionic currents that reduce excitation threshold and promote triggered activity; and (7) early after depolarizations generation in Pcell is mostly attributable to reactivation of I(NaL2), whereas I(CaL) plays this role in Vcell. Steeper rate dependence of action potential and Ca(2+) transients, central peripheral heterogeneity of Ca(2+) cycling, and distinct ion channel profile underlie greater arrhythmic vulnerability of Pcell compared to Vcell.

Coffee consumption and risk of heart failure in men: an analysis from the Cohort of Swedish Men

BACKGROUND

A previous study found that consuming 5 or more cups of coffee per day was associated with increased incidence of heart failure (HF). We sought to evaluate this association in a larger population.

METHODS

We measured coffee consumption using food frequency questionnaires among 37,315 men without history of myocardial infarction, diabetes, or HF. They were observed for HF hospitalization or mortality from January 1, 1998, until December 31, 2006, using record linkage to the Swedish inpatient and cause of death registries. Cox proportional hazards models adjusted for age, dietary, and demographic factors were used to calculate incidence rate ratios (RR) and 95% confidence intervals (CIs).

RESULTS

For 9 years of follow-up, 784 men experienced an HF event. Compared to men who drank <or=1 cup of coffee per day (unadjusted rate 29.9 HF events/10,000 person-years), RR were 0.87 (95% CI 0.69-1.11, unadjusted rate 29.2/10,000 person-years) for 2 cups/d, 0.89 (95% CI 0.70-1.14, unadjusted rate 25.1/10,000 person-years) for 3 cups/d, 0.89 (95% CI 0.69-1.15, unadjusted rate 25.0/10,000 person-years) for 4 cups/d, and 0.89 (95% CI 0.69-1.15, unadjusted rate 18.1/10,000 person-years) for >or=5 cups/d (P for trend in RR = .61).

CONCLUSIONS

This study did not support the hypothesis that high coffee consumption is associated with increased rates of HF hospitalization or mortality.

Cardiac Purkinje cells

Purkinje cells are specialized for rapid propagation in the heart. Furthermore, Purkinje fibers as the source as well as the perpetuator of arrhythmias is a familiar finding. This is not surprising considering their location in the heart and their unique cell ultrastructure, cell electrophysiology, and mode of excitation-contraction coupling. This review touches on each of these points as we outline what is known today about Purkinje fibers/cells.

Anti-arrhythmic effects of cyclopiazonic acid in Langendorff-perfused murine hearts

We investigated the effects of reducing sarcoplasmic reticular (SR) Ca(2+) stores using the Ca(2+)-ATPase inhibitor cyclopiazonic acid (CPA) in Langendorff-perfused mouse hearts exposed to different pro-arrhythmic agents all known to produce Ca(2+)-mediated arrhythmogenesis. CPA (100 and 150 nM) produced progressive (beginning over approximately 1 min) and significant (P<0.0001) reductions in peak amplitudes of Ca(2+) transients evoked by regular stimulation in isolated Fluo-3 loaded myocytes from F/F(0)=3.2+/-0.16 (n=12 cells) to 1.62+/-0.012 (n=6 cells) and 1.53+/-0.06 (n=12 cells), respectively, consistent with previous reports describing reductions of store Ca(2+) in other cell systems. The corresponding effects of CPA were then examined in intact hearts exposed to isoproterenol (100 nM), elevated extracellular [Ca(2+)] (5mM) and caffeine (1mM). All three agents produced ventricular tachycardia either when added alone or simultaneously with CPA during programmed electrical stimulation. However, arrhythmogenicity was not observed when such agents were added approximately 10 min after introduction of CPA. CPA thus antagonized this Ca(2+)-mediated arrhythmogenesis but only under circumstances of SR Ca(2+) depletion. These alterations in arrhythmogenic tendency took place despite an absence of alterations in electrogram and monophasic action potential characteristics. This was in sharp contrast to previous observations in murine, DeltaKPQ-Scn5a (LQT3) and KCNE1(-/-) (LQT5), systems where re-entry has been implicated in arrhythmogenesis.

Effect of caffeine on myocardial perfusion imaging using single photon emission computed tomography during adenosine pharmacologic stress

Approximately 6 million cardiac stress tests are performed annually in the United States, of which 2.4 million are pharmacologic stress tests using agents such as adenosine. Adenosine induces differential coronary hyperemia in normal coronary arteries versus coronary arteries with atherosclerosis, allowing single photon emission computed tomography (SPECT) imaging to identify reduced coronary flow in segments subtended by diseased coronary arteries. The potential attenuation of pharmacologic effects of adenosine in the presence of caffeine is why patients are routinely instructed to abstain from caffeine for 12 to 24 hours prior to administration of an adenosine stress test. Failure to abstain from caffeine results in cancellation or delaying of cardiac stress testing, resulting in procedural delays and its impact on patient throughput. Recent studies have evaluated such interaction and suggested a lack of clinically significant effect of caffeine on adenosine-induced hyperemia during myocardial SPECT imaging. This article reviews the clinical pharmacology of caffeine, adenosine, and dipyridamole and effect of caffeine on myocardial stress testing using adenosine and dipyridamole in clinical cardiovascular medicine. The limited published data are conflicting, but some recent publications suggest that myocardial perfusion SPECT imaging using adenosine may not be clinically significantly altered by routine consumption of caffeine, such as a cup of coffee. Although prospective randomized studies would be required to obtain a definitive answer to this question, it appears on the basis of some of the studies reviewed in this article that caffeine consumption prior to myocardial perfusion imaging may not necessitate cancellation or rescheduling of adenosine stress testing.

Cardiovascular challenges in toxicology

The diagnoses and subsequent treatment of poisoned patients manifesting cardiovascular compromise challenges the most experienced emergency physician. Numerous drugs and chemicals cause cardiac and vascular disorders. Despite widely varying indications for therapeutic use, many agents share a common cardiovascular pharmacologic effect if taken in overdose. Standard advanced cardiac life support protocol care of these patients may not apply and may even result in harm if followed. This chapter discusses com-mon cardiovascular toxins and groups them into their common mechanisms of toxicity. Multiple agents exist that result in human cardiovascular toxicity. The management of the toxicity of each agent should follow a rationale approach. The first step in the care of all poisoned patients focuses on good supportive care.

Massive Caffeine Overdose Requiring Vasopressin Infusion and Hemodialysis

INTRODUCTION

Massive caffeine overdose is associated with life-threatening hemodynamic complications that present challenges for clinicians. We describe the highest-reported serum concentration of caffeine in a patient who survived and discuss the first-reported use of vasopressin and hemodialysis in a caffeine-poisoned patient.

CASE REPORT

A 41-yr-old woman presented 3 h after ingesting approximately 50 g of caffeine. She subsequently underwent cardiopulmonary resuscitation and received multiple medications in an attempt to raise her blood pressure and control her heart rate without success. Vasopressin infusion increased her blood pressure to the point where hemodialysis could be performed. Despite ensuing multisystem organ failure, she survived and has made a complete recovery.

CONCLUSION

Hemodialysis and vasopressin infusions may be of benefit in the management of caffeine-intoxicated patients who fail to respond to standard therapies.

Caffeine-induced immobilization of gating charges in isolated guinea-pig ventricular heart cells

The effects of 10 mM caffeine (CAF) on intramembrane charge movements (ICM) were studied in isolated guinea-pig ventricular heart cells with the whole-cell patch-clamp technique. In the presence of CAF, the properties (voltage dependence, maximum Q(ON) [Q(max)], availability with voltage) of Q(ON) charge activated from -110 mV were barely affected. Following a 100 ms prepulse to -50 mV to decrease the participation of charges originating from Na channels, the voltage dependence of Q(ON) was shifted by 5 mV (negative component) and by 10 mV (positive component) towards negative potentials, and Q(max) was depressed by 16.5%. CAF drastically reduced in a time- and voltage-dependent manner Q(OFF) on repolarization to -50 mV, the effects being greater at positive potentials. CAF-induced Q(OFF) immobilization could be almost entirely removed by repolarization to voltages as negative as -170 mV. In these conditions, the voltage-dependence of Q(OFF) (repolarization to +30 to -170 mV) was shifted by 17 mV (negative component) and 30 mV (positive component) towards negative potentials, suggesting an interconversion into charge 2. Most of CAF effects were suppressed when the sarcoplasmic reticulum (SR) was not functional or when the cells were loaded with BAPTA-AM. We conclude that CAF effects on ICM are likely due to Ca(2+) ions released from the SR, and which accumulate in the subsarcolemmal fuzzy spaces in the vicinity of the Ca channels. Because CAF effects were more pronounced on Q(OFF) than on Q(ON) the channels have likely to open before Ca(2+) ions could affect their gating properties. It is speculated that such an effect on gating charges might contribute to the Ca-induced inactivation of the Ca current.

