Outbred Rat Strains Research Articles

Individual differences in renal ACE activity in healthy rats predict susceptibility to adriamycin-induced renal damage

In man, differences in angiotensin-converting enzyme (ACE) levels, related to ACE (I/D) genotype, are associated with renal prognosis. This raises the hypothesis that individual differences in renal ACE activity are involved in renal susceptibility to inflicted damage. Therefore, we studied the predictive effect of renal ACE activity for the severity of renal damage induced by a single injection of adriamycin in rats. Renal ACE activity (Hip-His-Leu cleavage by cortical homogenates) was determined by renal biopsy in 27 adult male Wistar rats. After 1 week of recovery, proteinuria was induced by adriamycin [1.5 mg/kg intravenously (i.v.) n = 18; controls, saline i.v. n = 9]. Proteinuria was measured every 2 weeks. After 12 weeks, rats were sacrificed and their kidneys harvested. As anticipated, adriamycin elicited nephrotic range proteinuria, renal interstitial damage and mild focal glomerulosclerosis. Baseline renal ACE positively correlated with the relative rise in proteinuria after adriamycin (r = 0.62, P<0.01), renal interstitial alpha-smooth muscle actin (r = 0.49, P<0.05), interstitial macrophage influx (r = 0.56, P<0.05), interstitial collagen III (r = 0.53, P<0.05), glomerular alpha-smooth muscle actin (r = 0.74, P<0.01) and glomerular desmin (r = 0.48, P<0.05). Baseline renal ACE did not correlate with focal glomerulosclerosis (r = 0.22, NS). In controls, no predictive values for renal parameters were observed. Individual differences in renal ACE activity predict the severity of adriamycin-induced renal damage in this outbred rat strain. This supports the assumption that differences in renal ACE activity predispose to a less favourable course of renal damage.

The stability of historical control data for common neoplasms in laboratory rats and the implications for carcinogenic risk assessment

Time-related changes in the incidences of spontaneous neoplasms in skin (fibroma and keratoacanthoma), thyroid (C-cell and follicular cell adenomas/carcinomas), uterus (stromal polyp), testes (Leydig cell tumor) and hemolymphoreticular system (mesenteric lymph node hemangioma and malignant granular lymphocytic leukemia) were assessed statistically in Wistar, Sprague–Dawley and F344 rats employed by the BASF, Germany and major European contract research organizations over the last 20 years. Negative trends ( 5 out of 80 cases) were observed for skin fibromas in F344 males, for follicular cell adenomas in Han Wistar females and in Sprague–Dawley males and females, and for follicular cell carcinomas in Sprague–Dawley males. Positive trends ( 8 out of 80 cases) were observed for skin keratoacanthomas in Han Wistar males, for C-cell adenomas in BASF Wistar males and females, for stromal polyps in Han Wistar and Sprague–Dawley females, and for mesenteric lymph node hemangiomas in Han Wistar and Sprague–Dawley males and in BASF Wistar females. In 67 out of 80 cases there were no statistically significant trends. Tumor drift was not common but occurred far more often in outbred rat strains (Wistar and Sprague–Dawley) than in the inbred rat strain (F344). This observation suggests that tumor predisposition is genetically determined, that tumor drift is primarily caused by genetic drift and that non-genotoxic carcinogens operate by facilitating the expression of tumor predisposition in target cells.

Long-term effects of a single exposure to stress in adult rats on behavior and hypothalamic–pituitary–adrenal responsiveness: comparison of two outbred rat strains

We have previously observed that a single exposure to immobilization (IMO), a severe stressor, caused long-term (days to weeks) desensitization of the response of the hypothalamic–pituitary–adrenal (HPA) axis to the homotypic stressor, with no changes in behavioral reactivity to novel environments. In contrast, other laboratories have reported that a single exposure to footshock induced a long-term sensitization of both HPA and behavioral responses to novel environments. To test whether these apparent discrepancies can be explained by the use of different stressors or different strains of rats, we studied in the present work the long-term effects of a single exposure to two different stressors (footshock or IMO) in two different strains of rats (Sprague–Dawley from Iffa-Credo and Wistar rats from Harlan). We found that both strains showed desensitization of the HPA response to the same (homotypic) stressor after a previous exposure to either shock or IMO. The long-term effects were higher after IMO than shock. No major changes in behavior in two novel environments (circular corridor, CC and elevated plus-maze, EPM) were observed after a single exposure to shock or IMO in neither strain, despite the fact that shocked rats showed a conditioned freezing response to the shock boxes. The present results demonstrate that long-term stress-induced desensitization of the HPA axis is a reliable phenomenon that can be observed with different stressors and strains. However, only behavioral changes related to shock-induced conditioned fear were found, which suggests that so far poorly characterized factors are determining the long-term behavioral consequences of a single exposure to stress.

