Concomitant toxicokinetics: techniques for and interpretation of exposure data obtained during the conduct of toxicology studies

Toxicokinetic analyses have become a routine component of preclinical toxicology studies with pharmaceutical candidates. Evaluation of plasma and/or tissue samples from animals used in toxicology studies (or concurrent satellite groups) provides information on dose proportionality, the potential for dose accumulation, and the sex and species differences in distribution and elimination. Toxicokinetic information is used by toxicologists, toxicology management, clinicians, institutional review boards, regulatory agencies to ensure that exposure has occurred in animal species to a sufficient extent to minimize the potential risk of toxicities in humans. The requirements for descriptive toxicokinetics change depending on the stage of development of new drug candidates. Early in development, documentation of exposure in 1 species and sex of laboratory animal might be enough to justify preliminary development costs and initiation of product development. Later in development, it becomes necessary to know how new drug candidates are distributed and eliminated following subchronic and chronic administration in multiple species and both sexes. Finally, knowledge of toxicokinetics is used to help establish doses in long-term oncogenicity studies. Scientific, public, and regulatory pressures have recently dictated that the number of animals used in toxicology studies be closely monitored and minimized. Toxicokinetic evaluation of new drug candidates by a staggered sampling design is now routinely performed in our laboratories to maximize information obtained while reducing animal use.

Cardiovascular and adrenal proliferative lesions in Fischer 344 rats induced by long-term treatment with type III phosphodiesterase inhibitors (positive inotropic agents), isomazole and indolidan

Male and female Fischer 344 rats were treated with the positive inotropic agents, isomazole or indolidan, in the diet for 104 weeks. The doses were 0.0, 11.5, 23.5, or 48.0 mg/kg and 0.0, 0.12, 0.40, or 1.3 mg/kg, respectively. Only 17% of the males treated with 48.0 mg/kg isomazole survived the duration of the study. The male component of the indolidan study was terminated at 22 months, with only 18% of the high-dose males surviving. Sixty-five percent of the males treated with 48.0 mg/kg isomazole and 70% of the males treated with 1.3 mg/kg indolidan were found to have severe periarteritis, often with thrombi located mainly in the mesenteric arteries. Fifty-four percent of the male rats treated with 48.0 mg/kg isomazole and 55% of the male rats treated with 1.3 mg/kg indolidan died from cardiovascular disease compared to 1-2% among the control males. Animals in the low- and middle-dose groups of both studies had a lower incidence of cardiovascular disease than did those in the high-dose group. Additional lesions associated with the long-term administration of both drugs were markedly increased incidence of adrenal medullary proliferative lesions (both hyperplasia and pheochromocytomas) and increased incidence of chronic progressive glomerulonephrosis. These lesions, like those in the cardiovascular system, occurred in a dose-dependent manner and were more frequent in males than in females. Treatment-related effects in these studies were judged to be related to the pharmacologic action of these compounds.

The influence of time of maternal exposure to 2,4,5,2',4',5'-hexachlorobiphenyl on its accumulation in their nursing offspring

2,4,5,2',4',5'-Hexachlorobiphenyl (6-CB) is mobilized from rodent tissues during the lipid depletion associated with food restriction or lactation, the latter condition resulting in the substantial elimination of the maternal body burden of the chemical to nursing offspring. The present study was undertaken to determine whether the rate and/or magnitude of accumulation of 6-CB in nursing offspring differed with time following PCB administration to the maternal animal. Female ICR mice were administered two doses of 6-CB. Group I animals received [14C]-6-CB as weanlings (15-20 g) followed by unlabeled 6-CB 5 weeks later, after mating, on Day 1 of gestation. Group II received unlabeled 6-CB as weanlings and [14C]-6-CB on Day 1 of gestation. Thus, 14C identified the mobilization and elimination of either the first or the second dose of 6-CB in the treatment groups (I = [14C]-6-CB, 6-CB; II = 6-CB, [14C]-6-CB). Both groups of animals retained approximately 80% of the administered radiolabeled dose. The tissue distribution of [14C]-6-CB in group II as a percentage of the body burden was not different from that in group I as determined from maternal tissue concentrations on Day 14 of gestation. The percentage of the maternal body burden of [14C]-6-CB accumulated in suckling offspring of group II mothers was significantly greater than that in group I offspring on Day 1 (I, 2.2 +/- 0.5%; II, 3.5 +/- 0.4%), Day 3 (I, 14.8 +/- 1.9%; II, 24.6 +/- 2.7%), Day 5 (I, 16.8 +/- 1.4%; II, 24.8 +/- 0.8%), and Day 12 (I, 32.3 +/- 0.5%; II, 45.5 +/- 1.7%) postpartum. This differential elimination was reflected in the t1/2 of elimination of the radiolabeled dose from parametrial fat during lactation, which was significantly longer in group I (14 days) than group II maternal animals (9 days). The observations that the last dose of 6-CB administered was the first to be mobilized from the whole animal, and that this was reflected in 6-CB release from parametrial fat, suggest that this highly lipophilic chemical is not homogeneously distributed within storage depots.