Differing effects of inotropic agents on length change deactivation of isolated rat myocardium

BACKGROUND

A rapid change in length of cardiac muscle during isometric contraction is followed by developed force that is less than appropriate for the new length because of deactivation of the contractile system. Length change deactivation may have favorable or unfavorable effects on cardiac function, depending on the circumstances under which it is produced.

METHODS

Left ventricular papillary muscles from male Sprague-Dawley rats were arranged for recording of isometric force. After each control or reference isometric contraction, a quick release-quick stretch V-step was applied to the following contraction. For each repetition of control and experimental contractions, the time of application of V-steps was increased by 20 ms until peak force was reached. Effects of these V-steps were assessed from ratios of peak redeveloped force to peak force in an isometric reference contraction. Slopes of plots of these ratios versus time after the onset of the contraction were used to quantify the effects of inotropic agents on deactivation.

RESULTS

Increasing calcium from 2.5 to 5.0 or 7.5 mM increased force by 12+/-4% (mean+/-SEM), did not change time to peak, and did not significantly alter the deactivation slope. Adding 5 mM epinephrine increased force by 16+/-5%, decreased time to peak by 34+/-3%, and increased the deactivation slope by 106+/-9% (P<0.001). Caffeine had variable effects on peak force, increased time to peak by 47+/-4%, and decreased the deactivation slope by 71+/-5% (P<0.001).

CONCLUSIONS

The quantitatively different effects of the three agents on length change deactivation slopes and time to peak force suggest a common mechanism, probably involving thin-filament cooperativity.

Integrative analysis of calcium cycling in cardiac muscle

The control of intracellular calcium is central to regulation of contractile force in cardiac muscle. This review illustrates how analysis of the control of calcium requires an integrated approach in which several systems are considered. Thus, the calcium content of the sarcoplasmic reticulum (SR) is a major determinant of the amount of Ca(2+) released from the SR and the amplitude of the Ca(2+) transient. The amplitude of the transient, in turn, controls Ca(2+) fluxes across the sarcolemma and thence SR content. This control of SR content influences the response to maneuvers that modify, for example, the properties of the SR Ca(2+) release channel or ryanodine receptor. Specifically, modulation of the open probability of the ryanodine receptor produces only transient effects on the Ca(2+) transient as a result of changes of SR content. These interactions between various Ca(2+) fluxes are modified by the Ca(2+) buffering properties of the cell. Finally, we predict that, under some conditions, the above interactions can result in instability (such as alternans) rather than ordered control of contractility.

Ca(2+) transients and Ca(2+) waves in purkinje cells : role in action potential initiation

Purkinje cells contain sarcoplasmic reticulum (SR) directly under the surface membrane, are devoid of t-tubuli, and are packed with myofibrils surrounded by central SR. Several studies have reported that electrical excitation induces a biphasic Ca(2+) transient in Purkinje fiber bundles. We determined the nature of the biphasic Ca(2+) transient in aggregates of Purkinje cells. Aggregates (n=12) were dispersed from the subendocardial Purkinje fiber network of normal canine left ventricle, loaded with Fluo-3/AM, and studied in normal Tyrode's solution (24 degrees C). Membrane action potentials were recorded with fine-tipped microelectrodes, and spatial and temporal changes in [Ca(2+)](i) were obtained from fluorescent images with an epifluorescent microscope (x20; Nikon). Electrical stimulation elicited an action potential as well as a sudden increase in fluorescence (L(0)) compared with resting levels. This was followed by a further increase in fluorescence (L(1)) along the edges of the cells. Fluorescence then progressed toward the Purkinje cell core (velocity of propagation 180 to 313 microm/s). In 62% of the aggregates, initial fluorescent changes of L(0) were followed by focally arising Ca(2+) waves (L(2)), which propagated at 158+/-14 microm/s (n=13). Spontaneous Ca(2+) waves (L(2)*) propagated like L(2) (164+/-10 microm/s) occurred between stimuli and caused slow membrane depolarization; 28% of L(2)* elicited action potentials. Both spontaneous Ca(2+) wave propagation and resulting membrane depolarization were thapsigargin sensitive. Early afterdepolarizations were not accompanied by Ca(2+) waves. Action potentials in Purkinje aggregates induced a rapid rise of Ca(2+) through I(CaL) and release from a subsarcolemmal compartment (L(0)). Ca(2+) release during L(0) either induced further Ca(2+) release, which propagated toward the cell core (L(1)), or initiated Ca(2+) release from small regions and caused L(2) Ca(2+) waves, which propagated throughout the aggregate. Spontaneous Ca(2+) waves (L(2)*) induce action potentials.

Ca2+ waves during triggered propagated contractions in intact trabeculae. Determinants of the velocity of propagation

During triggered propagated contractions, Ca2+ waves travel along cardiac trabeculae with a constant velocity (Vprop) ranging from 0. 34 to 5.47 mm/s. To explore the determinants of Vprop, we studied (1) the relationship between [Ca2+]i and Vprop and (2) the effect of low concentrations of caffeine on Vprop. Trabeculae were dissected from the right ventricle of rat hearts. [Ca2+]i was measured using electrophoretically injected fura-2 and an image-intensified CCD camera. Force was measured using a silicon strain gauge, and sarcomere length was measured using laser diffraction techniques. After induction of reproducible Ca2+ waves by trains of electrical stimuli (2.5 Hz) at 21.9+/-0.2 degrees C, the number of stimuli or [Ca2+]o was varied in 9 trabeculae. In 5 trabeculae, the effects of caffeine (0.1 to 1.0 mmol/L) at [Ca2+]o of 2.2+/-0.3 mmol/L were determined. All images were recorded under stable conditions of wave propagation. The increment in [Ca2+]i during the last electrically stimulated transient (DeltaCaT) and [Ca2+]i just before onset of the Ca2+ waves (CaD) were used to estimate the Ca2+ loading of the sarcoplasmic reticulum (SR) and the myoplasm, respectively. The ratio (DeltaCaW/DeltaCaT) of the [Ca2+]i increment during the waves (DeltaCaW) to DeltaCaT was used to estimate the probability of opening of the SR-Ca2+ release channel during wave propagation. As a result of an increase of the number of stimuli or [Ca2+]o, Vprop increased in proportion to (1) DeltaCaT (r=0.82); (2) CaD (r=0.88); (3) DeltaCaW (r=0.85); and (4) DeltaCaW/DeltaCaT (r=0.74). The addition of caffeine (</=0.3 mmol/L) increased Vprop for any DeltaCaT and any DeltaCaW, revealing an increased sensitivity of Vprop to DeltaCaT and DeltaCaW. In contrast, caffeine had little effect on the relationship between Vprop and CaD and no effect on that between Vprop and DeltaCaW/DeltaCaT. These results suggest that both the cellular Ca2+ loading and open probability of the SR-Ca2+ release channels determine the velocity of propagation of Ca2+ waves.

