Pathophysiological relationships between the biological and immunological activities of luteinizing hormone

Analysis of the impact of intravenous LH pulses versus continuous LH infusion on testosterone secretion during GnRH-receptor blockade

Gonadotrophin-releasing hormone (GnRH) pulsatility is required for optimal luteinizing hormone (LH) secretion, but whether LH pulsatility is required for physiological testosterone (T) secretion is not known. To test the postulate that pulses of recombinant human (rh) LH stimulate greater T secretion than continuous infusion of the same dose, a potent selective GnRH antagonist was administered overnight to 19 healthy men ages 18-49 yr. Subjects then received saline or rhLH intravenously continuously or as 6-min pulses intravenously every 1 or 2 h at the same total dose. Blood was sampled every 10 min for 10 h to quantify T responses. For the four interventions, the descending rank order of mean LH and mean T concentrations was 1-h = 2-h rhLH pulses > continuous rhLH > saline (P < 10(-3)). Plateau LH and T concentrations correlated positively (R(2) = 0.943, P = 0.029) as did LH concentrations and LH half-lives (R(2) = 0.962, P = 0.019). Percentage pulsatile T secretion assessed by deconvolution analysis (Keenan DM, Takahashi PY, Liu PY, Roebuck PD, Nehra AX, Iranmanesh A, Veldhuis JD. Endocrinology 147: 2817-2828, 2006) was the highest (P = 0.019), and half-time to attain peak T concentrations was the shortest (P < 10(-6)), for 1-h rhLH pulses. Approximate entropy (a pattern-regularity measure) revealed more orderly T secretion for 1- than 2-h rhLH pulses (P = 0.0076). Accordingly, a pulsatile LH signal, while not obligatory to maintain mean T concentrations, controls the mean plasma LH concentration and determines quantifiable patterns of T secretion. These data introduce the question whether blood T patterns in turn supervise distinctive target-tissue responses.

Older men exhibit reduced efficacy of and heightened potency downregulation by intravenous pulses of recombinant human LH: a study in 92 healthy men

Direct sampling of the human spermatic veins has disclosed concomitant LH and testosterone (T) pulses, suggesting pulsatile LH concentration-dependent stimulation of T secretion. However, studies to date have examined this hypothesis using only pharmacological stimulation with hCG. The present study tests the hypothesis that age is marked by decreased T secretory responses to repeated near-physiological iv pulses of recombinant human LH administered in a Clinical Translational Science Center. Participants included 92 healthy men aged 18-75 yr with BMI 18-34 kg/m(2). The contribution of endogenous LH pulses was minimized by combined injection of a selective GnRH receptor antagonist sc and successive pulses of biosynthetic LH iv. A new analytical dose response model was applied to estimate the properties of exogenous LH's drive of T secretion. Regression of LH-T dose response potency estimates on age showed that the efficacy of pulses of biosynthetic LH progressively decreased with age (P = 0.014, r = 0.26). Testis sensitivity to exogenous LH pulses also declined with age (P = 0.011, r = 0.27). Moreover, estimated Leydig cell downregulation by LH pulses rose significantly with age (P = 0.039, r = 0.22). These outcomes were selective, since the recovery potency of infused LH was not affected by age but was reduced by increasing BMI (P = 0.011, r = 0.27). Assuming stable bioactivity of infused recombinant human LH, these novel data indicate that factors associated with age and BMI attenuate LH efficacy and testis sensitivity and augment Leydig cell downregulation in healthy men.

Analytical construct of reversible desensitization of pituitary-testicular signaling: illustrative application in aging

