Caffeine induces a transient inward current in cultured cardiac cells

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.

Abnormalities of calcium metabolism and myocardial contractility depression in the failing heart

Heart failure (HF) is characterized by molecular and cellular defects which jointly contribute to decreased cardiac pump function. During the development of the initial cardiac damage which leads to HF, adaptive responses activate physiological countermeasures to overcome depressed cardiac function and to maintain blood supply to vital organs in demand of nutrients. However, during the chronic course of most HF syndromes, these compensatory mechanisms are sustained beyond months and contribute to progressive maladaptive remodeling of the heart which is associated with a worse outcome. Of pathophysiological significance are mechanisms which directly control cardiac contractile function including ion- and receptor-mediated intracellular signaling pathways. Importantly, signaling cascades of stress adaptation such as intracellular calcium (Ca(2+)) and 3'-5'-cyclic adenosine monophosphate (cAMP) become dysregulated in HF directly contributing to adverse cardiac remodeling and depression of systolic and diastolic function. Here, we provide an update about Ca(2+) and cAMP dependent signaling changes in HF, how these changes affect cardiac function, and novel therapeutic strategies which directly address the signaling defects.

Mechanisms of calcium transient and action potential alternans in cardiac cells and tissues

Alternation of cardiac action potential duration (APD) from beat to beat and concurrent alternation of the amplitude of the calcium transient are regarded as important arrhythmia mechanisms. These phenomena are causally interrelated and can be reliably evoked by an increase in beat frequency or by ischemia. The first part of this historical review deals with the physiology of APD alternans. Sections recounting the evolution of knowledge about calcium-activated ion currents and calcium transient alternans are interspersed among sections describing the growth of the so-called “restitution hypothesis,” which involves time-dependent recovery of potassium channels (including their passage through pre-open states) as a function of diastolic interval. Major developments are generally in chronological order, but it is necessary to move back and forth between the two theories to respect the overall time line, which runs from about l965 to the present. The concluding two sections deal with the pathophysiology of calcium transient and APD alternans during ischemia, which may be the basis for out-of-hospital cardiac arrest during the initial stages of acute myocardial infarction.

Caffeine-activated large-conductance plasma membrane cation channels in cardiac myocytes: characteristics and significance

Caffeine-activated, large-conductance, nonselective cation channels (LCCs) have been found in the plasma membrane of isolated cardiac myocytes in several species. However, little is known about the effects of opening these channels. To examine such effects and to further understand the caffeine-activation mechanism, we carried out studies using whole-cell patch-clamp techniques with freshly isolated cardiac myocytes from rats and mice. Unlike previous studies, thapsigargin was used so that both the effect of opening LCCs and the action of caffeine were independent of Ca2+ release from intracellular stores. These Ca2+-permeable LCCs were found in a majority of the cells from atria and ventricles, with a conductance of ∼370 pS in rat atria. Caffeine and all its direct metabolic products (theophylline, theobromine, and paraxanthine) activated the channel, while isocaffeine did not. Although they share some similarities with ryanodine receptors (RyRs, the openings of which give rise to Ca2+ sparks), LCCs also showed some different characteristics. With simultaneous Ca2+ imaging and current recording, the localized fluorescence increase due to Ca2+ entry through a single opening of an LCC (SCCaFT) was detected. When membrane potential, instead of current, was recorded, SCCaFT-like fluorescence transients (indicating single LCC openings) were found to accompany membrane depolarizations. To our knowledge, this is the first report directly linking membrane potential changes to a single opening of an ion channel. Moreover, these events in cardiac cells suggest a possible additional mechanism by which caffeine and theophylline contribute to the generation of cardiac arrhythmias.

Electrogenic Na+/Ca2+-exchange of nerve and muscle cells

The plasma membrane Na(+)/Ca(2+)-exchanger is a bi-directional electrogenic (3Na(+):1Ca(2+)) and voltage-sensitive ion transport mechanism, which is mainly responsible for Ca(2+)-extrusion. The Na(+)-gradient, required for normal mode operation, is created by the Na(+)-pump, which is also electrogenic (3Na(+):2K(+)) and voltage-sensitive. The Na(+)/Ca(2+)-exchanger operational modes are very similar to those of the Na(+)-pump, except that the uncoupled flux (Na(+)-influx or -efflux?) is missing. The reversal potential of the exchanger is around -40 mV; therefore, during the upstroke of the AP it is probably transiently activated, leading to Ca(2+)-influx. The Na(+)/Ca(2+)-exchange is regulated by transported and non-transported external and internal cations, and shows ATP(i)-, pH- and temperature-dependence. The main problem in determining the role of Na(+)/Ca(2+)-exchange in excitation-secretion/contraction coupling is the lack of specific (mode-selective) blockers. During recent years, evidence has been accumulated for co-localisation of the Na(+)-pump, and the Na(+)/Ca(2+)-exchanger and their possible functional interaction in the "restricted" or "fuzzy space." In cardiac failure, the Na(+)-pump is down-regulated, while the exchanger is up-regulated. If the exchanger is working in normal mode (Ca(2+)-extrusion) during most of the cardiac cycle, upregulation of the exchanger may result in SR Ca(2+)-store depletion and further impairment in contractility. If so, a normal mode selective Na(+)/Ca(2+)-exchange inhibitor would be useful therapy for decompensation, and unlike CGs would not increase internal Na(+). In peripheral sympathetic nerves, pre-synaptic alpha(2)-receptors may regulate not only the VSCCs but possibly the reverse Na(+)/Ca(2+)-exchange as well.

Clinical phenotype and functional characterization of CASQ2 mutations associated with catecholaminergic polymorphic ventricular tachycardia

BACKGROUND

Four distinct mutations in the human cardiac calsequestrin gene (CASQ2) have been linked to catecholaminergic polymorphic ventricular tachycardia (CPVT). The mechanisms leading to the clinical phenotype are still poorly understood because only 1 CASQ2 mutation has been characterized in vitro.

METHODS AND RESULTS

We identified a homozygous 16-bp deletion at position 339 to 354 leading to a frame shift and a stop codon after 5aa (CASQ2(G112+5X)) in a child with stress-induced ventricular tachycardia and cardiac arrest. The same deletion was also identified in association with a novel point mutation (CASQ2(L167H)) in a highly symptomatic CPVT child who is the first CPVT patient carrier of compound heterozygous CASQ2 mutations. We characterized in vitro the properties of CASQ2 mutants: CASQ2(G112+5X) did not bind Ca2+, whereas CASQ2(L167H) had normal calcium-binding properties. When expressed in rat myocytes, both mutants decreased the sarcoplasmic reticulum Ca2+-storing capacity and reduced the amplitude of I(Ca)-induced Ca2+ transients and of spontaneous Ca2+ sparks in permeabilized myocytes. Exposure of myocytes to isoproterenol caused the development of delayed afterdepolarizations in CASQ2(G112+5X).

CONCLUSIONS

CASQ2(L167H) and CASQ2(G112+5X) alter CASQ2 function in cardiac myocytes, which leads to reduction of active sarcoplasmic reticulum Ca2+ release and calcium content. In addition, CASQ2(G112+5X) displays altered calcium-binding properties and leads to delayed afterdepolarizations. We conclude that the 2 CASQ2 mutations identified in CPVT create distinct abnormalities that lead to abnormal intracellular calcium regulation, thus facilitating the development of tachyarrhythmias.

Microdomains of intracellular Ca2+: molecular determinants and functional consequences

Calcium ions are ubiquitous and versatile signaling molecules, capable of decoding a variety of extracellular stimuli (hormones, neurotransmitters, growth factors, etc.) into markedly different intracellular actions, ranging from contraction to secretion, from proliferation to cell death. The key to this pleiotropic role is the complex spatiotemporal organization of the [Ca(2+)] rise evoked by extracellular agonists, which allows selected effectors to be recruited and specific actions to be initiated. In this review, we discuss the structural and functional bases that generate the subcellular heterogeneity in cellular Ca(2+) levels at rest and under stimulation. This complex choreography requires the concerted action of many different players; the central role is, of course, that of the calcium ion, with the main supporting characters being all the entities responsible for moving Ca(2+) between different compartments, while the cellular architecture provides a determining framework within which all the players have their exits and their entrances. In particular, we concentrate on the molecular mechanisms that lead to the generation of cytoplasmic Ca(2+) microdomains, focusing on their different subcellular location, mechanism of generation, and functional role.