Sensitivity to drug effects on prepulse inhibition in inbred and outbred rat strains

Genetic differences in the neurochemical regulation of PPI in rats may help clarify the neural basis of inherited PPI differences in neuropsychiatric disorders. We reported and characterized substantial heritable differences in sensitivity to PPI-disruptive effects of DA agonists in outbred Sprague Dawley (SDH) versus Long–Evans (LEH) rats. Other strains might yield large group separations and facilitate studies of the neural basis for these strain differences; inbred strains might also allow us to map genes associated with differential PPI sensitivity. Sensitivity to the PPI-disruptive effects of the DA agonist apomorphine (APO) and the NMDA antagonist phencyclidine (PCP) were compared across inbred and outbred strains. APO sensitivity was greatest in SDH and buffalo rats, but the effect in buffalo rats was complicated by significant APO-induced startle suppression. PPI APO sensitivity was least in ACI and LEH rats; F344s exhibited intermediate sensitivity and Lewis rats showed a nonlinear dose response (sensitivity at low but not higher doses). PPI APO insensitivity in ACI rats developed over time, with ACI pups exhibiting robust sensitivity. Substantial strain differences were observed in short-interval (10–30 ms) prepulse effects, and APO-induced increases in short-interval PPI occurred in SDH, LEH, and Lewis rats, but not in F344, ACI, or buffalo rats. Sensitivity to PPI-disruptive effects of PCP was generally greater in outbred than inbred rats. These findings identify strains suitable for comparisons of PPI neural circuitry and others for whom such comparisons would be complex and perhaps less informative.

Heritable differences in the effects of amphetamine but not DOI on startle gating in albino and hooded outbred rat strains

Sensorimotor gating, measured by prepulse inhibition (PPI) of the startle reflex, is reduced in schizophrenia patients and in rats treated with dopamine (DA) agonists. Strain and substrain differences in the sensitivity to the PPI-disruptive effects of DA agonists may provide insight into the basis for human population differences in sensorimotor gating. We reported heritable differences in sensitivity to the PPI-disruptive effects of the D1/D2 agonist apomorphine (APO) in Harlan Sprague–Dawley (SDH) and Long–Evans (LEH) rats, offspring (F1) of an SDH×LEH cross, and subsequent offspring (N2) of an SDH×F1 cross. In this study, we assessed the neurochemical specificity of this heritable phenotype across parental SDH and LEH strains, and their F1 and N2 offspring, based on their sensitivity to the PPI-disruptive effects of the indirect DA agonist d-amphetamine (AMPH) and the 5HT2A agonist DOI. AMPH sensitivity followed a gradient of SDH>N2>F1>LEH, consistent with past findings with APO. DOI sensitivity did not differ across strains or generations. These findings demonstrate that the heritable phenotype in this model is not specific to a particular compound (APO), and reflects physiological differences in the DAergic, but not serotonergic, regulation of PPI.

The effects of chronic mild stress on male Sprague–Dawley and Long Evans rats: I. Biochemical and physiological analyses

The chronic unpredictable mild stress (CMS) is a paradigm developed in animals to model the relatively minor and unanticipated irritants that lead to a state of anhedonia in some individuals. However, the effectiveness of CMS is sometimes difficult to establish, for which unique strain sensitivities has been attributed as one contributing factor. These considerations led us to design the present study, which was an investigation of the corticosterone response to CMS in two outbred rat strains—Sprague–Dawley and Long Evans. Animals were exposed to one of two conditions—control or CMS—for 3 weeks during which body weight and fecal count were regularly monitored. At the end of this period, blood was sampled at a variety of time intervals following induction of a brief restraint stressor. First, a significant effect of CMS on corticosterone levels was evident at time 0 (prior to the application of the acute restraint stressor) in both strains. Second, the typical quadratic pattern of stressor-elicited fluctuations in this measure was similar in both Sprague–Dawley and Long Evans rats, with consistently elevated levels for the first hour following exposure to the acute stressor; near baseline values were observed at 2 h. However, only in the Long Evans strain were CMS related values much less than that observed in the control group after restraint stress. Third, both strains showed a reduced weight gain in the CMS groups relative to control groups. Fourth, spleen and adrenal weights were similar across all groups. Fifth, fecal counts remained stable across weeks of treatment in all groups with the exception of the Long Evans rats exposed to CMS; in this group, average counts were systematically reduced over the treatment period. We conclude that a history of chronic stress significantly blunts corticosterone levels in Long Evans but not Sprague–Dawley rats following exposure to an acute stressor. Physiological indices however are less influenced by this experience, at least when the exposure is limited to 3 weeks.

