Workgroup report: Implementing a national occupational reproductive research agenda--decade one and beyond

The initial goal of occupational reproductive health research is to effectively study the many toxicants, physical agents, and biomechanical and psychosocial stressors that may constitute reproductive hazards in the workplace. Although the main objective of occupational reproductive researchers and clinicians is to prevent recognized adverse reproductive outcomes, research has expanded to include a broader spectrum of chronic health outcomes potentially affected by reproductive toxicants. To aid in achieving these goals, the National Institute for Occupational Safety and Health, along with its university, federal, industry, and labor colleagues, formed the National Occupational Research Agenda (NORA) in 1996. NORA resulted in 21 research teams, including the Reproductive Health Research Team (RHRT). In this report, we describe progress made in the last decade by the RHRT and by others in this field, including prioritizing reproductive toxicants for further study; facilitating collaboration among epidemiologists, biologists, and toxicologists; promoting quality exposure assessment in field studies and surveillance; and encouraging the design and conduct of priority occupational reproductive studies. We also describe new tools for screening reproductive toxicants and for analyzing mode of action. We recommend considering outcomes such as menopause and latent adverse effects for further study, as well as including exposures such as shift work and nanomaterials. We describe a broad domain of scholarship activities where a cohesive system of organized and aligned work activities integrates 10 years of team efforts and provides guidance for future research.

An occupational reproductive research agenda for the third millennium

There is a significant public health concern about the potential effects of occupational exposure to toxic substances on reproductive outcomes. Several toxicants with reported reproductive and developmental effects are still in regular commercial or therapeutic use and thus present potential exposure to workers. Examples of these include heavy metals, organic solvents, pesticides and herbicides, and sterilants, anesthetic gases, and anticancer drugs used in health care. Many other substances are suspected of producing reproductive or developmental toxicity but lack sufficient data. Progress has been limited in identifying hazards and quantifying their potencies and in separating the contribution of these hazards from other etiologic factors. Identifying the causative agents, mechanisms by which they act, and any potential target populations, present the opportunity to intervene and protect the reproductive health of workers. The pace of laboratory studies to identify hazards and to underpin the biologic plausibility of effects in humans has not matched the pace at which new chemicals are introduced into commerce. Though many research challenges exist today, recent technologic and methodologic advances have been made that allow researchers to overcome some of these obstacles. The objective of this article is to recommend future directions in occupational reproductive health research. By bridging interdisciplinary gaps, the scientific community can work together to improve health and reduce adverse outcomes.

Regulation of cyclic guanosine 3',5'-monophosphate in interstitial cells of the rat testis

The present studies were performed to evaluate the role of luteinizing hormone (LH) and calcium on cyclic guanosine 3',5'-monophosphate (cyclic GMP) formation in rat testicular interstitial cells. Dose related increases were observed in both intra-and extra-cellular cyclic GMP when various concentrations of LH (1, 10, 100 mlU/ml) were added to the purified Leydig cells. Cyclic GMP was also increased significantly with calcium ionophore A23187 at concentrations of 0.1, 1 and 10 microgram/ml. Adding LH concomitantly with A23187 resulted in a synergistic response and greatly enhanced cyclic GMP formation. When interstitial cells were incubated in calcium-free medium with 1 mM EGTA, the effects of LH and 123187 on cyclic GMP formation were abolished. These data suggest that LH and A23187 can increase cyclic GMP formation in interstitial cells and this response requires extracellular calcium.

The aging Leydig cell: 2. Two distinct populations of Leydig cells and the possible site of defective steroidogenesis

Using metrizamide gradient centrifugation two populations of Leydig cells were found in both 60-90 day-old and 24 month-old rats. Cells from both Band 2 (B2) and Band 3 (B3) responded to LH stimulation with increased cyclic AMP formation; however, only B3 cells produced significant amounts of testosterone. Cells from both B2 and B3 of the old rats synthesized less cyclic AMP and testosterone than cells from their younger counterparts. In response to LH stimulation, 0.01 - 1.0 mIU/ml, no appreciable difference of cyclic AMP formation could be detected between young and old Leydig cells. Maximal testosterone production occurred when 1 mIU/ml LH was used. Only when LH concentration was increased to 10 and 100 mIU/ml, did young Leydig cells produce significantly more cyclic AMP than old Leydig cells. After addition of 5X10(-7)M of pregnenolone or progesterone to the incubation medium, both young and old Leydig cells produced comparable amounts of testosterone. These results demonstrate no impairment of old rat Leydig cells to synthesize testosterone from pregnenolone and progesterone.

Morning and evening human Leydig cell responses to hCG

The present 72 h study compares the patterns of 17α-hydroxyprogesterone (17OHP) and testosterone responses to im hCG (40 IU/kg) injected at 09.00 h and on different occasions at 17.00 h in eight normal men. Basal a.m. steroid concentrations were significantly higher for both steroids (15% for testosterone and 52% for 17OHP). A peak (at 2 h and 3 h) and statistically significant nadir (at 5 h, 5.5 h and 6 h) response was found for each steroid in the a.m. studies, but in the p.m. studies the two steroids rose significantly at 2 h and remained plateaued through 6 h. Between 5 h and 6 h, and for the remainder of the morning and evening studies, the respective steroid concentrations and the patterns to the changes were the same. Between 6 h and 24 h both steroids rose further. 17OHP peaked at 24 h and 48 h, 3.2 times basal and declined at 72 h. Testosterone rose further between 24 h and 48 h, and was unchanged at 72 h. At the latter two times it averaged 2.2 fold basal values. hCG was detectable at 1 h, rose progressively to highest levels at 24 h, and declined thereafter — in similar fashion in the a.m. and p.m. studies.

