Molecular basis for pacemaker cells in epithelia

Intercellular signaling is highly coordinated in excitable tissues such as heart, but the organization of intercellular signaling in epithelia is less clear. We examined Ca2+ signaling in hepatoma cells expressing the hepatocyte gap junction protein connexin32 (cx32) or the cardiac gap junction protein cx43, plus a fluorescently tagged V-1a vasopressin receptor (V1aR). Release of inositol 1,4,5-trisphosphate (InsP(3)) in wild type cells increased Ca2+ in the injected cell but not in neighboring cells, while the Ca2+ signal spread to neighbors when gap junctions were expressed. Photorelease of caged Ca2+ rather than InsP. resulted in a small increase in Ca2+ that did not spread to neighbors with or without gap junctions. However, photorelease of Ca2+ in cells stimulated with low concentrations of vasopressin resulted in a much larger increase in Ca2+, which spread to neighbors via gap junctions. Cells expressing tagged V1aR similarly had increased sensitivity to vasopressin, and could signal to neighbors via gap junctions. Higher concentrations of vasopressin elicited Ca2+ signals in all cells. In cx32 or cx43 but not in wild type cells, this signaling was synchronized and began in cells expressing the tagged V1aR. Thus, intercellular Ca2+ signals in epithelia are organized by three factors: 1) InsP(3) must be generated in each cell to support a Ca2+ signal in that cell; 2) gap junctions are necessary to synchronize Ca2+ signals among cells; and 3) cells with relatively increased expression of hormone receptor will initiate Ca2+ signals and thus serve as pacemakers for their neighbors. Together, these factors may allow epithelia to act in an integrated, organ-level fashion rather than as a collection of isolated cells.