Trapidil enhances the slowly activating delayed rectifier potassium current and suppresses the transient inward current induced by catecholamine in Guinea pig ventricular myocytes

We examined the electrophysiological effects of trapidil on the ionic currents influencing the repolarization and on the transient inward current (ITi) that can cause triggered arrhythmia using the whole-cell patch-clamp technique in guinea pig ventricular myocytes. Trapidil shortened the action potential duration (APD) and increased the delayed rectifier potassium current (IK) in a concentration-dependent manner. The effect of trapidil on the rapidly and slowly activating components of IK (IKr and IKs, respectively) was studied by the envelope of tails test. Trapidil failed to affect IKr and selectively enhanced IKs. Trapidil increased the amplitude of the L-type Ca2+ current (ICa,L), with an acceleration of its inactivation, whereas isoproterenol, a beta-adrenoceptor agonist, increased the amplitude of the ICa,L in a different manner. Isoproterenol activated ITi; however, trapidil not only failed to facilitate ITi but also suppressed isoproterenol-induced ITi. The inhibitory effect of trapidil on isoproterenol-induced ITi is at least partly via a reduction of Ca2+ overload through an acceleration of ICa,L inactivation and/or a sarcoplasmic reticulum (SR) Ca channel modulation. These results suggest that trapidil does not prolong the QT interval and has an antiarrhythmic effect on arrhythmias elicited by triggered activity secondary to Ca2+ overload at much higher concentrations than clinical concentration.

Calstabin deficiency, ryanodine receptors, and sudden cardiac death

Altered cardiac ryanodine receptor (RyR2) function has an important role in heart failure and genetic forms of arrhythmias. RyR2 constitutes the major intracellular Ca2+ release channel in the cardiac sarcoplasmic reticulum (SR). The peptidyl-prolyl isomerase calstabin2 (FKBP12.6) is a component of the RyR2 macromolecular signaling complex. Calstabin2 binding to RyR2 is regulated by PKA phosphorylation of Ser2809 in RyR2. PKA phosphorylation of RyR2 decreases the binding affinity for calstabin2 and increases RyR2 open probability and sensitivity to Ca2+-dependent activation. In heart failure, a majority of studies have found that RyR2 becomes chronically PKA hyper-phosphorylated which depletes calstabin2 from the channel complex. Calstabin2 dissociation causes a diastolic SR Ca2+ leak contributing to depressed intracellular Ca2+ cycling and decreased cardiac contractility. Missense mutations linked to genetic forms of exercise-induced arrhythmias and sudden cardiac death also cause decreased calstabin2-binding affinity and leaky RyR2 channels. We review the importance of calstabin2 for RyR2 function and excitation-contraction coupling, and discuss new observations that implicate dysregulation of calstabin2 binding as a central mechanism for abnormal calcium cycling in heart failure and triggered arrhythmias.

Spatial heterogeneity of action potential alternans during global ischemia in the rabbit heart

Cardiac ischemia causes beat-to-beat fluctuation in action potential duration (APD) alternans, which leads to T wave alternans and arrhythmias. Occurrence of APD alternans that is out of phase at two sites is especially important, but most APD alternans studies have involved rapid pacing of normal myocardium rather than ischemia. To determine the spatial features of APD alternans during ischemia, blood-perfused rabbit hearts were stained with 4-[beta-[2(di-n-butylamino)-6-napthyl]vinyl]pyridinium (di-4-ANEPPS) and imaged with a high-resolution camera. Hearts were perfused with oxygenated Tyrode solution at 37 degrees C for staining and then switched to a 50:50% blood/Tyrode mixture. Hearts were paced from the right ventricle at 3/s, and made ischemic by stopping flow for 6 min. Images of 10,000 pixels were obtained at 300 frames/s. Motion artifact was controlled by immobilization and by manual selection of undistorted single-pixel records. Upstroke propagation and conduction isochrones were displayed by computerized image processing. APD alternans was demonstrated in six of seven hearts, and was out of phase in different regions of the image in three hearts. The largest spatial variation in the onset of depolarization to 50% repolarization (APD50) was 155%. This caused beat-to-beat reversal of repolarization. An alternans map could be constructed for well-immobilized portions of the image. There were discrete regions of APD alternans separated by a boundary, as occurs with intracellular Ca2+ concentration alternans. Pixels as close together as 1.1 mm showed an APD alternans that was out of phase. The out-of-phase APD alternans was not due to conduction alternans, as shown by upstroke intervals and conduction isochrones. This contrasts with rapid pacing, where a causal relationship appears to exist. These new observations suggest distinct mechanisms for the genesis of arrhythmias during ischemia.

Calcium and cardiac arrhythmias: DADs, EADs, and alternans

Rapid progress has been made in understanding the molecular mechanisms by which calcium ions mediate certain cardiac arrhythmias. Principal advances include imaging of cytosolic calcium in isolated cells and in intact tissues, use of fluorescent indicators and monophasic action potentials to record membrane potentials in isolated tissue, and sequencing of the genes that encode critical ion channel proteins. In this review, five types of arrhythmias are discussed where calcium ion currents, or currents controlled by calcium, appear to be responsible for arrythmogenesis. These include: (1) the delayed afterpotential that occurs in conditions of intracellular calcium overload such as digitalis toxicity; (2) the early afterdepolarization that occurs when action potential duration is prolonged; (3) the slowly conducted calcium-dependent action potential (the slow response) in the SA and AV nodes; (4) the phenomenon of calcium transient alternans during ischemia, which is related to action potential duration alternans and t-wave alternans; (5) catecholamine-induced cardiac arrhythmias in families with mutations of the sarcoplasmic reticulum calcium-release channel. For each type of arrhythmia, the clinical implications of emerging knowledge are discussed. An especially important issue is whether ventricular fibrillation during acute coronary artery occlusion is due to calcium transient alternans. Ventricular fibrillation due to acute ischemia is an important subset of the 400,000 sudden cardiac deaths that occur annually in the U.S. Certain drugs, including beta blockers, fish oils, verapamil, and diltiazem, seem to specifically prevent ventricular fibrillation in this setting, and in most cases an effect of the drug on cytosolic calicum appears to be involved.

Local Ca(2+) signaling and EC coupling in heart: Ca(2+) sparks and the regulation of the [Ca(2+)](i) transient

The elementary event of Ca(2+) release in heart is the Ca(2+) spark. It occurs at a low rate during diastole, activated only by the low cytosolic [Ca(2+)](i). Synchronized activation of many sparks is due to the high local [Ca(2+)](i) in the region surrounding the sarcoplasmic reticulum (SR) Ca(2+) release channels and is responsible for the systolic [Ca(2+)](i) transient. The biophysical basis of this calcium signaling is discussed. Attention is placed on the local organization of the ryanodine receptors (SR Ca(2+) release channels, RyRs) and the other proteins that underlie and modulate excitation-contraction (EC) coupling. A brief review of specific elements that regulate SR Ca(2+) release (including SR lumenal Ca(2+) and coupled gating of RyRs) is presented. Finally integrative calcium signaling in heart is presented in the context of normal heart function and heart failure.

Spatial heterogeneity of calcium transient alternans during the early phase of myocardial ischemia in the blood-perfused rabbit heart

BACKGROUND

Optical mapping of cytosolic calcium transients in intact mammalian hearts is now possible using long-wavelength [Ca(2+)](i) indicators. We propose that beat-to-beat [Ca(2+)](i) transient alternans during ischemia may lead to spatial and temporal heterogeneity of calcium-activated membrane currents.

METHODS AND RESULTS

To test this hypothesis, isolated rabbit hearts were loaded with the fluorescent [Ca(2+)](i) indicator, rhod-2 AM, and imaged at 300 frames/sec during blood-perfused ischemic trials. High-quality [Ca(2+)](i) transients were recorded in each of 8 hearts.[Ca(2+)](i) transient alternans was never present in control records but occurred in each of the hearts during ischemia, with onset after 2 to 4 minutes. Alternans was confined to circumscribed regions of the heart surface 5 to 15 mm across. Multiple regions of alternans were found in most hearts, and regions that were out of phase with one another were found in 6 hearts. Quantitative maps of alternans were constructed by calculating an alternans ratio. This ratio behaved as a continuous variable that reached a maximum value in the center of the regions with alternans.

