Effect of lutropin and cycloheximide on lutropin receptors and cyclic AMP production in Leydig tumour cells in vitro

Mutations of gonadotropins and gonadotropin receptors: elucidating the physiology and pathophysiology of pituitary-gonadal function

The recent unraveling of structures of genes for the gonadotropin subunits and gonadotropin receptors has provided reproductive endocrinologists with new tools to study normal and pathological functions of the hypothalamic-pituitary-gonadal axis. Rare inactivating mutations that produce distinctive phenotypes of isolated LH or FSH deficiency have been discovered in gonadotropin subunit genes. In addition, there is a common polymorphism in the LHbeta subunit gene with possible clinical significance as a contributing factor to pathologies of LH-dependent gonadal functions. Both activating and inactivating mutations have been detected in the gonadotropin receptor genes, a larger number in the LH receptor gene, but so far only a few in the gene for the FSH receptor. These mutations corroborate and extend our knowledge of clinical consequences of gonadotropin resistance and inappropriate gonadotropin action. The information obtained from human mutations has been complemented by animal models with disrupted or inappropriately activated gonadotropin ligand or receptor genes. These clinical and experimental genetic disease models form a powerful tool for exploring the physiology and pathophysiology of gonadotropin function and provide an excellent example of the power of molecular biological approaches in the study of pathogenesis of diseases.

An investigation of glucose uptake in relation to steroidogenesis in rat testis and tumour Leydig cells

The mechanisms of the requirement of glucose for steroidogenesis were investigated by monitoring the uptake of the glucose analogue 2-deoxy-D-glucose by rat testis and tumour Leydig cells. The characteristics of glucose transport in both of these cell types were found to resemble those of the facilitated-diffusion systems for glucose found in most other mammalian cells. The Leydig cells took up 2-deoxy-D-glucose but not L-glucose, and the uptake was inhibited by both cytochalasin B and forskolin. In the presence of luteinizing hormone, the rate of 2-deoxy-D-glucose uptake by both cell types was increased by approx. 50%. In addition to D-glucose, it was shown that the Leydig cells could also utilize 3-hydroxybutyrate or glutamine to maintain steroidogenesis.

Construction of a Leydig cell line synthesizing testosterone under gonadotropin stimulation: a complex endocrine function immortalized by cell hybridization

Hybridization between a mouse Leydig tumor cell line, MA-10, which produces cyclic AMP and progesterone under human chorionic gonadotropin (hCG) stimulation, and freshly isolated mouse Leydig cells gave rise to 54 hybrid clones, one of which, LK17, was capable of hCG-stimulated testosterone production. Subcloning of this hybrid resulted in the emergence of a subclone, K9, whose testosterone production is more than 10 times that of parent clone LK17, after hCG stimulation, with an ED50 of 37 pM. Testosterone synthesis by K9 cells was multiplied by 25 after gonadotropin stimulation, and binding of hCG declined after prolonged exposure to the hormone. These similarities with murine Leydig cells in primary culture make the K9 clone an attractive alternative for physiological studies.

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

The phorbol ester 12-O-tetradecanoylphorbol 13-acetate (TPA) was shown to mimic luteinizing hormone (LH; lutropin) in causing desensitization of LH-mediated cyclic AMP production in tumour Leydig cells. However, there were differences between LH- and TPA-induced desensitization: (1) TPA induced a more rapid effect than LH; (2) adenosine did not inhibit TPA-induced desensitization, whereas it completely inhibited the LH-induced desensitization; (3) adenylate cyclase activity in plasma membranes from TPA-desensitized cells was not decreased, whereas similar preparations from LH-desensitized cells lost their response to LH and to LH plus guanosine 5'-[beta gamma-imido]triphosphate; TPA-, but not LH-, treated cells had a decreased capacity to respond to cholera toxin and forskolin. These results indicate that LH and phorbol esters induce desensitization of adenylate cyclase in rat tumour Leydig cells by different mechanisms.