Correlation of intact sensibility and neuropathic pain-related behaviors in eight inbred and outbred rat strains and selection lines

In some rat strains, total hindpaw denervation triggers autotomy, a behavior of self mutilation presumably related to neuropathic pain. Partial sciatic ligation (PSL) in rats produces tactile allodynia and heat hyperalgesia but not autotomy. Our aims in this study were to examine: (1) whether sensibility of intact rats to noxious and non-noxious stimuli is strain-dependent; (2) whether sensibility of intact rats could predict levels of autotomy, or of allodynia and hyperalgesia in the PSL model; and (3) whether autotomy levels are correlated with levels of allodynia or hyperalgesia. Here we report that in two inbred rat strains (Lewis and Fisher 344), two outbred rat strains (Sabra and Sprague–Dawley) and four selection lines of rats (Genetically Epilepsy-Prone Rats, High Autotomy, Low Autotomy and Flinders Sensitive Line), tactile sensitivity and response duration to noxious heat of intact animals were strain-dependent. Levels of autotomy following hindpaw denervation and of allodynia and hyperalgesia in the PSL model were also strain-dependent. Thus, these traits are determined in part by genetic factors. Sensory sensibility of intact rats was not correlated with levels of autotomy following total denervation, or allodynia and hyperalgesia following partial denervation. We suggest that preoperative sensibility of intact rats is not a predictor of levels of neuropathic disorders following nerve injury. Likewise, no correlation was found between autotomy, allodynia and hyperalgesia, suggesting that neuropathic pain behaviors triggered by nerve injury of different etiologies are mediated by differing mechanisms.

A new rat model of type 2 diabetes: The fat-fed, streptozotocin-treated rat

This study was initiated to develop an animal model of type 2 diabetes in a non-obese, outbred rat strain that replicates the natural history and metabolic characteristics of the human syndrome and is suitable for pharmaceutical research. Male Sprague-Dawley rats (n = 31), 7 weeks old, were fed normal chow (12% of calories as fat), or high-fat diet (40% of calories as fat) for 2 weeks and then injected with streptozotocin (STZ, 50 mg/kg intravenously). Before STZ injection, fat-fed rats had similar glucose concentrations to chow-fed rats, but significantly higher insulin, free fatty acid (FFA), and triglyceride (TG) concentrations (P < .01 to .0001). Plasma insulin concentrations in response to oral glucose (2 g/kg) were increased 2-fold by fat feeding (P < .01), and adipocyte glucose clearance under maximal insulin stimulation was significantly reduced (P < .001), suggesting that fat feeding induced insulin resistance. STZ injection increased glucose (P < .05), insulin (P < .05), FFA (P < .05), and TG (P < .0001) concentrations in fat-fed rats (Fat-fed/STZ rats) compared with chow-fed, STZ-injected rats (Chow-fed/STZ rats). Fat-fed/STZ rats were not insulin deficient compared with normal chow-fed rats, but had hyperglycemia and a somewhat higher insulin response to an oral glucose challenge (both P < .05). In addition, insulin-stimulated adipocyte glucose clearance was reduced in Fat-fed/STZ rats compared with both chow-fed and Chow-fed/STZ rats (P < .001). Finally, Fat-fed/STZ rats were sensitive to the glucose lowering effects of metformin and troglitazone. In conclusion, Fat-fed/STZ rats provide a novel animal model for type 2 diabetes, simulates the human syndrome, and is suitable for the testing of antidiabetic compounds. Copyright © 2000 by W.B. Saunders Company