CONCLUSIONS

These results demonstrate marked spatial heterogeneity of the [Ca(2+)](i) transient during the early phase of ischemia, which could produce electrical instability and arrhythmias in large mammalian hearts.

Intracellular Ca2+ oscillations drive spontaneous contractions in cardiomyocytes during early development

Activity of cardiac pacemaker cells is caused by a balanced interplay of ion channels. However, it is not known how the rhythmic beating is initiated during early stages of cardiomyogenesis, when the expression of ion channels is still incomplete. Based on the observation that early-stage embryonic stem cell-derived cardiomyocytes continuously contracted in high extracellular K+ solution, here we provide experimental evidence that the spontaneous activity of these cells is not generated by transmembrane ion currents, but by intracellular [Ca2+]i oscillations. This early activity was clearly independent of voltage dependent L-type Ca2+ channels and the interplay between these and ryanodine sensitive Ca2+ stores. We also show that intracellular Ca2+ oscillations evoke small membrane depolarizations and that these can trigger L-type Ca2+ channel driven action potentials.

Sodium/calcium exchange: its physiological implications

The Na+/Ca2+ exchanger, an ion transport protein, is expressed in the plasma membrane (PM) of virtually all animal cells. It extrudes Ca2+ in parallel with the PM ATP-driven Ca2+ pump. As a reversible transporter, it also mediates Ca2+ entry in parallel with various ion channels. The energy for net Ca2+ transport by the Na+/Ca2+ exchanger and its direction depend on the Na+, Ca2+, and K+ gradients across the PM, the membrane potential, and the transport stoichiometry. In most cells, three Na+ are exchanged for one Ca2+. In vertebrate photoreceptors, some neurons, and certain other cells, K+ is transported in the same direction as Ca2+, with a coupling ratio of four Na+ to one Ca2+ plus one K+. The exchanger kinetics are affected by nontransported Ca2+, Na+, protons, ATP, and diverse other modulators. Five genes that code for the exchangers have been identified in mammals: three in the Na+/Ca2+ exchanger family (NCX1, NCX2, and NCX3) and two in the Na+/Ca2+ plus K+ family (NCKX1 and NCKX2). Genes homologous to NCX1 have been identified in frog, squid, lobster, and Drosophila. In mammals, alternatively spliced variants of NCX1 have been identified; dominant expression of these variants is cell type specific, which suggests that the variations are involved in targeting and/or functional differences. In cardiac myocytes, and probably other cell types, the exchanger serves a housekeeping role by maintaining a low intracellular Ca2+ concentration; its possible role in cardiac excitation-contraction coupling is controversial. Cellular increases in Na+ concentration lead to increases in Ca2+ concentration mediated by the Na+/Ca2+ exchanger; this is important in the therapeutic action of cardiotonic steroids like digitalis. Similarly, alterations of Na+ and Ca2+ apparently modulate basolateral K+ conductance in some epithelia, signaling in some special sense organs (e.g., photoreceptors and olfactory receptors) and Ca2+-dependent secretion in neurons and in many secretory cells. The juxtaposition of PM and sarco(endo)plasmic reticulum membranes may permit the PM Na+/Ca2+ exchanger to regulate sarco(endo)plasmic reticulum Ca2+ stores and influence cellular Ca2+ signaling.

Calmodulin kinase inhibition prevents development of the arrhythmogenic transient inward current

Although it is widely accepted that afterdepolarizations initiate arrhythmias when action potentials are prolonged, the underlying mechanisms are unclear. In this study, we tested the hypothesis that action potential prolongation would raise intracellular calcium and thereby activate the arrhythmogenic transient inward current (Iti). Furthermore, given that Iti can be activated by sarcoplasmic reticulum Ca2+ release, we tested the hypothesis that inhibition of calmodulin (CaM) kinase would prevent Iti. Isolated rabbit ventricular myocytes were studied with whole-cell-mode voltage clamp. Stimulation with a prolonged action potential clamp, under near-physiological conditions, increased [Ca2+]i. Iti was reproducibly induced in 60 of 60 cells, but Iti was not seen with the use of a shorter action potential waveform (n=12). Iti was associated with a secondary elevation in [Ca2+]i. When [Ca2+]i buffering was enhanced by dialysis with BAPTA (20 mmol/L, n=9), no Iti was present. The Na+/Ca2+ exchanger was likely responsible for Iti, because Iti was inhibited by the Na+/Ca2+ exchanger inhibitory peptide XIP (10 micromol/L, n=6), but not by an inactive scrambled peptide (10 micromol/L, n=5) or by the Cl- current antagonist niflumic acid (10 to 40 micromol/L, n=9). Activator Ca2+ from the sarcoplasmic reticulum was essential for development of Iti, because it was prevented by pretreatment with ryanodine (10 micromol/L, n=6) or thapsigargin (1 micromol/L, n=6). Two different CaM kinase inhibitory peptides (n=16) and a CaM inhibitory peptide (n=4) completely suppressed Iti. These results are consistent with the hypothesis that CaM kinase plays a role in arrhythmias related to increased [Ca2+]i.

The control of Ca release from the cardiac sarcoplasmic reticulum: regulation versus autoregulation

This review discusses the mechanism and regulation of Ca release from the cardiac sarcoplasmic reticulum. Ca is released through the Ca release channel or ryanodine receptor (RyR) by the process of calcium-induced Ca release (CICR). The trigger for this release is the L-type Ca current with a small contribution from Ca entry on the Na-Ca exchange. Recent work has shown that CICR is controlled at the level of small, local domains consisting of one or a small number of L-type Ca channels and associated RyRs. Ca efflux from the s.r. in one such unit is seen as a 'spark' and the properties of these sparks produce controlled Ca release from the s.r. A major factor controlling the amount of Ca released from the s.r. and therefore the magnitude of the systolic Ca transient is its Ca content. The Ca content depends on both the properties of the s.r. and the cytoplasmic Ca concentration. Changes of s.r. Ca content and the Ca released affect the sarcolemmal Ca and Na-Ca exchange currents and this acts to control cell Ca loading and the s.r. Ca content. The opening probability of the RyR can be regulated by various physiological mediators as well as pharmacological compounds. However, it is shown that, due to compensatory changes of s.r. Ca, modifiers of the RyR only produce transient effects on systolic Ca. We conclude that, although the RyR can be regulated, of much greater importance to the control of Ca efflux from the s.r. are effects due to changes of s.r. Ca content.

Caffeine restriction has no role in the management of patients with symptomatic idiopathic ventricular premature beats

OBJECTIVE

To assess the role of caffeine restriction in the management of patients with symptomatic idiopathic ventricular premature beats.

DESIGN

A randomised, double blind, 6 week intervention trial incorporating dietary caffeine restriction, caffeinated coffee, and decaffeinated coffee.

SETTING

Cardiac outpatient clinic.

PATIENTS

13 patients with symptomatic frequent idiopathic ventricular premature beats.

MAIN OUTCOME MEASURES

Weekly measures of serum caffeine concentration, coffee consumption, visual analogue score of palpitations, and 24 hour ventricular premature beat frequency.

RESULTS

The interventions achieved significant alterations in serum caffeine concentrations (P < 0.001) which correlated with coffee consumption (r = 0.70; P < 0.001). Visual analogue palpitation scores showed a small, but significant correlation with ventricular premature beat frequencies (r = 0.34; P = 0.003). However, there were no significant changes in palpitation scores or ventricular premature beat frequencies during the intervention weeks and no significant correlations were found between these variables and serum caffeine concentrations.

CONCLUSIONS

Caffeine restriction has no role in the management of patients referred with symptomatic idiopathic ventricular premature beats.

