Characterization of the homologous and heterologous desensitization of rat Leydig-tumour-cell adenylate cyclase

Abstract

The homologous and heterologous desensitization of rat Leydig-tumour-cell adenylate cyclase induced by lutropin (LH) was characterized with the aid of forskolin and cholera toxin. Forskolin stimulated cyclic AMP production in a dose-dependent manner, with linear kinetics up to 2h. Forskolin also potentiated the action of LH on cyclic AMP production, but was only additive with cholera toxin. Preincubation of rat Leydig tumour cells with LH (1.0 micrograms/ml) for 1 h produced a desensitization of the subsequent LH (1.0 micrograms/ml)-stimulated cyclic AMP production, whereas the responses to cholera toxin (5.0 micrograms/ml), forskolin (100 microM), LH plus forskolin or cholera toxin plus forskolin were unaltered. In contrast, preincubation with LH for 20h produced a desensitization to all the stimuli tested. When rat Leydig tumour cells were preincubated for 1h with forskolin or dibutyryl cyclic AMP, the only subsequent response that was significantly altered was that to LH plus forskolin after preincubation with forskolin. However, preincubation for 20h with forskolin or dibutyryl cyclic AMP induced a desensitization to all stimuli subsequently tested. LH produced a rapid (0-1h) homologous desensitization, which was followed by a slower (2-8h)-onset heterologous desensitization. Forskolin and dibutyryl cyclic AMP were only able to induce heterologous desensitization. The rate of desensitization induced by either forskolin or dibutyryl cyclic AMP was similar to the rate of heterologous desensitization induced by LH. These results demonstrate that in purified rat Leydig tumour cells LH produces an initial homologous desensitization of adenylate cyclase that involves a cyclic AMP-independent lesion at or proximal to the guanine nucleotide regulatory protein (G-protein). This is followed by heterologous desensitization, which can also be induced by forskolin or dibutyryl cyclic AMP, thus indicating that LH-induced heterologous desensitization of rat Leydig-tumour-cell adenylate cyclase involves a cyclic AMP-dependent lesion that is after the G-protein.

Similarities and differences in phorbol ester- and luteinizing-hormone-induced desensitization of rat tumour Leydig-cell adenylate cyclase

“…As reported previously (Dix et al, , 1984, incubation with LH causes a desensitization of tumour Leydig cells, which results in a decreased cyclic AMP production in response to a second challange with LH. This desensitization is dependent on time (Fig.…”

“…it has the characteristics of heterologous desensitization. However, it would be expected that a decrease in response to forskolin would have been obtained (Dix et al, 1984), which was not the case (Rebois & Patel, 1985).…”

“…With respect to the mechanism of LH-induced desensitization, it was found in previous studies that the lesion probably occurred at the level of the receptor interaction with the GTP regulator protein (N.) Dix et al, , 1984, i.e. the receptor and/or part of N. that interacts with the LH receptor is modified so that activation of N. is decreased.…”

“…It was grown in Fischer rats, and the cells were isolated and purified on Percoll density gradients as previously described (Dix et al, 1984). Cells with a density of 1.070 g/ml were used in all studies.…”

“…It was found that they produced a small stimulation of steroidogenesis when added alone, but an inhibition of submaximally LH-stimulated steroidogenesis (but notdibutyryl cyclicAMP-stimulated steroidogenesis; Themmen et al, 1986b). In the normal testis and tumour Leydig cell, studies have been carried out in our laboratory into the mechanism of desensitization and on the internalization and recycling of the LH receptor (Dix & Cooke, 1981Dix et al, , 1984Habberfield et al, 1986). In the studies reported here, and similarly to the above, we have also found that phorbol esters mimic LH-induced desensitization in the tumour Leydig cell.…”