402. Genetics and neurobiology of sensorimotor gating: the gain in brain comes mainly in the strain

Sensorimotor gating of the startle reflex, measured by prepulse inhibition of startle (PPI), is impaired in humans with specific neuropsychiatric disorders, and in laboratory animals after manipulations of dopamine, serotonin or glutamate activity within specific cortico-striato-pallido-pontine (CSPP) circuitries. PPI is being used widely to understand the neurobiology of “gating” disorders, and to develop novel antipsychotics based on their ability to reverse experimentally-induced PPI deficits. As PPI studies have increased exponentially in the past decade, differences in drug effects across laboratories were identified that first prompted concern regarding potential interpretative flaws, and later stimulated efforts to understand methodological contribution to these apparent inconsistencies. An awareness has emerged that “baseline” levels of PPI differ significantly across specific outbred rat strains, and within strains, across specific suppliers. Differences in PPI sensitivity to pharmacologic manipulations have also been identified across rat strains and substrains. Differences in both baseline PPI and PPI drug sensitivity may reflect genetic differences within CSPP circuitries that regulate PPI, since they exist even when rats are raised in identical environments, and are tested very early in development. Inheritance patterns are being studied by selective cross-breeding and development of inbred lines based on pharmacologic sensitivity. Strategies are used to identify the neural substrates of strain differences, and to insure that strain differences do not reflect inherited differences in peripheral sensory systems. Ultimately, “inconsistencies” identified in PPI studies across laboratories promise to help understand the genetics of sensorimotor gating deficits in schizophrenia and other disorders.

Strain Differences in Neuropathic Hyperalgesia

The purpose of this study was to investigate strain-related differences in the onset and maintenance of thermal hyperalgesia following the induction of peripheral nerve injury in two inbred strains of rats (Fischer 344 and Lewis) and two outbred strains of rats (Sprague–Dawley and Wistar). Neuropathic pain was induced via unilateral ligation of the left sciatic nerve with chromic gut sutures. A plantar analgesia meter was used to measure paw-withdrawal latency from the ligated vs. unligated hind paws of inbred vs. outbred strains of rats to investigate strain-related differences in nerve injury-induced thermal hyperalgesia. The results demonstrated no significant effects of animal strain on presurgical paw-withdrawal latency values. Following the sciatic nerve ligation (SNL) surgery, a significant hyperalgesic response was elicited from the Sprague–Dawley and Wistar rats (outbred strains) for at least 28 days. Conversely, data analyses from the inbred strains failed to demonstrate significant hyperalgesic responses to peripheral nerve injury, with the exception of postsurgical day 10. These data emphasize the importance of considering the strain of the rat being investigated before extrapolating the results from animals experiments to treatment strategies for humans with chronic neuropathic pain.

Regeneration of the rat sciatic nerve into allografts made acellular through chemical extraction

The aim of this study was to develop a procedure by which myelin and Schwann cells could be removed from a peripheral nerve while the basal lamina tubes, remained intact, and to test if such preparations could be used as allografts for the repair of a gap in the continuity of the rat sciatic nerve. We found that extraction with the detergents Triton X-100 and deoxycholate resulted in acellular nerve segments with preserved basal lamina tubes, here defined as the tubes which surrounds the axon/Schwann cell units. The morphology of the acellular nerve segments was revealed by scanning electron microscopy, teasing, immunohistochemistry and electrophoresis. Such grafts when allografted between two outbred rat strains, were found to support outgrowth of axons and migration of Schwann cells, which reoccupied the empty basal lamina tubes without excessive signs of inflammation. This new paradigm offers a possible solution to the major shortcomings of autologous nerve grafts, i.e., the requirement to sacrifice a healthy nerve and the shortage of graft material available for repair.

Genetic Selection of Alcohol Preference Can Be Countered by Conditioning Processes

Twenty-four P rats and 24 NP rats were conditioned to consume 10% w/v alcohol in daily 0.5-h limited access periods using a modified version of Samson’s sucrose fading procedure. Both P and NP rats demonstrated a strikingly similar day-to-day pattern of alcohol intakes. For the most part, P rats drank more than NP rats, but by the middle of the fourth month of drinking, P and NP rats were drinking equivalent amounts of alcohol. Both P and NP rats consumed alcohol in amounts similar to two outbred strains of rats (Wistar and Sprague–Dawley) previously conditioned to drink alcohol in this laboratory.