Cardiac myofibrillar and sarcoplasmic reticulum function are not depressed in insulin-resistant JCR:LA-cp rats

Depressed myofibrillar Ca2+-ATPase activity and sarcoplasmic reticulum (SR) Ca2+ uptake are important mechanisms that are responsible for the cardiac dysfunction exhibited by insulin-deficient (type I) diabetic animals. The JCR:LA-cp rat is a model for type II non-insulin-dependent diabetes mellitus (NIDDM). This rat is insulin resistant, obese, and has high levels of circulating glucose, cholesterol, insulin, and triglycerides. The purpose of this study was to determine whether changes in cardiac myofibrillar, SR, and cardiomyocyte function exist in this model of type II diabetes. Myofibrils and SR were isolated from hearts by differential centrifugation. Surprisingly, we found that myofibrillar Ca2+-ATPase activities were unaltered in these animals. Ca2+ uptake in isolated SR fractions was increased in diabetic cp/cp rats, whereas Ca2+-ATPase activity and ryanodine binding were unchanged. Cardiomyocytes isolated from hearts of control and experimental animals had similar active cell shortening and intracellular Ca2+ concentration under basal conditions and in response to caffeine. Our data argue against the presence of a cardiomyopathy in this diabetic model and suggest that insulin may be an important factor in the cardiomyopathy observed in type I diabetic models.

Effects of intracoronary caffeine on left ventricular mechanoenergetics in Ca2+ overload failing rat hearts

How different the effects of caffeine on cardiac mechanoenergetics in failing hearts are from those of normal hearts remains to be fully elucidated. First we successfully instituted a new experimental model of acute mild heart failure in the rat by 0.005 mM Ca2+ Tyrode perfusion. These failing hearts neither decreased left ventricular end-systolic pressure nor increased left ventricular end-diastolic pressure, indicating unchanged left ventricular mechanics. However, their myocardial mitochondrial respiratory function examined by respiratory control index (RCI) and oxygen consumption rate in state III (State III O2) was significantly depressed compared with normal hearts. From these results, we judged that this Ca2+ protocol could make mild Ca2+ overload acute failing hearts and that this model would be appropriate for comparing the effects of caffeine on cardiac mechanoenergetics between normal hearts and these failing hearts. We investigated the effects of caffeine on cardiac mechanoenergetics above a concentration of 0.05 mM that corresponds to the maximum blood concentration after a healthy human subject drinks a cup of coffee or tea. We obtained results indicating that caffeine depressed left ventricular systolic and diastolic functions and decreased a measure of total mechanical energy per beat in terms of systolic pressure-volume area (PVA) more severely in these failing hearts at concentrations (20-fold higher than the concentration in a cup of coffee) lower than those in normal hearts. This result implies that these acute failing hearts are Ca2+ overloaded.

Caffeine decreases intracellular free Mg2+ in isolated adult rat ventricular myocytes

Caffeine has been extensively used to study intracellular Ca2+ control and contraction-relaxation in cardiomyocytes. The effects of caffeine on intracellular free Mg2+ concentration, [Mg2+]i, were studied in isolated adult rat ventricular myocytes by fluorescent techniques using mag-fura-2. Basal [Mg2+]i was 0.62 +/- 0.02 mM, n = 54, in quiescent cells and 0.73 +/- 0.02 mM, n = 23, in electrically-stimulated adult rat ventricular myocytes. Caffeine, 20 mM, significantly decreased [Mg2+] in both quiescent (-0.17 +/- 0.01 mM) and electrically-stimulated (-0.16 +/- 0.01 mM) adult ventricular myocytes. Ryanodine, a blocker for Ca(2+)-release channels of the sarcoplasmic reticulum, did not have any effect on basal [Mg2+]i, 0.67 +/- 0.02 mM nor on caffeine-induced reduction in [Mg2+]i, -0.16 +/- 0.01 mM in quiescent cardiomyocytes or electrically-stimulated cells; 0.74 +/- 0.03 mM and -0.11 +/- 0.03 mM, respectively. Ruthenium red, an inhibitor of mitochondrial Ca2+ uptake, also failed to alter basal [Mg2+]i, or caffeine-induced reduction in [Mg2+], in either quiescent or electrically-stimulated cells. The effects of caffeine on [Mg2+]i, may be important in considering the use of this drug to study contraction/function studies in heart cells.

Cardiac mechanical and electrophysiologic modulations of guinea-pig by caffeine and thapsigargin

1. The effects of caffeine and thapsigargin on the contractile force and the action potential in guinea-pig papillary muscles were examined. 2. Caffeine (1 to 10 mM) initially increased contractile force in a concentration-dependent manner. Subsequently, 1 mM caffeine decreased it as compared with precaffeine level (but not significantly). At 5 mM or 10 mM, caffeine also decreased contractile force, but the decrease was still positive as compared with control level. 3. Exchange to low [Ca]o (0.9 mM) or high [K]o (8 mM) decreased steady-state value during exposure to 1 mM caffeine. Addition of 1 microM thapsigargin (TG) decreased the steady-state value during exposure to 1 mM caffeine, but enhanced it with 5 mM and 10 mM caffeine. TG (1 microM) alone increased the force. 4. In electrophysiologic, studies, caffeine shortened the action potential duration (APD) in a concentration-dependent manner. In the presence of caffeine (1 mM), high [K]o shortened APD and decreased the action potential amplitude and resting potential. 5. These results suggest that in the presence of caffeine and/or thapsigargin calcium overload might not occur in the left ventricular papillary muscles of the guinea-pig heart.

Caffeine induces ventricular tachyarrhythmias possibly due to triggered activity in rabbits in vivo

Caffeine induces delayed afterdepolarizations (DADs) and triggered activity in isolated cardiac tissue. We investigated the ability of caffeine to induce triggered ventricular arrhythmias in rabbits in vivo. During continuous infusion of caffeine at doses of 0.3 or 1.0 mg/kg per min, ventricular pacing was performed with 50 stimuli with a cycle length of 220 msec (basic pacing train) every 5 min until ventricular tachycardia (VT) was induced. The effects of programmed stimulation and pharmacologic agents on the induction of ventricular ectopic beats (VEBs) were examined. Pacing protocols were carried out in the presence of vagal-induced slowing of sinus rhythm. VT was induced by a basic pacing train during the infusion of caffeine at 1.0 mg/kg per min, but not at 0.3 mg/kg per min. An increase in the pacing rate or the number of stimuli resulted in 1) a decrease in the first postpacing interval, and 2) an increase in the number of postpacing VEBs. Induction of VT was suppressed by intravenous bolus injections of verapamil, propranolol and adenosine. At the time of the initial induction of VT, the plasma concentration of caffeine was 87 +/- 2 micrograms/ml and the plasma level of norepinephrine increased from 666 +/- 166 pg/ml at baseline to 1121 +/- 245 pg/ml. These results suggest that catecholamine-associated triggered activity may be responsible for caffeine-induced VT.

Buffering of calcium influx by sarcoplasmic reticulum during the action potential in guinea-pig ventricular myocytes

1. Intracellular [Ca2+] ([Ca2+]i) transients, monitored by the fluorescent Ca2+ indicator, indo-1, and twitch contractions elicited by action potentials, by voltage clamp pulses or by rapid, brief pulses of caffeine, were measured in guinea-pig single ventricular myocytes. Experiments were designed to determine whether and to what extent the trans-sarcolemmal Ca2+ influx is immediately sequestered by the sarcoplasmic reticulum (SR). 2. Rapid, brief (100-200 ms) pulses of caffeine onto a rested myocyte elicited a [Ca2+]i transient and a contraction. Following exposure to specific SR inhibitors, ryanodine (100 nM) or thapsigargin (200 nM), the rapid application of caffeine onto a rested myocyte failed to elicit changes in [Ca2+]i or in cell length, indicating that caffeine increases [Ca2+]i by specifically discharging Ca2+ from the SR. In the absence of these inhibitors, a second pulse of caffeine, within 3 min following a prior pulse, failed to elicit a [Ca2+]i transient or contraction, indicating that a caffeine pulse depletes the SR releasable Ca2+ pool. 3. Following Ca2+ depletion of the SR by double caffeine pulses at rest, an electrical stimulation elicited a slow increase in [Ca2+]i, and, after a delay, a small, slow twitch contraction. The simultaneous application of caffeine and electrical stimulation of cells in which the SR was Ca2+ depleted elicited [Ca2+]i transients with an increased rate of rise and a larger amplitude (53 +/- 8 and 63 +/- 9% respectively; mean +/- S.E.M., n = 21) than those elicited by electrical stimulation alone. 4. Whether caffeine affected the L-type calcium current (ICa) elicited by electrical stimulation was determined under whole-cell voltage clamp. A caffeine pulse delivered at the onset of a depolarizing voltage clamp step also increased the rates of rise and the amplitudes of the [Ca2+]i transients and twitch contractions in cells in which the SR was depleted of Ca2+. However, Ca2+ influx via ICa decreased when caffeine was pulsed in conjunction with the voltage clamp, as the peak ICa was either unchanged or decreased while its inactivation was consistently accelerated. 5. Because the stimulation-dependent trans-sarcolemmal Ca2+ influx via ICa is not increased by a caffeine pulse, the augmentation of the rates of rise and the amplitudes of the electrically stimulated [Ca2+]i transients by caffeine pulsed in conjunction with the electrical stimulation in cells in which the SR had been depleted of Ca2+ indicates that a portion of Ca2+ influx during depolarization in the absence of caffeine is rapidly buffered by the SR.(ABSTRACT TRUNCATED AT 400 WORDS)