The effect of diet and litter size on the elimination of 2,4,5,2',4',5'-[14C]hexachlorobiphenyl from lactating mice

It was shown that 2,4,5,2',4',5'-hexachlorobiphenyl (6-CB) administered to adult female mice accumulated in their nursing offspring more rapidly than a dose administered to weanling mice when treated animals were bred at equivalent ages. This suggested that the PCB was eliminated from the maternal animal relative to its time of sequestration into storage depots. Using a model which more closely approximates conditions during human lactation, the influence of a high-fat diet and decreased litter size on this phenomenon was examined. Female ICR mice were treated with 4 mg/kg [14C]-6-CB as 13-g weanlings (dW) at 3 weeks of age or as adults (dA) at 11 weeks of age. All animals were mated at 11 weeks of age. On Day 1 of pregnancy, mice were placed on a low-fat (11.5% of the total calories) or high-fat (43.8% of total calories) diet. At parturition, litters were adjusted to either two or eight within each diet group. Elimination of maternal 6-CB was determined by assessing radioactivity in offspring carcasses on Day 15 of gestation or Day 1, 3, 5, 10, or 15 postpartum. Consumption of a high-fat diet significantly extended the t1/2 of elimination of 6-CB from mothers nursing a litter of two in the dW group (low fat = 7.3 days; high fat = 12.4 days) and in both the dW and dA groups nursing litters of eight (dW: low fat = 4.6 days; high fat = 6.8 days; and dA: low fat = 1.8 days; high fat = 3.0 days). Within diet and group, reducing litter size to two also significantly decreased the rate of elimination of 6-CB from maternal animals. 6-CB was eliminated to offspring more rapidly from the dA group when compared to the dW group regardless of diet in animals nursing litters of eight. This relationship was not observed in maternal animals nursing litters of two. In general, exposure to a high-fat diet increased the t1/2 of elimination of 6-CB from maternal animals. Reducing litter size had a greater influence and also masked the ordered mobilization of 6-CB relative to its time of sequestration. Thus, under experimental conditions which more closely mimic the human situation, 6-CB was eliminated less rapidly in the rodent, and sequential 6-CB doses were equally available to nursing offspring.

Transfer of 2,4,5,2',4',5'-hexachlorobiphenyl across the in situ perfused guinea pig placenta

The transplacental crossover of 14C-2,4,5,2',4',5'-hexachlorobiphenyl (6-CB) from the maternal circulation to the fetal side of the placenta was examined in intact fetuses and following the in situ perfusion of the guinea pig placenta. Fetal, late pregnant, and nonpregnant female guinea pig lipoprotein profiles and the association of 6-CB with these plasma constituents were also determined in vivo. Very low density lipoprotein (VLDL) concentrations were 10-fold higher in fetal than in maternal plasma, and the great majority of 6-CB which was transferred to intact fetuses became associated with this plasma fraction. 6-CB was found primarily in association with low density lipoproteins (LDL) in nonpregnant animals. In the late pregnant guinea pig, 6-CB became primarily associated with plasma protein in spite of circulating protein concentrations lower than those seen in the nonpregnant state. No differences in the levels of the three plasma lipoprotein classes were observed between pregnant and nonpregnant animals. It was found that an amount of 6-CB similar to that found in intact litter mates crossed the perfused placenta over the same time period. Despite the much higher VLDL concentrations on the fetal side of the placenta and the association of 6-CB with VLDL in intact fetuses, addition of 1,000 mg/dl VLDL to the 5.4% bovine serum albumin perfusion medium failed to influence the magnitude of 6-CB crossover. 6-CB crossover was influenced by protein concentration in the perfusion media in a concentration-dependent fashion. It is hypothesized that 6-CB and free fatty acids traverse the placenta and are retained by the fetus via similar mechanisms.

