The effects of elevated blood lead levels and succimer chelation therapy on physical growth in developing rhesus monkeys.

Seventy-two female rhesus monkeys were randomly assigned to three lead exposure conditions (none, birth to 1 year, birth to 2 years). In a completely crossed design, the lead-exposed and control monkeys were randomized to placebo or chelation therapy which began at 1 year of age. Dosing was conducted daily beginning on day 8 postpartum. The lead dose levels were adjusted biweekly to gradually elevate the blood lead level of each monkey to a target of 1.69-1.93 micromol/L (35-40 microg/dL). Succimer (or placebo) was administered orally (30 mg/kg/day for 5 days and 20 mg/kg/day for 14 additional days) for a total 19-day treatment regimen. There were two separate chelation regimes at 53 and 65 weeks of age. Succimer therapy in combination with lead abatement reduced blood lead levels significantly faster than lead abatement alone; however, that advantage disappeared once succimer therapy was discontinued. Weight, crown-rump length, and head circumference were measured regularly. Growth in weight, length, and head circumference did not vary significantly as a function of blood lead levels. Succimer chelation therapy did not significantly affect weight, length, or head circumference through 2 years of age.

Oral succimer decreases the gastrointestinal absorption of lead in juvenile monkeys.

Although succimer (Chemet, meso-2,3-dimercaptosuccinic acid, DMSA) is considered to be a safe and effective chelating agent for the treatment of lead poisoning in humans, there is concern that it may increase the gastrointestinal (GI) absorption and retention of Pb from exposures suffered concurrent with treatment. This concern is justified because the availability of Pb-safe housing during outpatient treatment with oral succimer is limited. We used a juvenile nonhuman primate model of moderate childhood Pb intoxication and a sensitive double stable Pb isotope tracer methodology to determine whether oral succimer chelation affects the GI absorption and whole-body retention of Pb. Infant rhesus monkeys (n = 17) were exposed to Pb daily for 1 year postpartum to reach and maintain a target blood lead (BPb) level of 35-40 microg/dL. Animals were administered succimer (n = 9) or vehicle (n = 8) over two successive 19 day succimer treatment regimens beginning at 53 and 65 weeks of age. The present study was conducted over the second chelation regimen only. Animals received a single intravenous (iv) dose of stable (204)Pb tracer (5 microg, 24.5 nmol) followed by a single oral dose of stable (206)Pb tracer (72.6 microg, 352 nmol) immediately before chelation, in order to specifically evaluate GI Pb absorption and whole-body Pb retention with treatment. We collected complete urine and fecal samples over the first 5 days and whole blood over the first 8 days of treatment for analyses of stable Pb isotopes using magnetic sector inductively-coupled plasma mass spectrometry. Results indicate that succimer significantly reduced the GI absorption of Pb (vehicle, 64.9% +/- 5.5; succimer, 37.0% +/- 5.8; mean +/- SEM). Succimer also significantly increased the urinary excretion of endogenous Pb by approximately 4-fold over the vehicle treatment, while endogenous fecal Pb excretion was decreased by approximately 33%. Finally, although succimer reduced the whole-body retention of endogenous Pb by approximately 10% compared to vehicle, the majority (77%) of the administered internal dose of Pb tracer was retained in the body when assessed after 5 days of treatment. These data do not support the concern that succimer treatment increases GI Pb absorption.

The effects of succimer chelation therapy on auditory function in rhesus monkeys.

Sixty-six female rhesus monkeys were randomly assigned to three lead exposure conditions (none, from birth to 1 year, and from birth to 2 years) by two chelation treatment (succimer and no succimer) conditions. Blood lead levels were maintained at 35-40 microg/dl beginning shortly after birth and continuing for 1 or 2 years postnatally. There were two separate chelation regimes: 53 and 65 weeks of age. Lead and lead-vehicle dosing were discontinued while succimer was administered. Succimer (or placebo) was administered orally at a dose of 30 mg/kg/day (divided into three doses per day) for 5 days and for 14 additional days at 20 mg/kg/day (divided into two doses per day) for a total 19-day treatment regimen. Auditory function was assessed in these monkeys at least 1 year after lead intake had been discontinued. The outcome measures included tympanometry to assess middle ear function, OAEs to assess cochlear function, and ABRs to assess the auditory nerve and brainstem pathways. There were no significant differences as a function of succimer treatment for any of the tympanometric variables measured. Suprathreshold and threshold distortion product otoacoustic emissions were comparable among the succimer and vehicle groups. However, there was a nonsignificant trend to smaller amplitude distortion products at the highest frequencies assessed (6.4-10.0 kHz). Finally, the auditory evoked response at levels from the auditory nerve to the cerebral cortex did not significantly differ as a function of succimer treatment.