The rapid desensitization of glucagon-stimulated adenylate cyclase is a cyclic AMP-independent process that can be mimicked by hormones which stimulate inositol phospholipid metabolism

Treatment of intact hepatocytes with glucagon, TH-glucagon [( 1-N-alpha-trinitrophenylhistidine, 12-homoarginine]glucagon), angiotensin or vasopressin led to a rapid time- and dose-dependent loss of the glucagon-stimulated response of the adenylate cyclase activity seen in membrane fractions isolated from these cells. Intracellular cyclic AMP concentrations were only elevated with glucagon. All ligands were capable of causing both desensitization/loss of glucagon-stimulated adenylate cyclase activity and stimulation of inositol phospholipid metabolism in the intact hepatocytes. Maximally effective doses of angiotensin precluded any further inhibition/desensitizing action when either glucagon or TH-glucagon was subsequently added to these intact cells, as has been shown previously for the phorbol ester TPA (12-O-tetradecanoylphorbol 13-acetate) [Heyworth, Wilson, Gawler & Houslay (1985) FEBS Lett. 187, 196-200]. Treatment of intact hepatocytes with these various ligands caused a selective loss of the glucagon-stimulated adenylate cyclase activity in a washed membrane fraction and did not alter the basal, GTP-, NaF- and forskolin-stimulated responses. Angiotensin failed to inhibit glucagon-stimulated adenylate cyclase activity when added directly to a washed membrane fraction from control cells. Glucagon GR2 receptor-stimulated adenylate cyclase is suggested to undergo desensitization/uncoupling through a cyclic AMP-independent process, which involves the stimulation of inositol phospholipid metabolism by glucagon acting through GR1 receptors. This action can be mimicked by other hormones which act on the liver to stimulate inositol phospholipid metabolism. As the phorbol ester TPA also mimics this process, it is proposed that protein kinase C activation plays a pivotal role in the molecular mechanism of desensitization of glucagon-stimulated adenylate cyclase. The site of the lesion in desensitization is shown to be at the level of coupling between the glucagon receptor and the stimulatory guanine nucleotide regulatory protein Gs, and it is suggested that one or both of these components may provide a target for phosphorylation by protein kinase C.

Diphtheria toxin prevents catecholamine desensitization of A431 human epidermoid carcinoma cells

We proposed that a rapidly turning over protein, induced in response to catecholamine stimulation of C6-2B rat astrocytoma cells, inhibits subsequent hormonal activation of adenylate cyclase. Studies upon which our hypothesis is based and confirmatory work in a variety of other cell lines and in vivo have utilized actinomycin D and cycloheximide to inhibit RNA and protein synthesis, respectively. These inhibitors, however, are not specific and have been reported also to interfere with other cellular processes. Diphtheria toxin is a specific protein synthesis inhibitor that acts only by ADP-ribosylating elongation factor 2, thus preventing peptide chain elongation. We thus tested whether diphtheria toxin could prevent catecholamine-induced desensitization in A431 human epidermoid carcinoma cells. The toxin inhibited protein synthesis and altered the time course of isoproterenol-stimulated cAMP accumulation as did the less-specific protein synthesis inhibitor cycloheximide. Cellular cAMP content after a 30-min exposure to isoproterenol was similar in control and in toxin-treated cells. However, after 4 hr of treatment with isoproterenol, toxin-treated cells accumulated up to six times more cAMP than controls. When cells or cell-free adenylate cyclase preparations were rechallenged with agonists, toxin-mediated inhibition of protein synthesis prevented desensitization. These results show that diphtheria toxin, a specific inhibitor of protein synthesis, can interfere with the normal physiological regulation of cAMP metabolism in eukaryotic cells and provide compelling evidence that catecholamine stimulation of adenylate cyclase promotes the synthesis of a protein(s) that, in some way, inhibits hormone-stimulated adenylate cyclase.

Processing of human choriogonadotropin and its receptors by cultured pig Leydig cells. Role of cyclic AMP and protein synthesis

