Currents through ionic channels in multicellular cardiac tissue and single heart cells

Ionic channels are elementary excitable elements in the cell membranes of heart and other tissues. They produce and transduce electrical signals. After decades of trouble with quantitative interpretation of voltage-clamp data from multicellular heart tissue, due to its morphological complexness and methodological limitations, cardiac electrophysiologists have developed new techniques for better control of membrane potential and of the ionic and metabolic environment on both sides of the plasma membrane, by the use of single heart cells. Direct recordings of the behavior of single ionic channels have become possible by using the patch-clamp technique, which was developed simultaneously. Biochemists have made excellent progress in purifying and characterizing ionic channel proteins, and there has been initial success in reconstituting some partially purified channels into lipid bilayers, where their function can be studied.     

Identification of swelling-activated Cl<Superscript>-</Superscript> current in rabbit cardiac Purkinje cells

The presence and functional role of the swelling-activated Cl^- current (I_Cl(swell)) in rabbit cardiac Purkinje cells was examined using patch-clamp methodology. Extracellular hypotonicity (210 or 135 mOsm) activated an outwardly rectifying, time-independent current with a reversal potential close to the calculated Cl^- equilibrium potential (E_Cl). The magnitude of this current was related to tonicity of the superfusate. The current was blocked by 0.5 mM 4,4-diisothiocyanostilbene-2,2-disulfonic acid (DIDS). These features are comparable to those of I_Cl(swell) found in sinoatrial nodal, atrial, and ventricular myocytes. I_Cl(swell) activation at 210 and 135 mOsm depolarized the resting membrane potential with 6 and 10 mV and shortened the action potential by 18 and 33%, respectively. DIDS partially reversed I_Cl(swell)-induced action potential changes. We conclude that I_Cl(swell) is present in Purkinje cells and its activation leads to action potential shortening and resting membrane potential depolarization, both of which can promote the development of reentrant arrhythmias.Received 20 January 2004; received after revision 17 February 2004; accepted 25 February 2004     

Gamma-aminobutyric acid in Purkinje cells and motoneurones

Zusammenfassung Enzymatische Messungen von Gamma-Aminobuttersäure (GABA) ergaben in den isolierten Purkinje-Zellen der Katze eine viermal höhere GABA-Konzentration als in den Motoneuronen.     

Electrogenic hyperpolarization in canine cardiac Purkinje fibres exposed to calcium ionophores

Summary The Ca ionophores markedly enhance the increase of intracellular Ca occurring during Na-free perfusion and the hyperpolarization observed upon Na readmission may be due to rapid restoration of intracellular Na and resultant stimulation of both electrogenic sodium and calcium efflux.This work was supported by a grant-in-aid and a Senior Investigatorship from the New York Heart Association.     

Electrogenic hyperpolarization in canine cardiac Purkinje fibres exposed to calcium ionophores

The Ca ionophores markedly enhance the increase of intracellular Ca occurring during Na-free perfusion and the hyperpolarization observed upon Na readmission may be due to rapid restoration of intracellular Na and resultant stimulation of both electrogenic sodium and calcium efflux.     

The electrophysiological effects of ionophore X-537A on cardiac Purkinje fibres

Summary Using classical microelectrode techniques in canine cardiac Purkinje fibres, calcium ionophore X-537A was shown to shorten the action potential, hyperpolarizing the membrane and lowering the plateau, suggesting that intracellular calcium controls membrane permeability to potassium in this preparation.This work was supported by grant No. HL 05726 from the U.S. Public Health Service, National Heart Institute, Bethesda, Md.I thank Dr.N. Krasnow for his valuable advice in carrying out this work.     

Potassium currents in cardiac cells

The kinetic properties of the inwardly rectifying K current and the transient outward current in cardiac cells were investigated. In sheep Purkinje fibers superfused with Na-free K-free solution, time-dependent changes in the conductance of the inward rectifier are described. In patch clamp experiments the inward rectifier inactivates during hyperpolarization, as can be seen by a decrease in the open state probability. Using whole cell clamp on ventricular myocytes it is demonstrated that the inactivation during hyperpolarization is due to blocking of the channel by external Na, Mg and Ca. The channels responsible for the transient outward current in cow, sheep and rabbit Purkinje fibers are identified using single channel recording. It is demonstrated that in all three preparations the channels are K-selective. The channel in cow Purkinje cells has a large conductance and is regulated by voltage and internal Ca concentration. The channels identified in the sheep and rabbit cells have a much smaller conductance.     

Sodium and calcium action potentials in human anterior pituitary cells

Summary Human anterior pituitary cells derived from a somatotropin-secreting adenoma were capable of generating action potentials with Ca²⁺ and tetrodotoxin-sensitive Na⁺ components. A major fraction of the action current was carried by Na ions.We wish to thank Dr O. Sand for correcting the English and Miss Michiko Takano for technical assistance. We also acknowledge the collaboration of Dr S. Miyazaki in the electron microscope.     

