Modulation of responsiveness of the adenylate cyclase system in avian chondroprogenitor cells by pertussis toxin, PTH, and PGE2

Chondroprogenitor cells, derived from avian tibia epiphyseal growth plate, were cultured in vitro. Incubation of these cells with pertussis toxin augmented their cAMP response to parathyroid hormone (PTH), attenuated the response to forskolin, but did not modify the response to PGE2. Pertussis toxin modulation of the cAMP response was accompanied by ADP ribosylation of two proteins with molecular weights of 39 and 40 kD. Using specific antibodies, the 39 kD protein was identified as the inhibitory guanine nucleotide binding protein (Gi) of the adenylate cyclase system. The other ADP-ribosylated protein has not been identified. Preincubation of the chondroprogenitor cells with PTH or PGE2 resulted in time-dependent heterologous desensitization of the cAMP response to a second challenge of either hormone. The cells did not recover from the densensitization for at least 18 h after removal of the hormones. PTH and PGE2 treatment did not affect the cAMP response to forskolin and cholera toxin. The PTH-dependent cAMP production was also not altered by forskolin treatment. PTH homologous desensitization was not affected by pertussis toxin treatment, but the heterologous desensitization due to PGE2 was significantly attenuated. These results suggest that exposure of chondroprogenitor cells to PTH and PGE2 results in heterologous desensitization of the cAMP response. The desensitization is not due to changes in the adenylate cyclase activity. The pertussis toxin-sensitive G proteins are involved in the PTH heterologous rather than homologous desensitization of the cAMP response.