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Although the heart rapidly adapts cardiac output to match the body's circulatory demands, the regulatory mechanisms ensuring that sufficient ATP is available to perform the required cardiac work are not completely understood. Two mechanisms have been suggested to serve as key regulators: (1) ADP and Pi concentrations--ATP utilization/hydrolysis in the cytosol increases ADP and Pi fluxes to mitochondria and hence the amount of available substrates for ATP production increases; and (2) Ca2+ concentration--ATP utilization/hydrolysis is coupled to changes in free cytosolic calcium and mitochondrial calcium, the latter controlling Ca2+-dependent activation of mitochondrial enzymes taking part in ATP production. Here we discuss the evolving perspectives of each of the putative regulatory mechanisms and the precise molecular targets (dehydrogenase enzymes, ATP synthase) based on existing experimental and theoretical evidence. The data synthesis can generate novel hypotheses and experimental designs to solve the ongoing enigma of energy supply-demand matching in the heart.

The 'mitochondrial permeability transition', characterized by a sudden induced change of the inner mitochondrial membrane permeability for water as well as for small substances (/=1.5 kDa), has been known for three decades. Research interest in the entity responsible for this phenomenon, the 'mitochondrial permeability transition pore' (mPTP), has dramatically increased after demonstration that it plays a key role in the life and death decision in cells. Therefore, a better understanding of this phenomenon and its regulation by environmental stresses, kinase signalling, and pharmacological intervention is vital. The characterization of the molecular identity of the mPTP will allow identification of possible pharmacological targets and assist in drug design for its precise regulation. However, despite extensive research efforts, at this point the pore-forming core component(s) of the mPTP remain unidentified. Pivotal new genetic evidence has shown that components once believed to be core elements of the mPTP (namely mitochondrial adenine nucleotide translocator and cyclophilin D) are instead only mPTP regulators (or in the case of voltage-dependent anion channels, probably entirely dispensable). This review provides an update on the current state of knowledge regarding the regulation of the mPTP.

The mitochondrial permeability transition (MPT) pore complex is a key participant in the machinery that controls mitochondrial fate and, consequently, cell fate. The quest for the pore identity has been ongoing for several decades and yet the main structure remains unknown. Established "dogma" proposes that the core of the MPT pore is composed of an association of voltage-dependent anion channel (VDAC) and adenine nucleotide translocase (ANT). Recent genetic knockout experiments contradict this commonly accepted interpretation and provide a basis for substantial revision of the MPT pore identity. There is now sufficient evidence to exclude VDAC and ANT as the main pore structural components. Regarding MPT pore regulation, the role of cyclophilin D is confirmed and ANT may still serve some regulatory function, although the involvement of hexokinase II and creatine kinase remains unresolved. When cell protection signaling pathways are activated, we have found that the Bcl-2 family members relay the signal from glycogen synthase kinase-3 beta onto a target at or in close proximity to the pore. Our experimental findings in intact cardiac myocytes and neurons indicate that the current "dogma" related to the role of Ca2+ in MPT induction requires reevaluation. Emerging evidence suggests that after injury-producing stresses, reactive oxygen species (but not Ca2+) are largely responsible for the pore induction. In this article we discuss the current state of knowledge and provide new data related to the MPT pore structure and regulation.

The inward rectifier current I(K1) is tightly regulated regionally within the heart, downregulated in heart failure, and genetically suppressed in Andersen syndrome. We used in vivo viral gene transfer to dissect the role of I(K1) in cardiac repolarization and maintenance of the resting membrane potential (RMP) in guinea pig ventricular myocytes. Kir2.1 overexpression boosted Ba(2+)-sensitive I(K1) by more than 100% (at -50mV), significantly shortened action potential durations (APDs), accelerated phase 3 repolarization, and hyperpolarized RMP compared with control cells (nongreen cells from the same hearts and green cells from GFP-transduced hearts). The dominant-negative Kir2.1AAA reduced I(K1) by 50-90%; those cells with less than 80% reduction of I(K1) exhibited prolonged APDs, decelerated phase 3 repolarization, and depolarization of the RMP. Further reduction of I(K1) resulted in a pacemaker phenotype, as previously described. ECGs revealed a 7.7% +/- 0.9% shortening of the heart rate-corrected QT interval (QTc interval) in Kir2.1-transduced animals (n = 4) and a 16.7% +/- 1.8% prolongation of the QTc interval (n = 3) in Kir2.1AAA-transduced animals 72 hours after gene delivery compared with immediate postoperative recordings. Thus, I(K1) is essential for establishing the distinctive electrical phenotype of the ventricular myocyte: rapid terminal repolarization to a stable and polarized resting potential. Additionally, the long-QT phenotype seen in Andersen syndrome is a direct consequence of dominant-negative suppression of Kir2 channel function.