Are stretch-sensitive channels in molluscan cells and elsewhere physiological mechanotransducers?

Single-channel recordings of dozens of cell types, including invertebrate (molluscan) and vertebrate heart cells, reveal stretch-sensitive ion channels. The physiological roles of these channels are undoubtedly diverse but it is usually assumed that the roles they play are related to the channels' mechanosensitive gating. Whether this assumption is valid remains to be seen. Attempts to connect the single-channel observations with the mechanical aspects of physiological or developmental processes are discussed. In the case of molluscan cells, recent work suggests that their stretch channels have physiological functions unrelated to mechanosensitive gating.     

Are stretch-sensitive channels in molluscan cells and elsewhere physiological mechanotransducers?

Single-channel recordings of dozens of cell types, including invertebrate (molluscan) and vertebrate heart cells, reveal stretch-sensitive ion channels. The physiological roles of these channels are undoubtedly diverse but it is usually assumed that the roles they play are related to the channels' mechanosensitive gating. Whether this assumption is valid remains to be seen. Attempts to connect the single-channel observations with the mechanical aspects of physiological or developmental processes are discussed. In the case of molluscan cells, recent work suggests that their stretch channels have physiological functions unrelated to mechanosensitive gating.     

Role of granular cells in the placement and distribution of synapses between ascending fibers and Purkinje cells of the rat cerebellum]

The study of rats irradiated during development shows that the establishment of the one-to-one relationship in the adult between climbing fibers and Purkinje cells which follows the immature multiply innervated stage mainly depends on granule cells formed before day 8. Climbing fibers ending upon a multiply innervated cell are intermingled on the same dendritic segments.     

Induced pluripotent stem cells for cardiac repair

Myocardial stem cell therapies are emerging as novel therapeutic paradigms for myocardial repair, but are hampered by the lack of sources for autologous human cardiomyocytes. An exciting development in the field of cardiovascular regenerative medicine is the ability to reprogram adult somatic cells into pluripotent stem cell lines (induced pluripotent stem cells, iPSCs) and to coax their differentiation into functional cardiomyocytes. This technology holds great promise for the emerging disciplines of personalized and regenerative medicine, because of the ability to derive patient-specific iPSCs that could potentially elude the immune system. The current review describes the latest techniques of generating iPSCs as well as the methods used to direct their differentiation towards the cardiac lineage. We then detail the unique potential as well as the possible hurdles on the road to clinical utilizing of the iPSCs derived cardiomyocytes in the emerging field of cardiovascular regenerative medicine.     

Selective representation of vibrissa at the level of cerebellar Purkinje cells by way of climbing fibers in rats]

In the Rat peripheral information from snout vibrissae are transmitted to cerebellar Purkinje cells of Crus I and II by both mossy and climbing fibers. The climbing fiber pathway is organized in a very precise manner, a particular Purkinje cell being preferentially activated by a single vibrissa.     

HCN channels contribute to the intrinsic activity of cochlear pyramidal cells

A hyperpolarization-activated current recorded from the pyramidal cells of the dorsal cochlear nucleus was investigated in the present study by using 150- to 200-m-thick brain slices prepared from 6- to 14-day-old Wistar rats. The pyramidal neurones exhibited a slowly activating inward current on hyperpolarization. The reversal potential of this component was –32 ± 3 mV (mean ± SE, n = 6), while its half-activation voltage was –99 ± 1 mV with a slope factor of 10.9 ± 0.4 mV (n = 27). This current was highly sensitive to the extracellular application of both 1 mM Cs⁺ and 10 M ZD7288. The electrophysiological properties and the pharmacological sensitivity of this current indicated that it corresponded to a hyperpolarization-activated non-specific cationic current (I_h). Our experiments showed that there was a correlation between the availability of the h-current and the spontaneous activity of the pyramidal cells, suggesting that this conductance acts as a pacemaker current in these neurones. Immunocytochemical experiments were also conducted on freshly isolated pyramidal cells to demonstrate the possible subunit composition of the channels responsible for the genesis of the pyramidal h-current. These investigations indicated the presence of HCN1, HCN2 and HCN4 subunits in the pyramidal cells.Received 15 May 2003; received after revision 26 June 2003; accepted 21 July 2003     

Localization of cholesterol in the Golgi apparatus of cardiac muscle cells

Summary Filipin (a polyene) interacts with cholesterol in membranes, producing distinctive deformations that can be detected by freeze-fracture. The distribution of filipin-induced deformations in the Golgi apparatus of cardiac myocytes suggests a role for this organelle in the transformation of cholesterol-poor membrane to cholesterol-rich membrane.Acknowledgments. This work was supported by a grant (No. 779) from the British Heart Foundation. I thank Dr E. Massey and Mr A. Slade for their help. Filipin was generously donated by Upjohn Ltd, Crawley, Sussex, U.K.