Volitional consumption of ethanol by fawn-hooded rats: Effects of alternative solutions and drug treatments

Behavioral and neurochemical measures of brain 5-hydroxytryptamine (5-HT) function in the Fawn-Hooded rat are abnormal relative to outbred strains of rats. Fawn-Hooded rats freely drink large amounts of 10% ethanol in the presence of water and have been proposed to be an animal model for studies related to alcoholism. In this study, Fawn-Hooded rats were given solutions of ethanol increasing in concentration from 3% to 30% (w/v in tap water) over 10 days with tap water in a second drinking tube and a third tube left empty. The solutions of ethanol that produced maximal drinking with a preference (ml ethanol/ml total fluid) near 50% ranged from 5% to 13%, which became: the fixed individual concentrations for each rat. After a 5-day baseline period the rats were offered a solution in the third drinking tube of either 0.5% aspartame or chocolate Ultra SlimFast (diluted with water 2: 1). The chocolate drink, but not aspartame, significantly reduced the consumption of alcohol by 73%. For the drug experiments, the rats were given successive 4-day periods of: baseline drinking; drug or saline injections b.i.d.; and a posttreatment period. Neither ipsapirone, a 5-HT 1a partial agonist, nor naltrexone injected inhibited the intakes of ethanol solutions. Treatment with 2.5 mg/kg of amperozide, a 5-HT 2 antagonist, decreased the consumption of ethanol by 38%, but also caused a decrease in consumption of food. These results show a pattern of drinking of increasing concentrations of ethanol different than other strains of rats. Because ethanol intakes of the Fawn-Hooded rat decline precipitously when offered palatable chocolate drink and fail to respond to drugs known to decrease human ethanol intake, this strain may not be a valid model for testing the effects of centrally acting drugs on the consumption of ethanol.

Type XI and II Collagen-Induced Arthritis in Rats: Characterization of Inbred Strains of Rats for Arthritis-Susceptibility and Immune-Responsiveness to Type XI and II Collagen

To determine the relationship between susceptibility to bovine type XI and II (BXI and BII) collagen-induced arthritis, we immunized 14 inbred and one outbred strains of rats with BXI and BII. Susceptibility to BXI-arthritis corresponded largely with susceptibility, or resistance, to BII-arthritis. LEW, BB, WF, DA, and WKY were readily susceptible to BXI- and BII-arthritis. Likewise, BII-resistant F344 and BN rats were BXI-resistant. Some strains responded differently to BXI and BII. BUF and COP, which are moderately susceptible to BII, were BXI-resistant, whereas the BII-resistant rats, DA.1N and WF.1N, were partially susceptible to BXI. (F344 x BN) F1 hybrids responded to both collagens suggesting gene complementation. Arthritis occurred in all strains producing the highest titer antisera (LEW, WF and BB). Antibody responses to BXI and BII were generally commensurate within individual strains. DA were susceptible to arthritis but produced low levels of antibody comparable to BN rats which were arthritis-resistant. BXI and BII-susceptibility was variable in rats producing intermediate antibody responses. Antibodies to RXI were detected in all BXI-immunized rats, whereas antibodies to RV and RII were uniformly weaker. DTH to RXI and RII was strong in both groups of rats, correlating poorly with arthritis and antibody responses. These studies show that phenotypic susceptibility to BXI- and BII-arthritis are largely concordant among inbred rat strains but clear differences exist in certain strains; multiple genes are likely involved.

Differences in response to an acoustic startle stimulus among forty-six rat strains

The startle response to intense environmental stimuli is found in most, if not all, animal species. Previous studies have shown that selected rodent strains differ in startle response to an acoustic stimulus, suggesting genetic variability. The current experiment was designed to extend these differences to a wider range of rat strains. Forty-six inbred and outbred rat strains were studied. These strains exhibited differences in both the mean amplitude of the startle response and the rate of habituation to startle stimuli over repeated trials. In addition, there was a significant relationship between these measures. These results suggest that robust phenotypic differences in startle response exist among rat strains. As previous investigations have demonstrated a direct relationship between the startle response and other behavioral end points, the use of strain differences in startle response may be an effective way to determine genetic contributions to specific behavioral responses.

Differential establishment and survival of Hymenolepis diminuta in syngeneic and outbred rat strains.

