Asymmetric voltage dependence of embryonic cardiac gap junction channels

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Abstract
The voltage dependence of junctional conductance (Gj) and the unitary channel behavior of junctions in most pairs of 3-day, 7-day, and 18-day embryonic chick heart cells are symmetrical, i.e., they are independent of the direction of polarization of junctional potential (Vj). With either cell depolarized relative to its neighbor, unitary channel events have a maximal unit conductance (yj) near 240 pS and five substates at nearly equal 40-pS increments down to near 40 pS (6, 9). Using the dual patch-clamp technique, we demonstrate here that, in a fraction of such cell pairs, Vj-dependent channel kinetics are asymmetric. Depolarization of one cell causes a larger and faster voltage-dependent decline in Gj than the same depolarization of the other cell. In a typical asymmetric preparation, depolarization of the strongly Vj-dependent side caused an immediate series of 47 +/- 16 pS closing steps in single-channel current (ij), followed by virtual cessation of channel activity. After depolarization of the less Vj-sensitive side, channel activity (56 +/- 13 pS) continues for many seconds. The large-conductance states (160-240 pS) observed in the electrically symmetric junctions were absent from the asymmetric preparations. In these cell pairs, connexin (Cx) 42, Cx43, and Cx45 could be immunolocalized at the junctional surfaces. We postulate that the asymmetry of voltage dependence in some cell pairs results from a preponderance of heterochannels formed from these different connexins. The frequency of asymmetric pairs obtained from 3-day, 7-day, and 18-day embryonic hearts was 50% (4/8), 24% (6/25), and 12.5% (1/8), suggesting that the fraction of heterochannels in the junctions decreases with cardiac development.