Human surrogates for injury biomechanics research.

This article reviews the attributes of the human surrogates most commonly used in injury biomechanics research. In particular, the merits of human cadavers, human volunteers, animals, dummies, and computational models are assessed relative to their ability to characterize the living human response and injury in an impact environment. Although data obtained from these surrogates have enabled biomechanical engineers and designers to develop effective injury countermeasures for occupants and pedestrians involved in crashes, the magnitude of the traffic safety problem necessitates expanded efforts in research and development. This article makes the case that while there are limitations and challenges associated with any particular surrogate, each provides a critical and necessary component in the continued quest to reduce crash-related injuries and fatalities.

Gonadal steroid hormone regulation of the somatotropic axis during puberty in humans Mechanisms of androgen and estrogen action.

The adolescent growth spurt is associated with a sex steroid hormone-dependent rise in GH production; both androgens and estrogens are implicated as positive regulators of the somatotropic axis during puberty. The issue is complicated by the fact that testosterone may act both directly via the androgen receptor and indirectly, after its aromatization to 17beta-estradiol, through the estrogen receptor. Recently, a number of investigators have studied the effects of the administration of androgen and estrogen receptor antagonists, as well as nonaromatizable androgens, on GH secretion. These reports suggest that estrogen receptor-dependent processes play a facilitatory role in the pubertyassociated rise in GH secretion. If androgen receptor-mediated events are involved in the control of the somatotropic axis, their role is likely inhibitory. A hypothalamic site of action of the sex steroids is postulated.

Pedestrian injuries: viscoelastic properties of human knee ligaments at high loading rates.

OBJECTIVE: Accidents involving pedestrians are very common, and often lead to severe injuries to the lower extremities. In a large portion of pedestrian-automobile collisions, knee ligament injuries are sustained. In this study, the viscoelastic properties of the four major human knee ligaments were investigated at loading rates representative for pedestrian-automobile collisions. METHODS: Bone-ligament-bone specimens were tested in knee distraction loading. The collateral ligaments and the separate functional bundles of the cruciate ligaments were tested in the anatomical position corresponding to a fully extended knee. A series of step-and-hold tests and ramp tests at different rates were conducted to characterize the time-dependent behavior of the knee ligaments for deformation rates associated with the pedestrian impact loading environment. The quasi linear viscoelastic (QLV) theory was used to describe the structural response of the knee ligaments and averaged parameters for this model were determined. RESULTS: The QLV theory was found to be applicable for the time range that is relevant for pedestrian-automobile collisions. The structural behavior of the knee ligaments was found to be particularly rate-sensitive for high elongation rates, as occur during these collisions. The ligament stiffness was found to increase with age for both the collateral ligaments and with weight for the medial collateral ligament. CONCLUSIONS: For the loading conditions that are relevant for pedestrian-automobile collisions, the use of the QLV model for the description of the mechanical behavior of knee ligaments is appropriate. The rate-sensitivity is particularly important for these extreme loading conditions. The relaxation behavior was found to be consistent between different ligament types and samples. Variations due to donor anthropometry were found predominantly for the instantaneous elastic behavior.

Somatostatin inhibition of growth hormone secretion by somatotropes from male, female, and androgen receptor-deficient rats: evidence for differing sensitivities.