We have examined the process by which human choriogonadotropin/luteinizing hormone (hCG/LH) receptors are regulated in cultured porcine Leydig cells. Treatment of Leydig cells with human choriogonadotropin, cholera toxin, forskolin and cyclic 8-bromoAMP (8-BrcAMP) produced a loss of surface receptors without modification of the binding affinity. This negative regulation of the number of receptors mediated by maximal concentrations of hCG was higher than that induced by the other agents. The extent of receptor loss in cells treated with increasing concentrations of hCG was highly correlated with their capacity to stimulate cAMP production. However, there was little correlation between down-regulation and cAMP production of these cells treated by hCG plus forskolin or cholera toxin plus forskolin, where a synergistic cAMP production was obtained. Following exposure of Leydig cells to both hCG and 8-BrcAMP, the surface receptor disappearance began after an initial lag period of about 6-8 h. Thereafter a 50% loss of surface receptor was observed in the next 8-h incubation. Monensin with hCG shortens this lag period before initiation of receptor loss. Kinetic studies with 125I-hCG, in the presence or absence of monensin, showed that the half-life of the receptor-bound hormone complexes at the cell surface was 10.5 h and 8 h respectively. Therefore, the steady state of the surface receptor during the lag phase of 8 h is probably related to recycling of internalized receptors and/or translocation of performed receptors. Cycloheximide and actinomycin D inhibit hCG-mediated and 8-BrcAMP-mediated down-regulation. Cycloheximide lengthens ligand-receptor complexes at the surface by slowing down the rate of internalization (half-life of 20 h), but this mechanism is not enough per se to explain the effect of cycloheximide. Pulses of hCG or 8-BrcAMP for 4 h and 8 h sufficed to induce nearly maximal down-regulation. However, it was possible to attenuate this triggering effect by adding cycloheximide after pulse of the cells. Thus, even after removal of the triggering agent (hCG or 8-BrcAMP), the loss of surface receptor could be triggered by a protein-sensitive signal. Taken as a whole these results indicate that a coordinated interaction is involved in the cell-surface hCG/LH receptor regulation. The apparent steady state of the number of receptors during the first hours of stimulation passed through a reuptake of internalized receptors.(ABSTRACT TRUNCATED AT 400 WORDS)

Adenosine potentiates lutropin-stimulated cyclic AMP production and inhibits lutropin-induced desensitization of adenylate cyclase in rat Leydig tumour cells

The action of adenosine on lutropin (LH)-stimulated cyclic AMP production and LH-induced desensitization of adenylate cyclase in rat Leydig tumour cells was investigated. Adenosine and N6-(phenylisopropyl)adenosine caused a dose-dependent potentiation of LH-stimulated cyclic AMP production at concentrations (0.01-10 microM) which alone did not produce an increase in cyclic AMP production. However, 2-deoxyadenosine had no effect either alone or in combination with LH on cyclic AMP production. The potentiation produced by adenosine was unaffected by concentrations of the specific nucleoside-transport inhibitor dipyridamole, which inhibited [3H]adenosine uptake by up to 90%. The phosphodiesterase inhibitor 3-isobutyl-l-methylxanthine, but not RO-10-1724, inhibited the adenosine-induced potentiation. In the presence of adenosine, the kinetics of LH-stimulated cyclic AMP production were linear with time up to 2h, compared with those with LH alone, which showed a characteristic decrease in rate of cyclic AMP production after the first 15-20 min. Consistent with the altered kinetics, adenosine also inhibited the LH-induced desensitization of adenylate cyclase. These results suggest that adenosine has effects on rat tumour Leydig cells through receptors on the external surface of the plasma membrane. This receptor has characteristics similar to those of the R-type receptors, which have been shown either to stimulate or to inhibit adenylate cyclase. However, the effects of adenosine in the present studies does not involve a direct inhibition or activation of adenylate cyclase, but may involve an as yet undefined receptor-mediated modulation of adenylate cyclase.

Intratesticular regulation of testosterone secretion: comparison of the effects and interactions of hCG, an LHRH agonist and testicular interstitial fluid on Leydig cell testosterone secretion in vitro

