Connexins and secretion.

Four decades ago, a serendipitous finding revealed that a membrane-impermeable tracer could be rapidly exchanged between salivary gland cells. Since this first demonstration of direct cell–cell communication in a secretory system, gap junctions, connexins, and coupling have been shown to be obligatory attributes of all multicellular glands. The distribution of various connexin isoforms has now been mapped in most of these organs and shown to differ in endocrine and exocrine systems, because of a specific transcriptional control of the connexin genes. Many studies have provided further circumstantial evidence of a role for connexin-dependent signaling in the secretory function of a variety of glands. This function has been directly demonstrated in vivo in only a few systems, in which the mechanism linking connexin signaling to secretion has started to be elucidated. However, this mechanism, and implications for the physiological and pathophysiological function of secretory cells remains to be unraveled.