Luteinizing hormone (LH) administered in pharmacological amounts downregulates Leydig cell steroidogenesis. Whether reversible downregulation of physiological gonadotropin drive operates in vivo is unknown. Most of the analytical models of dose-response functions that have been constructed are biased by the assumption that no downregulation exists. The present study employs a new analytical platform to quantify potential (but not required) pulsatile cycles of LH-testosterone (T) dose-response stimulation, desensitization, and recovery (pulse-by-pulse hysteresis) in 26 healthy men sampled every 10 min for 24 h. A sensitivity-downregulation hysteresis construct predicted marked hysteresis with a median time delay to LH dose-response inflection within individual T pulses of 23 min and with median T pulse onset and recovery LH sensitivities of 1.1 and 0.10 slope unit, respectively (P < 0.001). A potency-downregulation model yielded median estimates of one-half maximally stimulatory LH concentrations (EC(50) values) of 0.66 and 7.5 IU/l for onset and recovery, respectively (P < 0.001). An efficacy-downregulation formulation of hysteresis forecasts median LH efficacies of 20 and 8.3 ng·dl(-1)·min(-1) for onset and offset of T secretory burst, respectively (P = 0.002). Segmentation of the LH-T data by age suggested greater sensitivity, higher EC(50) (increased LH potency), and markedly (2.7-fold) attenuated LH efficacy in older individuals. Each of the three hysteresis models yielded a marked (P < 0.005) reduction in estimated model residual error compared with no hysteresis. In summary, model-based analyses allowing for (but not requiring) reversible pituitary-gonadal effector-response downregulation are consistent with a hypothesis of recurrent, brief cycles of LH-dependent stimulation, desensitization, and recovery of pulsatile T secretion in vivo and an age-associated reduction of LH efficacy. Prospective studies would be required to prove this aging effect.

Aging and hormones of the hypothalamo-pituitary axis: gonadotropic axis in men and somatotropic axes in men and women

Neuroendocrinology of the aging (male) gonadal and (male and female) somatotropic axes will be reviewed. A companion chapter discusses reproductive hormonal changes in aging women. Both the gonadal and growth-hormone/insulin-like growth factor (GH/IGF-I) axes function as ensembles. The ensembles comprise tripartite interactions among the brain (hypothalamus), anterior pituitary gland (gonadotrope and somatotrope cells) and target organs (testis, liver, muscle, fat and brain). Compelling evidence indicates that combined hypothalamic and gonadal adaptations operate in the reproductive axis of older men, and multiple hypothalamic adaptations prevail in the GH axis of elderly men and women. Evolving investigative methods allow more precise parsing of the particular mechanisms that subserve such age-related changes, and suggest novel interventional strategies to evaluate the physiological impact of the dynamic alterations discerned in aging individuals.

Age attenuates testosterone secretion driven by amplitude-varying pulses of recombinant human luteinizing hormone during acute gonadotrope inhibition in healthy men

CONTEXT

Whether testosterone (Te) depletion in aging men reflects deficits in the testis, hypothalamus, and/or pituitary gland is unknown.

OBJECTIVE

Our objective was to quantify the impact of age on gonadal Te secretion driven by amplitude-varying pulses of recombinant human LH (rhLH) in the absence of confounding by endogenous hypothalamo-pituitary signals.

DESIGN

This was a double-blind, placebo-controlled study.

SETTING

The setting was an academic medical center.

SUBJECTS

Fifteen healthy community-dwelling men ages 22-78 yr were included in the study.

INTERVENTION

Saline or four separate rhLH doses were each infused twice iv in randomized order as one pulse every 2 h over 20 h to stimulate Te secretion, after LH secretion was suppressed by a GnRH-receptor antagonist, ganirelix.

MAIN OUTCOME

LH and Te concentrations were determined in blood samples collected every 5 min. Maximal and minimal (as well as mean) Te responses were regressed linearly on age to reflect LH peak and nadir (and average) effects, respectively.

RESULTS

The ganirelix/rhLH paradigm yielded serum LH concentrations of 4.6 +/- 0.22 IU/liter (normal range 1-9). By regression analysis, age was associated with declines in rhLH pulse-stimulated peak and nadir (and mean) concentrations of total Te (P = 0.0068), bioavailable Te (P = 0.0096), and free Te (P = 0.013), as well as lower Te/LH concentration ratios (P < 0.005). Deconvolution analysis suggested that the half-life of infused LH increases by 12%/decade (P = 0.044; R(2) = 0.28).

CONCLUSIONS

Infusion of amplitude-varying pulses of rhLH during gonadal-axis suppression in healthy men unmasks prominent age-related deficits in stimulated total (39%), bioavailable (66%), and free (63%) Te concentrations, and a smaller age-associated increase in LH half-life. These data suggest that age-associated factors reduce the efficacy of LH pulses.

