Electrical conduction is very rapid and highly anisotropic in atrial fiber bundles, such as the crista terminalis. In contrast to left ventricular myocardium in which the ratio of longitudinal to transverse conduction velocities is approximately 3, propagation velocity in the crista terminalis is approximately 10 times greater in the longitudinal than in the transverse direction. To elucidate potential determinants of these distinct conduction properties, we characterized structural and molecular features of intercellular coupling in the crista terminalis and left ventricular myocardium of the canine heart. Analysis of the number and spatial orientation of myocyte interconnections at gap junctions revealed that a typical left ventricular myocyte was connected to 11.3 +/- 2.2 other myocytes. Approximately equal numbers of connections occurred between ventricular myocytes juxtaposed in side-to-side and end-to-end orientation. In contrast, a typical myocyte of the crista terminalis was connected to only 6.4 +/- 1.7 other cells (P < .05), but nearly 80% of these connections occurred between cells oriented in an end-to-end configuration. In comparison with the ventricular pattern, this spatial distribution of connections would limit intercellular current transfer between laterally apposed cells and thereby enhance anisotropy of conduction velocity in the longitudinal and transverse directions. Ultrastructural analysis showed that crista terminalis myocytes were connected by numerous small gap junctions that occurred in relatively simple, straight intercalated disks. Northern blot analysis showed approximately equivalent amounts of mRNAs encoding the gap junction channel proteins connexin43 and connexin45 but approximately four times more connexin40 mRNA in crista terminalis than in the left ventricle.(ABSTRACT TRUNCATED AT 250 WORDS)

Positron emission tomographic myocardial viability imaging appears to identify patients at increased risk of having an adverse cardiac event or death. Patients with impaired left ventricular function and positron emission tomographic evidence for jeopardized myocardium appear to have the most benefit from a revascularization procedure.

Although most cardiac gap junctions contain connexin40, connexin43 and connexin45, the relative amounts of each of these connexins vary considerably in cardiac tissues with different conduction properties.

Electrical propagation in the normal heart occurs via intercellular transfer of current at gap junctions. Alterations in intercellular coupling in the diseased heart are critical in the pathogenesis of reentrant ventricular arrhythmias. Until recently only a single gap junction protein was known to couple cardiac myocytes. We have now identified and sequenced two additional distinct gap junction proteins (connexins) expressed in the mammalian heart. The sequences differ in their predicted cytoplasmic regulatory domains. Expression of all three connexins by canine ventricular myocytes has been confirmed by Northern blotting and by immunohistochemistry with connexin-specific antisera. Immunoelectron microscopy confirmed that all three connexins are localized to myocyte gap junctions. The presence of multiple connexins in myocyte gap junctions suggests novel mechanisms for regulating cardiac electrical coupling.

Electrical activation of the heart requires transfer of current from one discrete cardiac myocyte to another, a process that occurs at gap junctions. Recent advances in knowledge have established that, like most differentiated cells, individual cardiac myocytes express multiple gap junction channel proteins that are members of a multigene family of channel proteins called connexins. These proteins form channels with unique biophysical properties. Furthermore, functionally distinct cardiac tissues such as the nodes and bundles of the conduction system and atrial and ventricular muscle express different combinations of connexins. Myocytes in these tissues are interconnected by gap junctions that differ in tissue-specific manner in terms of their number, size, and three-dimensional distribution. These observations suggest that both molecular and structural aspects of gap junctions are critical determinants of the anisotropic conduction properties of different cardiac tissues. Expression of multiple connexins also creates the possibility that "hybrid" channels composed of more than one connexin protein type can form, thus greatly increasing the potential for fine control of intercellular ion flow and communication within the heart.