[Ca2+]i-dependent membrane currents in guinea-pig ventricular cells in the absence of Na/Ca exchange

Transient inward currents (Iti) during oscillations of intracellular [Ca2+] ([Ca2+]i) in ventricular myocytes have been ascribed to Na/Ca exchange. We have investigated whether other Ca2+-dependent membrane currents contribute to Iti in single guinea-pig ventricular myocytes, by examining membrane currents during [Ca2+]i oscillations and during caffeine-induced Ca2+ release from the sarcoplasmic reticulum in the absence of Na+. Membrane currents were recorded during whole-cell voltage clamp and [Ca2+]i measured simultaneously with fura-2. In the absence of Na/Ca exchange, i.e., with Li+, Cs+ or N-methyl-D-glucamine (NMDG+) substituted for Na+, the cell could be loaded with Ca2+ by repetitive depolarizations to +10 mV, resulting in spontaneous [Ca2+]i oscillations. During these oscillations, no inward currents were seen, but instead spontaneous Ca2+ release was accompanied by a shift of the membrane current in the outward direction at potentials between -40 mV and +60 mV. This [Ca2+]i-dependent outward current shift was not abolished when NMDG+ was substituted for internal monovalent cations, nor was it sensitive to substitution of external Cl-. It was however, sensitive to the blockade of ICa by verapamil. These results suggest that the transient outward current shift observed during spontaneous Ca2+ release represents [Ca2+]i-dependent transient inhibition of ICa. Similarly, during the [Ca2+]i transients induced by brief caffeine (10 mM) applications, we could not detect membrane currents attributable to a Ca2+-activated nonselective cation channel, or to a Ca2+-activated Cl- channel; however, transient Ca2+-dependent inhibition of ICa was again observed. We conclude that neither the Ca2+-activated nonselective cation channel nor the Ca2+-activated Cl- channel contribute significantly to the membrane currents during spontaneous [Ca2+]i oscillations in guinea-pig ventricular myocytes. However, in the voltage range between -40 mV and +60 mV Ca2+-dependent transient inhibition of ICa will contribute to the oscillations of the membrane current.

Two components of [Ca2+]i-activated Cl- current during large [Ca2+]i transients in single rabbit heart Purkinje cells

1. Single Purkinje cells, enzymatically isolated from rabbit ventricle, were studied under whole-cell voltage clamp conditions and internally perfused with the fluorescent Ca2+ indicator fura-2(100 microM). 2. Ca2+ release from the sarcoplasmic reticulum was either induced by external application of caffeine or occurred spontaneously in Ca2+i-overloaded cells. Membrane currents accompanying these Ca(2+)-release signals were studied at steady membrane potentials. 3. [Ca2+]i transients were accompanied by transient membrane currents. In the absence of Na(+)-Ca2+ exchange, two current components could be observed. The first component peaked well before the [Ca2+]i transient (Ifast) and relaxed before peak [Ca2+]i. The second component, on the other hand, peaked at the time when [Ca2+]i was maximal (Islow). 4. In symmetrical Cl- solutions both current components had a reversal potential close to O mV. A reduction of external or internal [Cl-] shifted this reversal potential in accordance with the change of the Cl- equilibrium potential. 5. Each [Ca2+]i transient was accompanied by Ifast. Properties of Ifast suggest that this current component is the [Ca2+]i-dependent Cl- current, ICl(Ca), previously observed during depolarizing pulses. 6. Islow was only detected in cells that displayed a large [Ca2+]i transient with or without elevated resting [Ca2+]i. 7. It is concluded that during large [Ca2+]i transients a slow component of ICl(Ca) can be activated. This second component may arise from the same channel population as the previously described fast component and be related to the presence of spatial and temporal inhomogeneities of [Ca2+]i. Alternatively, this current component may arise from a different Cl- channel population with a different Ca2+ sensitivity.

Caffeine-induced decreases in the inward rectifier potassium and the inward calcium currents in rat ventricular myocytes

The effects of high (20 mM) concentrations of caffeine were studied on the transmembrane voltage and currents in rat single ventricular myocytes by the whole cell configuration of the patch clamp technique. Rapid application of caffeine released Ca2+ from the sarcoplasmic reticulum and induced a Ni(2+)-sensitive transient inward current with concomitant change of the transmembrane voltage from -72.6 +/- 0.4 to -68.0 +/- 0.6 mV (n = 4). Maintained application of caffeine lengthened the action potential duration (APD90) from 66.7 +/- 16.9 to 135.1 +/- 34.1 ms (n = 4) and depressed the amplitude of both the inward rectifier potassium and the inward calcium currents. It is concluded that these effects of caffeine should be recognized when it is used as a tool to study electromechanical coupling.

Spontaneous myocardial calcium oscillations: are they linked to ventricular fibrillation?

The physiological oscillation of cytosolic [Ca2+] that underlies each heart beat is generated by the sarcoplasmic reticulum (SR) in response to an action potential (AP) and occurs relatively synchronously within and among cells. When the myocardial cell and SR Ca2+ loading become sufficiently high, the SR can also generate spontaneous, i.e., not triggered by sarcolemmal depolarization, Ca2+ oscillations (S-CaOs). The purpose of this review is to describe properties of S-CaOs in individual cells, myocardial tissue, and the intact heart, and to examine the evidence that may link S-CaOs to the initiation or maintenance of ventricular fibrillation (VF). The SR Ca2+ release that generates S-CaOs occurs locally within cells and spreads within the cell via Ca(2+)-induced Ca2+ release. The localized increase in cytosolic [Ca2+] due to S-CaOs may equal that induced by an AP and causes oscillatory sarcolemmal depolarizations of cells in which it occurs. These oscillatory depolarizations are due to Ca2+ activation of the Na/Ca exchanger and of nonspecific cation channels. Asynchronous occurrence of diastolic S-CaOs among cells within the myocardium causes inhomogeneity of diastolic SR Ca2+ loading; this leads to inhomogeneity of the systolic cytosolic [Ca2+] transient levels in response to a subsequent AP, which leads to heterogeneity of AP repolarization, due to heterogeneous Ca2+ modulation of the Na/Ca exchanger, nonspecific cation channels, and of the L-type Ca2+ channel. In a tissue in which asynchronous S-CaOs are occurring in diastole, the subsequent AP temporarily synchronizes SR Ca2+ loading and release within and among cells. Varying extents of synchronized S-CaOs then begin to occur during the subsequent diastole. The partial synchronization of this diastolic S-CaOs among cells within myocardial tissue produces aftercontractions and diastolic depolarizations. When S-CaOs are sufficiently synchronized, the resultant depolarizations summate and can be sufficient to trigger a spontaneous AP.S-CaOs occurrence within some cells during a long AP plateau also modulates the removal of voltage inactivation of L-type Ca2+ channels and increases the likelihood for "early afterdepolarizations" to occur in myocardial tissue. S-CaOs have an apparent modulatory role in the initiation of VF in the Ca2+ overload model and in the reflow period following ischemia. Likewise, in non-a priori Ca2+ overloaded hearts, S-CaOs modulate the threshold for VF induction (induced typically by alternating current) but may not be essential for VF induction.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Potential arrhythmogenic role of cyclic adenosine monophosphate (AMP) and cytosolic calcium overload: Implications for prophylactic effects of beta-blockers in myocardial infarction and proarrhythmic effects of phosphodiesterase inhibitors

Activation of the adrenergic nervous system appears to play a crucial role in the genesis of fatal arrhythmias associated with the very early stages of acute myocardial infarction. The second messenger of beta-adrenergic catecholamine stimulation, cyclic adenosine monophosphate (AMP), has established arrhythmogenic qualities, acting by an increase in cytosolic calcium, which potentially has three adverse electrophysiologic effects. First, stimulation of the transient inward current by excess oscillations of cytosolic calcium can invoke delayed afterdepolarizations, so that triggered automaticity can develop in otherwise quiescent ventricular muscle. Second, cyclic AMP can evoke calcium-dependent slow responses in depolarized fibers, so that conditions for reentry are favored. Third, excess cytosolic calcium can cause intercellular uncoupling with conduction slowing. Focal changes in cyclic AMP and cytosolic calcium promote the development of ventricular fibrillation. Beta-adrenergic blockade can limit the formation of cyclic AMP in ischemic tissue. Furthermore, by reducing sinus tachycardia it can lessen cytosolic calcium overload. Hence, beta-adrenergic blockade helps to prevent ventricular fibrillation in the early stages of acute myocardial infarction and protects from sudden death in the postinfarction phase. In congestive heart failure, abnormalities of cytosolic calcium patterns exist with cytosolic calcium overload. It is proposed that the adverse effects of phosphodiesterase inhibitors on the mortality rate in patients with congestive heart failure can be explained by increased rates of formation of cyclic AMP and the development of calcium-dependent arrhythmias. Because calcium is the ultimate messenger of cyclic AMP-induced arrhythmias and because cytosolic calcium is increased in heart failure, it will be difficult to develop positive inotropic agents that are free of the risk of sudden death.