Resistance of hypogonadic patients with mutated GnRH receptor genes to pulsatile GnRH administration

We have studied a kindred with three siblings with isolated hypogonadotropic hypogonadism caused by compound heterozygote mutations in the GnRH receptor gene. The disorder was transmitted as an autosomal recessive trait. The R262Q mutation in intracellular loop 3 of the receptor was associated with a mutation in the third transmembrane domain of the receptor, A129D, that has never been described before. This A129D mutation results in a complete loss of function, indicated by the lack of inositol triphosphate (TP3) 3 production by transfected Chinese hamster ovary (CHO) cells after GnRH stimulation. The two brothers had microphallus and bilateral cryptorchidism and were referred for lack of puberty, whereas their sister had primary amenorrhea and a complete lack of puberty. Their basal gonadotropin concentrations were below the reference range, and their endogenous LH secretory patterns were abnormal, with a low-normal frequency of small pulses or no apparent LH pulse. Pulsatile GnRH administration (10 microg/pulse every 90 min for 40 h) resulted in increased mean LH without any significant changes in testosterone levels in the two brothers, whereas the LH secretory profile of their sister remained apulsatile. Larger pulses of exogenous GnRH (20 microg every 90 min for 24 h) caused the sister to produce recognizable low amplitude LH pulses. The concentrations of free alpha-subunit significantly increased in all patients during the pulsatile GnRH administration. Thus, these hypogonadal patients are partially resistant to pulsatile GnRH administration, suggesting that they should be treated with gonadotropins to induce spermatogenesis or ovulation rather than with pulsatile GnRH.

Establishment of an in vitro bioassay and radio receptor assay for LH/CG in human sera using immortalized granulosa cells transfected with LH/CG receptor

Levels of gonadotropic hormones in human sera or urine are routinely measured by radioimmunoassay or by enzyme-linked immunoassay (ELISA), which determine the immunoactivity of the hormone, but not its biological activity. We have utilized immortalized stable steroidogenic granulosa cells, which express 5-10 times more of the luteinizing hormone/chorionic gonadotropin (LH/CG) receptors than the primary cells, to develop a biological assay and radioreceptor assay for this hormone. We found that stimulation of cells expressing LH/CG receptor with increasing doses of human LH or human CG resulted in a dose-dependent increase of cAMP and progesterone with an ED(50) of 30 and 57 mlU/mL, respectively. These dose-response data served as calibration curves for measuring the gonadotropin bioactivity in human serum samples at concentrations as low as 1-5 mlU/mL. We found a close correlation between LH levels measured by enzyme immunoassay (EIA) and the in vitro bioassay in normal cycling and menopausal women, as well as in normal adult men. Also, a close correlation was found between the EIA and the in vitro biological assay of hCG in pregnant women. In addition, we have developed a radioreceptor assay (RRA) for this hormone using enriched cell membranes of the appropriate cell line, which corresponds well to both the EIA and the bioassay in human sera. Deglycosylated hCG was fully active in RRA, but failed to activated cAMP response in these cells, demonstrating the importance of the bioassay in the biologically inactive form of gonadotropins. We believe this novel in vitro bioassay of gonadotropic hormones will serve as a useful tool for a more comprehensive set of assays that will determine not only the amount, but also the possible modulation in bioactivity of the gonadotropin associated with gonadal failure and miscarriage.

Basal and stimulated gonadotropin levels in the perimenopause

OBJECTIVE

We investigated whether perimenopausal menstrual cycle irregularity is associated with increased gonadotropin immunoactivity, bioactivity, or the bioactivity/immunoactivity ratio at baseline and after short-term stimulation with gonadotropin-releasing hormone.

STUDY DESIGN

Subjects consisted of four groups: (1) young regular cycling women (< 35 years old), older women (> 35 years) with (2) regular or (3) irregular menstrual cycles, and (4) postmenopausal women. Gonadotropin-releasing hormone stimulation tests (100 micrograms intravenous gonadotropin-releasing hormone) were performed in the National Institute of Mental Health outpatient clinic during the follicular phase of the menstrual cycle or randomly in postmenopausal women.

