Recording gap junction currents

Abstract

The heart acts as a functional syncytium that allows coordinated contraction and ejection of blood. The prerequisite of this behaviour is successful propagation of the action potential from cell to cell. There is broad consensus, that the transfer of an action potential from cell to cell is realized via gap junction channels. These channels can be considered to be low resistance pathways that allow current, ions, and small molecules (molecules <1000 D or with an ionic radius <0.8-1.0 nm; there is a slightly higher conductivity for cations over anions) to pass from one cell to the next. Gap junction channels (approx. length 100-150 Å, approx. pore width 12.5 Å, bridging the 20 Å gap between the cells) are dodecamers composed from proteins that are termed connexins. Each of the neighbouring cells provides a hexameric hemi-channel, a connexon consisting of 6 connexins. Two of these connexons dock to each other via their extracellular loops thereby forming the complete gap junction channel. There exist a number of isoforms of the connexins, which all belong to one protein family comprising two groups (group I: Cx26, Cx30, Cx30.3, Cx31, Cx31.1, Cx32; group II: Cx33, Cx37, Cx38, Cx40, Cx42, Cx43, Cx45, Cx50, Cx56; for details see: Bennett et al. 1995; Bruzzone 2001). A connexin consists of an intracellular N-terminal, 4 transmembrane domains, 2 extracellular and 1 intracellular loops and an intracellular C-terminal. The C-terminal is the most variable part of a connexin and differs in length and amino acid sequence between the various connexin isoforms. Connexins are defined by their molecular weight, i.e. Cx43 means a connexin of 43 kD molecular weight. The C-terminal is the most variable part of a connexin and contains consensus sequences that can be phosphorylated by protein kinases (for detailed information and species variability see Dhein 1998). Such phosphorylation reactions have been shown to play a role in regulation of channel conductance and of its degradation. In the heart muscle there are mainly three subtypes of connexins, Cx40, Cx43 and Cx45. Cx43 is the most abundant connexin, while Cx40 is mainly found in atrial tissue and in the conduction system. Cx45 has been detected predominantly during early development of the heart (for more details see Dhein 1998, 1998a). © Springer-Verlag Berlin Heidelberg 2005.