To investigate the role of somatostatin (SRIF) in regulating sexually dimorphic GH secretion, we used a reverse hemolytic plaque assay and acutely dispersed somatotropes from age-matched normal male, normal female, and androgen receptor-deficient, testicular feminized (Tfm) rats. Hemolytic plaques were developed after a 90-min incubation in the presence of GH antiserum, 10 nM GH-releasing hormone (GHRH), and the following concentrations of SRIF: 0, 0.001, 0.003, 0.01, 0.03, 0.1, 0.3, 1, 3, 10, 30, and 100 nM. Additional studies were performed with 0 or 100 nM SRIF in the absence of GHRH. The absolute number of somatotropes (x10(6); mean +/- SEM) recovered from the pituitaries of Tfm rats (1.73 +/- 0.18) was significantly greater than that from the males (1.11 +/- 0.13; P = 0.01); the number from female rats (1.30 +/- 0.15) was not different from that of either male or Tfm animals. GHRH-stimulated GH secretion, as estimated by the mean GH plaque area (micron2 x 10(4); mean +/- SEM) in the absence of SRIF, was greater for somatotropes from male rats (3.36 +/- 0.41) than that for either Tfm (2.27 +/- 0.32; P = 0.02) or female (1.78 +/- 0.24; P = 0.001) rats; values for the latter two groups did not differ. However, mean GH plaque areas for each group during maximal SRIF inhibition in either the presence or absence of GHRH were indistinguishable from each other and from mean plaque areas obtained under basal conditions. As demonstrated by a lesser EC50 value (0.04 +/- 0.02 nM; mean +/- SEM), somatotropes from female rats were more sensitive to the inhibitory effect of SRIF than were those from either male (EC50 = 1.82 +/- 0.45; P = 0.0001) or Tfm (EC50 = 0.74 +/- 0.22, P = 0.0001) rats; values for the latter two groups were indistinguishable. These observed differences suggest that gender and/or the gonadal hormone environment may be important determinants of the inhibitory effects of SRIF on GH secretion by the somatotrope. While these gender-associated differences may represent effects of the gonadal hormones directly on the somatotrope, they could reflect modulation of the secretion of hypothalamic SRIF and/or GHRH by the prevailing gonadal hormone environment. Such gender-related differences may contribute to the overall sex-dependent patterns of GH secretion in the intact animal.

Failure of gonadotropin-releasing hormone (GnRH) pulses to increase luteinizing hormone beta messenger ribonucleic acid in GnRH-deficient female rats.

Gonadotropin subunit gene transcription and messenger RNA (mRNA) levels are differentially regulated by GnRH pulse frequency and amplitude in the male rat. The rapid changes of subunit mRNA levels and LH and FSH secretion during the estrous cycle, particularly the rapid rise in LH-beta subunit mRNA on proestrus afternoon, suggest that physiological changes in the pattern of GnRH action may also be important in female rats. However, in the absence of a GnRH-deficient female model the role of varying GnRH stimulation remains to be determined. We have characterized a GnRH-deficient model by administering the alpha-adrenergic antagonist phenoxybenzamine (PBZ) to ovariectomized (OVX) rats. Initial experiments showed that PBZ given 24 h earlier abolished the afternoon LH surge in OVX estradiol (E2) replaced rats whereas LH responses to exogenous GnRH were preserved. A PBZ regimen of 15 mg/kg ip at OVX followed by 10 mg/kg at 24 h and 5 mg/kg at 48 h prevented the increase in alpha, LH-beta, and FSH-beta mRNAs and LH and FSH secretion for 72 h post-OVX. LH and FSH responses to GnRH pulses were preserved suggesting that PBZ blocked the post-OVX increase in hypothalamic GnRH secretion. The suppressive effect of PBZ appeared to be specific to the hypothalamic-pituitary-ovarian axis as plasma PRL, TSH, and corticosterone were not decreased compared to controls. We have used this GnRH-deficient OVX female model to investigate the effects of exogenous GnRH pulses on subunit mRNA expression. GnRH pulses (5-250 ng/30 min for 12-24 h) were administered via an intraatrial catheter beginning 24 h after OVX and the first PBZ injection (OVX+PBZ+saline pulses to controls). Expression of alpha and FSH-beta mRNAs and LH and FSH secretion were increased by GnRH pulse doses of 5-25 ng to values similar to or greater than those in OVX controls though the higher doses of GnRH/pulse did not increase FSH-beta mRNA or plasma FSH. However, LH-beta mRNA levels were not increased by GnRH pulses. GnRH pulses were also given to rats replaced with proestrus concentrations of estradiol alone or in combination with progesterone (P). Again, no demonstrable increases in LH-beta mRNA expression were observed. alpha-mRNA concentrations were further increased in the presence of E2 alone, and P in combination with E2, produced an augmented response of FSH-beta subunit mRNA. These data suggest that ovarian steroid hormones act directly on the gonadotrope to augment alpha and FSH-beta mRNA responses to GnRH.(ABSTRACT TRUNCATED AT 400 WORDS)

Gonadotropin-releasing hormone (GnRH) pulse pattern regulates GnRH receptor gene expression: augmentation by estradiol.

