Single canine cardiac ryanodine receptor channels were incorporated into planar lipid bilayers. Single-channel currents were sampled at 1–5 kHz and filtered at 0.2–1.0 kHz. Channel incorporations were obtained in symmetrical solutions (20 mM HEPES-Tris, pH 7.4, and pCa 5). Unitary Ca2+ currents were monitored when 2–30 mM Ca2+ was added to the lumenal side of the channel. The relationship between the amplitude of unitary Ca2+ current (at 0 mV holding potential) and lumenal [Ca2+] was hyperbolic and saturated at ∼4 pA. This relationship was then defined in the presence of different symmetrical CsCH3SO3 concentrations (5, 50, and 150 mM). Under these conditions, unitary current amplitude was 1.2 ± 0.1, 0.65 ± 0.1, and 0.35 ± 0.1 pA in 2 mM lumenal Ca2+; and 3.3 ± 0.4, 2.4 ± 0.2, and 1.63 ± 0.2 pA in 10 mM lumenal Ca2+ (n > 6). Unitary Ca2+ current was also defined in the presence of symmetrical [Mg2+] (1 mM) and low [Cs+] (5 mM). Under these conditions, unitary Ca2+ current in 2 and 10 mM lumenal Ca2+ was 0.66 ± 0.1 and 1.52 ± 0.06 pA, respectively. In the presence of higher symmetrical [Cs+] (50 mM), Mg2+ (1 mM), and lumenal [Ca2+] (10 mM), unitary Ca2+ current exhibited an amplitude of 0.9 ± 0.2 pA (n = 3). This result indicates that the actions of Cs+ and Mg2+ on unitary Ca2+ current were additive. These data demonstrate that physiological levels of monovalent cation and Mg2+ effectively compete with Ca2+ as charge carrier in cardiac ryanodine receptor channels. If lumenal free Ca2+ is 2 mM, then our results indicate that unitary Ca2+ current under physiological conditions should be <0.6 pA.

The mitochondrial ATP-sensitive K ؉ (mitoKATP) channel plays a central role in protection of cardiac and neuronal cells against ischemia and apoptosis, but its molecular structure is unknown. Succinate dehydrogenase (SDH) is inhibited by mitoK ATP activators, fueling the contrary view that SDH, rather than mitoK ATP, is the target of cardioprotective drugs. Here, we report that SDH forms part of mitoK ATP functionally and structurally. Four mitochondrial proteins [mitochondrial ATP-binding cassette protein 1 (mABC1), phosphate carrier, adenine nucleotide translocator, and ATP synthase] associate with SDH. A purified IM fraction containing these proteins was reconstituted into proteoliposomes and lipid bilayers and shown to confer mitoK ATP channel activity. This channel activity is sensitive not only to mitoK ATP activators and blockers but also to SDH inhibitors. These results reconcile the controversy over the basis of ischemic preconditioning by demonstrating that SDH is a component of mitoK ATP as part of a macromolecular supercomplex. The findings also provide a tangible clue as to the structural basis of mitoKATP channels.A lthough prolonged ischemia results in severe damage to myocardial and neuronal cells and ultimately cell death, brief ischemic episodes can recruit a mechanism that can protect against future ischemic insults. The identification of this endogenous autoprotective mechanism, known as ischemic preconditioning (IPC), has sparked considerable interest given its potential to mitigate cellular injury in heart attack and stroke (1-3). The molecular basis of IPC remains unclear. Recent evidence suggests that the mitochondrial ATP-sensitive K ϩ channel (mitoK ATP ) is a key player in the process of IPC and is an inhibitor of apoptosis.The mitoK ATP channel was first identified in 1991 from single-channel recordings of the mitochondrial inner membrane (IM) (4). Subsequent studies (5, 6) revealed that the antihypertensive agent diazoxide is an agonist of mitoK ATP channels and protects against cardiac cell death by a mechanism like that of IPC but without the need for conditioning ischemia. These observations led to the notion that mitoK ATP channels protect against ischemia by inhibiting apoptosis and necrosis, possibly as a consequence of altered mitochondrial K ϩ or Ca 2ϩ homeostasis (7-9). Nevertheless, the existence of mitoK ATP channels has been questioned (10, 11). Inhibitors of the Krebs cycle enzyme succinate dehydrogenase (SDH) have been shown to mimic the process of IPC (12). Furthermore, some drugs that activate mitoK ATP channels also inhibit SDH (13), leading to the alternative hypothesis that respiratory inhibition, rather than channel activity, underlies IPC (10). The controversy has been fostered by the lack of a molecular identity for mitoK ATP channels. Such ignorance has also stymied rational drug development of compounds to prevent or delay ischemic injury.Given the functional and pharmacological overlap between SDH and mitoK ATP channels, we hypothesized that SDH either intera...

The glycolytic product pyruvate has beneficial effects on cardiac contractile function. The postulated cellular mechanisms underlying the positive inotropic effect of pyruvate, however, are contradictory or have remained elusive. Therefore, we studied the effects of pyruvate on cardiac Ca 2+ regulation, intracellular pH (pH i ) and flavoprotein oxidation using fluorescence confocal microscopy in intact and permeabilized rat ventricular myocytes and single channel recordings from rat cardiac ryanodine receptors (