The disposition and elimination of two sequential doses of 2,4,5,2',4',5'-hexachlorobiphenyl

It has been suggested based on fecal elimination data that a second dose of 2,4,5,2',4',5'-hexachlorobiphenyl (6-CB) is eliminated more rapidly from male rats than a first dose of 6-CB due to a differential accumulation of the two 6-CB doses within individual adipocytes. This phenomenon was further examined and extended to assess tissue levels of the compound in male rats which received either 0.6 mg/kg unlabeled or 14C-6-CB iv (dose 1). Fourteen days later, those animals which had received unlabeled 6-CB were administered 0.6 mg/kg 14C-6-CB. One-half of those rats which had received 14C-6-CB as dose 1 were administered unlabeled 6-CB after 14 days, and the remainder did not receive a second dose. The fecal excretion of radiolabeled 6-CB from rats treated only with 14C-6-CB was 5.3% of the total dose from days 14 through 28 post-treatment. Those rats receiving 14C-6-CB followed by unlabeled 6-CB excreted 4.4% of the total radiolabeled dose. Animals treated with 14C-6-CB as the second dose excreted 9% of their total radiolabeled dose within 14 days of treatment and those receiving only 14C-6-CB excreted 11% over the same time interval. Urinary elimination of radioactivity was negligible in all treatment groups. The pattern of distribution of 14C-6-CB in tissues was similar among all treatment groups. These data suggest that a second dose of 6-CB is not distributed differently from the first in growing male rats.

Potential mechanisms for redistribution of polychlorinated biphenyls during pregnancy and lactation

Female mice treated with 14C-2,4,5,2',4',5'-hexachlorobiphenyl (6-CB) two weeks prior to mating eliminated virtually their entire body burden of the compound through milk during one lactation cycle. 6-CB was shown to distribute among rat and human plasma lipoproteins and protein in vitro. It was readily transferred among plasma constituents and its distribution was related to the triacylglycerol:protein ratio in plasma. At one hour following its intravenous administration to virgin rats, 6-CB was primarily distributed to LDL. With the hypertriglyceridemia of late pregnancy, more than 70% of circulating 6-CB was associated with VLDL. VLDL is a major substrate for mammary gland lipoprotein lipase which is elevated during lactation. When 6-CB was complexed with human VLDL and injected i.v. into late pregnant mice, mammary gland concentrations of 6-CB exceeded those of adipose tissue at all sacrifice times between 5 min and 6 h. No differences between adipose tissue and mammary gland concentrations of 6-CB were observed with Emulphor:ethanol:saline as vehicle until 6 h. Isolated hepatocytes were capable of secreting protein and triacylglycerol in the form of VLDL into serum-free media. Eighty percent of 6-CB released from hepatocytes was in association with VLDL, with the remainder in association with protein. Adipocytes isolated from epididymal fat pads of male rats which were pretreated with 6-CB released progressively less radioactivity to incubation media with time after treatment even though PCB content of these cells increased. 6-CB may not be evenly distributed among adipocyte lipids.

Influence of lipolysis on the mobilization of 2,4,5,2'4',5'-hexachlorobiphenyl from adipocytes in vitro

Epididymal adipocytes, isolated from rats pretreated with [14C]-2,4,5,2',4',5'-hexachlorobiphenyl (6-CB), were utilized to examine the relationship between the mobilization of lipid and 6-CB and to determine whether 6-CB was differentially associated with subcellular organelles over time as has been demonstrated for newly synthesized lipid. Lipolysis, induced by the presence of 8 X 10(-7) M isoproterenol (ISO) for 50 min, depleted approximately 1% of total cellular triacylglycerols (TG) regardless of time from treatment with 6-CB. The percentage of cellular 6-CB released from adipocytes to incubation buffer infranatants was not correlated with the magnitude of lipolysis produced over the 50-min incubation period; nor was the percentage of 6-CB released to the buffers correlated with the length of the incubation period, regardless of the presence of ISO. Although adipocytes responded similarly to lipolytic stimuli independent of time (days) since 6-CB treatment, significant decreases were found in the percentage of 6-CB released from adipocytes over time. The in vitro labeling of this newly synthesized TG in fat cells with [U-14C]glucose or [1-14C]palmitate demonstrated that TG was differentially distributed among adipocyte organelles. Newly synthesized TG was also the first to be mobilized under lipolytic stimulus. 6-CB was not released in a similar fashion, since radioactivity associated with the chemical levels of [14C]-6-CB and glucose-derived 14C in buffers were not correlated over time. 6-CB was found to redistribute to all available lipid pools during the subcellular fractionation procedure and thus did not resemble TG. However, it is possible that 6-CB may exist in equilibrium among organelle fractions and that it becomes sequestered within the nonsoluble lipid compartment with time, thus decreasing its appearance in the soluble buffer infranatants over the experimental time course.