The effects of inhibitors of sarcoplasmic reticulum function on the systolic Ca2+ transient in rat ventricular myocytes

1. The effects of thapsigargin, ryanodine and caffeine were examined on systolic Ca2+ transients in indo-1-loaded rat ventricular myocytes. 2. Thapsigargin (1-10 microM) decreased the magnitude of the Ca2+ transient. This was accompanied by a decrease of the rate constant of decay of the transient. 3. Ryanodine (1-10 microM) decreased the magnitude of the Ca2+ transient. Initially there was no change in the rate of decay but further reduction of the magnitude was accompanied by a slowing. 4. Caffeine (0.5-10 mM) decreased the magnitude of the Ca2+ transient and its rate of decay. These effects were graded with caffeine concentration. 5. For a given submaximal reduction of the magnitude of the Ca2+ transient, the effect on the rate of decay was greatest for thapsigargin, least for ryanodine and intermediate for caffeine. 6. The above data are reproduced by a model in which all three agents decrease the magnitude of the Ca2+ transient by decreasing the calcium content of the sarcoplasmic reticulum (SR) (thapsigargin by inhibiting the Ca2+ pump and ryanodine and caffeine by increasing the leak of Ca2+ from the SR). The decreased contribution of the SR will thereby slow relaxation. The fact that thapsigargin inhibits the SR Ca2+ pump accounts for the observation that, for a given decrease of amplitude, it has more effect than the other agents on the rate of decay. The difference between caffeine and ryanodine is suggested to arise because caffeine potentiates Ca2+ release from the SR and thereby attenuates the effect of the decreased SR calcium content on the magnitude of the Ca2+ transient.

Multiple effects of caffeine on calcium current in rat ventricular myocytes

Caffeine exerts a number of different effects on L-type calcium current in rat ventricular myocytes. These include: (1) a slowing of inactivation that is comparable to, but not additive to, that produced by prior treatment of the cells with ryanodine (a selective sarcoplasmic reticulum Ca2+ releaser) or high concentrations of intracellular 1,2-bis[2-aminophenoxy]ethane-N,N,N',-N'-tetraacetic acid (BAPTA) (a fast Ca2+ chelator), (2) a stimulation of peak ICa that is comparable to, but not additive to that produced by prior treatment with isobutylmethylxanthine (a selective phosphodiesterase inhibitor), and (3) a dose-dependent decrease of peak ICa that is not prevented by pretreatment with any of these agents. None of the caffeine actions could be mimicked or prevented by administration of 8-phenyltheophylline, a specific adenosine receptor antagonist. We conclude that only the slowing of ICa inactivation is due to caffeine's ability to deplete the sarcoplasmic reticulum of calcium. The stimulatory effect of caffeine on peak ICa is probably due to phosphodiesterase inhibition, while caffeine's inhibitory effect on ICa is independent of these processes and could be a direct effect on the channel. The multiplicity of caffeine actions independent of its effects on the sarcoplasmic reticulum lead to the conclusion that ryanodine, though slower acting and essentially irreversible, is a more selective agent than caffeine for probing sarcoplasmic reticulum function and its effects on other processes.

Caffeine depression of spontaneous activity in rabbit sino-atrial node cells

1. Effects of caffeine on the action potentials and the membrane currents in spontaneously beating rabbit sino-atrial (SA) node cells were examined using a two-microelectrode technique. 2. Cumulative administrations of caffeine (1-10 mM) caused a negative chronotropic effect in a concentration-dependent manner, which was not modified by atropine (0.1 microM). At 10 mM, caffeine increased the amplitude and prolonged the duration of action potentials significantly; the other parameters were unaffected. 3. In 3 of 16 preparations, caffeine (5 mM) elicited arrhythmia. At high Ca2+ (8.1 mM), caffeine (5 mM) increased the incidence of arrhythmia. 4. Caffeine (0.5-10 mM) enhanced the slow inward current, but at 10 mM decreased the enhanced peak current by 5 mM. The hyperpolarization-activated inward current was also enhanced by caffeine, but 10 mM caffeine decreased the current peak as compared with that at 5 mM. In addition, caffeine inhibited the delayed rectifying outward current in a concentration-dependent manner, accompanied by a depressed activation curve without any shift in the half-maximum activation voltage. 5. Caffeine elevated the cytoplasmic Ca2+ level in the SA node cells loaded with Ca(2+)-sensitive fluorescent dye (fura-2). 6. These results suggest that caffeine enhances and/or inhibits the ionic currents and elicits arrhythmia due to the induction of cellular calcium overload.

Ca transients in cardiac myocytes measured with a low affinity fluorescent indicator, furaptra

Intracellular calcium ion ([Ca2+]i) transients were measured in single rat ventricular myocytes with the fluorescent indicator furaptra. Cells were voltage clamped with a single patch electrode containing the K+ salt of furaptra and fluorescence at 500 nm was measured during illumination with 350 and 370 nm light. Depolarizing voltage-clamp pulses elicited [Ca2+]-dependent fluorescent transients in 30 of 33 cells tested. The peak change in [Ca2+]i elicited by 50-ms depolarizations from -70 to +10 mV was 1.52 +/- 0.25 microM (mean +/- SEM, n = 7). The size of the [Ca2+]i transient increased in response to 10 microM isoproterenol, prolongation of the depolarization, and increasing pipette [Na+]. Because furaptra is sensitive to Ca2+ and Mg2+, changes in [Mg2+]i during the [Ca2+]i transient could not be measured. Instead, a single-compartment model was developed to simulate changes in [Mg2+] during [Ca2+] transients. The simulations predicted that a 2 microM [Ca2+] transient was accompanied by a slow increase in [Mg2+] (14-29 microM), which became larger as basal [Mg2+] increased (0.5-2.0 mM). The [Mg2+] transient reached a peak approximately 1 s after the peak of the [Ca2+] transient with the slow changes in [Mg2+] dominated by competition at the Ca2+/Mg2+ sites of Troponin. These changes in [Mg2+], however, were so small and slow that they were unlikely to affect the furaptra fluorescence signal at the peak of the [Ca2+]i transient. The [Ca2+]i transient reported by furaptra appears to be larger than that reported by other Ca2+ indicators; however, we conclude this larger transient is at least as accurate as [Ca2+]i transients reported by the other indicators.