GnRH acts via a single cell surface receptor (GnRH-R), and the number of pituitary GnRH-R increases on proestrus, after gonadectomy, or in response to pulsatile GnRH in the rat. Estradiol (E2) is known to exert a transient positive action to increase GnRH-R number, and the rise in plasma E2 contributes to initiation of the midcycle LH surge. The present study was designed to determine the effect of GnRH pulse amplitude and frequency on GnRH-R messenger RNA (mRNA) levels and to assess the relative contributions of GnRH and gonadal steroids to increasing GnRH-R gene expression. These studies were conducted in vivo using previously characterized GnRH-deficient male (castrate testosterone-replaced) and ovariectomized phenoxybenzamine-treated female models. To investigate the effect of GnRH pulse amplitude, adult male and female rats received GnRH iv (5-250 ng/pulse at 30-min intervals; saline pulses to controls) for 12 or 24 h. In males, GnRH-R mRNA was increased by all pulse doses, with maximal effects (3-fold) at 5-25 ng/pulse. In contrast, only lower doses (5-10 ng/pulse) were effective in females (2-fold increase). In a subsequent study, GnRH pulses (25 ng for males; 10 ng for females) were given at 8-, 30-, or 240-min intervals for 12 or 24 h. Some animals received a continuous GnRH infusion (200 ng/h). In males, GnRH-R mRNA levels were stimulated by all GnRH pulse intervals (maximal after 30-min pulses), whereas continuous GnRH was ineffective. In females, only 30- and 240-min pulse intervals increased GnRH-R mRNA levels, with faster (8-min) pulses or continuous GnRH being ineffective. To determine the relative roles of ovarian steroids and GnRH, ovariectomized phenoxybenzamine-treated female animals received GnRH (10 ng/pulse, 30-min interval), E2 (via sc implants; plasma E2 levels, approximately 50 pg/ml), or their combination for 12-24 h (saline pulses to controls). In the absence of E2, GnRH-R concentrations fell by 70% between 12-24 h. E2 alone tended to increase GnRH-R mRNA at 12 h, with a 2-fold rise observed after 24 h. Pulsatile GnRH alone increased GnRH-R mRNA by 50% at 12 h (compared to saline-pulsed controls; P < 0.05) and by 6-fold after 24 h. When GnRH and E2 were combined, the magnitude of the increase (vs. saline controls) was greater than that seen for either GnRH or E2 alone.(ABSTRACT TRUNCATED AT 400 WORDS)

Augmented hypothalamic proopiomelanocortin gene expression with pubertal development in the male rat: evidence for an androgen receptor-independent action.

To investigate the mechanism(s) during pubertal development by which androgens alter hypothalamic proopiomelanocortin (POMC) gene expression and beta-endorphin content, we used the technique of in situ hybridization histochemistry and the androgen-insensitive testicular feminized (Tfm) rat. We evaluated POMC mRNA levels in the arcuate nuclei and periarcuate regions of 12 coronal brain slices from prepubertal (age, 30 days) and adult (age, 60 days) normal male and Tfm rats (n = 4 for each group). Hybridizations were performed using an 35S-radiolabeled oligonucleotide probe complementary to a 30-base sequence within POMC mRNA. The tissue sections were sequentially exposed to x-ray film and photographic emulsion with subsequent analysis by both densitometry and computer-assisted grain counting. beta-Endorphin was measured in hypothalamic tissue blocks from similar animals in each of the four experimental groups. The results of densitometry and grain counting were consistent and revealed an increase in POMC mRNA with pubertal development in both the male and Tfm animals. The concentration of hypothalamic beta-endorphin was greater for the adult Tfm animals than for all other groups, which did not differ from each other. These results suggest that androgens may stimulate POMC gene transcription by their action through estrogen receptors after conversion by aromatase. Alternatively, additional pubertal factors may be responsible for act directly through their respective receptors to alter translation, posttranslational processing, or secretion of beta-endorphin, resulting in diminished intracellular hypothalamic peptide concentration.

Androgen receptor blockade with flutamide enhances growth hormone secretion in late pubertal males: evidence for independent actions of estrogen and androgen.