Spontaneous sarcoplasmic reticulum Ca2+ release leads to heterogeneity of contractile and electrical properties of the heart

The cytosolic Ca2+ (Cai) oscillation generated by the sarcoplasmic reticulum (SR) in response to an action potential (AP) occurs relatively synchronously within and among cells. The SR can also generate spontaneous Cai oscillations (S-CaOs), i.e., not triggered by sarcolemmal depolarization. The local increase in Cai due to S-CaOs is equivalent to that induced by an AP. Heterogeneity of diastolic Cai caused by asynchronous S-CaOs among cells within myocardial tissue leads to heterogeneous myofilament activation, the summation of which produces a Ca(2+)-dependent component to diastolic tone. The local increases in Cai due to S-CaOs also cause oscillatory sarcolemmal depolarizations due to Ca2+ modulation of the Na/Ca exchanger and of non-specific cation channels. Thus, inhomogeneous levels of diastolic Cai may lead to heterogeneity in cell coupling and thus may also affect the impulse conduction. The magnitude of the S-CaOs induced diastolic tonus and membrane depolarization varies with the extent to which S-CaOs are synchronized; partially synchronized S-CaOs following an AP induced SR Ca2+ release produce an aftercontraction and after depolarization. When local S-CaOs is sufficiently synchronized within the cell the resultant depolarization summates and can be sufficient to trigger spontaneous AP. Inhomogeneity of diastolic SR Ca2+ loading and sarcomere lengths within individual cardiac cells due to S-CaOs leads to inhomogeneous systolic Cai levels and sarcomere length inhomogeneities in response a subsequent AP; this heterogeneity compromises the systolic contraction amplitude. Heterogeneity of systolic Cai among cells due to diastolic S-CaOs also leads to heterogeneity of AP repolarization times, due, to heterogeneous Cai modulation of the Na/Ca exchanger, the non-specific cation channel and of the L type sarcolemmal Ca2+ channel. S-CaOs occurrence during a long AP plateau may also modulate the removal of voltage inactivation of L type Ca2+ channels and affect the likelihood of the occurrence of "early after depolarizations." Thus, as a single entity, S-CaOs may be implicated in diverse manifestations of heart failure--impaired systolic performance, increased diastolic tonus and an increased probability for the occurrence of arrhythmias.

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.

Alterations of ionic currents after reoxygenation in isolated cardiocytes of guinea-pigs

Single myocytes were isolated from ventricles of adult guinea-pig hearts. The patch-clamp technique in the whole-cell configuration was used to study ionic currents. Experiments were performed in an experimental chamber that allowed the cells to be exposed to a sufficiently low O2 pressure to cause metabolic inhibition after 4-35 min (mean 14.1 min, n = 20), which was indicated by the appearance of a large time-independent K current. Reoxygenation about 1 min after the first extra outward current was observed caused this current to vanish completely within 2-6 s if the calcium inside the pipette was buffered to negligible values with 20 mmol/l EGTA. With only 10 microM EGTA in the pipette, reoxygenation was followed by an arrhythmogenic period of 10-150 s duration, which was dominated by three types of event: (a) transient inward currents (Iti) developed during the first 5-10 s (26 cells); (b) the net current was increased by a factor of 1.9 +/- 0.4 (mean +/- SD, n = 17) yielding a reversal potential for the increased component of -77 +/- 4 mV (mean +/- SD, n = 4); and (c) the Ca current decreased by 20%-100% within the first 5-10 s. At the end of the arrhythmogenic period, Iti vanished, the net current recovered completely, and the Ca current recovered partially. At -45 mV, increasing preceding depolarization enlarged the amplitude of both the Iti and the net current, Iti being about four times more increased than the net current. The suppression of the Ca current was independent of the phase of the preceding Iti. We conclude that in isolated cardiocytes, after the induction of an anoxia-induced K current, reoxygenation causes a period of up to 150 s of cytosolic Ca overload, during which Iti is triggered, the net current is enhanced, and the Ca current is suppressed.

Reoxygenation-induced arrhythmogenic transient inward currents in isolated cells of the guinea-pig heart

Transient inward currents (Iti), activated by a rise in intracellular Ca concentration, are believed to trigger cardiac arrhythmias in reperfused hearts. In this report, Iti in isolated cardiocytes from the guinea-pig were evoked by reoxygenation following a period of anoxia of between 4 min and 35 min. Reoxygenation was performed 1 min after the full development of an anoxia-induced time-independent K current. This current disappeared within 2-6 s and in the following 10 s Iti developed to maximum amplitude. Iti were evoked using a constant pulse pattern (holding potential Vh = -45 mV; test potential Vt = +10; pulse duration 350 ms; frequency 1 Hz). In more than 95% of the cells, Iti at the holding potential Iti (-45 mV) declined with a time constant of tau = 670 +/- 240 ms (mean +/- SD, n = 17). In two cells, undamped oscillatory currents were observed. The amplitude of Iti (-45 mV) was proportional to the amplitude and duration of the preceding depolarizing test pulse. Test pulses of long duration (500 ms and 1000 ms, mean +/- SD) to potentials positive to +10 mV produced slowly decaying tail currents (tau = 391 +/- 51 ms, mean +/- SD), which superimposed with Iti (-45 mV). The current/voltage relationship of Iti peaked between -30 mV and -10 mV and approximated zero at the most positive potentials, i.e. no reversal of Iti was found up to +80 mV. Using double-pulse protocols (prepulse potential +40 mV), Iti were enhanced at potentials negative to -30 mV and were also present in the range of the normal resting potential of ventricular heart cells. The instantaneous current-voltage relationship was monotone between -50 mV and +40 mV. Because of the dependence of Iti on the preceding depolarization, the instantaneous current-voltage relationship provides more reliable information on the voltage dependence of Iti. The interval between two subsequent Iti (-45 mV) values was 237 +/- 35 ms (mean +/- SD, n = 27) and depended on the amplitude of Iti (-45 mV) to increase by 5.2 +/- 0.5% (mean +/- SD) per 100 pA decrease in Iti (-45 mV). A simple noise analysis showed that if one assumes that ionic channels are responsible for the generation of Iti (-45 mV), their unitary conductance cannot exceed 0.36 pS. We conclude that reoxygenation-induced Iti are triggered by a cyclic release of Ca from the sarcoplasmic reticulum and provide evidence that they are mediated by the electrogenic Na/Ca exchanger.(ABSTRACT TRUNCATED AT 400 WORDS)

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.

Adrenergic stimulation of sea urchin sperm cells

The noradrenergic agonists norepinephrine and isoproterenol elicit greater stimulatory swimming responses in sea urchin spermatozoa than epinephrine. The beta-blocker atenolol induces an even greater motile rate, while the alpha-blocker phentolamine has only a moderate effect, it also causes a minimal reduction in the sperm cells' response to atenolol. Caffeine increases the motility but to a lesser degree than 8-Br-cAMP. In drug interaction assays, both caffeine and 8-Br-cAMP depress the adrenergic effects. Agents that affect access of calcium to the flagellar apparatus (verapamil and trifluoperazine) depress the motility below the level of the controls when incubated separately with the sperm suspensions and counteract the stimulation due to atenolol. Adrenergic modulation of sperm motility thus appears to be both a calcium-dependent and a cyclic nucleotide-dependent process.

