Physiology and pharmacology of gap junctions.

In embryonic and adult organisms, most tissues contain intercellular channels that connect each cell with all the neighbors that it contacts (as in liver) or divide the tissue into coupled subsets (as in various regions of the nervous system). These channels provide cytoplasmic continuity with regard to ions and small molecules, and their aggregates belong to a single ultrastructurally recogniz­ able class: gap junctions (cf. 98). Recently a number of specific treatments have been found that operate to open and close, i.e. gate, the channels; some of these treatments, and others, operate over a longer time course to affect the formation or disappearance of the channels. The intent of this review is to summarize the static and dynamic characteristics of gap junctions obtained using electrophysiological techniques. Although we highlight diversity among electrophysiological properties of junctions from different tissues, our message is that there are general regulatory mechanisms that act on this class of membrane channels. One role that gap junctions presumably play is linked to their permeability to relatively large molecules. Nonarthropod gap junctions are permeable to spher­ oid molecules at least as large as 1000 daltons, whereas arthropod junctions apparently allow flux of molecules 50% larger in diameter (cf. 104). (A problem with measurements of channel size has been the absence of a suffi­ ciently large range of spherical probes with higher molecular weights and the absence of quantitative permeability data for determining selectivity for the