The effects of early lead exposure on auditory function in rhesus monkeys.

Thirty-one female rhesus monkeys were randomly assigned to three lead exposure conditions (none, birth to 1 year, and birth to 2 years). Blood lead levels were maintained at 35-40 microg/dl beginning shortly after birth and continuing for 1 or 2 years postnatally. Auditory function was assessed in these monkeys at least 1 year after exposure to lead. The outcome measures included tympanometry to assess middle ear function, otoacoustic emissions (OAEs) to assess cochlear function, and auditory brainstem-evoked responses (ABRs) to assess the auditory nerve and brainstem pathways. There were no significant differences among the three experimental groups for any of the tympanometric variables measured suggesting no effect of lead exposure on middle ear function. Suprathreshold and threshold distortion product OAEs (DPOAEs) were comparable among the three groups. Finally, the auditory-evoked response at levels from the auditory nerve to the cerebral cortex did not significantly differ as a function of lead exposure. The lead exposure in this study had little effect on auditory function.

Succimer and the reduction of tissue lead in juvenile monkeys.

The extent to which succimer (2,3-dimercaptosuccinic acid, DMSA) chelation reduces target organ lead (Pb) levels, including the skeleton, relative to the cessation of Pb exposure is a primary consideration in evaluating its efficacy for reducing toxicity in children. Here, we utilized a rhesus monkey model of childhood Pb exposure and a sensitive stable (204)Pb isotope tracer methodology to determine the efficacy of succimer for reducing Pb in blood, liver, and skeletal tissues from chronic (>/=1 year) versus short-term (3-4 days) Pb exposures. Specific attention was paid to the efficacy of succimer treatment compared to the cessation of Pb exposure. Infant rhesus monkeys (n = 48) were exposed to Pb daily for 1 year or >1 year postpartum to reach and maintain a target blood Pb level of 35-40 microg/dL. Two successive 19-day succimer treatment regimens were administered at 53 and 65 weeks of age (30 mg/kg/day x 5 days followed by 20 mg/kg/day x 14 days). Blood was collected over the course of treatment, and liver and bone biopsy samples were collected on days 0, 5, and 20, relative to the start of treatment (day 0). Complete 24-h urine collections were conducted over the course of treatment. Results of the first chelation indicate that a single regimen of succimer treatment led to significant reductions in blood and liver Pb levels, relative to the placebo group. However, the cessation of Pb exposure alone (i.e., placebo) also led to significant reductions in blood and liver compared to pretreatment levels. Neither succimer nor the cessation of Pb exposure had a significant impact on bone lead levels. Blood Pb levels in the succimer-treated group rebounded within 5 days after treatment ended, becoming comparable with levels in the placebo group from that point on. Results from the second chelation indicate that succimer treatment is essentially equally efficacious in reducing blood Pb at moderate (20 microg/dL) levels where exposures ended >3 months previously and more elevated (40-50 microg/dL) levels where exposures ended just prior to treatment, relative to the placebo treatment. Finally, similar overall outcomes were observed for tissue Pb from recent exposures (i.e., (204)Pb tracer levels), indicating little or no apparent difference in the chelation of Pb from recent (3-4 days) versus long-term exposures. These data demonstrate that succimer does not reduce skeletal Pb levels, and they show that the efficacy of succimer for reducing blood Pb levels does not persist beyond the completion of treatment due to posttreatment rebounds in blood Pb from endogenous sources. They also demonstrate the relative benefit of eliminating Pb exposures, which serves to underscore the importance of primary prevention of Pb exposure. The extent to which these data reflect the efficacy of succimer for reducing neurocognitive impairment is not yet known, although those data are forthcoming.

Lead effects on neurobehavioral development in the neonatal rhesus monkey (Macaca mulatta).

Effects of lead exposure on behavioral development during the first month of postnatal life were examined in rhesus monkeys using a multi-item assessment scale developed for the evaluation of neonatal rhesus monkeys. Lead was administered daily beginning at day 8 postpartum at levels that produced blood lead levels of about 20 microg/dl by week 4 (n = 48); controls were treated identically but given vehicle only (n = 24). All monkeys were tested once a week for the first 4 weeks postpartum. The first principal component explained a substantial portion of the variance and was relatively consistent across ages for both groups. Analyses of the individual items and of both conceptually derived and empirically defined summary scores yielded no significant effects of lead. Furthermore, there were no systematic relationships between blood lead level and performance on the test. Correlation coefficients indicated more similarity across age for control monkeys than for lead-exposed monkeys suggesting that continuity of development, as measured by this test, was disrupted by lead. The relationship between outcome on these early assessments and later behavior will be explored in subsequent studies of these monkeys.