Comparison of sodium-calcium exchanger and transient inward currents in single cells from rabbit ventricle

1. Whole-cell voltage-clamp measurements have been made in rabbit ventricular myocytes under conditions in which both Na(+)-Ca2+ exchanger currents (IEX, slow tails) and transient inward currents (ITI or TI) can be recorded. A number of experimental manoeuvres have been used in an attempt to separate or dissociate these two currents. 2. As expected, partial inhibition of the Na(+)-K+ pump by application of 0.54 mM [K+] Tyrode solution or 10(-5) M-strophanthidin induced TI currents which were recorded in the presence of IEX slow tails. 3. Complete inhibition of the Na(+)-K+ pump with zero [K+] Tyrode solution resulted in larger and more frequent TIs but smaller IEX tails. 4. A somewhat similar dissociation between ITI and IEX was observed when NaCl was reduced to 37.5 mM by using LiCl to replace NaCl. This inhibited the Na(+)-Ca2+ exchanger current, but induced ITI. 5. Transient inward currents and IEX tails could also be separated by selected patterns of stimulation (voltage-clamp depolarizations): following the second pulse of a pair of stimuli, IEX was significantly reduced whereas the TIs increased in size and frequency. 6. Additional experimental tests involving changes in external divalent ions could also separate these two currents. Increasing [Ca2+]o 3-fold increased the TIs without changing IEX. Shortly after [Ca2+]o was replaced with either [Ba2+]o or [Sr2+]o the TIs were blocked but IEX was unchanged. Application of MnCl2 (1 mM) and elevation of [K+]o inhibited IEX but did not significantly change the TI currents. 7. Application of caffeine (5-10 mM) or ryanodine (2 x 10(-6) M) blocked the TI currents at times when the IEX tails were not changed. 8. In combination these results suggest that even though both IEX and ITI are triggered (activated) by increases in [Ca2+]i, these two currents are distinct. IEX is generated by electrogenic Na(+)-Ca2+ exchange, while the TI currents may be due to Ca2(+)-activated cation-selective channels in the sarcolemma.

Nisoldipine inhibition of sodium influx into aorta from aldosterone-salt-hypertensive rats

The purpose of this study was to determine whether increased sodium (Na) influx into the aorta was associated with aldosterone-salt hypertension in the rat and, if present, to determine what mechanisms contributed to the increase. Basal 24Na influx was elevated in aorta from the hypertensive rats (2.21 +/- 0.10 mmol/l cell H2O/min, n = 25) compared with control-salt rats (1.75 +/- 0.04 mmol/l cell H2O/min, n = 24). The calcium (Ca) antagonist nisoldipine inhibited the Na influx into aorta from hypertensive rats in a concentration-dependent manner. At 10 nM nisoldipine, the Na influx in hypertensive rats (1.52 +/- 0.14 mmol/l cell H2O/min, n = 10) was similar to control rats (1.66 +/- 0.18 mmol/l cell H2O/min, n = 7). The basal Na influx in aorta from hypertensive rats was not altered by dichlorobenzamil or ethylisopropylamiloride, selective inhibitors of Na-Ca and Na-H exchange, respectively. The Na influx was 2.21 +/- 0.10, 2.03 +/- 0.24, and 2.11 +/- 0.19 mmol/l cell H2O/min for basal (n = 25), dichlorobenzamil (n = 4), and ethylisoproisopropylamiloride (n = 11), respectively. Inhibition of Na influx in hypertensive rats by 0.1 microM nisoldipine (delta Na influx = -0.72 +/- 0.18 mmol/l cell H2O/min, n = 9) was not significantly altered when applied with dichlorobenzamil (-0.72 +/- 0.21 mmol/l cell H2O/min, n = 4) or ethylisopropylamiloride (-0.55 +/- 0.15 mmol/l cell H2O/min, n = 11). These agents did not alter Na influx in control aorta.(ABSTRACT TRUNCATED AT 250 WORDS)

Caffeine affects four different ionic currents in the bull-frog sympathetic neurone

1. Ionic mechanisms related to the caffeine-induced current (Icaffeine) were examined in the single isolated sympathetic neurones of the bull-frog. We used the 'concentration-jump' technique in combination with intracellular perfusion and a rapid external solution change, under single-electrode voltage-clamp conditions. 2. Icaffeine was pharmacologically separated into a tetraethylammonium (TEA)-sensitive transient outward current (ITO), a picrotoxin (PTX)-sensitive transient inward current (ITI) and a TEA- and PTX-insensitive sustained inward current (ISI). At low concentrations of caffeine, a sustained outward current (ISO) was observed instead of ISI. 3. All components of Icaffeine were abolished by intracellular perfusion of 30 mM-EGTA. Pre-treatment with A23187 or ryanodine or the simultaneous application of procaine either reduced or abolished all the components of Icaffeine in a dose-dependent manner. The concentration causing 50% inhibition (IC50) was 10(-8) M for A23187 and 2 mM for procaine. 4. The peak response of ITO increased abruptly at caffeine concentrations between 3 and 6 mM followed by saturation above 30 mM. A notch was observed on the rising phase of ITO. 5. The reversal potential (Ecaffeine) of ITO shifted 58 mV for a tenfold change of the extracellular K+ concentration. External application of TEA blocked ITO with an IC50 of 1 mM. ITO was relatively insensitive to apamin, 4-aminopyridine and muscarine. 6. In external solution containing 2 mM-Ca2+, ITO induced by 10 mM-caffeine recovered completely within 3 min from a previous exposure to caffeine. In the absence of extracellular Ca2+, there was little such recovery. A 5 min treatment in a Ca2+-free solution reduced ITO induced by the first application of caffeine by 5%. With a continuous application of 3 mM-caffeine, the amplitude of ITO induced by 10 mM-caffeine reduced in 1 min, and showed a partial recovery in 3 min. The amplitude of ITO increased by increasing the concentration of intracellular Cl-. 7. ITI was activated around the peak of ITO and was rapidly inactivated. ITI was evoked at caffeine concentrations of about 6-10 mM. When the intracellular Cl- concentration was changed, the amplitude of ITI behaved like a Cl- electrode. The Ecaffeine of ITI was close to the Cl- equilibrium potential (ECl). 8. ISI was a 'plateau' response and persisted for over 3 min. ISI was due to a decrease in K+ conductance. In the presence of muscarine (3 x 10(-5) M), ISI was occluded.(ABSTRACT TRUNCATED AT 400 WORDS)

Effect of ischemia on calcium-dependent fluorescence transients in rabbit hearts containing indo 1. Correlation with monophasic action potentials and contraction

The effects of acute global ischemia on cytosolic calcium transients were studied in perfused rabbit hearts loaded with the fluorescent calcium indicator indo 1. Indo 1-loaded hearts were illuminated at 360 nm, and fluorescence was recorded simultaneously at 400 and 550 nm from the epicardial surface of the left ventricle. The F400/F550 ratio was calculated by an analog circuit, which allowed cancellation of optical motion artifact. Resulting calcium transients demonstrated a rapid upstroke and slow decay similar to those recorded in isolated ventricular myocytes. Global ischemia rapidly suppressed contraction, but it produced a concurrent increase in the systolic and diastolic levels of the calcium transients, together with an increase in the duration of the peak. The effects of ischemia were reversed by reperfusion, inhibited by verapamil, and mimicked by perfusion of nonischemic hearts with acidified (CO2-rich) solution. In addition to elevation of the calcium transients, ischemia caused a pattern of intracellular calcium alternans that was discernible after 2-3 minutes. The pattern of alternans was stable at a given epicardial site, but it could be out of phase at different sites. Similar nonuniformities were observed in contraction strength and in the duration of monophasic action potentials recorded immediately adjacent to the fiber-optic probe. Abnormalities in intracellular calcium may be a causal factor in the loss of electrical and mechanical synchrony in the acutely ischemic heart.