Exogenous and endogenous sex steroid hormones influence GH secretion. To test the relative importance of androgens in the enhancement of GH secretion, we administered flutamide (a potent androgen receptor blocker) to six late pubertal males. Blood samples for GH (and LH) were obtained at 10-min intervals for 24-h periods after 3 days of flutamide and during the untreated state. Waveform-specific, multiple-parameter deconvolution analysis was employed to assess secretory and elimination dynamics for GH. Androgen receptor blockade was confirmed by significant increases in 24-h mean LH concentrations and in total 17 beta-estradiol and free testosterone levels in the serum. Mean serum GH concentrations (24-h) also increased (P < 0.001) during androgen receptor blockade (mean +/- SEM, 2.9 +/- 0.3 vs. 1.8 +/- 0.3 micrograms/L); this was associated with an increased (P < 0.001) GH production rate [152 +/- 15 vs. 93 +/- 16 micrograms/liter of distribution volume (Lv)/24 h]. The enhanced GH secretion during flutamide administration was a result of both increased mass of GH released per secretory burst (12.0 +/- 1.4 vs. 8.4 +/- 1.0 micrograms/Lv; P < 0.005) and increased maximal rate of GH secretion (0.39 +/- 0.04 vs. 0.30 +/- 0.03 micrograms/Lv/min; P < 0.05), as well as a small increase in the number of detectable secretory bursts (12 +/- 1 vs. 10 +/- 1/24 h; P < 0.05). There was no significant change in either the serum half-life of GH or in the half-duration of GH secretory bursts during androgen receptor blockade. We speculate that the augmentation of GH secretion observed during antagonism of androgen action in late pubertal males is a result of increased stimulation of estrogen receptor-mediated pathways. Alternatively, androgens may exert a tonic inhibition of GH secretion which can be abolished by androgen receptor blockade.

Estrogen receptor blockade with tamoxifen diminishes growth hormone secretion in boys: evidence for a stimulatory role of endogenous estrogens during male adolescence.

The increase in GH production during the male adolescent growth spurt has been attributed to both androgen and estrogen receptor-mediated processes. To evaluate the role of endogenous estrogens in the control of GH secretion, we administered the estrogen receptor antagonist tamoxifen to 10 late pubertal males. Blood samples were obtained for GH determination at 10-min intervals on 2 occasions during the last 24 h of a 4-day course of either tamoxifen or placebo. Waveform-specific, multiple parameter deconvolution analysis was employed to assess GH secretory and elimination dynamics. Estrogen receptor blockade resulted in a significant (P < 0.05) diminution in mean 24-h serum GH concentrations, from 3.9 +/- 1.0 (placebo; mean +/- SEM) to 2.7 +/- 0.6 micrograms/L (tamoxifen). This was associated with a significant (P < 0.01) decline in the GH production rate [237 +/- 55 vs. 155 +/- 33 micrograms/L GH distribution volume (Lv).24 h]. Furthermore, this reduction in GH secretion was the result of significant decreases in both the maximal GH secretory rate (0.46 +/- 0.08 vs. 0.34 +/- 0.06 microgram/Lv.min; P < 0.01) and, to a smaller degree, GH secretory burst number (16 +/- 1 vs. 14 +/- 1/24 h; P < 0.05). There was also a trend toward reduced mass of GH secreted per burst (13.3 +/- 2.5 vs. 10.3 +/- 2.0 micrograms/Lv; P = 0.06). No significant alterations in either GH elimination t1/2 or GH secretory burst half-duration were observed during estrogen receptor antagonism. Tamoxifen treatment was associated with a significant (P < 0.05) decrease in plasma insulin-like growth factor-I concentrations. However, total and free testosterone, 17 beta-estradiol, insulin-like growth factor-binding protein-3, and pooled 24-h LH concentrations were not significantly changed by short term blockade of estrogen action. We postulate that endogenous estrogens play a facilitatory role in neuroendocrine control of the somatotropic axis during puberty in boys. Tamoxifen blocks this estrogen-dependent stimulation of GH secretion without altering the hormone elimination t1/2. Furthermore, we speculate that any stimulatory role of androgens on GH secretion is exerted primarily through the estrogen receptor after aromatization.