Abstract —The cardiac delayed rectifier potassium current mediates repolarization of the action potential and underlies the QT interval of the ECG. Mutations in either of the two molecular components of the rapid delayed rectifier ( I K,r ), HERG and KCNE2, have been linked to heritable or acquired long-QT syndrome. Mechanisms whereby mutations of KCNE2 produce fatal cardiac arrhythmias characteristic of long-QT syndrome remain unclear. In this study, we characterize functional interactions between HERG and KCNE2 with a view to defining underlying mechanisms for action potential prolongation and long-QT syndrome. Whereas coexpression of hKCNE2 with HERG alters both kinetics and density of ionic current, incorporation of these effects into a quantitative model of the action potential predicts that only changes in current density significantly affect repolarization. Thus, the primary functional consequence of hKCNE2 on action potential morphology is through modulation of I K,r density, as predicted by the model. Mutations associated with long-QT syndrome that result only in modest changes of gating kinetics may be epiphenomena or may modulate action potential repolarization via interaction with alternative pore-forming potassium channel α subunits.

Abstract Membrane current abnormalities have been described in human heart failure. To determine whether similar current changes are observed in a large animal model of heart failure, we studied dogs with pacing-induced cardiomyopathy. Myocytes isolated from the midmyocardium of 13 dogs with heart failure induced by 3 to 4 weeks of rapid ventricular pacing and from 16 nonpaced control dogs did not differ in cell surface area or resting membrane potential. Nevertheless, action potential duration (APD) was significantly prolonged in myocytes isolated from failing ventricles (APD at 90% repolarization, 1097±73 milliseconds [failing hearts, n=30] versus 842±56 milliseconds [control hearts, n=25]; P 2+ current and whole-cell Na + current did not differ in cells from failing and from control hearts, but significant differences in K + currents were observed. The density of the inward rectifier K + current (I K1 ) was reduced in cells from failing hearts at test potentials below −90 mV (at −150 mV, −19.1±2.2 pA/pF [failing hearts, n=18] versus −32.2±5.1 pA/pF [control hearts, n=15]; P K1 was also reduced in cells from failing hearts (at −60 mV, 1.7±0.2 pA/pF [failing hearts] versus 2.5±0.2 pA/pF [control hearts]; P 2+ -independent transient outward current (I to ) was dramatically reduced in myocytes isolated from failing hearts compared with nonfailing control hearts (at +80 mV, 7.0±0.9 pA/pF [failing hearts, n=20] versus 20.4±3.2 pA/pF [control hearts, n=15]; P to kinetics or single-channel conductance. A reduction in the number of functional I to channels was demonstrated by nonstationary fluctuation analysis (0.4±0.03 channels per square micrometer [failing hearts, n=5] versus 1.2±0.1 channels per square micrometer [control hearts, n=3]; P to by 4-aminopyridine in control myocytes decreased the notch amplitude and prolonged the APD. Current clamp–release experiments in which current was injected for 8 milliseconds to reproduce the notch sufficed to shorten the APD significantly in cells from failing hearts. These data support the hypothesis that downregulation of I to in pacing-induced heart failure is at least partially responsible for the action potential prolongation. Because the repolarization abnormalities mimic those in cells isolated from failing human ventricular myocardium, canine pacing-induced cardiomyopathy may provide insights into the development of repolarization abnormalities and the mechanisms of sudden death in patients with heart failure.

The pacemaker cells of the heart initiate the heartbeat, sustain the circulation, and dictate the rate and rhythm of cardiac contraction. Circulatory collapse ensues when these specialized cells are damaged by disease, a situation that currently necessitates the implantation of an electronic pacemaker. Here we report the use of viral gene transfer to convert quiescent heart-muscle cells into pacemaker cells, and the successful generation of spontaneous, rhythmic electrical activity in the ventricle in vivo. Our results indicate that genetically engineered pacemakers could be developed as a possible alternative to implantable electronic devices.