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Notes: Introduction: Electrophysiologic heterogeneity across the ventricular wall is a result of differential transmural expression of various ion channel proteins that underlie the different action potential waveforms observed in epicardial, midmyocardial, and endocardial regions. Cardiac connexins mediate cell-to-cell communication, are critical for normal impulse propagation, and play a role in electrophysiologic remodeling in disease states. However, little is known about the transmural distribution of cardiac gap junction proteins. Methods and Results: Connexin expression in epicardium, midmyocardium, and endocardium was assessed immunohistochemically in mouse and rat hearts. The total connexin protein content within different ventricular regions was measured by immunoblotting. Connexin43 is twice as abundant in midmyocardium and endocardium compared with epicardium in the mouse but not in the rat. Connexin45 is expressed equally across the left ventricular wall. Conclusion: Epicardial myocytes express significantly less Cx43 and therefore may be less well coupled than midmyocardial and endocardial myocytes. A transmural gradient of connexin43 expression across the left ventricular free wall likely results in differences in the stoichiometry of connexins expressed in different regions of the heart. (J Cardiovasc Electrophysiol, Vol. 15, pp. 1-6, June 2004)

Notes: Na+ Channel Activity in C×43-Defident Myocytes. Introduction: Dynamic interplay between active and passive electrical properties of cardiac myocytes is based on interrelationships between various channels responsible for depolarizing and repolarizing ionic currents and intercellular conductances. Mice with targeted disruption of the connexin43 (C×43) gene have hearts completely devoid of C×43, the principal gap Junctional protein expressed in mammalian hearts. Methods and Results: To determine whether cardiac myocytes that develop in an abnormal environment of reduced intercellular coupling have altered active membrane properties, we studied whole cell action potentials, Na+ channel currents, and Na+ channel expression and distribution via immunoblotting and confocal immunofluorescence in neonatal ventricular myocytes isolated from C×43 wild-type, heterozygous, and homozygous null hearts. Action potential morphology, peak Na+ current, activation and inactivation kinetics, and Na+ channel protein expression and distribution were not different among myocytes isolated from wild-type, heterozygous, or null hearts. Active membrane properties and Na+ channel activity were completely normal in C×43-deficient myocytes isolated from hearts that have been shown to exhibit markedly reduced C×43 expression, gap junction number, and epicardial conduction delay. Conclusion: Despite a genetic inability to produce C×43 and a developmental history that culminates in marked gross cardiac morphologic abnormalities, premature death, and myocardial inexcitability ex vivo, cardiac Na+ channel distribution and function appear to he normal in C×43 null hearts. Although intimate structural and functional interrelationships have been described between ion channels and gap junction channels, expression and function of Na+ channels is not affected by the absence of C×43.

Notes: Davidson has argued that the phenomenon of malapropism shows that languages thought of as social entities cannot be prior in the account of communication. This may be taken to imply that Dummett's belief, that language is prior in the account of thought, cannot be retained. This paper criticises the argument that takes Davidson from malapropism to the denial of the priority of language in the account of communication. It argues, against Davidson, that the distinction between word meaning and what speakers mean by words, used by Davidson to account for metaphor, suffices to account for malapropism. The reasons for the failure of Davidson's argument are used to throw light on what is meant by the priority of language over thought, and to argue that the priority thesis is ambiguous.

Notes: Dummett has offered an argument which begins with certain criteria of adequacy for any account of the way in which communication functions and which ends with normative and revisionary conclusions concerning our logical practice. This argument, which hinges on Dummett's criticisms of holism, is inadequate as it stands, for the holist can give an adequate description of the functioning of communication. There is a plausible defence of intuitionism to be extracted from Dummett's writing, but it should be recognised that it has a normative starting point and does not follow from the criteria of adequacy with which Dummett apparently begins.