Mechanisms for the release and redistribution of 2,4,5,2',4',5'-hexachlorobiphenyl (6-CB) from hepatic tissues in the rat

The translocation of 6-[14C]CB from rat hepatic tissues to various media was studied employing in situ hepatic perfusion and primary hepatocyte culture techniques. 6-[14C]CB release from the hepatic tissues of female rats pretreated with 2 microCI 6-CB was dependent on the relative proportion of perfusate buffer components. Approximately 10% of hepatic 6-CB was released into buffer containing either 4% BSA or 4% BSA and 100 mg/dl exogenous human very low density lipoproteins (VLDL). 6-CB release was significantly increased under simulated hyperlipidemic conditions (400 mg VLDL/dl). Release declined when BSA was eliminated or replaced with Dextran. The distribution of 6-CB between the triacylglycerol (TG)-rich VLDL and the protein buffer components was found to be dependent on the ratio of TG:protein. Under hyperlipidemic perfusate conditions, approximately 83% of the 6-CB associated with the BSA fraction. Under normolipidemic conditions, 99% of the 6-CB associated with BSA. The concentration of 6-CB in TG was greatly increased under hyperlipidemic conditions. Thus, 6-CB distribution under simulated normolipidemic conditions could not be explained by saturation of the VLDL fraction. Approximately 15% of 6-CB was released from hepatocytes prepared from late pregnant or age-matched control rats. Eighty percent of 6-CB was associated with VLDL secreted from hepatocytes. The TG:protein ratio in culture media was approximately 1: 6 while ratios of 1:20 or 1:600 occurred in the perfusion studies. These data suggest that 6-CB may be released from hepatic tissues in association with newly synthesized TG, but that once in the circulation, its distribution is dependent on the ratio of TG to protein present.

Mechanism of enhanced parathion/paraoxon toxicity during pregnancy in the mouse

The mechanism of enhanced parathion/paraoxon toxicity during pregnancy was examined. Enhanced toxicity following exposure to paraoxon in the pregnant mouse as determined by cholinesterase suppression was observed at 0.10 and 0.58 mg/kg after ip administration on Day 19 of gestation. However, there were no significant differences in cholinesterase activity between pregnant animals and virgin controls after either po or iv paraoxon. Higher systemic and lower hepatic levels of parathion were demonstrated in pregnant mice following ip administration of parathion (5 mg/kg). Data herein also suggest that during pregnancy, larger quantities of paraoxon bypass initial liver detoxification after ip dosing. The mechanism of increased toxicity of parathion/paraoxon during pregnancy may result from alterations in absorption from the peritoneal cavity.

Influence of pregnancy on the hepatic metabolism of parathion

The effect of pregnancy on the hepatic metabolism of parathion was examined. The in vitro rate of hepatic microsomal activation of parathion to paraoxon was significantly reduced in mice at 19 days of gestation when compared to nonpregnant controls. Total hepatic metabolism of parathion was determined during in situ perfusion of livers from pregnant and nonpregnant mice. Levels of parathion, paraoxon, and p-nitro-phenol in the perfusate after 45 min of perfusion did not differ significantly between livers from the pregnant and nonpregnant groups. These data indicate that total hepatic metabolism of these three compounds is not altered in pregnancy despite a decrease in specific activity for parathion activation.

The effect of pregnancy and lactation on the disposition of [2,4,2',4'-14C]tetrachlorobiphenyl in the mouse

