The presence of gap junctions between osteoblastic cells has been previously reported. For this study we used the rat osteosarcoma cell line UMR 106, which expresses the osteoblastic phenotype, as a model to characterize further the nature, physiology, and regulation of gap junctions. Northern blot analysis identified a 3.0-kilobase RNA species corresponding to the gap junction protein connexin 43. The presence of two other connexin RNA species (26 and 32) could not be detected by this method in these cells. The identified connexin RNA was amplified by reverse transcription coupled to polymerase chain reaction; the sequence of the amplified product appears identical to the sequence of a cloned rat heart connexin 43 gene. After treatment with PTH, forskolin, and 8-Br-cAMP (a cAMP analog), the levels of connexin 43 RNA in UMR 106 cells increased. Further evidence for the role of PTH and cAMP in the physiology of gap junctions in these cells was obtained with Lucifer yellow dye transfer experiments. Gap-junctional intercellular communication increased in response to PTH and forskolin (an inducer of adenylate cyclase activity). Expression of connexin 43 RNA increased severalfold in response to PTH in a concentration- and time-dependent fashion. Connexin 43 RNA and its PTH-mediated stimulation were also observed in several other osteoblastic cell lines. The roles of PTH and forskolin in regulating the physiological state of gap junctions were confirmed in primary cultures of rat calvaria osteoblasts. © 1992 by The Endocrine Society.