Otoacoustic emission, evoked potential, and behavioral auditory thresholds in the rhesus monkey (Macaca mulatta).

Distortion product otoacoustic emission (DPOAE), auditory brainstem evoked response (ABR), and behavioral thresholds were recorded in a group of 15 adult rhesus monkeys with normal auditory function. DPOAE thresholds were recorded with stimulus parameters selected to maximize signal-to-noise ratio. Additional averaging at the lowest frequencies ensured comparable noise levels across frequencies. DPOAE thresholds decreased with increasing frequency (f(2)=0.5-16 kHz) and at 16 kHz were close to 0 dB SPL. ABR thresholds were best from 1 through 16 kHz (32-38 dB peSPL); higher at 0.5 (45 dB peSPL), 24 (39 dB peSPL), and 30 kHz (49 dB peSPL). At all levels including threshold, the early ABR waves (II and I) were more prominent at the high frequencies while the later waves (IV and V) were more prominent at the low frequencies. The behavioral thresholds recorded were similar to those reported by other researchers although elevated by about 10 dB presumably because of the complexity of the threshold task. DPOAE and ABR thresholds can be reliably and efficiently recorded in the rhesus monkey and provide information concerning site of processing in the auditory pathway not directly available from behavioral data.

Efficacy of succimer chelation for reducing brain lead in a primate model of human lead exposure.

The extent to which succimer (meso-2,3-dimercaptosuccinic acid [DMSA], Chemet) reduces brain lead (Pb) levels may be a primary consideration in evaluating its efficacy for reducing neurotoxicity. Clinical research in this area has been hampered by the need to use blood Pb levels as the index of treatment efficacy, despite the fact that brain Pb level is the exposure parameter of greater relevance to cognitive outcomes. Here, a nonhuman primate model of human Pb exposure was used to determine: (1) The efficacy of oral succimer for reducing brain Pb derived from chronic or recent exposures, and (2) The extent to which blood Pb levels reflect brain Pb prior to and following chelation. Adult rhesus monkeys were chronically exposed to Pb orally for 5 weeks to reach and maintain a target blood Pb level of 35-40 microg/dL. Chelation of Pb from recent exposures was assessed using a stable (204)Pb isotope tracer administered over 4 days prior to treatment. Immediately prior to chelation, a prefrontal cortex (PFC) biopsy was collected to determine pretreatment brain Pb levels. Subsequently, monkeys were assigned to vehicle (n = 5) or succimer (n = 6, 30 mg/kg/day x 5 days followed by 20 mg/kg/day x 14 days) groups. Blood and brain PFC, frontal lobe (FL), hippocampus (H), and striatum (S) were analyzed for total Pb and (204)Pb tracer concentrations by magnetic sector inductively coupled plasma-mass spectrometry. There were no measurable differences in brain Pb concentrations between the succimer and vehicle groups, indicating that succimer treatment was not efficacious in reducing brain Pb levels. In contrast, the cessation of Pb exposure significantly reduced brain (PFC) Pb ( approximately 34%) when compared to pretreatment levels (succimer and vehicle groups). Pb concentrations also varied among brain regions (PFC > FL approximately H > S). Finally, pretreatment PFC Pb concentrations were significantly correlated with the integrated blood Pb level (AUC) over the Pb exposure period, but not with the single pretreatment blood Pb collected concurrently with the PFC biopsy. Following treatment, blood Pb levels correlated only with Pb in the PFC, and not the other brain regions measured (FL, H, S). These data indicate that, under the conditions of this study, succimer treatment did not reduce brain Pb levels beyond the cessation of Pb exposure alone. Moreover, a single blood Pb measurement may be a poor predictor of brain Pb levels, reflecting limitations in the use of blood Pb level as an indicator of treatment efficacy.

Soybeans versus animal sources of rumen-undergradable protein and fat for early lactation dairy cows.

Fifteen multiparous Holstein cows averaging 39 DIM were used in a replicated 5 x 5 Latin square design to compare roasted soybeans and animal by-products as supplements for increasing the RUP content of lactation diets based on alfalfa. The control diet contained roasted soybeans and was formulated to meet requirements for RUP. Rumen-undegradable protein in the diet was increased by including additional roasted soybeans or animal by-product proteins. To achieve diets with similar amounts of fat, we added tallow, hydrolyzed tallow fatty acids, or partially hydrogenated tallow to the control diet and to diets containing animal by-product proteins. Control diets and diets with high RUP were estimated to be 5.8 and 7.6% RUP (DM basis). The RDP of all diets was estimated to be approximately 11.3% of DM. Milk yield was increased by additional dietary RUP. Yield of FCM was greater for cows receiving animal byproducts than for those fed additional roasted soybeans. Cows fed additional RUP yielded more milk protein, but milk protein percentage was decreased. Milk fat percentage was reduced by additional dietary RUP, but fat yield was not affected. Nutrient digestibility was greatest for diets containing animal byproducts. Cows fed tallow yielded more milk, FCM, and protein that did those fed hydrolyzed tallow fatty acids or partially hydrogenated tallow. Lactation performance can be enhanced by supplementing animal by-products or roasted soybeans to diets based on alfalfa and formulated to meet NRC recommendations for RUP.