The disposition of 150 mg/kg 2,4,2',4'-tetrachlorobiphenyl (4-CB) administered intraperitoneally as a function of nonpregnant body weight was studied in virgin, late pregnant, and early postpartum mice and their offspring. Highest concentrations of the polychlorinated biphenyl (PCB) were observed in adipose tissue and mammary gland regardless of reproductive state. Concentrations of 4-CB equivalents in the tissues examined, however, differed among the three groups, possibly due to the alterations in lipid deposition/mobilization associated with pregnancy and lactation. Approximately 20% of 14C activity was eliminated from carcasses of virgin mice at 4 days after 4-CB administration. No significant decrease in carcass 14C content was observed during this 4-day interval in late pregnant animals. Minimal transplacental transfer of 14C activity occurred (approximately 1% of total maternal body burden) but 4-CB was rapidly eliminated in breast milk to nursing offspring. Ninety percent of total carcass 14C activity was eliminated from lactating mice over a 4-day period, approximately 75% of which could be accounted for in neonatal carcasses. This indicates milk to be the preferred route of elimination for this PCB congener in postpartum animals. Virtually all radioactivity recovered from milk comigrated with parent compound following thin-layer chromatographic analysis. A comparison between hepatic microsomal ethoxycoumarin-O-deethylase activity (ECOD) and liver concentrations of 14C activity showed that pregnant animals were less responsive to the inducing effects of 4-CB than virgin or postpartum mice. This diminution in response may be, in part, responsible for the lack of elimination of 4-CB equivalents from the late pregnant animal during the 4-day experimental period.

The enhancing effect of spawning on elimination of a persistent polychlorinated biphenyl from female yellow perch

Distribution and elimination of 2,5,2',5'-tetrachloro[14C]biphenyl (4-CB) were studied for 6 months after exposing sexually mature female yellow perch to the compound in water and transferring them to flowing 4-CB-free water. Perch that were exposed in January spawned in May, and the study was terminated in June. During the first 41/2 months after exposure, the t1/2 for whole-body elimination was 22 weeks, primarily by elimination of 4-CB from the viscera and carcass. During spawning, enhanced elimination (t1/2 less than 0.7 weeks) was due to the voiding of eggs containing 4-CB. After spawning, whole-body elimination returned to a slower rate (t1/2 = 16.3 weeks). Prior to the enhancement in 4-CB elimination rate during spawning, there was a redistribution of 4-CB residues within the body of the perch which was characterized by a transfer of 4-CB residues from primarily the carcass and viscera to eggs. Two weeks after exposure, 30% of the initial 4-CB body burden was distributed to the eggs, whereas just prior to spawning, about 50% was present in this tissue. These findings demonstrate that egg maturation and spawning result in a significant reduction in the body burden of a persistent polychlorinated biphenyl in a lean-fish species.

Distribution of 2,4,5,2',4',5'-hexachlorobiphenyl among lipoproteins during pregnancy and lactation in the rat

The mechanism by which polychlorinated biphenyls are transferred from adipose tissue to the mammary gland during late pregnancy and lactation is unknown. Lipoproteins were investigated as a possible vehicle of transport. 2,4,5,2',4',5'-[14C] Hexachlorobiphenyl (6-CB) distribution among very low-, low- and high-density lipoproteins and the protein-rich bottom fraction was examined in virgin controls, in pregnant animals on days 9 and 18 of gestation, in mothers on day 10 of lactation and in suckling pups. Plasma obtained 1 hr after an i.v. injection of 6-CB was separated into the various lipoprotein fractions and their lipid composition analyzed. With advancing pregnancy, there was a shift in 6-CB distribution from the higher to lower density lipoproteins. A linear increase was observed in the amount of very low-density lipoprotein as well as the proportion of plasma 6-CB associated with it. Corresponding decreases occurred in the proportion of 6-CB associated with low-density lipoprotein and the bottom fraction. During late pregnancy, very low-density lipoprotein became the primary carrier of 6-CB in vivo. The pattern of 6-CB distribution among lipoproteins on day 10 of lactation resembled that on day 9 of pregnancy. Suckling pups exhibited the highest proportion of plasma 6-CB in low- and high-density lipoprotein. The shifts in the distribution of 6-CB during pregnancy and lactation were related to changes in the lipid constituents of plasma and the individual lipoproteins.