RESULTS

Perimenopausal women had baseline follicle-stimulating hormone and luteinizing hormone levels and stimulated follicle-stimulating hormone levels (area under the curve) that were similar to those of postmenopausal women and significantly greater than those of control (younger and older) women. Postmenopausal women had significantly greater baseline levels of luteinizing hormone bioactivity than did the other three groups. The bioactivity/immunoactivity ratio in the postmenopausal women was significantly greater than those in the perimenopausal and older cycling women, which were similar. No change in the bioactivity/immunoactivity ratio was seen after gonadotropin-releasing hormone stimulation in any group.

CONCLUSIONS

Although the perimenopause is associated with increases in baseline and stimulated gonadotropin levels similar to those seen after the menopause, significantly increased baseline luteinizing hormone bioactivity and the bioactivity/immunoactivity ratio are seen only after the menopause.

Sensitive immunoassay and in vitro bioassay demonstrate constant bioactive/immunoreactive ratio of luteinizing hormone in healthy boys during the pubertal maturation

The quality of serum LH was assessed during pubertal maturation in boys by measuring immunoreactive (I) LH by a time-resolved immunofluorometric assay (IFMA, Delfia), and bioactive (B) LH by a sensitized in vitro bioassay. Seven samples were collected at 3-mo intervals from 14 healthy boys (median starting age 11.8 y) during pubertal maturation from Tanner stage I-III or II-IV (n = 7 for each). The mouse Leydig cell in vitro bioassay was sensitized 10-fold, to 0.05-0.1 IU/L, by including 1.5 mumol/L of forskolin in the incubation medium. The I- and B-LH levels showed good linear correlation throughout the concentration range analyzed. Mean I-LH increased between the pubertal stages I-IV from 0.42 to 2.24 IU/L and that of B-LH from 1.35 to 5.04 IU/L. No concomitant change occurred in the B-LH/I-LH (B/I) ratio, which was 2.84 +/- 0.54 in stage I and 2.58 +/- 0.48 in stage IV (mean +/- SEM, n = 7). Although the B/I ratios of LH varied from 0.59 to 5.85 in the samples analyzed, the intraindividual variation was small (mean coefficient of variance, 22%). In conclusion, IFMA and sensitized in vitro bioassay showed in healthy boys a similar 4-5-fold increase in the mean LH concentration during pubertal maturation, with no concomitant change in the B/I ratio. The sensitized in vitro bioassay of LH is useful for analysis of the low peripubertal LH levels. The good correlation between the I-LH and B-LH levels, and the lack of change in LH B/I ratio, indicate that IFMA correctly estimates the LH levels upon evaluation of pubertal maturation.

Less acidic forms of luteinizing hormone are associated with lower testosterone secretion in men on haemodialysis treatment

OBJECTIVE

Men with chronic renal failure treated by haemodialysis have raised levels of bioactive LH (B-LH) and immunoreactive LH (I-LH) but reduced B-LH:I-LH (B:I) ratio and testosterone (T) secretion. This study investigated the LH isoform distribution in serum from normal adult males and males on regular haemodialysis treatment.

DESIGN

Four blood samples (2 ml) were obtained at 15-minute intervals from a group of men on regular haemodialysis treatment. These samples were part of a larger pulse profile series and showed no evidence of LH pulsatility. The serum was pooled for each individual patient. Blood (10 ml) was also drawn randomly from healthy male volunteers. The sera were chromatofocused on a 4-ml mono-P column attached to a fast performance liquid chromatography system. This procedure separates the LH isoforms according to their isoelectric point. The pH gradient was between pH 7 and pH 4.

PATIENTS

The five men with chronic renal failure were aged between 18 and 40 years and had been on haemodialysis for a mean of 10 months (5-20). They were sampled the night prior to a dialysis session. The five normal healthy volunteers had never had any endocrine disorder diagnosed.

MEASUREMENTS

An immunoradiometric assay and a commercially available (Delfia) immunofluorimetric assay were employed for detection of LH in the sera and in chromatofocusing fractions. B-LH and testosterone were also measured in the sera.