Characteristics of ryanodine-induced tetani in the perfused rat heart. Tetanic tension is not the highest force that cardiac muscle can generate

The aim of the present study was to elucidate the conditions required to obtain tetanic contractions in rat intact heart and to investigate whether tetanic tension was actually the maximal tension that isolated rat heart is able to generate. Experiments were performed on isolated rat hearts (Langendorff technique) perfused at constant coronary flow (8-9 ml/min). Rapid repetitive stimulation (400 to 3000 pulses/min) failed to elicit a fused tetanus. The first twitch that occurred at the end of the rapid stimulation period was a potentiated beat (PSP) of significantly greater magnitude than that of the regular twitch. This potentiation declined in successive beats. When rapid electrical stimulation (600 to 3000 pulses/min) was applied to hearts treated with 5 x 10(-6) M ryanodine, the result was a fused and steady tetanic tension. Ryanodine suppressed PSP. Tetanic tension could be graded by stepwise increase of [Ca2+]o from 0.25 to 5 mM. Maximal tetanic tension occurred at a [Ca2+]o between 3.85 and 5 mM. At any of the [Ca2+]o, tetanic tension was significantly greater than the tension of the twitch obtained at approximately the natural frequency of rat heart in the intact animal (250 beats/min) but it did not differ significantly from the twitch obtained at 100 beats/min. Moreover, the tension of PSP at 0.25 and 1.35 mM [Ca2+]o was significantly greater than the maximal tetanic tension that could be obtained. Similar results to that obtained with ryanodine, were obtained in additional experiments in which caffeine was used to evoke tetanic contraction.(ABSTRACT TRUNCATED AT 250 WORDS)

Cytosolic calcium and myofilaments in single rat cardiac myocytes achieve a dynamic equilibrium during twitch relaxation

1. Single isolated rat cardiac myocytes were loaded with either the pentapotassium salt form or the acetoxymethyl ester (AM) form of the calcium-sensitive fluorescent probe, Indo-1. The relationship of the Indo-1 fluorescence transient, an index of the change in cytosolic calcium [Ca2+]i concentration, to the simultaneously measured cell length during the electrically stimulated twitch originating from slack length at 23 degrees C was evaluated. It was demonstrated that even if the Ca2+ dissociation rate from Indo-1 was assumed to be as slow as 10 s-1, the descending limb ('relaxation phase') of the Indo-1 fluorescence transient induced by excitation under these conditions is in equilibrium with the [Ca2+]i transient. Additionally, the extent of Indo-1 loading employed did not substantially alter the twitch characteristics. 2. A unique relationship between the fluorescence transient and cell length was observed during relaxation of contractions that varied in amplitude. This was manifest as a common trajectory in the cell length vs. [Ca2+]i phase-plane diagrams beginning at the time of cell relengthening. The common trajectory could also be demonstrated in Indo-1 AM-loaded cells. The Indo-1 fluorescence-length relation defined by this common trajectory is steeper than that described by the relation of peak contraction amplitude and peak fluorescence during the twitch contractions. 3. The trajectory of the [Ca2+]i-length relation elicited via an abrupt, rapid, brief (200 ms) pulse of caffeine directly onto the cell surface or by 'tetanization' of cells in the presence of ryanodine is identical to the common [Ca2+]i-length trajectory formed by electrically stimulated contractions of different magnitudes. As the [Ca2+]i and length transients induced by caffeine application or during tetanization in the presence of ryanodine develop with a much slower time course than those elicited by electrical stimulation, the common trajectory is not fortuitous, i.e. it cannot be attributed to equivalent rate-limiting steps for the decrease of [Ca2+]i and cell relengthening. 4. The [Ca2+]i-length relation defined by the common trajectory shifts appropriately in response to perturbations that have previously been demonstrated to alter the steady-state myofilament Ca2+ sensitivity in skinned cardiac fibres. Specifically, the trajectory shifts leftward in response to an acute increase in pH or following the addition of novel myofilament calcium-sensitizing thiadiazinone derivatives; a rightward shift occurs in response to an acute reduction in pH or following the addition of butanedione monoxime.(ABSTRACT TRUNCATED AT 400 WORDS)

The inhibitory action of caffeine on calcium currents in isolated intestinal smooth muscle cells

The patch-clamp method has been used to investigate the action of caffeine on the calcium current (ICa) in single isolated smooth muscle cells of the guinea-pig ileum. Caffeine (10 mM) substantially inhibited ICa. This effect occurred in a biphasic manner and it was not due either to activation of additional ionic currents of opposite direction nor to inhibition of phosphodiesterase activity. It strongly depended upon the ethylenebis-(oxonitrilo)tetraacetate (EGTA) concentration in the pipette solution. When there was K+ in the pipette solution, application of caffeine evoked a transient Ca-dependent K+ current and an abrupt and transient increase in the frequency of channel openings. Such well-known blockers of Ca release as procaine and ruthenium red strongly decreased ICa. Ryanodine had only little effect on ICa, but application of caffeine in the presence of ryanodine led to a complete and irreversible inhibition of ICa. The results of experiments involving different EGTA concentrations and comparison of the time courses of all caffeine-induced phenomena clearly indicated that only the initial, transient component of the ICa inhibition by caffeine was related to a Ca-dependent inactivation of Ca channels, evoked as a result of Ca release from intracellular stores. The tonic component of ICa inhibition was probably due to a direct blocking action of caffeine on Ca channels.

Effects of pretreatment with caffeine or ryanodine on the myocardial response to simulated ischaemia

1. The cytoplasmic calcium concentration ([Ca]c) of rat isolated atrial myocardium was assessed with the dye indo-1. Dye-loaded atria were superfused with physiological salt solution and excited with radiation at 360 nm, while epifluorescence emissions were collected simultaneously at 400 nm and 500 nm. The ratio of these emissions was used as a measure of [Ca]c. 2. Dye-loaded atria showed a phasic rise and fall in [Ca]c with each applied electrical pacing stimulus. The amplitude of these oscillations was reduced by the presence of caffeine (10(-3)-10(-2) M) or of ryanodine (10(-8)-10(-6) M) in a concentration-dependent manner. 3. Atria superfused with a solution the composition of which resembled that found extracellularly in regions of myocardial ischaemia rapidly lost systolic increments in [Ca]c, while end-diastolic [Ca]c values gradually rose. 4. Pretreatment with caffeine (10(-2) M) or ryanodine (10(-7) M) protected atria against the rise in end-diastolic [Ca]c that occurred when the tissue was exposed to conditions of simulated ischaemia.

Intracellular calcium and ventricular fibrillation. Studies in the aequorin-loaded isovolumic ferret heart

To elucidate the role of changes in [Ca2+]i in the induction of ventricular fibrillation (VF), Ca2+i signals, epicardial electrical potentials, and isovolumic left ventricular pressure were simultaneously recorded in isolated intact ferret hearts loaded with aequorin, a bioluminescent protein. When the preparations were perfused with 3 microM acetylstrophanthidin and 8 mM Ca2+, or with a low Na+ solution (18 mM Na+, 100 mM Li+), spontaneous transitions to the VF state were consistently observed within a short period of time. The initiation of spontaneous VF was preceded by development of a Ca2+i overload state, coincidental with the ascending phase of diastolic Ca2+i oscillations, and was followed by further elevation in Ca2+i levels, which were associated with augmented Ca2+i oscillations of a saw-toothed pattern. Pretreatment with 10 microM ryanodine, which blocked Ca2+i oscillations in the preparation, did not eliminate inducibility of VF by means of AC electrical stimulations; however, VF no longer occurred spontaneously, and the threshold for VF induction increased markedly. In the absence of a state of Ca2+i overload, spontaneous defibrillation occurred within a minute after the initiation of VF. We conclude that 1) VF can be induced in the absence of Ca2+i oscillations; however, 2) Ca2+i oscillations play a crucial role as a trigger for VF and therefore are an important determinant of the vulnerability to VF; and 3) the augmented Ca2+i oscillations after the transition to VF state may support the maintenance of this type of arrhythmia.