Transport of 2,4,5,2',4',5'-hexachlorobiphenyl by lipoproteins in vivo

Lipoproteins are currently being recognized as transport vehicles for lipophilic drugs and xenobiotic chemicals in plasma. The weight of in vitro evidence suggests lipoproteins as the principal carries of 2,4,5, 2',4',5'-hexachlorobiphenyl (6-CB) in plasma from normolipidemic rats and humans. The present study examined the in vivo distribution of 6-CB among lipoproteins as well as the influence of time on the absolute amount and proportion of 6-CB associated with each density fraction. Plasma obtained between 1 min and 24 hr after an iv injection of 6-[14C]CB was separated into very low density lipoprotein (VLDL), low density lipoprotein (LDL), and high density lipoprotein (HDL) fractions by sequential ultracentrifugation. The in vivo results corroborate the in vitro data which suggest LDL to be a major transport vehicle for 6-CB in plasma. However, the preference of 6-CB for LDL existed for only a short time following injection. From 6 to 24 hr after administration of 6-CB, there was a shift in the distribution of the PCB from LDL to HDL and the remaining protein-rich bottom fraction. By 24 hr, the proportion of 6-CB in LDL had declined from 80% of the plasma concentration to 30%, while that in HDL had doubled. Furthermore, the amount of 6-CB in the bottom fraction accounted for 35% of the radioactivity in plasma at 24 hr as opposed to less than 5% up to 1 hr after administration. The absolute contents of 6-CB in both HDL and the bottom fraction also increased during the later time points. Analysis of the decay curves of 6-CB among the various lipoproteins further substantiated a change in the distribution of 6-CB over time. The decay of 6-CB in LDL most closely resembled its disappearance from plasma. The content of 6-CB remaining in plasma at 24 hr was equally distributed among LDL, HDL, and the bottom fraction. Changes in lipoprotein composition during the 24-hr period could not explain 6-CB redistribution, since there were no significant differences in the proportion of constituents comprising VLDL, LDL, HDL, or the bottom fraction.

Biotransformation of chemicals by fish: an overview

Evidence accumulated in recent years indicates that many species of fish possess most of the hepatic drug-metabolizing enzyme activities that have been demonstrated to exist in higher vertebrates. Although much of the early work with fish involved in vitro studies, it is now apparent that fish are capable of many phase I and II biotransformation reactions in vivo as well, but the rate and pathways for a given compound may vary widely among species of fish. In several instances, it has been demonstrated that the rate of biotransformation of certain compounds is appreciable. Biotransformation may be important in protecting fish from the toxic effects of certain chemicals by converting them to less biologically active chemicals. On the other hand, evidence is mounting that certain species of fish can react in such a way in vivo so as to produce active carcinogens from chemicals which are not themselves carcinogenic.

The effect of monooxygenase inducing agents on the incorporation of [35S]methionine into hepatic microsomal protein of rainbow trout (Salmo gairdneri)

Treatment of rainbow trout (Salmo gairdneri) with 150 mg/kg BNF resulted in an increase in hepatic microsomal monooxygenase activity as assessed by ECOD and EROD when compared to those activities in corn oil-pretreated animals. Administration of 100 mg/kg 2,4,5,2',4',5'-hexachlorobiphenyl (6-CB) to trout had no significant effect on these catalytic activities or on BeND. The amount of radioactivity in hepatic microsomes at 24, 48 or 72 hr following the administration of 75 muCi of [35S]methionine was consistently higher in animals pretreated with BNF than in those treated with corn oil or 6-CB. Autoradiography/fluorography of electrophoretograms demonstrated the appearance of at least three radiolabeled bands in the 50,000-60,000 mol. wt range in solubilized microsomes from BNF-treated fish which were not present in microsomes from control animals or fish treated with 6-CB. These data indicate that the stimulation of hepatic microsomal catalytic activities observed following the administration of 3-MC-type agents to rainbow trout is due, at least in part, to induction of enzyme(s) rather than activation of existing enzyme(s). These results further support the observation that fish appear to be non-responsive to phenobarbital-type inducing agents.

The effect of estradiol benzoate on the association of 2,4,5,2',4',5'-hexachlorobiphenyl with rainbow trout plasma lipoproteins

The mechanism by which persistent polychlorinated biphenyls (PCBs) are redistributed from maternal storage depots to developing oocytes during estrogen-dependent vitellogenesis in rainbow trout is unknown. Since PCBs have been shown to be associated with mammalian plasma lipoproteins, and since estrogens alter lipoprotein profiles in fish, the in vitro interaction of 14C-2,4,5,2',4',5'-hexachlorobiphenyl (6-CB) with lipoproteins was assessed in corn oil- and estradiol benzoate (EB)-pretreated sexually immature rainbow trout. Serum was collected and pooled, and aliquots were incubated with 0.04 nmoles 6-CB for 48 hours at 12 degrees C. Lipoproteins were separated by density flotation. Radioactivity, as well as, triacylglycerol, cholesterol, phospholipid phosphorus and protein content were assessed in each fraction. 6-CB was found to be associated with all lipoprotein classes, as well as the protein-rich bottom fraction. EB administration resulted in dose-dependent alterations in lipoprotein profiles and a shift in 6-CB association from protein and lipoproteins of higher density to those of lower density. A significant positive correlation was observed between the triacylglycerol or cholesterol content of the fractions and the percentage of 6-CB which was distributed in that fraction.

