Patch-clamp measurements of gap-junction channels in cultured cells

Direct intercellular communication in most tissues is made possible by proteinaceous pores called gap-junction channels. These channels bridge the extracellular gap between apposed cells and connect their intracellular compartments both electrically and metabolically. The extracellular parts of two hemichannels - the connexons - are linked thus forming a communicating gap-junction channel. A connexon is a hexamer of protein subunits which are members of the connexin family. Since connexin 32 (Cx32) was the first gap-junction channel protein to be sequenced from hepatocytes, it serves as a reference to which all other gap-junction proteins are compared. The individual channel conductance may vary between 25 and 150 pS. Gap-junction channels of some tissues are more voltage sensitive (e.g. liver) than others (e.g. heart). The question whether these differences in electrical properties may be attributed to the different connexins being expressed in these tissues is still unanswered. Several approaches to resolve this problem will be discussed in this contribution, all are based on double whole-cell patch-clamp measurements using isolated cell pairs, as follows: (1) Cells with two different channel conductances perfused with anti connexin antibodies to specifically block one channel species; (2) Cells with only one connexin species selected by immunological characterization; (3) Weakly coupled HeLa cells transfected with specific connexin genes, a method which resulted in better correlations between connexin type and single channel properties.