Effects of aluminium on electrical and mechanical properties of frog atrial muscle

1. The effects of aluminium on membrane ionic currents were studied in single cardiac myocytes. Most of the work was done on frog atrial cells, but some experiments were also carried out on single cells isolated from rabbit ventricles and atria. 2. The effects of aluminium on the force of contraction of frog atrial trabeculae were also investigated. 3. Aluminium was prepared from AlCl3 as a stock 0.5 M solution which has a pH of 3.5. Before each experiment, this solution was added to the control solution, to give a final concentration of 20-100 micrograms ml-1 aluminium (0.75-3.75 mM AlCl3). The solutions were brought to a pH of 7.4 or 7.6. at which they consist of a mixture of amorphous aluminium hydroxides and a very small amount of soluble ionic aluminium complexes: free aluminium cations (less than 10 pM), aluminohydroxide anions (less than 8 microM). The addition of this suspension reduced the peak inward calcium currents in single rabbit atrial and ventricular cells and in frog atrial cells. In the latter, the peak current was reduced (at + 10 mV) to 45% of control (mean of 9 cells). This effect was reversible upon washout, and was obtained at all membrane potentials, with no shift of the calcium current voltage relationship along the voltage axis. 4. Aluminium also reduced the time-dependent potassium current IK. This reduction was observed at all membrane potentials. For example, at + 10 mV, the mean reduction of IK (n = 9) was to 69% of the control amplitude. This effect, which was very difficult to reverse, was not due to IK rundown. The fully activated current-voltage relationships (obtained by standard 'tail' analysis) showed that the effect of aluminium was due mainly to a decrease in conductance and not to a shift in the activation range of IK. The mean voltage of half activation was shifted by 8 mV in the depolarizing direction (n = 5). 5. The background potassium current IK1 was also slightly but consistently changed in a complex fashion, with an outward shift at membrane potentials positive to -60 mV. For example, at a membrane potential of -40mV, the mean shift was by 22 + 4pA. At more negative potentials, there was an inward shift in the current amplitudes. For example, for steps to -I00 mV the current elicited was larger (more inward) by 53 pA (mean value, n = 10). The reversal potential was slightly shifted (<10 mV) in the hyperpolarizing direction. 6. The force of contraction of frog atrial trabeculae was altered by aluminium in a complex manner, which showed marked seasonal variation. During most of the year, 50-100,ug ml-1 aluminium caused a biphasic change, with an early small and consistent decrease, followed by a large increase in twitch amplitude. For a short period corresponding to the (local) winter months the sensitivity to aluminium was greatly enhanced. Aluminium lOOupgml-1 totally abolished contraction (n = 5), while a lower concentration (20,ug ml- 1) produced a sustained reduction in the force of contraction. Similar biphasic and seasonal responses have been reported to be induced by lanthanum. 7. The biphasic changes in twitch amplitude were independent of the transmembrane sodium gradient. Aluminium produced the same effects when 90% of the extracellular sodium was replaced by lithium. Caffeine (5 mM) attenuated or even inverted the positive inotropic effect of aluminium. These results imply that aluminium alters the release of calcium from intracellular, caffeine-sensitive stores. This could be effected either by augmenting the amount released during each activation, and/or by increasing the loading of stores prior to release.

Electrophysiologic effects of exogenous phosphocreatine in cardiac tissue: potential antiarrhythmic actions

The cellular electrophysiologic effects of exogenous phosphocreatine (PCr) were analyzed to ascertain its purported antiarrhythmic properties during myocardial ischemia and reperfusion. Transmembrane potentials were recorded from isolated guinea pig papillary muscles and Purkinje fibers studied in vitro. Under control, normoxic conditions, 10 mmol/L PCr significantly increased the action potential duration (measured at 90% of repolarization) in ventricular muscle by 14.6 +/- 3.3 msec and the effective refractory period by 11.5 +/- 3.8 msec (both p less than 0.01). Under ischemic-like conditions (hypoxia, lactic acidosis, elevated [K+]o, zero substrate) PCr had no effect. Phosphocreatinine, a related compound that is not a direct substrate in the creatine kinase reaction, acted similarly to PCr suggesting that alterations induced by PCr did not involve a change in the energy state of cells. However, PCr reduced free [Ca2+]o by nearly 20%, and its electrical effects under normoxic conditions could be largely reversed by a concomitant 20% increase in [Ca2+]o. In Purkinje fibers superfused with low [K+]o-Tyrode's solution to elicit conditions of Ca2+ overload, delayed afterdepolarizations and triggered responses were reversibly inhibited by PCr. These data suggest that the antiarrhythmic effects of PCr in situ may involve prolongation of the effective refractory period in nonischemic tissue or attenuation of membrane changes elicited by Ca2+ overload in ischemic cells. The mechanism by which PCr produces these effects may be related in part to changes in extracellular Ca2+ composition.

A mechanism for the effects of caffeine on Ca2+ release during diastole and systole in isolated rat ventricular myocytes

1. The fluorescent indicator Indo-1 was used to measure both [Ca2+]i and [caffeine]i in single ventricular myocytes. 2. Caffeine (at concentrations of 1 mM or above) produced a transient increase of resting [Ca2+]i attributed to the release of Ca2+ ions from the sarcoplasmic reticulum (SR). Simultaneous measurement of [caffeine]i showed that the Ca2+ release only began when [caffeine]i had risen to about 1 mM. Subsequently the rate of release was a steep function of [caffeine]i. It is suggested that this results from a positive feedback as the Ca2+ released activates further release. 3. If external Ca2+ was removed the release of Ca2+ produced by caffeine was delayed such that [caffeine]i rose to a greater concentration before release was initiated. This suggests that an increase of [Ca2+]i increases the efficacy of caffeine to release Ca2+ ions from the SR. 4. Lower concentrations of caffeine (50-500 microM) had no effect on diastolic [Ca2+]i. In contrast they increased systolic [Ca2+]i and contraction. This increase was most obvious if the systolic contraction had previously been decreased either by reducing [Ca2+]o from 1 to 0.25 mM or (in voltage-clamped cells) by decreasing the magnitude of the depolarizing pulse. 5. If the exposure to caffeine was prolonged, this increase of systolic [Ca2+]i and contraction was completely transient. On removal of caffeine, systolic [Ca2+]i and contraction decreased to below control before recovering. 6. During these transient changes of systolic [Ca2+]i and contraction there was no change of the sarcolemmal Ca2+ current. 7. It is suggested that the increase of systolic [Ca2+]i is due to caffeine increasing the fraction of the SR Ca2+ content released during the twitch. 8. The above results concerning both diastolic and systolic [Ca2+]i can be explained by a model in which caffeine increases the affinity with which Ca2+ ions activate Ca2(+)-induced Ca2+ release. At high enough [caffeine], the threshold [Ca2+]i for regenerative Ca2(+)-induced Ca2+ release will be reduced to below the resting [Ca2+]i thus producing a diastolic increase of [Ca2+]i. At lower [caffeine] the threshold is higher than resting [Ca2+]i and caffeine only serves to enhance the release produced during systole.

The effects of ryanodine and caffeine on Ca-activated current in guinea-pig ventricular myocytes

1. Action potentials from guinea-pig single ventricular myocytes were interrupted by application of a 300 ms voltage clamp to -40 mV in order to evoke the Ca-activated tail current which is thought to be carried by Na:Ca exchange. Stimulation frequency was 1 Hz and temperature 36 degrees C. 2. The actions of ryanodine (1 microM and 10 microM) and caffeine (1 mM and 10 mM) on Ca-activated tail currents were investigated. 3. Exposure to 10 mM caffeine and ryanodine reduced tail currents associated with very abbreviated (12 ms duration) action potentials and greatly reduced the difference between first and steady-state tail currents at this action potential duration. These observations were interpreted in terms of suppression of Ca release from the sarcoplasmic reticulum (SR) stores. 4. Tail current decay during the voltage clamp is thought to reflect the fall in [Ca]i which accompanies muscle relaxation. Current decay is dependent on Ca extrusion via Na:Ca exchange and on Ca accumulation by the SR stores. Time constants of tail current decay were seen to decrease with increasing action potential duration. This relationship was not affected by 1 mM caffeine or 1 microM ryanodine. Ryanodine at 10 microM and 10 mM caffeine abolished this relationship and increased the time constants of current decay. An increase in the time constant of tail current decay was thought to reflect a reduction in the rate of Ca accumulation by the sarcoplasmic reticulum. 5. The actions of caffeine and ryanodine on the Ca-activated tail currents are consistent with a dose-dependent leakage of Ca from the SR Ca stores. The Ca-activated tail current appears to be a useful tool in the study of Ca homeostasis.