Influence of pregnancy on parathion toxicity and disposition

The effects of pregnancy and lactation on the toxicity and distribution of parathion and paraoxon were examined. Signs of cholinergic stimulation were more intense in pregnant mice when compared to virgin controls after administration of parathion or its active metabolite, paraoxon. Cholinesterase activity and tissue levels of parathion and paraoxon were determined in mice at 19 days of gestation or Day 19 postpartum after administration of a single dose of 5 mg/kg parathion or 0.58 mg/kg paraoxon. Plasma (pseudo) cholinesterase activity was consistently lower in treated pregnant mice. Total brain cholinesterase was also suppressed to a greater degree in pregnant mice after treatment with parathion or paraoxon when compared with virgin animals treated similarly. In addition, when equal quantities of paraoxon (32 micrograms) were administered to both pregnant and virgin animals, total brain cholinesterase was significantly less in pregnant mice. Administration of parathion to lactating mice on Day 19 postpartum did not result in any significant differences in plasma or brain cholinesterase activity when compared to that in virgin animals. Pregnant mice treated with 5 mg/kg parathion demonstrated higher concentrations of both parathion and paraoxon in blood and brain than similarly treated virgin controls which correlated with the enhanced cholinesterase inhibition. Decreased ability to detoxify paraoxon was also demonstrated by a significant reduction in serum paraoxonase activity during pregnancy.

Characteristics of 2,4,5,2',4',5'-hexachlorobiphenyl distribution among lipoproteins in vitro

The uptake, distribution, and transfer of 2,4,5,2',4',5'-hexachlorobiphenyl (6-CB) were examined in vitro with human and rat whole blood, plasma, and lipoprotein fractions. 6-CB distribution between plasma and erythrocytes as well as among lipoproteins was determined following sedimentation of erythrocytes and ultracentrifugal fractionation of plasma. In both rat and human whole blood, 70 to 75% of 6-CB partitioned into plasma and 25 to 30% into erythrocytes. The uptake of 6-CB into plasma was extremely rapid and the rate of uptake was found to be dependent upon temperature. The distribution of 6-CB among lipoproteins was relatively homogeneous with 20 to 30% being distributed in very low-density lipoproteins (VLDL, d = 0.95-1.006 g/ml), 15 to 20% in low-density lipoproteins (LDL, d = 1.006-1.063 g/ml), and 15 to 25% in high-density lipoproteins (HDL, d = 1.063-1.21 g/ml). Over 25% of 6-CB was found in the remaining bottom fraction. In addition, each isolated fraction when incubated alone with 6-CB was shown capable of uptake. The relative proportion of 6-CB among the lipoproteins was independent of the level taken up by plasma. 6-CB was also found to transfer among lipoproteins. This exchange of 6-CB proved to be dependent upon the concentrations of both protein and triacylglycerol in the incubations. Two proteins in the bottom fraction (Bf), albumin and a steroid binding globulin, were capable of competing with the lipoproteins for 6-CB uptake.

The effect of sex steroids and pregnenolone-16 alpha-carbonitrile on the hepatic microsomal monooxygenase system of rainbow trout (Salmo gairdneri)

Estradiol benzoate (EB-5 mg/kg) or testosterone propionate (TP-50 mg/kg) administration to sexually immature rainbow trout resulted in an increase in liver weight to body weight ratios, and a diminution in hepatic microsomal benzphetamine-N-demethylation (BeND), ethoxycoumarin- and ethoxyresorufin-O-deethylations (ECOD, EROD) and cytochrome(s) P-450 content when compared to corn oil-pretreated controls. A low dose of TP (2 mg/kg) caused an increase in cytochrome(s) P-450 content but had no effect on the selected monooxygenase activities. EB administration prior to treatment with 150 mg/kg beta-naphthoflavone caused a dose-dependent decrease in the magnitude of induction of cytochrome(s) P-450 and associated catalytic activities. These data suggest that the sex differences in monooxygenation observed in prespawning trout are mediated via the sex steroids and that fish may respond differently to inducers depending on their reproductive state at the time of exposure. Administration of the synthetic steroid, pregnenolone-16 alpha-carbonitrile (PCN), resulted in an increase in BeND and ECOD but had no effect on EROD or cytochrome(s) P-450 content.