RESULTS

Hormone data (mean +/- SEM for normal and renal subjects respectively) were 15.5 IU/l +/- 1.2 and 26.9 +/- 7.2 (B-LH), 6.0 IU/l +/- 0.3 and 16.5 +/- 4.8 (irmaLH), 5.7 +/- 0.5 and 13.6 +/- 4.8 (fluorLH), 25.2nmol/l +/- 2.0 and 12.1 +/- 1.2 (T). The serum B:I ratios were 2.6 +/- 0.1 and 2.6 +/- 0.2 (controls, irmaLH and fluorLH respectively) and 1.7 +/- 0.1 and 2.1 +/- 0.1 (chronic renal failure group). Recovery of LH from the column was 111 +/- 12% (mean +/- SEM) by IRMA and 104 +/- 7% by IFMA for the ten FPLC runs. The median pI for the LH distribution measured by both assays was in the region 6.54-6.40 for subjects with chronic renal failure and 6.09-5.95 for controls. Median pI was negatively correlated to the B-LH:irmaLH (P < 0.0001) and B-LH:fluorLH ratios (P = 0.002) in the serum. Furthermore, the proportion of isoforms recovered in the pH region 6.25-5.50 increased with increasing T levels in the serum (P < 0.004).

CONCLUSION

The distribution of LH in serum of men on haemodialysis is more basic than in normal men. The greater the proportion of more acidic LH species, particularly those with a pI of between 5.50-6.25, the higher the ratio of LH bioactivity to immunoactivity and consequently testosterone levels.

Physiological oestradiol administration does not change the biological to immunological ratio of LH in serum from post-menopausal women

OBJECTIVE

We investigated the effect of chronic physiological concentrations of oestradiol on the bioactivity (B) and immunoactivity (I) of LH in serum using radioimmunoassay (RIA) and a sensitive immunoradiometric (IRMA) assay.

DESIGN

Twelve post-menopausal, hysterectomized women received a 50-mg subcutaneous E2 implant, in the absence of progestogen treatment. Pretreatment serum samples were obtained on two occasions for measurement of E2 and FSH by radioimmunoassay and for measurement of LH by bioassay (BIO), immunoradiometric assay and radioimmunoassay. Further serum samples were obtained monthly for 6 months for assay as above.

RESULTS

The serum E2 level (basal levels < 70 pmol/l) rose following implant (P < 0.01) peaking at 1 month (mean 454, range 356-580 pmol/l) and remaining within premenopausal physiological levels for 6 months. There were falls in FSH, LH-RIA, LH-IRMA and LH-BIO (P < 0.001). There was no change in the B:I ratio for B:I-RIA or B:I-IRMA for LH which were 1.8 and 3.3, respectively pretreatment and 1.7 and 2.9 after 1 month's treatment.

CONCLUSION

The treatment of post-menopausal women with 50-mg E2 subcutaneous implants produces physiological premenopausal E2 levels for at least 6 months. There is no change in the B:I ratio of LH using either the RIA or IRMA during chronic E2 treatment.

The significance of a single serum LH measurement in women with cycle disturbances: discrepancies between immunoreactive and bioactive hormone estimates

OBJECTIVE

We evaluated the significance of single serum LH estimates (as assessed by radiometric assay (IRMA) and Leydig cell in-vitro bioassay (BIO)) for the diagnosis of polycystic ovary syndrome (PCOS) in women with infertility and cycle abnormalities.

DESIGN

Hormonal and clinical comparisons between subgroups were made based on classification according to (a) rigid clinical and endocrine (excluding LH) characteristics of PCOS, (b) elevated IRMA-LH concentrations, (c) BIO-LH levels. In addition, androgen modulation of LH biopotency was studied in these patients.

PATIENTS

Ninety-nine women presenting at our infertility Unit with oligo/amenorrhoea.

MEASUREMENTS AND RESULTS

Of the total study group, 35 women were diagnosed positive as PCOS and 42 showed elevated IRMA-LH levels. Only 51% (n = 18) of PCOS patients showed elevated IRMA-LH levels, and in PCOS significantly higher levels of BIO-LH, androstenedione, oestrone, and BIO/IRMA-LH ratios were found as compared to non-PCOS patients. In the group with elevated IRMA-LH only 43% (n = 18) of subjects were diagnosed as PCOS, and no difference in BIO/IRMA-LH ratios was found. With increasing BIO-LH levels the probability of PCOS rises sharply (P < 0.001), whereas this probability is of only marginal significance (P < 0.06) for IRMA-LH. In the total study group a correlation is observed between serum testosterone (T) levels and IRMA-LH (r = 0.47), and BIO-LH (r = 0.51) concentrations. This correlation is absent comparing serum T and BIO/IRMA-LH ratios (r = 0.15).