The stimulatory effects on Leydig cell testosterone secretion of a polypeptide(s) factor present in testicular interstitial fluid (IF) were compared with those of hCG and an LHRH agonist (LHRH-A). The actions of IF and LHRH-A were similar in showing (1) a delayed onset of action, (2) enhancement of testosterone production in response to a maximally stimulating concentration (5 nM) of hCG, and (3) near cessation of stimulation following their removal from the incubation medium. However, addition of an LHRH antagonist blocked only the actions of LHRH-A. Moreover, IF continued to stimulate testosterone production up to at least 20 h either on its own or in the presence of 5 nM hCG, whereas the stimulatory effects of LHRH-A disappeared beyond 6 h. IF was also able to enhance testosterone production in response to LHRH-A or in response to hCG + LHRH-A. IF enhanced testosterone production over 4-20 h in response to all doses of hCG and increasing concentrations of IF caused dose-dependent increments in the rate of hCG (5 nM) stimulated testosterone production. With submaximally stimulating doses of hCG or with LHRH-A alone, the stimulatory effect of IF was more or less additive, whereas with maximally stimulating doses of hCG the effect of IF was clearly synergistic. Thus, whereas the rate of testosterone production by Leydig cells in response to 5 nM hCG declined progressively from 4 to 20 h, addition of IF attenuated or prevented this decline. These findings have implications with respect to the physiological control of intratesticular testosterone levels and with respect to the regulation of steroidogenesis.(ABSTRACT TRUNCATED AT 250 WORDS)

Rapid in vitro desensitization of the testosterone response in rat Leydig cells by sub-active concentrations of porcine luteinizing hormone

We have studied in rat Leydig cells, the effect of sub-active concentrations of porcine LH on the subsequent stimulation of the cAMP and testosterone production by a sub-maximal concentration of pLH or hCG. We found that extremely low concentrations of pLH (0.01-2.0 ng/ml) were able to induce rapidly a partial but highly significative desensitization of the testosterone response without affecting the cyclic AMP response. These data indicate that desensitization of the steroidogenic response might be due to some lesion beyond cAMP formation or at the level of one discrete compartment of cyclic AMP, directly involved in the control of steroidogenesis. Moreover, our data strongly suggest that the basal circulating concentrations of LH can exert an inhibitory control on the testosterone response to LH pulses in vivo.

Modulation of cultured mouse Leydig cells adenylate cyclase by forskolin and hCG

The diterpene, forskolin, stimulated cAMP accumulation about 15-fold over basal levels in purified mouse Leydig cells; however, it remained far less potent than hCG. Simultaneous addition of forskolin and hCG resulted in a striking synergistic stimulation of cAMP production. In contrast, forskolin-enhanced testosterone accumulation was never synergistic with that produced by maximal concentrations of hCG. hCG (3 X 10(-9) M) lowered about 6-fold the ED50 for forskolin-elicited cAMP accumulation and increased the maximal response to forskolin about 16-fold. Conversely, forskolin 10(-6) M) reduced the ED50 for hCG 2-fold but had a much smaller effect (2-3-fold) on maximal response. Moreover, pretreatment with hCG induced only a homologous desensitization of adenylate cyclase, whereas the enzyme became partially resistant to both hCG and forskolin in cells pretreated with forskolin. The homologous hCG-induced desensitization and the partial heterologous one induced by forskolin suggest that more than the catalytic unit of the cyclase is required for the diterpene activation.

A transplantable rat Leydig cell tumor—2. Adenylyl cyclase activation by prostaglandin E1, isoproterenol and glucagon

The responsiveness of the membrane bound adenylyl cyclase (AC) system to various hormones and hormone analogues in a transplantable rat Leydig cell tumor (H-540) has been investigated and compared with that of interstitial tissue from normal adult rats. The results can be summarized as follows: The AC of the Leydig cell tumors was stimulated by isoproterenol, epinephrine, norepinephrine, dopamine, glucagon, PGE1, PGF2 alpha and LH/hCG. The AC of the Leydig cell tumors exhibited a much greater response to PGE1 than the AC in normal interstitial tissue (5-fold and 2-fold stimulation, respectively). The AC of the Leydig cell tumours also revealed a somewhat higher response to isoproterenol than the AC of interstitial tissue (2.5-fold and 2-fold, respectively). Adenylyl cyclase in both tissues were equally stimulated by glucagon (2-fold). The LH response in the Leydig cell tumors was only half of that found in normal Leydig cells. This study indicates, that in spite of the fact that tumor Leydig cells respond to LH and produce testosterone, the response pattern of the AC is different from that of normal Leydig cells.