Ionic currents contributing to the action potential in single ventricular myocytes of the guinea pig studied with action potential clamp

With the action potential clamp procedure we studied the contribution of various ionic currents to the action potential in single ventricular myocytes. Action potentials were elicited by a current pulse through the suction pipette and recorded by a computer. A representative action potential was then repetitively replayed to the same cell under voltage-clamp conditions. Successive pharmacological blocks of ionic currents allowed for the first time the measurement of the contribution of the L-type calcium current (ICa) and the [Ca2+]i-activated currents as well as the potassium current to the action potential. Experiments using caffeine as a tool to increase calcium release from the sarcoplasmic reticulum supported the idea that INaCa contributes to the plateau during the second half of the action potential and even lasts into diastole, whereas strong elevation of the intracellular [Ca]i during the action potential additionally activated the non-specific cation channel.

Effects of caffeine and ryanodine on delayed afterdepolarizations and sustained rhythmic activity in 1-day-old myocardial infarction in the dog

Caffeine and ryanodine are known to modulate oscillatory release of Ca2+ from the sarcoplasmic reticulum. The effects of caffeine and ryanodine on delayed afterdepolarizations (DADs) and sustained rhythmic activity in subendocardial Purkinje fibers surviving 1-day-old myocardial infarction in the dog were studied with standard microelectrode techniques. In preparations that showed sustained rhythmic activity, a high concentration of caffeine (10 mM) and ryanodine (10(-7) and 10(-6) M) slowed and terminated the sustained rhythmic activity and markedly suppressed DADs. An increase in the temperature of the tissue bath from 37 degrees to 39 degrees C did not change these results. In quiescent normal and infarcted preparations, a low concentration of caffeine (0.5 mM) differentially induced DADs in ischemic but not in normal Purkinje fibers, increased the amplitude of existing DADs, and brought subthreshold DADs to threshold potential that caused triggered activity. Our results are consistent with the hypothesis that triggered activity arising from DADs characterizes the sustained rhythmic activity in endocardial preparations 1 day after infarction and indicate an important role for the sarcoplasmic reticulum in the genesis of DADs and triggered activity in this model.

Action potential duration alternans in dog Purkinje and ventricular muscle fibers. Further evidence in support of two different mechanisms

An abrupt shortening of cycle length causes action potential duration (APD) alternation in both canine Purkinje (P) and ventricular (V) muscle fibers. Our recent study suggested that APD alternans is determined by the process controlling APD during electrical restitution in P but not in V fibers. In the latter, alternans was attributed to changes in the availability of intracellular calcium [Ca2+]i. We examined this hypothesis further with the following pharmacologic probes known to alter restitution or action of [Ca2+]i: tetradotoxin (0.5-3.0 microM), lidocaine HCl (2.0-12.0 micrograms/ml), sotalol (10 microM), nicorandil (10-20 microM), 4-amino-pyridine (0.5 microM), ryanodine (10 microM), caffeine (2 mM), and ARL 115 BS (100 microM). Alternans in P fibers persisted under all studied conditions but varied in magnitude depending on the time constant and amplitude of restitution. In V fibers, the magnitude of alternans did not correlate with APD changes during restitution, and APD alternans was associated with the alternans of action potential shape and alternans of developed tension. Alternans in V was suppressed by caffeine at 2.0 mM [Ca2+]o when tension was increased and by ryanodine at 1.0 mM [Ca2+]o when tension was decreased. Alternans in V was not altered by changes in [Ca2+]o within the range of 1.0-4.0 mM; by ARL 115 BS, a compound that increases myofibrillar sensitivity to calcium; or by any other pharmacologic probes. We concluded that in P fibers, APD alternans was determined by the factors controlling APD in the absence of alternans; V fibers posses an independent mechanism of alternans linked to alternans of tension and controlled by [Ca2+]i; in V fibers, alternans could be suppressed by both positive and negative inotropic interventions; and calcium released from sarcoplasmic reticulum plays an important role in the V alternans.

Calcium-sensitive and insensitive transient outward current in rabbit ventricular myocytes

1. A suction pipette whole-cell voltage-clamp technique was used to record membrane currents and potentials of isolated ventricular myocytes from rabbit hearts. 2. Transient outward current (Ito) was activated by voltage steps positive to -20 mV, increasing in amplitude with further depolarization to reach a maximum around +70 mV. The current attained its peak within 10 ms and then it inactivated for 100-200 ms. 3. A large portion of Ito still remained after the calcium current (ICa) was blocked when depolarizing pulses were applied at a frequency of 0.1 Hz or less. Therefore, this current component is referred to as calcium-insensitive Ito or It. 4. It showed voltage- and time-dependent inactivation similar to that observed in Purkinje fibres and other cardiac preparations. 5. The reversal potential of It depended on external K+ concentration, [K+]o, with a slope of 32 mV per 10-fold change in the presence of a normal [Na+]o (143 mM), while the slope was 48 mV per 10-fold change in low [Na+]o (1.0 mM). 6. It was completely inhibited by 2-4 mM-4-aminopyridine. Ito in the presence of ICa was also partially blocked by 4-aminopyridine and the remainder was abolished by 5 mM-caffeine. 7. The calcium-insensitive and caffeine-sensitive Ito differed in their decay rates as well as in their recovery time courses. The former was predominantly available at a slow pulsing rate, while the latter increased its amplitude with high-frequency depolarization. 8. The caffeine-sensitive Ito was inhibited by a blockade of ICa, by replacing Ca2+ with Sr2+, by external application of ryanodine and by internal application of EGTA. This indicates that the current is calcium-sensitive and is dependent on increased myoplasmic Ca2+ through Ca2+ influx via the sarcolemma and Ca2+ release from the sarcoplasmic reticulum. The current is therefore designated as IK, Ca. 9. The physiological functions of IK, Ca and It are indicated by their contribution to ventricular repolarization at fast and slow heart rates, respectively.

Comparison between effects of caffeine and ryanodine on electromechanical coupling in myocardium of hibernating chipmunks: role of internal Ca stores

1. To clarify the cause of uncoupling of Ca influx through Ca channels and the contractility of the myocardium in hibernating chipmunks, the electromechanical effects of two different internal Ca store inhibitors, caffeine and ryanodine, and a cardiotonic agent, isoprenaline, were investigated in papillary muscles of hibernating animals. 2. Ryanodine (10(-6) M), an inhibitor of internal Ca release, abolished the contraction with a marked inhibition of the action potential plateau (APp). In such preparations, an increase in Ca influx induced by isoprenaline (5 x 10(-8) M) failed to augment the contraction, indicating uncoupling of Ca influx and contraction. 3. In ryanodine pretreated preparations, 10 mM caffeine produced an early phase of APp, but did not affect the contraction abolished by ryanodine, while a higher concentration of caffeine (25 mM) markedly increased the contraction with an augmentation of the electrical response. 4. In the absence of ryanodine, caffeine (5 mM) almost abolished the contraction with a greater inhibition of APp. In such preparations, isoprenaline greatly increased the contraction with an augmentation of the early phase of APp. 5. These effects were not significantly affected by additional application of ryanodine, but were inhibited by nifedipine, a Ca channel blocker. 6. These observations suggest that in cardiac muscles of hibernating animals, lack of the positive inotropic effect of isoprenaline may be attributed to a rapid and effective sequestration of increased cytoplasmic Ca through Ca influx by internal stores, probably by enhancement of their ability to take up Ca.