The short-term infusion of ovine corticotropin-releasing hormone does not alter luteinizing hormone concentrations in young adult men.

Chronic stress leads to suppression of the hypothalamic-pituitary-gonadal (HPG) axis with decreased plasma LH concentrations. This is believed to be due to the influence of elevated levels of endogenous CRH mediated via the endogenous opiate peptide receptor. Efforts to reproduce this phenomenon with exogenous CRH have produced varied results depending on the dose and route of administration of CRH as well as on the species, gonadal state, and endogenous opiate peptide system tone of the experimental subjects. In humans, conflicting results for CRH-induced suppression of the HPG axis exist for women, and the issue has not been addressed sufficiently in men. We, therefore, studied the effects of a 4-h infusion of ovine CRH (oCRH) on LH secretion in 11 healthy, nonobese young adult men (age range, 20-33 yr). Subjects were admitted to the General Clinical Research Center on 4 occasions in randomized order. They underwent blood sampling for LH at 10-min intervals from 1800-0600 h. From 2200-0200 h, subjects received one of the following iv infusion protocols in blinded fashion: a normal saline (NS) bolus and NS infusion, a naloxone (NAL) bolus (4 mg) and NAL infusion (2 mg/h), a NS bolus and oCRH infusion (1 microgram/kg.h; maximum, 75 micrograms/h), and a NAL bolus and both NAL and oCRH infusions, using the above-mentioned doses. For each time point, serum LH values from the four experimental conditions were compared by one-way ANOVA with repeated measures; the paired t test was applied post-hoc. This experimental model is predicted to have a beta-error of less than 0.10 for identifying a 1.0 U/L change in LH levels. As expected, NAL was associated with a transient, but significant, rise in serum LH concentrations compared to those in the NS control. On the other hand, oCRH administration did not result in any significant alteration in either basal or NAL-stimulated LH levels. We conclude that exogenous oCRH administration does not significantly alter pituitary secretion of LH in healthy men. We speculate that any suppressive effect of CRH on the HPG axis occurs at the level of the hypothalamus.

Estimation of daily cortisol production and clearance rates in normal pubertal males by deconvolution analysis.

To investigate daily cortisol production and clearance rates in a group (n = 18) of normal unstressed pubertal males, we applied deconvolution analysis to serum cortisol concentrations obtained every 20 min for 24 h. Subject-specific characterization of adrenocortical secretory episodes, cortisol production rate, and serum hormone half-life for nine early pubertal (Tanner I or II; early) and nine late pubertal (Tanner IV or V; late) subjects was undertaken to assess potential roles of sexual maturation and changing gonadal steroid hormone concentrations on glucocorticoid physiology. The estimated cortisol production rate for the early group [16.8 +/- 1.3 mumol/m2 x day (6.1 +/- 0.4 mg/m2 x day)] was indistinguishable from that of the late subjects [14.8 +/- 1.4 mumol/m2 x day (5.3 +/- 0.5 mg/m2 x day)]. No differences were observed between the two pubertal groups in the secretory burst frequency and half-duration, mass of cortisol released per secretory episode, average maximal rate of hormone secretion, and serum cortisol half-life. A significant diurnal pattern of cortisol secretion was observed for all subjects manifest by nyctohemeral variations in the frequency of adrenocortical secretory bursts, the amplitude (maximal rate of cortisol secretion) and the mass of cortisol released per secretory episode. Maximum serum hormone concentrations occurred between 0706 and 1114 h. We conclude that in normal pubertal males: 1) cortisol production rates as estimated by deconvolution analysis are in agreement with other recent independent isotopic estimates, but are lower than many previous estimates; 2) the rise in serum gonadal steroid hormone levels is unassociated with alterations in the production rate or metabolic clearance of cortisol; and 3) increased secretory burst frequency, increased amplitude (maximal rate of cortisol secretion attained within each secretory event), and increased mass of cortisol released per adrenocortical secretory episode give rise to the normal diurnal rhythm of circulating cortisol.

Characterization of pulsatile secretion and clearance of plasma cortisol in premature and term neonates using deconvolution analysis.