A transplantable rat Leydig cell tumor--1. LH and prolactin receptors and effects of the endocrine status of the host animal

We have examined the binding capacity and properties (affinity, specificity) of LH and prolactin (Prl) receptors in a transplantable rat Leydig cell tumor (H-540) grown in intact, castrated and hypophysectomized rats. LH receptors in adult rat testis and Prl receptors in the rat ventral prostate were examined simultaneously for comparison. The results can be summarized as follows: The qualitative properties (affinity, specificity) of LH and Prl receptors in tumor Leydig cells appear to be identical to those of corresponding receptors in non-tumor tissues. The levels of LH receptors in tumor Leydig cells are only some 1% of that present in normal Leydig cells from adult rats. Tumor Leydig cells grown in hypophysectomized rats had even lower levels of LH receptors; ca. 1/3 of that found in tumors from intact rats. The levels of Prl receptors in the tumor Leydig cells are almost as high as in normal Leydig cells from adult rats. In tumors grown in hypophysectomized rats, the levels of Prl receptors were much lower (ca. 20%) than in tumors from intact or castrated rats. There were great variations in the number of LH and Prl receptors in individual tumors, and there was a positive correlation (r = 0.88; P less than 0.01) between LH and Prl receptors in individual tumors. No differentiation toward a "LH receptor tumor" or "Prl receptor tumor" was observed. Thus, receptors for LH and Prl in tumor cells are qualitatively normal, but the number is greatly (LH) or moderately (Prl) reduced. These receptors in the tumor Leydig cells are stimulated by pituitary hormones.

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

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.

Desensitization of the cAMP system in mouse Leydig cells by hCG, cholera toxin, dibutyryl cAMP and cAMP: localization of the 'lesion' to the guanine nucleotide regulatory protein-adenylate cyclase complex

The mechanism of hCG-induced desensitization of the cAMP system was studied in Percoll-purified mouse Leydig cells. Pretreatment of Leydig cells with hCG resulted in a time- and dose-dependent decrease in the capacity of hCG-induced cAMP formation. Maximal desensitization (approximately 90%) was induced by only partial prior stimulation. Desensitization, however, was not observed without a prior increase in cAMP or testosterone production. Pretreatment of the cells with N6,O2'-dibutyryl cAMP (DBcAMP) also induced a dose- and time-dependent densensitization. cAMP was only effective in the presence of the phosphodiesterase inhibitor 1-methyl-3-isobutylxanthine (MIX). Cholera toxin desensitized the hormone-induced cAMP response as drastically as hCG. Cholera toxin was unable to reverse the refractory state induced by one of the agonists. hCG-induced desensitization was not associated with a loss in [125I]hCG binding or an increase in maximal phosphodiesterase activity, and appeared not to be dependent on protein synthesis. Membranes from hCG, cholera toxin of DBcAMP-desensitized cells showed an impaired adenylate cyclase activity in response to hCG, hCG plus beta-gamma-imidoguanosine 5'-triphosphate (GPPNP) and NaF. In conclusion, hCG-induced desensitization of the adenylate cyclase system in mouse Leydig cells can be mimicked by cholera toxin, DBcAMP and cAMP, indicating a cAMP-mediated process. The site of the 'lesion' has to be localized to the guanine nucleotide regulatory protein-adenylate cyclase complex rather than to its uncoupling from the hormone receptor.

Isolation and characterization of plasma membranes containing LH sensitive adenylate cyclase from a Leydig cell tumour

An LH sensitive adenylate cyclase from a tumour Leydig cell has been investigated. The plasma membranes, prepared by a 2 phase (dextran-polyethylene glycol) centrifugation method were found to have the following properties: In the presence of LH plus p(NH)ppG (guanosine 5'-beta, gamma-imido triphosphate) or fluoride ions, maximum adenylate cyclase activity was obtained in the plasma membranes with 4 to 6 mM Mg2+ plus 0.33 to 2 mM ATP. LH alone stimulated adenylate cyclase activity 2-fold when compared with basal activity and the time course of cyclic AMP production was linear up to 45 min. With GTP (10(-5)M) and GTP plus LH, adenylate cyclase activity was increased 3 and 6-fold, respectively, for up to 20 min and thereafter declined. In contrast p(NH)ppG (10(-5)M) and p(NH)ppG plus LH increased adenylate cyclase activity 7 and 14-fold which was maintained for at least 45 min. Fluoride ions increased the enzyme activity linearly over 45 min approx 18-fold. When GTP or p(NH)ppG were added alone there was a lag time of activation of approximately 10 min which was abolished by the addition of LH. GTP but not p(NH)ppG at concentrations greater than 10(-4) inhibited basal and LH stimulated adenylate cyclase when compared with 10(-5)M GTP. The tumour Leydig cell adenylate cyclase is thus essentially similar to other hormone sensitive somatic cells. The present study makes it feasible to prepare plasma membranes by a simple method from large quantities of pure Leydig cells.