Notes: Phospholipase D (PLD; EC 3.1.4.4) initiates phospholipid (PL) catabolism in plant cells and is also involved in signal transduction and retailoring of membrane PL. Phosphatidic acid (PA), the product of PLD hydrolysis of PL, increases in pericarp tissue during ripening of tomato (Lycopersicon esculentum Mill.) fruit, suggesting that increased PLD activity may be involved in loss of membrane function associated with ripening. However, a recent report showed a decline in soluble PLD activity in both normal and nonripening mutant fruit over the span that encompasses full ripening. To directly assess the role of PLD in tomato ripening, we have initiated a molecular genetic approach. Using a PLDα cDNA from castor bean as a probe, a PLDα cDNA (LEPLD2) was isolated from a tomato fruit library. It has an open reading frame of 2 421 nucleotides, predicted to encode a polypeptide of 807 amino acids, with a molecular mass of 91.9 kDa. These values are close to those of PLDαs from 11 plant species and LEPLD2 has ≥73% nucleotide sequence identity with PLDα cDNAs from castor bean and tobacco, as well as another tomato cDNA. LEPLD2 transcript was detected in all tissues of the tomato plant by RNA gel-blot analysis. Levels were very low in roots, low in stems, moderate in leaves, high in flowers and increased in fruit during development and ripening. Expression of LEPLD2 in Escherichia coli yielded phosphatidylcholine-hydrolyzing enzyme, and cells transformed with a pFLAG-MAC vector construct produced a FLAG-PLDα fusion protein that migrated close to the calculated 94 kDa on sodium dodecyl sulfate-polyacrylamide gel electrophoresis.

Notes: Connexin Phenotypes in the Human Heart. Introduction: Gap junction channels are major determinants of intercellular resistance to current flow between cardiac myocytes. Alterations in gap junctions may contribute to development of arrhythmia substrates in patients. However, there is significant interspecies variation in the types and amounts of gap junction subunit proteins (connexins) expressed in disparate regions of mammalian hearts. To elucidate determinants of conduction properties in the human heart, we characterized connexin phenotypes of specific human cardiac tissues with different conduction properties. Methods and Results: The distribution and relative abundance of Cx37, Cx40, Cx43, Cx45, and Cx46 were studied immunohistochemically using monospecific antibodies and frozen sections of the sinoatrial node and adjacent atria, the AV node and His bundle, the bundle branches, and the left and right ventricular walls. Patterns of expression of these connexins in the human heart differed from those in previous animal studies. Sinus node gap junctions were small and sparse and contained Cx45 and apparently smaller amounts of Cx40 but no Cx43. AV node gap junctions were also small and contained mainly Cx45 and Cx40 hut, unlike the sinus node, also expressed Cx43. Atrial gap junctions were larger than nodal junctions and contained moderate amounts of Cx40, Cx43, and Cx45. Junctions in the bundle branches were the largest in size and contained abundant amounts of Cx40, Cx43, and Cx45. Gap junctions in ventricular myocardium contained mainly Cx43 and Cx45; only a very small amount of ventricular Cx40 was detected in subendocardial myocyte junctions and endothelial cells of small to medium sized intramural coronary arteries. Minimal Cx37 and Cx46 immunoreactivity was detected between occasional atrial or ventricular myocytes. Conclusions: The relative amounts of individual connexins and the number and size of gap junctions vary greatly in specific regions of the human heart with different conduction properties. These differences likely play a role in regulating cardiac conduction velocity. Differences in the connexin phenotypes of specific regions of the human heart and experimental animal hearts must he considered in future experimental or modeling studies of cardiac conduction.

Notes: Structure of Sinus Node Gap Junctions. Introduction: To elucidate the role of tissue structure as a determinant of the unique conduction properties of the sinus node. we compared the spatial distribution of intercellular connections at gap junctions in the sinus node to the more rapidly conducting crista terminalis and left ventricle, which have been studied previously. Methods and Results: Samples of four canine sinus nodes were prepared for electron microscopy. The total number and spatial orientation of neighboring myocytes connected by ultrastructurally identified intercalated disks and gap junctions to nine randomly selected index cells were determined by sequentially examining subserial sections. Sinus node cells were sparsely interconnected compared to the extent of interconnections observed previously in other tissues. A typical sinus node cell was connected to only 4.8 ± 0.7 neighbors compared with 11.3 ± 2.2 cells in the left ventricle and 6.4 ± 1.7 cells in the crista terminalis. Sinus node interconnections occurred at small intercalated disks that usually connected cells in partial side-to-side and end-to-end juxtaposition. In contrast, left ventricular myocytes are interconnected at large intercalated disks that adjoin many cells in pure side-to-side and end-to-end orientations. Crista terminalis myocytes are connected primarily in end-to-end fashion. The aggregate gap junction profile length per unit myocyte area was 26.5 times greater in the left ventricle and 5.0 times greater in the crista terminalis than in the sinus node. Conclusion: Sinus node myocytes exhibit small, sparsely distributed gap junctions that interconnect cells in complex patterns of lateral and terminal apposition. These structural features are consistent with the unique conduction properties of the sinus node.

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