Experimental Hymenolepis diminuta infection was carried out in inbred strains of rats (F344/N, JAR-2, LOU/M, TM, DA and DA-bg/bg) and outbred Wistar rats. All strains became infected with this cestode, but clear strain-dependent variation in the susceptibility to H. diminuta infection was observed. Marked differences in worm persistence and worm weight were found at 6 weeks post-infection in TM and DA rats. These strains would be useful to clarify the interactions between H. diminuta and its rat host.

Comparison of sensitivity and alcohol consumption in four outbred strains of rats

Differences in alcohol consumption and in sensitivity to the effects of ethanol were investigated in four outbred rat strains: Fischer 344, Long-Evans, Sprague-Dawley and Wistar. Alcohol consumption was measured in all four strains in three separate subgroups for each strain, using three different concentrations of ethanol (5, 10 and 20% v/v). An intermittent forced alternate-day ethanol presentation procedure (ethanol as the sole fluid for one day followed by only water the next day), as well as a two-bottle choice paradigm, were employed for this purpose. Ethanol-induced hypothermia and motor impairment (tilting plane test) were used to assess sensitivity. Significant differences in alcohol consumption were found among these strains. The Long-Evans strain consumed the highest and Fischer 344 the lowest amount of ethanol. Wistar and Sprague-Dawley were intermediate. However, the strains did not differ in sensitivity to ethanol. Similarly, determination of sensitivity to ethanol on day 0 in separate groups of these four strains (same age and weight, and obtained at the same time from the same supplier) did not reveal graded differences in sensitivity (hypothermia and motor impairment) corresponding to differences in alcohol consumption. These results suggest that sensitivity does not correlate with alcohol consumption.

Intensity and precision of circadian wheel running in three outbred rat strains

The laboratory rat is widely used in many areas of neurobiologic and behavioral research. However, studies of circadian activity rhythms have usually employed other rodent species whose activity patterns are more amenable to measurement in the circadian paradigm. Nevertheless, the rat may be the optimal choice in certain investigations of circadian function because of the wealth of data on the neuroanatomy and neuropharmacology of this species. This study was undertaken to determine whether choice of rat strain could affect the quality of measured circadian activity data. Intensity of wheel running (wheel revolutions per day), precision of onset, and longevity of consistently interpretable running records were assessed in three common outbred rat stains, Long-Evans, Wistar, and Sprague-Dawley. Long-Evans rats showed greater intensity of wheel running while entrained and at four and eight weeks of free running in constant darkness. There was no difference in precision of running onset among the three strains. Thus, use of the Long-Evans strain, with its greater intensity of wheel running and similar precision of activity onset, may confer advantages in studies involving measurement of circadian activity rhythms in the rat.

Expression of glutamine/glutamic acid-rich proteins in rat submandibular glands

The expression of GRP transcripts was found to be highly specific to the rat submandibular gland. GRP cross-reactive species were detected in the saliva of both inbred and outbred rat strains. There was no evidence of GRP transcripts in RNA prepared from bovine, ovine, porcine or murine submandibular glands. Thus the GRPs differ from the family of PRPs that are expressed in several species. The restriction of GRP expression to the rat suggests a relatively recent origin for a functional GRP gene, presumably after rat-mouse divergence. The ontogeny of the relative steady-state levels of GRP transcripts was assessed by dot blot analysis. Maximal levels of RNA-encoding GRP were detected at 6 months; there was a significant age-related decline at both 12 and 18 months. There was, however, no significant age-related alteration in the size of transcripts which encode this protein family.

Net glucuronidation in different rat strains: importance of microsomal beta-glucuronidase.

The net glucuronidation of bilirubin (BR) has been determined in inbred and outbred rat strains and their subpopulations with similar glucuronosyltransferase (EC 2.4.1.17) activity but with different levels of beta-glucuronidase (beta G) (EC 3.2.1.31), or in which the level of beta G activity was reduced with D-glucaro-1,4-lactone. These studies demonstrated that outbred rat strains consist of two subpopulations that differ approximately 1.5- to two-fold in serum and liver beta G activity. Evidence is presented indicating that owing to its compartmentalization the lysosomal beta G, unlike the corresponding microsomal enzyme, is neither inhibited by glucarolactone nor accessible for hydrolysis of newly synthesized glucuronides. The ratio of glucuronidated to unconjugated BR 15 min after injection of albumin-bound BR into the tail vein appears to correlate negatively with the liver microsomal beta G activity. The results may be relevant to the relative risk to toxins, including carcinogens, and to their reduction by dietary intervention.