Guanine nucleotide mediated desensitization of adenylate cyclase in cell free preparations from a Leydig cell tumour

Cell free desensitization of a tumour Leydig cell plasma membrane adenylate cyclase has been demonstrated in the presence of guanine nucleotides. In experiments in which the membranes were pre-incubated with various nucleotides and LH, it was shown that this decreased adenylate cyclase activity was dependent on the presence of GTP and occurred both in the presence and absence of ATP. While pre-treatment with LH alone appeared to enhance subsequent adenylate cyclase activity, this hormone was able to potentiate the desensitizing effect of GTP. The desensitizing effect of GTP was not inhibited by sodium fluoride. In contrast, the GTP analogue p(NH)ppG (guanosine 5'beta, gamma-imido triphosphate) caused a persistent activation of the adenylate cyclase. GMP and guanosine also initially inhibited the adenylate cyclase activity, but this was entirely reversed by p(NH)ppG plus LH. GDP in addition to GTP caused desensitization but this was only partially reversed by p(NH)ppG plus LH. It is proposed that in similarity with the ovary (Bockaert et al. 1976; Ezra & Salomon 1982a) desensitization of Leydig tumour cell plasma membrane adenylate cyclase may involve a GTP-mediated phosphorylation step.

Catecholamine stimulation of testosterone production via cyclic AMP in mouse Leydig cells in monolayer culture

The effects of the beta-adrenergic hormone agonist, isoproterenol, on testosterone and cyclic AMP production in mouse Leydig cells in culture have been investigated. It was found that isoproterenol increased testosterone production on days 1, 2 and 3 of culture but not in freshly cultured cells. Cyclic AMP production was however increased on all days of culture. In subsequent studies carried out on day 2 of culture the amounts of testosterone formed during incubation with isoproterenol were 20-90% of those obtained with maximum stimulating levels of luteinizing hormone. The amounts of cyclic AMP formed were extremely low compared with those obtained with luteinizing hormone (22 +/- 5.3 and 2320 +/- 100 pmoles/10(6) cells/2 h respectively). Isoproterenol (10(-8) -10(-7) M) gave a significant increase in testosterone production and reached a maximum with 10(-6) M. Similar dose-response curves for cyclic AMP production were obtained. The stimulation of cyclic AMP and testosterone by isoproterenol was highly dependent on the presence of the phosphodiesterase inhibitor, methylisobutylxanthine. Propranolol blocked, in a dose-dependent manner, both isoproterenol-stimulated cyclic AMP and testosterone production. In the presence of excess luteinizing hormone no additional effects of isoproterenol were detected. Epinephrine also stimulated testosterone production. It is concluded that catecholamines stimulate testosterone production in mouse Leydig cells in monolayer culture and that this effect if mediated by cyclic AMP.

Stimulation and inhibition by LHRH analogues of cultured rat Leydig cell function and lack of effect on mouse Leydig cells

The effect of 2 luteinizing hormone-releasing hormone (LHRH) analogues(10-8-10-6 M) on the functional activity (testosterone and cyclic AMP production and [125I]hCG binding) of purified mouse Leydig cells in culture was examined. The analogues were found to have no significant effect on the cells over a period of 3 days. No specific binding of a labelled analogue to impure or pure mouse Leydig cells could be detected. In contrast high levels of specific binding to impure rat interstitial cells occurred. Centrifugation of the rat interstitial cells on 0-90% Percoll gradients showed that the LHRH analogue bound specifically to the active lutropin-responsive Leydig cells. The purified rat Leydig cells were cultured in the presence of LHRH analogue (ICI 118630) (10-7 M) and after an initial lag period (2h) a marked stimulation of testosterone production occurred over a 32-h period (up to 400 ng/10(6) cells). The response to LH alone increased with time in culture up to 10 h, and the LHRH analogue enhanced this LH-stimulated testosterone production. When the cells were cultured for longer time periods (24 h) the LHRH analogue was found to inhibit LH-stimulated testosterone production at all concentrations of LH used (p less than 0.01). The LHRH analogue had no consistent effect on LH-stimulated cyclic AMP production, although when added alone, cyclic AMP production was increased. These results show that LHRH analogues do not bind to or have any detectable effect on mouse Leydig cells in vitro. However, LHRH analogue does bind specifically to purified rat Leydig cells. After a short lag period the analogue stimulates testosterone production which turns to inhibition after 20 h in culture.