Pulsatile secretion of cortisol (F) has not been documented in the newborn infant. Using repeated blood sampling and deconvolution analysis, we investigated F secretion and elimination dynamics in a group of five premature (gestational age, 24-34 weeks) and five term neonates. These infants had required placement of an umbilical arterial cannula for monitoring respiratory status, but were otherwise clinically stable. Blood samples were obtained at 15-min intervals for a 6-h period. All plasma F determinations were 58 nmol/L (2.1 micrograms/dL) or more, and pulsatile F secretion was observed in all infants. No significant differences were noted between the two groups with regard to 6-h mean plasma F concentration [350 +/- 129 (premature) vs. 277 +/- 54 nmol/L (term)], plasma corticosteroid-binding globulin (14 +/- 0 vs. 13 +/- 1 mg/L), F secretory burst frequency (4 +/- 0 vs. 5 +/- 1 bursts/6 h), mass of F secreted per burst [760 +/- 480 vs. 310 +/- 100 nmol/Lv [Lv, liter of F distribution volume)], F production rate (FPR; 2.7 +/- 1.4 vs. 1.1 +/- 0.2 mumol/Lv.6 h), or plasma F half-life (45 +/- 6 vs. 56 +/- 4 min). However, the premature infants had a significantly longer F secretory burst half-duration (63 +/- 18 vs. 6.7 +/- 4.0 min; P < 0.01) and a significantly lower maximal F secretory rate (9.4 +/- 3.4 vs. 100 +/- 26 nmol/Lv.min; P < 0.02) than the term infants. Body surface area and body weight were inversely correlated with F secretory burst half-duration (r = -0.74 and -0.75, respectively); both were also positively correlated with the maximal F secretory rate (r = 0.66 and 0.72). The two most premature infants had significantly greater mean plasma F and FPR than the other three premature and all of the term infants. Extrapolating to 24 h and correcting for the distribution volume of F and body surface area, we estimate FPR to be approximately 17-24 mumol/m2.24 h (6.6-8.8 mg/m2.24 h) for newborn infants of 34 weeks or more gestational age. These values are consistent with newer estimates of FPR in older children and adults determined using either deconvolution analysis or stable isotope dilution methods.

Alterations in the pulsatile properties of circulating growth hormone concentrations during puberty in boys.

To investigate the mechanisms subserving physiological alterations in circulating GH concentrations during puberty, we assessed the GH pulse characteristics of 60 24-h serum GH profiles obtained from healthy male volunteers of normal stature (aged 7-27 yr) whose physical development spanned the entire pubertal range. Subjects were divided into five study groups based on degree of sexual maturation. The mean 24-h concentration of GH was greater in late pubertal boys than in all other groups (P less than 0.001). This elevation primarily reflected a greater size, rather than number, of GH pulses, whether assessed as mean GH pulse area (P = 0.004 vs. all other groups), mean GH pulse amplitude (P = 0.001), or sum of the GH pulse areas (P less than 0.001). GH pulse frequency was indistinguishable among all groups (P greater than 0.05). However, circadian GH rhythms varied significantly in amplitude and mean values (but not in phase) throughout puberty. Plasma insulin-like growth factor-I levels were greatest in the late pubertal boys (1.98 +/- 0.15 U/mL) and remained elevated in the postpubertal group (1.44 +/- 0.18). The mean value for the adult men (0.74 +/- 0.06) was indistinguishable from that of prepubertal boys (0.90 +/- 0.13). In addition, all assessed characteristics of GH pulses and circadian rhythms in adults were equal to or less than corresponding values in prepubertal boys. We conclude that circulating GH concentrations transiently increase during mid- to late puberty in normal boys, primarily through augmentation of the size of GH pulses, but return to or below prepubertal levels during early adulthood.

Reliability of estimates of pulsatile characteristics of luteinizing hormone and growth hormone release in women.