Age-dependent changes in the architecture of the sinus node comprise an increasing ratio between fibroblasts and cardiomyocytes. This change is discussed as a potential mechanism for sinus node disease. The goal of this study was to determine the mechanism through which non-excitable cells influence the spontaneous activity of multicellular cardiomyocyte preparations. Cardiomyocyte monolayers (HL-1 cells) or embryonic stem cell-derived cardiomyocytes were used as two-and three-dimensional cardiac pacemaker models. Spontaneous activity and conduction velocity (θ) were monitored by field potential measurements with microelectrode arrays (MEAs). The influence of fibroblasts (WT-fibs) was determined in heterocellular cultures of different cardiomyocyte and fibroblast ratios. The relevance of heterocellular gap junctional coupling was evaluated by the use of fibroblasts deficient for the expression of Cx43 (Cx43 −/− -fibs). The beating frequency and θ of heterocellular cultures depended negatively on the fibroblast concentration. Interspersion of fibroblasts in cardiomyocyte monolayers increased the coefficient of the interbeat interval variability. Whereas Cx43−/− -fibs decreased θ significantly less than WT-fibs, their effect on the beating frequency and the beat-to-beat variability seemed largely independent of their ability to establish intercellular coupling. These results suggest that electrically integrated, non-excitable cells modulate the excitability of cardiac pacemaker preparations by two distinct mechanisms, one dependent and the other independent of the heterocellular coupling established. Whereas heterocellular coupling enables the fibroblast to depolarize the cardiomyocytes or to act as a current sink, the mere physical separation of the cardiomyocytes by fibroblasts induces bradycardia through a reduction in frequency entrainment.

Postnatal maturation of the rat heart is characterized by major changes in the mechanism of excitation-contraction (E-C) coupling. In the neonate, the t tubules and sarcoplasmic reticulum (SR) are not fully developed yet. Consequently, Ca(2+)-induced Ca(2+) release (CICR) does not play a central role in E-C coupling. In the neonate, most of the Ca(2+) that triggers contraction comes through the sarcolemma. In this work, we defined the contribution of the sarcolemmal Ca(2+) entry and the Ca(2+) released from the SR to the Ca(2+) transient during the first 3 wk of postnatal development. To this end, intracellular Ca(2+) transients were measured in whole hearts from neonate rats by using the pulsed local field fluorescence technique. To estimate the contribution of each Ca(2+) flux to the global intracellular Ca(2+) transient, different pharmacological agents were used. Ryanodine was applied to evaluate ryanodine receptor-mediated Ca(2+) release from the SR, nifedipine for dihydropyridine-sensitive L-type Ca(2+) current, Ni(2+) for the current resulting from the reverse-mode Na(+)/Ca(2+) exchange, and mibefradil for the T-type Ca(2+) current. Our results showed that the relative contribution of each Ca(2+) flux changes considerably during the first 3 wk of postnatal development. Early after birth (1-5 days), the sarcolemmal Ca(2+) flux predominates, whereas at 3 wk of age, CICR from the SR is the most important. This transition may reflect the progressive development of the t tube-SR units characteristic of mature myocytes. We have hence directly defined in the whole beating heart the developmental changes of E-C coupling previously evaluated in single (acutely isolated or cultured) cells and multicellular preparations.

In cardiac research, single-cell experimental models have been extensively used to study the molecular mechanisms of intracellular Ca 2+ homeostasis. The results of these studies are usually extrapolated to the tissue level assuming that the phenomena studied at the cellular level are either similar in the intact organ, or only slightly modified by variables that exist at the whole-heart level. The validity of these assumptions has rarely been confirmed experimentally. Common obstacles associated with the study of intracellular Ca 2+ signals in beating hearts include motion artifacts and spatio-temporal limitations of the recording system. In this work, action potentials and intracellular Ca 2+ signals were measured in beating hearts from young rats, with spatio-temporal resolutions similar to cellular studies using a novel pulsed local-field fluorescence technique. This method was based on maximizing emitted fluorescence to increase the signal-to-noise ratio (S/N). The fluorescence emission of the indicator molecules was synchronized with brief (<1 ns), high-power (400 W) laser pulses, and the common mode noise of the fluorescence signal was differentially cancelled. To follow rapidly evolving signals, a highly sensitive and fast detection system was used (10 kHz). The spatial resolution was improved using a small (50-200 m diameter) multimode fiberoptic.Mechanical artifacts were effectively reduced by inserting the fiberoptic into a "floating" glass micropipette sealed to the heart wall with negative pressure. Our results demonstrate that local-field fluorescence microscopy offers an outstanding experimental approach for studying physiological signals at the whole-organ level with the high spatio-temporal resolution common to normal cellular approaches.

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A B S T R A C T Single-channel properties of dihydropyridine (DHP)-sensitive calcium channels isolated from transverse tubular (F-tube) membrane of skeletal muscle were explored. Single-channel activity was recorded in planar lipid bilayers after fusion of highly purified rabbit T-tube microsomes. Two populations of DHPsensitive calcium channels were identified. One type of channel (noninactivating) was active (2 I~M +-Bay K 8644) at steady-state membrane potentials and has been studied in other laboratories. The second type of channel (inactivating) was transiently activated during voltage pulses and had a very low open probability (Po) at steady-state membrane potentials. Inactivating channel activity was observed in 47.3% of the experiments (n = 84 bilayers). The nonstationary kinetics of this channel was determined using a standard voltage pulse (HP = -50 mV, pulse to 0 mV). The time constant (~) of channel activation was 23 ms. During the pulse, channel activity decayed (inactivated) with a • of 3.7 s. Noninactivating singlechannel activity was well described by a model with two open and two closed states. Inactivating channel activity was described by the same model with the addition of an inactivated state as proposed for cardiac muscle. The single-channel properties were compared with the kinetics of DHP-sensitive inward calcium currents (lc~) measured at the cellular level. Our results support the hypothesis that voltagedependent inactivation of single DHP-sensitive channels contributes to the decay of I~..