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CLASSICAL PERSPECTIVES |
1 Department of Physiology, University of Bern, Bühlplatz 5, CH-3012 Bern, Switzerland
Email: niggli{at}pyl.unibe.ch
What are the requirements to create a breakthrough in science? The late Silvio Weidmann, who sadly passed away on 11 July 2005, used to contemplate: ‘All it takes is the right person, at the right place, at the right time’. These three circumstances certainly came together when Silvio Weidmann himself spent a postdoctoral fellowship in the laboratory of A. L. Hodgkin and A. F. Huxley in Cambridge from 1948 to 1950, during the enthralling times of innovation and discovery of the basics of electrophysiology in squid axon. Inspired by this experience, and after returning to the Department of Physiology in Bern, Silvio continued his seminal studies on cardiac muscle. In the following years, much of Weidmann's research efforts focused on the understanding of the electrical phenomena and membrane currents of cardiac muscle, including the propagation of electrical signals across the entire heart. Many of his influential publications have had a long-lasting and far-reaching impact and still stand out as monuments. Several of them initiating entire research fields on their own, sprouting from pure cellular cardiac electrophysiology. Quite often these landmark papers not only made significant contributions to our understanding of cardiac function, but also reported major advances in instrumentation and the developments of new methods. In this short article, we will highlight two out of these many classical papers, and try to put them into perspective regarding their impact and relevance to ongoing research.
Before we start, let us have a look at the classical figure below, which shows a reproduction of one of the very early recordings of a canine cardiac action potential, captured with a photo camera from the screen of an oscilloscope, and published in Draper & Weidmann (1951). In this study, the resting membrane potential of cardiac muscle was carefully evaluated and found to be around –90 mV. Furthermore, the recorded action potentials were analysed for their amplitude and time course, comprising the now well known rapid upstroke, an initial overshooting spike, followed by a plateau that was terminated by a phase of repolarization. The propagation velocity of the electrical excitation was determined with two electrodes impaled. Subsequent to these early recordings and descriptions of the cardiac action potential, a thorough investigation of the sodium carrying system was performed.
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In the first paper to be highlighted, Silvio Weidmann examined the sodium current as the system underlying action potential depolarization and propagation of electrical excitation in cardiac tissue (Weidmann, 1955). In this elegant study, he assessed the voltage-dependent inactivation of the Na+ current many years before voltage-clamping approaches were, partly by himself, developed for application in cardiac muscle. By injecting different currents with the help of an ingenious electronic circuit, the resting potential could be chosen and ‘clamped’ at will. The upstroke velocity and the size of the voltage ‘overshoot’ were then used to derive Na+ current amplitudes under the respective voltage conditions. Taken together, these results allowed an early characterization of the voltage-dependent inactivation of the cardiac Na+ current, or the ‘Na+ carrying system’ as Weidmann carefully termed it at a time when Na+ channels had not yet been identified. However, to characterize the biophysics of Na+ current in heart cells remained a challenge for many years to come. Even with the introduction of single cardiac myocyte preparations and whole-cell configuration of the patch-clamp technique, cardiac Na+ currents have been notoriously challenging to investigate, due to their sheer amplitude which causes large voltage errors, resulting from the series resistance inherent in single-electrode voltage-clamp and patch-clamp approaches. Significant progress has been made in the meantime by the introduction of a low access-resistance electrode, pioneered by the laboratory of Harry Fozzard (Makielski et al. 1987), who had previously spent sabbaticals with Silvio Weidmann in Bern. Nowadays, many biophysical features of the Na+ channels are well described, and research on the Na+ current has evolved into a large enterprise investigating its role in conduction of cardiac electrical excitation, and, related to that, examining the pathophysiological consequences of inherited Na+ channel channelopathies and the associated arrhythmias, coupled with specific variants of long QT syndrome (Liu et al. 2003).
Silvio Weidmann was also concerned with the passive electrical properties of cardiac tissue: ‘A knowledge of the myoplasm resistance and the membrane resistance and capacity is important in discussions of cardiac excitation and conduction’. With Purkinje fibres of kid (young goat) heart he performed cable analyses using microelectrodes and multiple impalements (Weidmann, 1952). The findings were that the electrical length constant, 
, is much larger than the cell length and the internal longitudinal resistance (myoplasm in series with cell–cell contact) is much smaller than the membrane resistance. Hence, he reasoned that the resistance between neighbouring cells is low. At that time, cardiac muscle was thought to be a functional syncytium, a view inferred from arguments on ‘healing over’ (Engelmann, 1875). Silvio Weidmann modestly declared: ‘I just put Engelmann's concept on a quantitative basis’. In reality, he laid down the basis for what later on was called gap junction channels, at a time when no structural evidence was available.
