DISTINCT PATTERNS OF CONNEXIN EXPRESSION IN CANINE PURKINJE-FIBERS AND VENTRICULAR MUSCLE

Electrical conduction is more rapid in Purkinje fibers than in ventric ular muscle, which are distinct cardiac tissues that have different ac tive and passive electrophysiological properties. We have recently dem onstrated that canine myocardium contains multiple gap junction protei ns or connexins that form channels with unique electrophysiological pr operties. To determine whether differences in connexin expression may account, in part, for the characteristic conduction properties of Purk inje fibers and ventricular muscle, we assessed the amounts of mRNA fo r two connexins, Cx40 and Cx43, in these tissues obtained from canine hearts by Northern blot analysis and in situ hybridization. We also ch aracterized the distribution and relative abundance of these two conne xins in gap junctions with immunocytochemistry. A significantly greate r amount of Cx40 mRNA was observed in Purkinje fibers compared with ve ntricular muscle, a difference that was at least threefold according t o quantitative in situ hybridization (p<0.001) and densitometric analy sis of Northern blots. Purkinje fibers also demonstrated greater immun ostaining 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 demonstra ted a modest but not statistically significant increase in Cx43 mRNA i n Purkinje fibers compared with ventricular myocardittm. These results indicate that Purkinje fibers and ventricular muscle express distinct patterns of connexins. This tissue-specific pattern of connexin expre ssion could contribute to differences in the conduction properties of Purkinje fibers and ventricular muscle.