Transient inward current in guinea-pig atrial myocytes reflects a change of sodium-calcium exchange current

1. Enzymatically isolated, cultured myocytes from hearts of adult guinea-pigs were voltage clamped with a whole-cell patch-clamp technique. The pipette-filling solution for internal dialysis contained 65 mM-citrate and 50 microM-EGTA as Ca2+-chelating agents and 20 mM-Na+. Potassium channel currents were blocked by replacing this ion on both sides of the membrane by Cs+. 2. In the above conditions myocytes develop spontaneous transient inward currents (Iti) at constant negative membrane holding potentials. At a given membrane potential Iti can be recorded with constant amplitude and frequency for periods of up to ca. 40 min. A membrane current with similar properties can be evoked by superfusion of the cell with caffeine-containing (5-10 mM) solution. 3. Depolarization results in a reduction of Iti amplitude and a prolongation of its duration. After a step change of the membrane potential to ca. -10 mV or a less-negative level only one inward current change is observed. Thereafter the membrane current remains inward with regard to the instantaneous current at this membrane potential. Complete relaxation of Iti then is only observed after repolarization to a more-negative membrane potential. 4. The current change caused by sarcoplasmic Ca2+ release is inward in a range of membrane potentials between -90 and +75 mV. A reversal of Iti was never detected. 5. Both the instantaneous current-voltage (I-V) relation and voltage dependence of peak Iti display distinct outward rectification. Both I-V relations can be described by a formalism suggested for a membrane current caused by electrogenic Na+-Ca2+ exchange (INa, Ca) assuming a 3:1 stoichiometry and a single energy barrier in the electric field of the membrane. 6. An increase of the time integral of Iti at the holding potential is observed after depolarizations to positive membrane potentials, where the outward-rectifying current component is prominent. This supports the view that the outward current represents INa, Ca in the 'reverse mode', carrying Ca2+ ions into the cell. 7. After prolonged cell dialysis a run-down of Iti is observed. Since strong depolarizations in this condition still can cause inward currents upon repolarization, run-down is likely to reflect an impairment of sarcoplasmic reticulum function rather than an effect of cell dialysis on the exchanger. 8. We conclude that under the present conditions a membrane current is measured, which to a large extent determines the 'passive' I-V curve of the myocytes. This current is modified by a rise in Ca2+(i) following sarcoplasmic Ca2+ release.(ABSTRACT TRUNCATED AT 400 WORDS)

Synchronous occurrence of spontaneous localized calcium release from the sarcoplasmic reticulum generates action potentials in rat cardiac ventricular myocytes at normal resting membrane potential

Under certain conditions, spontaneous release of Ca2+ from the sarcoplasmic reticulum occurs in resting mammalian myocardium. In single rat ventricular myocytes, such spontaneous Ca2+ release appears localized rather than homogeneous. When the increase in cytosolic Ca2+ is present in a single locus within a cell, it causes a small depolarization, which, at the normal resting potential, is subthreshold for generating an action potential. However, when spontaneous Ca2+ release occurs simultaneously at more than a single discrete locus, the resultant sarcolemmal depolarization is augmented to levels that can induce an action potential, even when this depolarization begins at the normal resting membrane potential. Thus, the synchronous occurrence of multifocal localized increases in cytosolic Ca2+ due to spontaneous Ca2+ release from the sarcoplasmic reticulum within ventricular myocytes is a mechanism for "abnormal automaticity."

Effects of calcium release from sarcoplasmic reticulum on membrane currents in guinea pig atrial cardioballs

(1) Ca current (ICa) and membrane currents related to Ca-entry during activation of ICa have been studied in cultured atrial myocytes from hearts of adult guinea pigs by means of patch clamp pipettes. The pipettes were filled with solutions containing citrate (65 mM) as major Ca-chelating compound and Cs ions in order to block K currents. (2) In myocytes dialysed with such solutions a monophasic time course of inactivation of ICa is observed, which is 1-2 orders of magnitude slower as compared to studies on intact cardiac cells or cells dialysed with EGTA as only Ca-chelating compound. (3) During long-lasting or repetitive depolarization a second component of ICa inactivation, apart from the slow decay observed in cells dialysed with such solutions, can be seen. This component of inactivation is not related to the depolarization as such but to loading of the cells with Ca2+. Whenever the rapid component of inactivation occurs, a transient inward current (Iti) after repolarization to the holding potential (-40 to -50 mV) is recorded. Both, ICa inactivation and Iti can be mimicked by extracellular application of caffeine (5-10 mM), suggesting both current changes to be caused by a rise in Cai due to Ca release from sarcoplasmic reticulum. In the presence of caffeine the rapid component of ICa-inactivation and Iti are abolished. (4) In addition to ICa inactivation and activation of Iti sarcoplasmic Ca release causes openings of a novel ion channel with large conductance (greater than 200 pS), the function of which is unknown. (5) The results are consistent with the concept of Cai-dependent inactivation of Ca current, which can be caused either by Ca-entry or by Ca-release from the SR. The transient inward current is likely to reflect a process of Ca-removal from the cell, namely Na-Ca exchange.

Calcium- and voltage-activated plateau currents of cardiac Purkinje fibers

We have used the two-microelectrode voltage-clamp technique to investigate the components of membrane current that contribute to the formation of the early part of the plateau phase of the action potential of calf cardiac Purkinje fibers. 3,4-Diaminopyridine (50 microM) reduced the net transient outward current elicited by depolarizations to potentials positive to -30 mV but had no consistent effect on contraction. We attribute this effect to the blockade of a voltage-activated transient potassium current component. Ryanodine (1 microM), an inhibitor of sarcoplasmic reticulum calcium release and intracellular calcium oscillations in Purkinje fibers (Sutko, J.L., and J.L. Kenyon. 1983. Journal of General Physiology. 82:385-404), had complex effects on membrane currents as it abolished phasic contractions. At early times during a depolarization (5-30 ms), ryanodine reduced the net outward current. We attribute this effect to the loss of a component of calcium-activated potassium current caused by the inhibition of sarcoplasmic reticulum calcium release and the intracellular calcium transient. At later times during a depolarization (50-200 ms), ryanodine increased the net outward current. This effect was not seen in low-sodium solutions and we could not observe a reversal potential over a voltage range of -100 to +75 mV. These data suggest that the effect of ryanodine on the late membrane current is attributable to the loss of sodium-calcium exchange current caused by the inhibition of sarcoplasmic reticulum calcium release and the intracellular calcium transient. Neither effect of ryanodine was dependent on chloride ions, which suggests that chloride ions do not carry the ryanodine-sensitive current components. Strontium (2.7 mM replacing calcium) and caffeine (10 mM), two other treatments that interfere with sarcoplasmic reticulum function, had effects in common with ryanodine. This supports the hypothesis that the effects of ryanodine may be attributed to the inhibition of sarcoplasmic reticulum calcium release.

Effects of ryanodine and caffeine on contractility, membrane voltage, and calcium exchange in cultured heart cells

To investigate the mechanisms of action of ryanodine and caffeine, changes in mechanical and electrical activity caused by these agents were correlated with alterations in 45Ca fluxes and cell Ca contents in chick embryo ventricular cell monolayer cultures. Ryanodine (10(-10)-10(-5) M) irreversibly decreased contraction amplitude by 10-70% relative to control in a concentration-dependent manner with minimal effects on electrical activity. Ryanodine caused a slight decrease in rapid 45Ca uptake, but no change in total exchangeable calcium content or rapid 45Ca efflux. Caffeine (1-20 mM) caused a transient (less than 10 seconds) 5-12% increase in contraction amplitude followed by a sustained 9-76% decrease in contraction amplitude and a 10 mV decrease in diastolic membrane voltage. Caffeine caused a decrease in rapid 45Ca uptake, a decrease in total exchangeable calcium content, and an increase in rapid 45Ca efflux. These results suggest that caffeine produces a decrease in sarcoplasmic reticulum (SR) Ca2+ uptake, and/or an increase in SR Ca2+ release that eventually depletes the SR of Ca2+, presumably accounting for the negative inotropic effect. The ryanodine effects on contraction are more difficult to account for solely in terms of alterations of transsarcolemmal Ca2+ fluxes and Ca2+ contents. Our data indicate an important role for the SR in excitation-contraction coupling in cultured chick embryo ventricular cells and suggest that SR Ca2+ is part of the rapidly exchanging Ca2+ compartment noted in 45Ca flux studies.