CONCLUSIONS

Results presented in this study indicate that (1) women with infertility and oligo/amenorrhoea classified based on signs of PCOS or IRMA-LH levels, exhibit different clinical and endocrine characteristics, (2) only 51% of PCOS women exhibit elevated IRMA-LH concentrations, and only 43% of women with elevated IRMA-LH were diagnosed as PCOS, (3) IRMA-LH levels are a poor predictor of PCOS, whereas the predictive value of BIO-LH is better, (4) elevated BIO/IRMA-LH ratios in PCOS are dependent on alterations in BIO-LH, rather than IRMA-LH concentrations, and (5) no correlation was observed between serum T levels and BIO/IRMA-LH ratios.

Evaluation of testicular function

The evaluation of testicular function is based primarily on a detailed medical history, a careful physical examination, basal measurements of FSH, LH and testosterone and a routine semen analysis. In a patient with androgen deficiency, the diagnosis can often be made with these basic tests. The clinician will then decide on other investigations to localize the organic lesion and to plan further treatment for the patient. Sperm function tests are often performed in patients presenting with infertility. These newer tests may help to delineate the abnormality of the spermatozoa at each stage during the achievement of fertilizing capacity such as adequate forward motility, penetration of cervical mucus, acrosome reaction, development of hyperactivated motility, binding to the zona pellucida, and fusion with the oocyte. Currently, many of these sperm function tests depend on cumbersome bioassays with many limiting factors contributing to their availability, accuracy and precision. The development of biochemical tests as markers of sperm function may allow more precise definition of sperm functional abnormalities. With the continued improvement of computer-aided sperm analysis, objective motion parameter measurements are possible and morphological assessment are being developed. These newer objective methods of semen analysis have to be shown to be valuable in the clinical assessment of patients with testicular dysfunction.

Pattern of secretion of luteinizing hormone and testosterone in the sexually mature male turkey

Whether luteinizing hormone (LH) and testosterone (T) are secreted in pulsatile patterns was determined in sexually mature male turkeys. Turkeys were chronically cannulated and serially bled for three 8-hr periods covering the 24-hr day (14L:10D, n = 7, series B), or for two 12-hr periods covering the 24-hr day (14L:10D, n = 4, series C). Pulses of both LH and T occurred during both the light and dark portions of the 24-hr day. A portion of the secretory episodes of T, where the baseline level of LH was relatively low, was associated with prior peaks of LH secretion. Secretory episodes of T also occurred, where baseline levels of LH and T were both relatively high, without detection of prior peaks of LH. No differences were found between the photophase and scotophase portions of the photoperiod for either LH or T concentration. It is concluded that T is secreted in a pulsatile pattern in sexually mature male turkeys. However, LH is secreted in a pulsatile pattern only when baseline levels of both LH and T are relatively low. Neither LH nor T secretion is entrained by the photoperiod. Corticosterone was measured in hourly samples, but no changes in concentration occurred in association with the photoperiod.

Prolactin and growth hormone: molecular heterogeneity and measurement in serum

The pituitary hormones prolactin and growth hormone are related single-chain polypeptides. Both hormones exist in the circulation in several molecular forms, and this heterogeneity may account for some of the complex and sometimes contradictory actions, in vivo and in vitro, of both hormones. It may also lead to problems with quantitation by immunoassays and discrepancies between the results given by assays using different antibodies. Modified forms of the hormones may have markedly different activity in bioassays from that of the parent hormone, but the clinical significance of this is unclear. In this review we summarize what is known about the molecular heterogeneity of the hormones and briefly discuss the implications for clinical biochemists.