Resensitization of lutropin-desensitized tumour Leydig-cell adenylate cyclase with human erythrocyte membranes

Purified rat tumour Leydig cells were pretreated with or without lutropin (1 h at 32 degrees C). The plasma membranes were then isolated and the adenylate cyclase activity measured in the presence of freshly prepared or heat-inactivated (1 h at 60 degrees C) human erythrocyte membranes. In plasma membranes from control cells in the presence of heat-inactivated human erythrocyte membranes both guanosine 5'-[beta, gamma-imido]triphosphate (p[NH]ppG) plus lutropin and NaF caused a 45--50-fold increase in cyclic AMP production over 30 min compared with 12--13 fold p[NH[ppG and 2--3-fold with lutropin alone. In plasma membranes isolated from lutropin-pretreated cells the NaF- and the p[NH]ppG-stimulated cyclic AMP production rates were unchanged, but no effect of lutropin could be demonstrated with or without added p[NH]ppG. However, after mixing lutropin-desensitized Leydig tumour-cell plasma membranes with freshly prepared human erythrocyte plasma membranes, the adenylate cyclase activity in the presence of lutropin, p[NH]ppG, lutropin plus p[NH]ppG and NaF were similar to those of control cell plasma membranes treated in the same manner. The possible mechanisms of this reversal of lutropin-induced desensitization by human erythrocytes are discussed.

Desensitization of tumour Leydig cells by lutropin: evidence for uncoupling of the lutropin receptor from the guanine nucleotide-binding protein

Purified rat Leydig tumour cells were pretreated with lutropin and the effect on the subsequent response to lutropin was determined. Maximal cyclic AMP production was achieved with the same concentration of lutropin in control and lutropin-pretreated cells; however, the maximum stimulated level in pretreated cells was only 30% of controls. The sensitivity to lutropin was decreased in lutropin-pretreated cells [ED(50) (dose that produces a response that is 50% of the maximum response) 60+/-5.7ng/ml and 8+/-1.8ng/ml (mean+/-s.d., n=3) for controls], as was the rate of maximal cyclic AMP production (0.58, compared with 1.89pmol/10(6) cells per min for controls). However, cholera-toxin-stimulated cyclic AMP production was not decreased by lutropin pretreatment, and a potentiation was seen at all time points studied (up to 6h). Pre-incubation with lutropin caused a decrease in specific (125)I-labelled human choriogonadotropin binding; however, this decrease was abolished if the cells were washed under acidic conditions (pH3.0 for 2min at 4 degrees C), indicating that occupation but not loss of the lutropin receptors had taken place. The effect of pretreating the cells with lutropin on adenylate cyclase activity in purified plasma membranes was also investigated. In plasma membranes from control cells both guanosine 5'-[beta,gamma-imido]triphosphate [p(NH)ppG] plus lutropin and NaF plus lutropin caused a 50-60-fold linear increase in cyclic AMP production over 40min compared with 15-fold with p(NH)ppG and 6-fold with lutropin alone. In plasma membranes isolated from lutropin-treated cells the NaF-plus-lutropin- and the p(NH)ppG-stimulated cyclic AMP production rates were unchanged but no effect of lutropin could be demonstrated with or without added p(NH)ppG. In contrast the plasma membranes from dibutyryl cyclic AMP-treated cells had similar cyclic AMP production rates to control cells with all stimulants studied. The present evidence obtained from studies both with intact cells and with isolated plasma membranes indicates that the initial lutropin-induced desensitization of the rat Leydig tumour cell is due to a lesion in the hormone-receptor coupling to the guanine nucleotide regulatory protein. This process is apparently not mediated by cyclic AMP.