On the involvement of cyclic AMP and extracellular Ca2+ in the regulation of hormone release from rat pituitary tumour (GH3) cells in culture

Thyroliberin (TRH), dibutyryl cyclic AMP (db-cAMP), and 3-isobutyl-1-methylxanthine (MIX) had a stimulatory effect on prolactin (PRL) and growth hormone (GH) release from GH3 cells. Half-maximal and maximal effects were observed for TRH at 2.5 nM and 10 nM; for db-cAMP at 0.6 mM and 5 mM, respectively. MIX (0.1 mM-1 mM) induced a dose-dependent accumulation of cellular cyclic AMP, while the hormone release was already maximally stimulated at 0.1 mM MIX. The maximal effects on hormone release of TRH and db-cAMP, but not of TRH and MIX, were additive. The Ca2+ channel blockers Co2+ (5 mM) and verapamil (100 microM) and the Ca2+ chelator EGTA (4 mM) abolished the stimulatory effect of TRH (1 microM) on hormone release. Co2+ and verapamil, but not EGTA, inhibited the stimulatory effect of db-cAMP (5 mM) on hormone release. The inhibitory effects of Co2+ and verapamil on GH release were counteracted by the combination of TRH and db-cAMP. For PRL release Co2+, but not verapamil, was able to inhibit the combined action of TRH and db-cAMP. Co2+, verapamil, and EGTA eliminated the stimulatory effect of MIX (1 mM) on PRL release while only Co2+ and EGTA affected the GH release. Hormone release in the presence of MIX plus verapamil or EGTA, but not Co2+, was increased by TRH. The calmodulin antagonist trifluoperazine (TFP) at 30 microM inhibited basal hormone release and hormone release stimulated by TRH (1 microM), db-cAMP (5 mM), and MIX (1 mM). The Ca2+ ionophore A23187 (5 microM) had a stimulatory effect on basal hormone release which was abolished by 30 microM TFP.

Identification of Na-Ca exchange current in single cardiac myocytes

In cardiac muscle the exchange of intracellular Ca2+ for extracellular Na+ is an important transport mechanism for regulation of the intracellular free Ca2+ concentration [( Ca]i) and hence the contractile strength of the heart. Due to its stoichiometry of greater than or equal to 3:1 Na+/Ca2+ (refs 3,5), Na-Ca exchange is supposed to generate a current across the cell membrane. It is thought that such a current may contribute to cardiac action potential and physiological or pathological pacemaker activity. Although the occurrence of Na-Ca exchange is well documented, a membrane current generated by this transport has not been identified unequivocally. Previous attempts to detect such a current in multicellular preparations, for example, by measuring small current differences after varying the extracellular ionic composition, although providing evidence, did not rule out other possible interpretations. Here we demonstrate that a transient rise in [Ca]i caused by release of Ca from sarcoplasmic reticulum (SR) generates a membrane current in cardiac myocytes. The dependence of this current on the transmembrane gradients for Na+ and Ca2+ and on membrane potential meets the criteria for a current produced by electrogenic Na-Ca exchange. Cyclic activation of this current by release of Ca from the SR can cause maintained spontaneous activity, suggesting that Na-Ca exchange contributes to certain forms of cardiac pacemaking.

Frequency, amplitude, and propagation velocity of spontaneous Ca++-dependent contractile waves in intact adult rat cardiac muscle and isolated myocytes

Spontaneous contractile waves due to spontaneous calcium cycling by the sarcoplasmic reticulum occur in unstimulated bulk rat papillary muscle and single rat cardiac myocytes with intact sarcolemmal function. We used video analytic techniques to quantify the wave characteristics in both bulk muscle and myocytes; laser-light scattering techniques were also employed in muscle. In muscle bathed in physiological concentrations of calcium, the true periodicity of these waves was a fraction of 1 Hz and increased up to several hertz with increases in cell calcium. This was paralleled by an increase in the frequency of scattered laser light intensity fluctuations. In myocytes, a range of spontaneous contractile wave frequencies similar to that which occurred in the muscle was observed; it could be demonstrated that an increase in superfusate calcium concentrations (2-15 mM at 23 degrees C) increases the oscillation frequency but not amplitude. In both myocytes and muscle, low concentrations of caffeine (0.5 mM) and higher temperature increased the oscillation frequency but diminished their amplitude. However, the scattered light fluctuations did not change with temperature and decreased with caffeine. These results demonstrate that (1) the true frequency of spontaneous sarcoplasmic reticulum oscillations in the unstimulated rat muscle and single myocytes with intact sarcolemmal function is low, i.e., a fraction of a hertz; (2) with cell calcium loading, the oscillation frequency accelerates to those frequencies measured previously in the "calcium overload" state; (3) while scattered light fluctuations which sample myofilament motion are a sensitive, noninvasive method of detecting the oscillations in bulk muscle, they can be insensitive to the divergent changes in oscillation amplitude and frequency.

Paradoxical electromechanical effect of lanthanum ions in cardiac muscle cells

Although lanthanum ions (La+++) block calcium influx in cardiac cells, they may paradoxically accentuate the sodium-free contracture. We have therefore studied the effects of La+++ on the zero sodium response in chick embryonic myocardial cell aggregates. Zero sodium alone causes: (a) A maintained contracture; (b) Asynchronous localized contractions that are selectively inhibited by caffeine or ryanodine, and presumably reflect release of calcium from the sarcoplasmic reticulum; (c) A nonspecific conductance increase that is ascribable to calcium-activated ion channels. Addition of La+++ potentiates the sodium-free contracture, and causes similar potentiation of the localized contractions and the conductance increase. All three phenomena occur 5-10-fold faster in 1 mM La+++ than in sodium-free fluid alone. In contrast, when La+++ is combined with caffeine or ryanodine, the zero sodium response is suppressed. We conclude that the paradoxical effect of La+++ on the contracture is not due to calcium influx, but to enhancement, or disinhibition of intracellular calcium release. Relaxation of normal myocardium may involve control of spontaneous calcium release by lanthanum- and sodium-sensitive calcium transport across the surface membrane.

The effects of caffeine and ryanodine on the electrical activity of the canine coronary sinus

Cells of the coronary sinus of the canine heart can exhibit triggered activity which each action potential arises from a depolarizing after-potential that follows the previous action potential; an early after-hyperpolarization commonly precedes the delayed after-depolarization and both are increased in amplitude by the addition of noradrenaline. The delayed after-depolarization is thought to be caused by an inward current activated by a rise in intracellular Ca2+ that is, in turn, caused by Ca2+-induced release of Ca2+ from the sarcoplasmic reticulum (s.r.). The effects of caffeine and of ryanodine on the electrical activity of the coronary sinus were investigated because each of those agents is thought to affect the handling of intracellular Ca2+ by the s.r. The steady-state effect of exposure to 5 mM-caffeine is to cause the delayed after-depolarization to move much earlier in the cycle, and become too small to give rise to an action potential so that preparations cannot show triggered activity; moreover, if a burst of activity is in progress it is terminated by exposure to 5 mM-caffeine. Exposure to 0.5 mM-caffeine causes the delayed after-depolarization to move earlier in the cycle but to become larger so that triggered activity is more easily induced and longer lasting than in the absence of caffeine. Shortly after the addition (or wash-out) of 5 mM-caffeine the after-depolarization transiently resembles that seen in the presence of 0.5 mM-caffeine so that bursts of triggered activity may occur just after the addition or removal of 5 mM-caffeine. Exposure to 5 mM-caffeine abolishes early rapid repolarization (phase 1), shifts the plateau to a more positive level and retards the completion of repolarization. The effect on phase 1 is mimicked by exposure to solutions low in Cl-; the effect on the plateau is mimicked by exposure to 20 mM-tetraethylammonium (TEA); fibres exposed to solutions containing 20 mM-TEA and 21 mM-Cl- show action potentials very like those of fibres exposed to 5 mM-caffeine. If a fibre already exposed to a low Cl-, TEA-containing solution is then exposed to 5 mM-caffeine, no further change occurs in the action potential but the characteristic effects of caffeine on the after-depolarization appear. Exposure to ryanodine prevents the appearance of the delayed after-depolarization but leads to the appearance of an exceptionally long depolarizing after-potential that begins very early in diastole and, though waning, persists almost throughout diastole.(ABSTRACT TRUNCATED AT 400 WORDS)