Distinct patterns of connexin expression in canine Purkinje fibers and ventricular muscle.

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Abstract
Electrical conduction is more rapid in Purkinje fibers than in ventricular muscle, which are distinct cardiac tissues that have different active and passive electrophysiological properties. We have recently demonstrated that canine myocardium contains multiple gap junction proteins or connexins that form channels with unique electrophysiological properties. To determine whether differences in connexin expression may account, in part, for the characteristic conduction properties of Purkinje fibers and ventricular muscle, we assessed the amounts of mRNA for two connexins, Cx40 and Cx43, in these tissues obtained from canine hearts by Northern blot analysis and in situ hybridization. We also characterized the distribution and relative abundance of these two connexins in gap junctions with immunocytochemistry. A significantly greater amount of Cx40 mRNA was observed in Purkinje fibers compared with ventricular muscle, a difference that was at least threefold according to quantitative in situ hybridization (p < 0.001) and densitometric analysis of Northern blots. Purkinje fibers also demonstrated greater immunostaining intensity when incubated with anti-Cx40 antibodies than did ventricular muscle. In contrast, Cx43 mRNA and protein appeared to be abundant in both tissues. Quantitative in situ hybridization demonstrated a modest but not statistically significant increase in Cx43 mRNA in Purkinje fibers compared with ventricular myocardium. These results indicate that Purkinje fibers and ventricular muscle express distinct patterns of connexins. This tissue-specific pattern of connexin expression could contribute to differences in the conduction properties of Purkinje fibers and ventricular muscle.