Shortly thereafter, a communication from another lab came as a surprise (Sjöstrand & Andersson, 1954). Using high resolution electron microscopy, these authors demonstrated the existence of cardiac muscle cells morphologically distinct from each other. Was the histology in this tissue different from Purkinje fibres? To answer this question, Silvio Weidmann set out to study in a bundle of muscle cells the movement of potassium, the ion most important as electrical charge carrier. For this he developed a unique method which enabled measurement of the intercellular movement of radioactive potassium. A muscle strand of sheep heart was placed in a two-compartment chamber, one compartment being perfused with 42K+-Tyrode solution, the other one with normal Tyrode solution. After several hours, when steady-state diffusion was reached, the bundle was frozen, sliced and assayed for radioactivity. The analysis of the 42K+ distribution yielded a diffusion coefficient three times lower than that in aqueous solution. An additional quantitative analysis of the data was carried out by A. L. Hodgkin, who added an appendix to the paper. He used a mathematical model of K+ diffusion to support the conclusion that the permeability of the intercalated disk to 42K+ is 5000 times greater than the permeability of the cell membrane. These results implied that the cell–cell resistance is low.
Preliminary diffusion data were communicated early (Weidmann, 1960), and the comprehensive study was published in 1966 (Weidmann, 1966). This paper is a masterpiece for several reasons: the physiological concept is basic, the methodological approach unique, the technical skills demanding, the analysis tools extraordinary, the interpretation of data proper and the impact on future research significant. Reading this paper is highly recommended, and not only to young electrophysiologists.
Analysis of diffusion data at that time was complex and time consuming. In this situation, Silvio Weidmann eluded to the following solutions. On the one hand, he built himself electrical analogue circuits, i.e. networks of resistors and capacitors, to mimic the longitudinal and radial diffusion pathway. Gauging voltages, he succeeded in simulating the diffusion data in the non-steady-state and steady-state conditions and thus extract relevant parameters. On the other hand, for a mathematical treatment of the diffusion process he consulted A. L. Hodgkin, his previous mentor. This resulted in an appendix to the 1966 paper.
Until 1960, low resistance cell-to-cell contacts were assumed to occur solely in heart and smooth muscle. Today, we know that gap junctions exist in all tissues of the human body, with the exception of skeletal muscle. Hence, electrical and diffusional studies turned out to be pivotal in elucidating the biophysical properties of gap junctions and their channels. Over the last two decades, at least 19 different channel proteins in humans have been identified and their channels characterized (Harris, 2001). We now know that gap junction channels are voltage gated and are permeable to ions and small molecules. Not surprising, they participate in many biological functions. Recent evidence suggests that hemichannels, the precursors of gap junction channels, may be involved in auto/paracrine signalling (Ebihara, 2003). Hence, what has started out with Silvio Weidmann's concern about cable properties of cardiac tissue, has grown to an exciting and flourishing field of research.
Silvio Weidmann was not only an outstanding scientist, but also an amicable person with a good sense of humour. Hence, it might be appropriate to finish off this article with one of his memorable remarks: ‘Electrophysiologists never die, they only depolarize.’
Original classic papers
The original classic papers reviewed in this article and published in The Journal of Physiology can be accessed online at: DOI: 10.1113/jphysiol.2005.101550
http://jp.physoc.org/cgi/content/full/jphysiol.2005.101550/DC1
This material can also be found as part of the full-text HTML version available from http://www.blackwell-synergy.com
References
Draper MH & Weidmann S (1951). Cardiac resting and action potentials recorded with an intracellular electrode. J Physiol 115, 74–94.
Ebihara L (2003). New roles for connexons. News Physiol Sci 18, 100–103.
Engelmann T (1875). Über die Leitung der Erregung im Herzmuskel. Pflugers Arch 11, 465–480.[CrossRef]
Harris AL (2001). Emerging issues of connexin channels: biophysics fills the gap. Q Rev Biophys 34, 325–472.[Medline]
Liu H, Clancy C, Cormier J & Kass R (2003). Mutations in cardiac sodium channels: clinical implications. Am J Pharmacogenomics 3, 173–179.[CrossRef][Medline]
Makielski JC, Sheets MF, Hanck DA, January CT & Fozzard HA (1987). Sodium current in voltage clamped internally perfused canine cardiac Purkinje cells. Biophys J 52, 1–11.
Sjöstrand FS & Andersson E (1954). Electron microscopy of the intercalated discs of cardiac muscle tissue. Experientia 10, 369–370.[Medline]
Weidmann S (1952). The electrical constants of Purkinje fibres. J Physiol 118, 348–360.
Weidmann S (1955). The effect of the cardiac membrane potential on the rapid availability of the sodium-carrying system. J Physiol 127, 213–224.
Weidmann S (1960). Sheep heart; low resistance of intercalated discs to the movement of 42K. J Physiol 153, 32P.
Weidmann S (1966). The diffusion of radiopotassium across intercalated disks of mammalian cardiac muscle. J Physiol 187, 323–342.
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