Effect of prepartum and postpartum dietary energy on growth and lactation of primiparous cows.

Sixty-seven Holstein replacement heifers (19 mo) were fed a standard (59.7% TDN) or a high energy (69.3% TDN) diet until parturition. After parturition, primiparous cows were fed either 0 or 2.8% supplemental tallow for 150 d in a 2 x 2 factorial arrangement with prepartum treatments. High energy prepartum increased BW (693.5 vs. 663.7 kg) and body condition scores (3.72 vs. 3.55) at calving. Increased energy density of the diet prepartum did not affect milk yield or composition. Supplemental dietary fat postpartum increased milk yield approximately 1.5 kg/d, but the response was not observed until 7 wk postpartum. Heifers fed the standard diet prepartum and no supplemental fat postpartum had higher DMI than other treatments. Heifers fed high energy prepartum and supplemental fat postpartum lost the greatest BW and body condition from 1 to 5 wk postpartum. Heifers fed high energy diets prepartum had higher concentrations of blood NEFA, BHBA, and liver triglycerides. Increases in BW and body condition scores at calving above approximately 660 kg and 3.5, respectively, do not enhance lactation performance. When 2.8% supplemental fat was fed to primiparous cows, milk yield improved approximately 1.5 kg/d after wk 7 of lactation.

Lactational performance of dairy cows fed raw soybeans, with or without animal by-product proteins, or roasted soybeans.

Twelve multiparous Holstein cows averaged 10 wk postpartum and were used in a replicated 3 x 3 Latin square design to compare two feeding strategies for increasing the ratio of dietary undegradable to degradable protein. Treatments were raw soybeans, with or without meat and bone meal plus blood meal, and roasted soybeans as the primary protein supplements. Meat and bone meal and blood meal were fed at 4.0 and .9% of dietary DM, respectively. Basal diets were 30% alfalfa silage, 18% corn silage, and 52% corn-based concentrate mix. Diets were formulated to be isonitrogenous and isocaloric. Estimated undegradable protein contents, as a percentage of total CP, were 32.2, 36.2, and 34.3 for diets containing raw soybeans, raw soybeans plus animal by-product proteins, and roasted soybeans, respectively. The undegradable protein estimates were calculated from NRC values for basal feeds and from results of in vitro analysis of test protein supplements. Yields of milk and 3.5% FCM of cows receiving raw soybeans plus animal by-product proteins (45.5 and 43.4 kg/d) and roasted soybeans (44.7 and 42.7 kg/d) were greater than those of cows receiving raw soybeans alone (43.2 and 41.3 kg/d). Increasing the ratio of undegradable to degradable dietary protein also increased yields of milk protein and fat. No differences occurred in lactation performance among cows fed the two diets containing higher undegradable protein. The DMI was not influenced by treatment. Increasing the ratio of undegradable to degradable dietary protein by feeding animal by-product proteins or heated soybeans enhanced lactation performance.

Rumen fermentation and lactation performance of cows fed roasted soybeans and tallow.

Sixteen multiparous Holstein cows were in a replicated 4 x 4 Latin square design with 21-d periods to examine the effects of incremental tallow addition to diets containing whole roasted soybeans on rumen fermentation and lactational performance. Total mixed rations were fed for ad libitum intake and contained, on a DM basis, 33% alfalfa silage, 12% corn silage, 14% roasted soybeans, and 41% concentrate based on ground corn and soybean meal. Treatments were 0, 1, 2, or 3% supplemental tallow. Diets contained 20% CP and ranged from 1.68 to 1.82 Mcal NEL/kg of DM. The DMI, milk yield, milk protein and fat yields, milk fat percentage, rumen acetate: propionate ratio, and in situ forage DM disappearance did not differ among treatments. A small linear decrease occurred in milk protein percentage as tallow feeding was increased (2.89 to 2.86%). Tallow supplementation increased total VFA concentration in rumen fluid and resulted in a linear decrease in rumen pH (6.17 to 5.99). Supplementation of 1 to 3% tallow to diets containing 2.8% supplemental fat from whole roasted soybeans had minimal negative effects on rumen fermentation and did not influence lactational performance.