The 'peak and nadir' responses occur within the first few hours of gonadotrophin stimulation begun in the morning and appear to be related to the time of day and do not appear to be the result of a biosynthetic block. This suggests a diurnal variation of human Leydig cell response. With intense, probably maximal, testis stimulation each steroid shows the same respective pattern and circulating concentrations between 5 h and 72 h in both the a.m. and p.m. studies.

Testosterone and 17 OH-progesterone responses in men to 3 h LH infusions

LH is recognized as the Leydig cell stimulus for testosterone production. Yet earlier studies of bolus administered human pituitary LH did not result in testosterone elevations and spontaneous LH elevations infrequently cause recognizable parallel rises of testosterone in adult males. Therefore, 10 normal adult volunteers were given 3 h infusions of human pituitary LH A-3 (2 IU/kg) and sampling was continued for 3 h after the gonadotrophin was stopped. Testosterone, the precursor 17 OH-progesterone and LH were determined in blood samples. There was a mean 5.2-fold LH elevation. From a group baseline mean of 698 ng/100 ml ± 72 (± se) testosterone gradually rose and was significantly elevated at 90, 120 and 150 min (787 ± 67 P <0.03, 767 ± 60 P < 0.04, and 804 ± 69 P < 0.001, respectively). 17 OH-progesterone was determined on 9 subjects and averaged 142 ng/100 ml ± 16 before LH and gradually rose to 178 ± 18 at 120 min (P < 0.05) and then declined slightly at 150 and 180 min. Both steroids significantly declined between 180 min (end of LH infusion) and 240 min with no further significant concentration changes. The two men with the highest basal testosterone titers showed the smallest testosterone increases, but in both 17 OH-progesterone rose. In conclusion tonic LH stimulation caused mean testosterone and 17 OH-progesterone to peak at 150 min and at 120 min, respectively. Factors within the Leydig cell appear to limit increases of testosterone and 17 OH-progesterone during the first few hours of enchanced gonadotrophin stimulation.

The aging Leydig cell: 1. Testosterone and adenosine 3',5'-monophosphate responses to gonadotropin stimulation in rats

Plasma testosterone levels before and after a single injection of hCG were significantly lower in 24-month old rats than 60--90 day old animals (p less than 0.001). Even with repeated hCG administration for three weeks, plasma testosterone levels of old rats could not be restored to levels present in unstimulated young rats. In response to in vitro LH and 8-bromo-cyclic AMP stimulation, purified young Leydig cells produced significantly higher amounts of testosterone than Leydig cells from old rats. Maximal testosterone formation of the young Leydig cells in response to LH was 42.0 +/- 6.88 ng/10(6) cells, while cells from old rats produced only 16.8 +/- 3.69 ng/10(6) cells (p less than 0.01). However, the dose of LH at which one half maximal response (ED50) occurred was 0.1 mIU/ml for young Leydig cells and 0.05 mIU/ml for old Leydig cells. Basal and 1.0 mIU LH-stimulated cyclic AMP formation were comparable in both groups, but cyclic AMP formation in response to 10 mIU of LH was significantly less in the old rats (p less than 0.05). Present results demonstrate impaired steroidogenic capacity of old rats both in vivo and in vitro. Decreased testosterone response in old rats most likely is the consequence of understimulation of Leydig cells by gonadotropin; however, there appear to be additional intrinsic defects in old Leydig cells.

The effects of calcium ionophore A23187 on interstitial cell steroidogenesis

The present study was performed to evaluate the effects of calcium ionophore A23187 on adenosine 3',5'-monophosphate (cyclic AMP) and testosterone production in rat interstitial cells. Interstitial cells were incubated in Krebs-Ringer solution with varying amounts of luteinizing hormone, pregnenolone, or A23187. Cyclic AMP and testosterone were measured in the incubation medium after 4 h incubation. A23187 (0.01--10 microgram/ml) caused progressive increases of cyclic AMP formation (from 0.18 +/- 0.02 (S.E.) pmol/10(6) cells for the control of 0.42 +/- 0.02 pmol/10(6) cells, P less than 0.025), while testosterone production remained unaltered. When varying amounts of A23187 were added concomitantly with luteinizing hormone (5 IU/l), A23187 inhibited luteinizing hormone-induced steroidogenesis in a dose-dependent manner, but it had no effect on luteinizing hormone-induced cyclic AMP formation. When pregnenolone (10(-6) M) was added to the cells, testosterone formation increased from 1.50 +/- 0.22 to 8.46 +/- 1.65 ng/10(6) cells. A23187 (1 microgram/ml) had no discernable effect on the conversion of pregnenolone to testosterone. The main effect of increased cytosol calcium on steroidogenesis seems to be at the steps beyond adenylate cyclase-cyclic AMP. These results suggest that calcium is important for the conversion of cholesterol to pregnenolone, while the steps beyond pregnenolone are relatively independent of Ca2+.