Desensitisation of LH-stimulated cyclic AMP accumulation in isolated bovine luteal cells--effect of phorbol ester.

In this study, we have characterized the LH-mediated desensitisation of receptor-linked cAMP generation in bovine luteal cells. Furthermore, the possibility that protein kinase C could play a role in this process has been investigated. The results obtained, show that the preincubation of Percoll-purified bovine luteal cells with LH diminished the cAMP response during reincubation with LH, depending upon the duration of prior exposure to LH and the concentration of LH used in the first incubation. This desensitisation was specifically dependent upon the prior exposure of the cells to the hormone only, as preincubation with either forskolin or cholera toxin did not result in a desensitised cAMP response to subsequent LH stimulation. On the other hand, LH-desensitised cells retained undiminished responsiveness to restimulation with cholera toxin. Neither the maximum binding capacity nor the affinity of the LH-receptor was affected by exposure of the cells to a desensitising dose of LH. The results demonstrate that in bovine luteal cells, LH produces a homologous desensitisation of the cAMP response which is not mediated by cAMP and that a hormone-receptor interaction appears to be a prerequisite for this process. Preincubation of the cells with varying concentrations of the protein kinase C activator phorbol 12-myristate 13-acetate (PMA) did not result in any reduction of LH-induced cAMP response during reincubation. The affinity of LH-receptor was also not affected by PMA pretreatment. In contrast, PMA-pretreated cells consistently produced increased amounts of cAMP when challenged with any of the agonists, LH, cholera toxin or forskolin. The preincubation of the cells with LH in the presence of PMA appears to prevent, at least partially, the desensitising effect of LH. It is concluded that in bovine luteal cells there is no evidence for a role of protein kinase C in LH-induced desensitisation. On the contrary, PMA pretreatment increased the response of adenylate cyclase to a subsequent hormonal stimulation without changing the affinity of the receptors for the hormone. Either an attenuation of the inhibitory N protein or a direct activation of the catalytic unit of adenylate cyclase could be the explanation for the observed effects of PMA. However, available data at present do not offer a choice between the two possibilities.