Changes in the calcium current of rat heart ventricular myocytes during development

1. Calcium current (ICa) was recorded in single rat heart cells at two periods during development: (1) at 2-7 days post-partum (neonatal), and (2) at 6-8 weeks (adult). 2. We measured both transient and steady-state components of ICa and could describe ICa in terms of the steady-state activation (d infinity) and inactivation (f infinity) parameters, the channel reversal potential (Echannel) and a relative conductance parameter, gr. 3. In adult single cells, the application of ryanodine (10 microM), an agent known to alter the function of the sarcoplasmic reticulum (SR), abolished contraction rapidly and increased ICa. Ryanodine also produced a 13 mV shift in f infinity towards more positive potentials and altered its slope, while producing a small increase in gr but no effect on d infinity. In neonatal single cells, ryanodine (10 microM) had no significant effect on contraction, ICa, d infinity, f infinity, or gr. Caffeine (10 mM), a less specific agent widely used to investigate sarcoplasmic reticulum function, had actions similar to those of ryanodine. 4. In adult myocytes, when EGTA (10 or 20 mM) or bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid (BAPTA, 10 mM) were included in the pipette solution, contractions were rapidly abolished, while a small (4 mV) shift of f infinity to more positive potentials was seen. A large additional shift of f infinity was observed when ryanodine (10 microM) was added to the superfusion solution in the continued presence of EGTA or BAPTA. The alterations of ICa in EGTA (or BAPTA) plus ryanodine were the same as those seen in ryanodine alone. In neonatal cells, in contrast, when EGTA or BAPTA were included in the pipette solution we observed only a small effect on f infinity and the application of ryanodine had no effect. 5. Electron micrographs of our preparations show that the dissociated adult cells have sharp sarcolemmal borders, fully developed sarcomeres with T-tubules and sarcoplasmic reticulum membranes. In contrast, the neonatal cells that we use have few of these intracellular structures. Our observations in these preparations are consistent with the work of others (e.g. Penefsky, 1974; Hirakow & Gotoh, 1975; Ishikawa & Yamada, 1975; Legato, 1975; Hoerter, Mazet & Vassort, 1981). 6. Our data suggest that fully developed sarcoplasmic reticulum in rat heart cells can affect ICa.(ABSTRACT TRUNCATED AT 400 WORDS)

Use-dependent reduction and facilitation of Ca2+ current in guinea-pig myocytes

1. Action potentials, calcium currents (iCa) and cell contraction have been recorded from single guinea-pig myocytes during periods of stimulation from rest. Voltage clamp was carried out using a single microelectrode. Cell contraction was measured optically. All experiments were performed at 18-22 degrees C. 2. An inverse relationship was observed between cell contraction and action potential duration or iCa. Mixed trains of action potentials and voltage clamp pulses preserved this relationship. Long voltage clamp pulses induced negative 'staircases' of iCa and positive 'staircases' of cell contraction. A facilitation of iCa was observed during repetitive stimulation with clamp pulses of 100 ms duration or less and was accompanied by a decrease in cell contraction. 3. The voltage dependence of inward current staircases was found to depend on Ca2+ entry rather than membrane voltage for long voltage clamp pulses and was not affected by 30 mM-TEA or 50 microM-TTX. Current reduction was greatest at 0 mV (P less than 0.05) when iCa was largest. Changes in cell contraction during pulse trains showed a similar voltage dependence. The time constant of current staircases was only mildly voltage dependent. 4. Interference with normal cellular mechanisms for Ca2+ uptake and release by strontium, 1-5 mM-caffeine and 1 microM-ryanodine increased current staircases and could abolish iCa facilitation with short clamp pulses. 5. Variations in the level of Ca2+-dependent inactivation of iCa can explain many features of the changes in iCa during stimulation after rest. Long clamp pulses (or action potentials) may increase cell Ca2+ loading and inhibit iCa. Short clamp pulses reduce available Ca2+ for cell contraction and this may reflect a lowered myoplasmic Ca2+ level which allows facilitation of iCa.

Effects of acute iron loading on contractility and spontaneous beating rate of cultured rat myocardial cells

Cardiac dysfunction is a well known but poorly understood complication of iron overload. We have previously shown that cultured myocardial cells are able to assimilate large amounts of iron. In the present study, the effect of iron on the rate and amplitude of beating in monolayer cultures of rat ventricular myocytes was studied. Iron had negative chronotropic and inotropic effects, both reversible upon washout. The negative chronotropic effect developed earlier and could be reversed by adrenaline. The negative inotropic effect took longer to develop and was completely reversed by caffeine. Elevated [Ca++] also partially restored impaired contractility, while adrenaline or ouabain did not show any significant effect. These results indicate that iron toxicity in cultured heart cells impairs cellular function at both sarcolemmal and intracellular sites.

Stimulation of muscarinic receptors raises free intracellular Ca2+ concentration in rat ventricular myocytes

The effect of carbachol on free intracellular calcium concentration, ([Ca2+]i) and on intracellular hydrogen concentration (pHi) was determined from fluorescence signals obtained from rat ventricular myocytes. Application of carbachol (300 mumol/l) to quin2-loaded myocytes bathed in 2 mmol/l Ca2+-containing solution caused [Ca2+]i to increase within 7-10 minutes from 182 +/- 9 to 212 +/- 11 nmol/l (n = 4). Carbachol acted via stimulation of muscarinic receptors because atropine (1 mumol/l) either prevented or abolished the increase in [Ca2+]i. Carbachol also produced a positive inotropic effect in rat papillary muscles contracting isometrically at a frequency of 0.5 Hz and enhanced contracture in resting preparations in the presence of high extracellular Ca2+ concentration ([Ca2+]o) (20 mmol/l). The effect of carbachol on [Ca2+]i was dependent on [Ca2+]o. In the presence of 10 mmol/l [Ca2+]o, the increase in [Ca2+]i was about two times that elicited by carbachol when bath [Ca2+]o was 2 mmol/l. Reduction of [Ca2+]o to 50 mumol/l abolished the carbachol effect but did not prevent caffeine-induced Ca2+ release. The carbachol-induced rise in [Ca2+]i remained unchanged in the presence of either 10 mmol/l caffeine or 1 mumol/l ryanodine. In the absence of extracellular Na+ concentration [( Na+]o), carbachol no longer produced an increase in [Ca2+]i of cardiomyocytes and failed to enhance Na+-withdrawal contracture of the rat papillary muscle. In contrast to the effect on [Ca2+]i, carbachol did not produce any change in pHi as determined from fluorescence signals obtained from rat ventricular myocytes loaded with 2',7'-bis(carboxyethyl)-5,6-carboxyfluorescein.(ABSTRACT TRUNCATED AT 250 WORDS)

Maximal twitch tension in intact length-clamped ferret papillary muscles evoked by modified postextrasystolic potentiation

A modified test of postextrasystolic potentiation achieved with a brief episode of rapid pacing followed by a 6-second pause (RPP maneuver) was used to evoke maximal force in isolated intact ferret right ventricular papillary muscles. Maximal RPP tensions were examined under length-clamped conditions and compared with the steady-state forces obtained when further increases in [Ca2+]o, did not further increase force and to the tensions recorded at the point of saturation of force when similarly length-clamped muscles were subjected to caffeine-induced tetanization. The results show that the calculated maximal twitch tension achieved with RPP is comparable to the 25-35 g/mm2 observed in intact single skeletal muscle fibers. The study also shows that the beat-to-beat decay of the potentiated contraction is exponential. While the amount of the constant fractional beat-to-beat decay is a function of [Ca2+]o, it is not influenced by length. During the decay of potentiation, the ratio of the potentiation of any beat divided by that of the previous beat is a constant, called (X). With certain assumptions, it is shown that (X) is a measure of the fraction of activator calcium taken up by the sarcoplasmic reticulum in each beat and, in the steady state, the fraction of activator calcium that comes from the sarcoplasmic reticulum. The (X) amounted to 33%, 50%, and 65% when [Ca2+]o was 1.25, 2.50, and 5.0 mM, respectively. Thus, at 1.25 mM [Ca2+]o, some two thirds of the total calcium required to activate the myofilaments comes from the extracellular compartment during excitation and only one third is contributed via release from the sarcoplasmic reticulum. In the region of optimal myofilament overlap, RPP force-length curves are remarkably shallow and almost indistinguishable from the sarcomere length-tension relation observed in skinned single cardiac cells. Tetanus plateau tensions are significantly smaller than RPP forces at any length, and the slope of the tetanus force-length curves is greater than that obtained with RPP. Thus, and by exclusion, we also suggest that caffeine may exert significant downstream inhibitory effects.