The test-retest reliability of estimates of pulsatile LH and GH release was evaluated in 23 eumenorrheic women during the early follicular phase of the menstrual cycle. Each subject was studied during two successive or near-successive menstrual cycles by repetitive blood sampling every 10 min for 24 h. Pulsatile parameters for LH and GH release were identified and characterized using the Cluster pulse detection algorithm. For LH, no significant differences existed in any parameter mean between the two 24-h admissions. Correlation coefficients for consecutive 24-h studies ranged from r = 0.22 (P less than 0.32) for number of LH peaks to r = 0.79 (P less than 0.0001) for 24-h integrated LH values (area under the concentration vs. time curve). No significant mean differences in any parameter were observed for consecutive 24-h GH evaluations. Correlation coefficients for 24-h GH ranged from r = 0.25 (P less than 0.34) for nadir to r = 0.71 (P less than 0.002) for incremental peak increase. Cosinor analysis was used to determine significant 24-h variations in LH and GH concentrations. Statistically significant differences existed between admissions for the amplitude of the nyctohemeral LH rhythm and its acrophase (time at which maximal hormone value was attained), but no mean differences were found for mesor (mean concentration). Correlation coefficients for LH were r = 0.10 (P less than 0.65), r = 0.43 (P less than 0.08), and r = 0.78 (P less than 0.0001) for phase, amplitude, and mesor, respectively. No significant mean differences existed for any parameter of nyctohemeral GH rhythms. Correlation coefficients were r = -0.18 (P less than 0.52), r = 0.49 (P less than 0.72), and r = 0.14 (P less than 0.80) for 24-h GH amplitude, phase, and mesor, respectively. We conclude that comparisons of mean and integrated LH and GH concentrations over a 24-h interval in the early follicular phase of the menstrual cycle are reliable; however, certain pulsatile properties responsible for the achievement of the mean daily concentrations of LH and GH may be nonuniform from menstrual cycle to menstrual cycle. In addition, nonuniformities may exist in the nyctohemeral rhythms of serum concentrations of LH and GH in the adult woman between cycles when a single 24-h time series is the basis for the analysis.

Estrogen and testosterone, but not a nonaromatizable androgen, direct network integration of the hypothalamo-somatotrope (growth hormone)-insulin-like growth factor I axis in the human: evidence from pubertal pathophysiology and sex-steroid hormone replacement.

Activation of the gonadotropic and somatotropic axes in puberty is marked by striking amplification of pulsatile neurohormone secretion. In addition, each axis, as a whole, constitutes a regulated network whose feedback relationships are likely to manifest important changes at the time of puberty. Here, we use the regularity statistic, approximate entropy (ApEn), to assess feedback activity within the somatotropic (hypothalamo-pituitary/GH-insulin-like growth factor I) axis indirectly. To this end, we studied pubertal boys and prepubertal girls or boys with sex-steroid hormone deficiency treated short-term with estrogen, testosterone, or a nonaromatizable androgen in a total of 3 paradigms. First, our cross-sectional analysis of 53 boys at various stages of puberty or young adulthood revealed that mean ApEn, taken as a measure of feedback complexity, of 24-h serum GH concentration profiles is maximal in pre- and mid-late puberty, followed by a significant decline in postpubertal adolescence and young adulthood (P = 0.0008 by ANOVA). This indicates that marked disorderliness of the GH release process occurs in mid-late puberty at or near the time of peak growth velocity, with a return to maximal orderliness thereafter at reproductive maturity. Second, oral administration of ethinyl estradiol for 5 weeks to 7 prepubertal girls with Turner's syndrome also augmented ApEn significantly (P = 0.018), thus showing that estrogen per se can induce greater irregularity of GH secretion. Third, in 5 boys with constitutionally delayed puberty, im testosterone administration also significantly increased ApEn of 24-h GH time series (P = 0.0045). In counterpoint, 5 alpha-dihydrotestosterone, a nonaromatizable androgen, failed to produce a significant ApEn increase (P > 0.43). We conclude from these three distinct experimental contexts that aromatization of testosterone to estrogen in boys, or estrogen itself in girls, is likely the proximate sex-steroid stimulus amplifying secretory activity of the GH axis in puberty. In addition, based on inferences derived from mathematical models that mechanistically link increased disorderliness (higher ApEn) to network changes, we suggest that sex-steroid hormones in normal puberty modulate feedback within, and hence network function of, the hypothalamo-pituitary/GH-insulin-like growth factor I axis.

Dynamic bending tolerance and elastic-plastic material properties of the human femur.