Induction in rainbow trout of an acute phase (C-reactive) protein by chemicals of environmental concern

1. Rainbow trout, Salmo gairdneri, produce elevated amounts of a serum acute phase (C-reactive) protein (CRP) when administered a variety of chemicals of environmental importance. 2. Compounds administered in doses which induce the cytochrome(s) P450 catalytic enzymes in trout hepatic microsomes also induce serum CRP. 3. However, an interferon-inducing virus does not induce CRP. Interferon induction by the virus is not significantly inhibited by chemicals which induce trout cytochrome(s) P450. 4. Simultaneous administration of chemicals and virus or virus alone results in depression of P450 protein production and only minor induction of CRP. 5. Thus, as with mammals, a reciprocating relationship appears to exist between the hemeprotein monooxygenase and immune systems of this freshwater teleost, and C-reactive protein appears to fit the reciprocating scheme closer to the cytochromes P450 response.

Alterations in hepatic mixed-function oxidase activity of rainbow trout after acute treatment with polybrominated biphenyl isomers and FireMaster BP-6

The effects of FireMaster BP-6 and several pure isomers on hepatic microsomal mixed-function oxidase (MFO) activity in rainbow trout have been investigated. After one parenteral injection of these agents at 150 mg/kg and sacrifice 5 d later, there was no elevation of hepatic cytochrome P-450. The noncoplanar isomers 2,4,5,2',4',5'-hexa and 2,3,4,5,2',4',5'-heptabromobiphenyl did not increase any MFO activity generally associated with an increase in cytochrome P-450 levels, including the dealkylations of ethoxycoumarin and benzphetamine. The coplanar isomer 3,4,5,3',4',5'-hexabromobiphenyl produced an increase in O-deethylation of both ethoxycoumarin and ethoxyresorufin, suggesting enhancement of cytochrome P-448 associated activity, but produced no increase in benzphetamine N-dealkylation. The "mixed inducer" 2,4,5,3',4',5'-hexabromobiphenyl, produced a small but insignificant elevation of cytochrome P-448 associated MFO activities and no increase in cytochrome P-450 associated activities. At both 150 and 500 mg/kg, the commercial mixture of polybrominated biphenyls FireMaster BP-6 produced marked elevations of both ethoxycoumarin and ethoxy-resorufin dealkylations but had no effect on demethylation of benzphetamine. When microsomes from rainbow trout treated with FireMaster BP-6 were examined electrophoretically, A coomassie Blue-staining band at 57,000 daltons was intensified, as seen after treatment of fish with the cytochrome P-448 inducer beta-naphthoflavone. It is concluded that coplanar polybrominated isomers produce an induction of hepatic cytochrome P-448 associated activity, while noncoplanar isomers are ineffective as inducers of cytochrome P-450 related MFO activities in rainbow trout.

Comparative aspects of the disposition and metabolism of xenobiotics in fish and mammals

The number of studies in aquatic toxicology has rapidly increased due to the importance of fish as a food source and also because of the increasing awareness of long-term health hazards of aquatic pollution of humans as well as to aquatic flora and fauna. The bioaccumulation and persistence of certain chemicals in fish have been studied for some time but with the characterization of an active hepatic mixed function oxidase system, comparable in many respects to that found in mammals, the role of metabolism in the disposition, toxicity, and persistence of certain chemicals can be studied. The recent observation that rainbow trout and their eggs are very sensitive to aflatoxin B1, a carcinogen that apparently must be activated to a chemically reactive intermediate to exert its effect, has led to the development of fish being considered appropriate animal models for toxicity testing. Since most predictive toxicology in the past has been restricted to nonaquatic species, this review has attempted to describe some of the similarities and dissimilarities between mammals and fish in terms of absorption, distribution, metabolism, and excretion of xenobiotics.

The effects of pinealectomy on pituitary prolactin levels in Carassius auratus exposed to various photoperiod-temperature regimes

Pinealectomy has an influence on pituitary prolactin levels in goldfish maintained under certain photoperiod-temperature regimes. The effects of pinealectomy on pituitary prolactin content depend on photoperiod and time of sampling. Thus, differences between pinealectomized and sham operated animals may be due to a shift of a daily variation in pituitary prolactin levels. The pineal in fishes may thus be involved in photoperiod influences on daily prolactin rhythms.