Regulation of testicular function by insulin and transforming growth factor-beta

Hyperinsulinism is associated with disorders of androgen production in humans. We have studied the effects of insulin and insulin-like growth factor-1 on androgen production in vitro using a crude preparation of mouse Leydig cells incubated with luteinizing hormone in a serum-free medium. We found a positive correlation between testosterone production and the luteinizing hormone dose over 3 hours. Exposure of the cells for 1 hour to insulin (1 micrograms/ml) prior to the addition of luteinizing hormone significantly augmented the amount of testosterone produced in response to the gonadotropin when added after this preincubation. In contrast, prior exposure of the cells to proinsulin (30 micrograms/ml), insulin-like growth factor-1 (30 ng/ml), or epidermal growth factor-1 (1 micrograms/ml) did not influence the testosterone response to luteinizing hormone. Transforming growth factor-beta reduced the testosterone response to luteinizing hormone. Transforming growth factor-beta (1,000 pg/ml) blocked the insulin augmentation of luteinizing hormone-stimulated testosterone production. We conclude that insulin has an endocrine effect on testosterone production by mouse Leydig cells in vitro. Furthermore, the Leydig cell response to insulin is itself sensitive to interaction with transforming growth factor-beta which may operate as part of the paracrine control of Leydig cell function.

Luteinizing hormone pulsatility in men with damage to the germinal epithelium

Bioactive-LH (B-LH) was measured in plasma by in-vitro bioassay and immunoactive-LH (I-LH) by immunoassay at 10 min intervals for 6 h in five men after standard chemotherapy for Hodgkin's disease. Eleven normal men acted as controls. Follicle-stimulating hormone (FSH) was markedly raised in the treated patients (mean +/- SEM; 12.8 +/- 2.8 vs. 2.7 +/- 0.4 IU l-1, P less than 0.006) reflecting damage to the germinal epithelium. Bioactive (27.4 +/- 2.8 vs. 12.9 +/- 1.3 IU l-1) and I-LH (9.6 +/- 2.0 vs. 4.9 +/- 0.4 IU l-1) were elevated (P less than 0.006) in the patient group whilst testosterone levels (24.0 +/- 3.8 vs. 19.6 +/- 2.4 nmol l-1) were normal. The testosterone I-LH ratio, a putative index of Leydig cell dysfunction, was negatively correlated with FSH levels (r = -0.85, P less than 0.02). Bioactive and I-LH pulse peak amplitude were elevated, as were pulse maxima (P less than 0.05). In contrast, B-LH pulse frequency was similar between the patients (2 pulses per 6 h) and controls (median 2, range 1-3 pulses per 6 h) as was the I-LH pulse frequency (median 2, 1-2 pulses per 6 h in both groups). The mean B:I LH ratios were similar (2.94 +/- 0.09 vs. 2.63 +/- 0.14) in both groups, although the inter-pulse B:I ratio was increased (P less than 0.007) in the patient group.(ABSTRACT TRUNCATED AT 250 WORDS)

Attenuated release of biologically active luteinizing hormone in healthy aging men

To examine the biological quality and physiologically pulsatile mode of endogenous luteinizing hormone release in active, healthy aging men, we used the rat interstitial-cell testosterone in vitro bioassay to probe LH bioactivity in response to (a) endogenous gonadotropin-releasing hormone (GnRH) action (basal pulsatile bioactive LH secretion); (b) exogenous GnRH stimulation (10 micrograms IV pulses); and (c) inhibition of endogenous estrogen negative feedback (treatment with a nonsteroidal antiestrogen, tamoxifen). Basally, some healthy older men exhibited evidence of neuroendocrine dysfunction, reflected by irregular bursts of bioactive LH release followed by transiently low plasma bio:immuno (B:I) LH ratios. However, mean basal plasma bioactive LH concentrations, B:I ratios, and spontaneous LH pulse properties (peak frequency, amplitude, duration, and enhanced B:I ratios within LH peaks) were not altered in older men. On the other hand, augmentation of bioactive LH secretion and enhancement of plasma B:I ratios by pulsed injections of exogenous GnRH were either significantly reduced or absent in older men. In addition, although tamoxifen increased bioactive LH pulse frequency in both age groups and facilitated exogenous GnRH action in some subjects, older men increased their 12-h mean bioactive LH concentrations, B:I ratios, and bioactive LH peak amplitudes to a significantly lesser degree than young men. In summary, young and older healthy men exhibit similar mean basal plasma bioactive LH concentrations and spontaneous LH pulse properties. However, pituitary bioactive LH reserve is markedly attenuated in older men challenged with either exogenous GnRH or antiestrogen. Accordingly, we conclude that healthy aging men manifest an impaired secretory reserve for biologically active LH release.