Physiological role of gap-junctional hemichannels: Extracellular calcium-dependent isosmotic volume regulation

Hemichannels in the overlapping regions of apposing cells plasma membranes join to form gap junctions and provide an intercellular communication pathw ay. Hemichannels are also present in the nonjunctional regions of individua l cells and their activity is gated by several agents, including calcium. H owever, their physiological roles are unknown. Using techniques of atomic f orce microscopy (AFM), fluorescent dye uptake assay, and laser confocal imm unofluorescence imaging, we have examined the extracellular calcium-depende nt modulation of cell volume. In response to a change in the extracellular physiological calcium concentration (1.8 to less than or equal to 1.6 mM) i n an otherwise isosmotic condition, real-time AFM imaging revealed a signif icant and reversible increase in the volume of cells expressing gap-junctio nal proteins (connexins). Volume change did not occur in cells that were no t expressing connexins. However, after the transient or stable transfection of connexin43, volume change did occur. The volume increase was accompanie d by cytochalasin D-sensitive higher cell stiffness, which helped maintain cell integrity. These cellular physical changes were prevented by gap-junct ional blockers, oleamide and beta-glycyrrhetinic acid, or were reversed by returning extracellular calcium to the normal level. We conclude that nonga p-junctional hemichannels regulate cell volume in response to the change in extracellular physiological calcium in an otherwise isosmotic situation.