The objective of this study was to provide data on the structural tolerance and material properties of the human femur in dynamic bending. Fifteen (15) isolated femurs from eight (8) males were tested in either posterior-to-anterior or lateral-to-medial three-point bending. The failure moment was 458 +/- 95 Nm and did not differ significantly with loading direction. A method was developed to estimate the elastic-plastic material properties of the bone using both force-deflection data and strain gauge measurements. The bone material appeared to yield at about one third of the ultimate strain level prior to fracture. It is hoped that these data will aid in the development of injury criteria and finite element models for predicting injuries to pedestrians and vehicle occupants.

Nonlinear viscoelastic behavior of human knee ligaments subjected to complex loading histories.

The nonlinear viscoelastic structural response of the major human knee ligaments when subjected to complex loading histories is investigated, with emphasis on the collateral ligaments. Bone-ligament-bone specimens are tested in knee distraction loading, where the ligaments are in the anatomical position corresponding to a fully extended knee. Temporal nonlinearities for time scales in the range of 1

Exogenous androgen does not alter hypothalamic proopiomelanocortin gene transcript levels in the sexually immature male rat.

To investigate possible mechanisms whereby the augmentation of hypothalamic proopiomelanocortin (POMC) messenger ribonucleic acid (mRNA) levels occurs with pubertal development, we employed the techniques of testosterone administration and in situ hybridization histochemistry in sexually immature male rats. Six animals from each of the following groups were studied: (1) untreated controls (CTRL); (2) empty capsule (SHAM); (3) testosterone capsule (TEST), and (4) untreated adults (ADLT). Capsules were implanted at 21 days of age. Groups 1-3 were sacrificed at 35 days of life; group 4 at 55 days. Ventral prostate and seminal vesicle weights were obtained to assess the biologic effect of testosterone. Hybridizations were performed on coronal brain slices through the region of the arcuate nucleus using a 35S-labeled oligonucleotide probe complementary to a 30-base sequence within POMC mRNA. Anatomically matched tissue sections (11 per animal, from the retrochiasmatic region rostrally to the premammillary nucleus caudally) were exposed to x-ray film, followed by densitometric analysis. The mean serum testosterone concentration of the TEST group was significantly greater than that of the ADLT animals; values for the CTRL and SHAM rats were undetectable. The accessory sex organ weights of the ADLT animals were greater than those of the TEST rats; both values were greater than those of the CTRL and SHAM groups which were indistinguishable. Increased levels of hypothalamic POMC mRNA were observed in the male rat after pubertal development.(ABSTRACT TRUNCATED AT 250 WORDS)

Androgen-receptor blockade enhances pulsatile luteinizing hormone production in late pubertal males: evidence for a hypothalamic site of physiologic androgen feedback action.

To determine potential mechanisms by which androgens alter gonadotropin secretion and elimination in the pubertal male, we administered the potent nonsteroidal androgen receptor blocking agent, flutamide, to eight males with Tanner IV or V genital development. Venous blood samples were obtained every 10 min for 24 h and assayed for LH by a sensitive and high-precision fluorimmunoassay. Subjects were studied before and after the administration of flutamide. Deconvolution analysis was used to assess specific pulsatile LH secretory characteristics and estimate LH production and metabolic clearance rates quantitatively. After antagonism of endogenous androgen action, mean 24-h serum LH concentrations increased significantly. An increased mean 24-h LH production rate, without evident changes in serum LH half-life, accounted for the increase in average serum LH levels. The increased daily secretion rate of LH was in turn due to both an augmented mass of LH released per secretory episode and increased frequency of secretory events. There was no demonstrable change in the maximal rate of LH secretion attained within each secretory event. Serum concentrations of total testosterone, free-testosterone, and 17 beta-estradiol all increased during blockade of androgen action. Administration of the antiandrogen had no measurable effect on the pituitary response to a single maximally effective dose of exogenous gonadotropin releasing hormone (GnRH). These results indicate that, in the late pubertal male, endogenous androgen exerts negative feedback control of gonadotropin secretion primarily at a hypothalamic site reflected by regulation of the frequency of pulsatile LH secretion.(ABSTRACT TRUNCATED AT 250 WORDS)