temporal changes in plasma levels of catecholamines, lactate, glucose

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after intravenous injection of cocaine (5 mg/kg) or saline and .... (Pfizer, New York, NY) solution (in 0.5% ethanol). ... In this cage, the rats were allowed to rest for 90 ..... However, this blocking action cannot be the only reason for ... this problem, as evidenced by the lack of increase in ... release of calcium from sarcoplasmic.
Cocaine and exercise: temporal changes in plasma of catecholamines, lactate, glucose, and cocaine

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DONG H. HAN, K. PATRICK KELLY, GILBERT W. FELLINGHAM, AND ROBERT K. CONLEE Departments of Physical Education and Statistics, Brigham Young University, Provo, Utah 84602 Han, Dong H., K. Patrick Kelly, Gilbert W. Fellingham, and Robert K. Conlee. Cocaine and exercise: temporal changes in plasma levels of catecholamines, lactate, glucose, and cocaine. Am. J. Physiol. 270 (Endocrinol. Metab. 33): E438-E444, 1996.-To determine the combined sympathoadrenal effects of cocaine and exercise in awake animals, rats were assigned to one of four treatment groups: salinerest (SR), saline-exercise (SE), cocaine-rest (CR), and cocaineexercise (CE). Venous blood samples from jugular catheters were obtained at -40, O-4, 7, 10, 13, 16, 19, 26, and 36 min after intravenous injection of cocaine (5 mg/kg) or saline and the simultaneous onset of a 16-min treadmill run (26 m./min, 10% grade). CE increased plasma epinephrine (24.2 nM at 16 min), norepinephrine (28.0 nM at 10 min), and lactate (11.2 mM at 4 min) to levels 2-5 times greater than either treatment (SE and CR) alone (P < 0.05) and 11-35 times higher than SR. Blood glucose values were significantly depressed in CE (- 33% vs. SE) but increased in CR (+26% vs. SR). Plasma cocaine peaked , we had waited 20-50 min after injection of cocaine (12.5 mg/kg ip) before sampling the blood. However, in resting rats, the cocaine-induced plasma catecholamine response has been reported to peak in ~15 min (13). In addition, our samples were obtained after anesthesia, whereas the data of Chieuh and Kopin (6) were obtained from awake animals. More importantly, our

Animal care, preparation, and surgery. Male SpragueDawley rats weighing between 250 and 275 g (Sasco Labs, Omaha, Nebraska) were housed individually in a temperature (23-25”C)and light (12:12-h light-dark cycle, light 0700-1900)-controlled room. The animals were allowed standard laboratory rat chow and water ad libitum. Upon arriving, animals were allowed to adjust to their new environment for 2-3 days. Thereafter, all animals began a daily running program on a motor-driven rodent treadmill. Over a 2-wk period, intensity and duration of running were gradually increased from 13 m/min and 0% grade until the animals were running 20 mm/day at a speed of 26 m/min and 10% grade. They continued to train at this intensity for 3-4 wk until the day of the experiment. This protocol is standard procedure to accustom the animals to running on the treadmill to reduce variability in running performance in unaccustomed animals. Before surgical implantation of the venous catheters (4 days), tap water was replaced with a 0.025% oxytetracycline*HCl (Pfizer, New York, NY) solution (in 0.5% ethanol). At the time of surgery, animals were anesthetized with a 1 ml/kg ip dose of ketamine and acepromazine maleate mixture (90 mg and 1 mg/ml, respectively). All animals were implanted with a polyurethane (Braintree Scientific, Braintree, MA) catheter (0.040 in. OD X 0.025 in. ID) through the right jugular vein following the method of Harms and Ojeda (12). This catheter was used for blood sampling. A second polyurethane catheter (0.025 in. OD X 0.012 in. ID) was placed through the left jugular vein to accommodate intravenous injections. The smaller size of the second catheter was selected so that it would allow adequate venous return from the head region (25). Catheters were implanted

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IN OUR EARLY WORK (1,2), we noted that intraperitoneal

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COCAINE AND EXERCISE at least 5 days before the experiment and kept patent with 40% polyvinylpyrrolidone in 1,000 U/ml heparin solution. Testing and blood sampling procedure. On the day of the experiment, each animal was randomly assigned to one of four experimental groups (8-13 animals/group): saline-rest (SR), cocaine-rest (CR), saline-exercise (SE), and cocaineexercise (CE). The indwelling catheters of each animal were connected to extension tubes as follows: Silicone tubing (0.037 in. OD X 0.02 in. ID) for the right jugular catheter and polyurethane tubing (0.025 in. OD X 0.012 in. ID) for the left catheter. The rat was then transferred into a cage with a dark Plexiglas cover through which catheter extension tubes were exteriorized. In this cage, the rats were allowed to rest for 90 min before the first blood samples were obtained. After the first sample was obtained, the rats assigned to the exercise groups (SE and CE) were then transferred to the treadmill and were allowed another 40 min of rest before a second blood sample was obtained. Animals assigned to the resting groups (SR or CR) remained in the cage and did not donate a second resting sample. At the start of the exercise test, cocaine (cocaine=HCl; Sigma, St. Louis, MO; 5 mg/kg) or saline (1 ml/kg) was injected intravenously over a 40- to 60-s time period during which the speed of the treadmill was slowly increased up to 26 m/min and 10% grade. This intensity was maintained for 16 min. After 16 min, the treadmill was stopped, and the rats recovered for 20 min. Resting animals received similar injections but remained in their isolated cages. We administered the cocaine intravenously as opposed to intraperitoneally to ensure a more rapid and uniform delivery of the drug. When the drug is administered intraperitoneally, the drug itself can delay its own absorption by its vasoconstrictive effects at the site of injection (27). Furthermore, the dose of 5 mg/kg administered intravenously is consistent with the protocol of other groups attempting to assess the cardiovascular effects of the drug (4,13). Blood (400 ul) was drawn through the right catheter, as described by Steffens (30), at l-4, 7, 10, 13, 16, 19,26, and 36 min after injection. After each blood draw, blood was replaced with fresh titrated blood (blood-4% sodium citrate, 9: 1) drawn from the venous cannula of drug-free unstressed donor animals. At the end of the experiment, animals were killed with an intravenous overdose of pentobarbital sodium. Blood was drawn into a heparinized (5 ul, 10,000 units) syringe and immediately transferred into an ice-cold (4°C) microcentrifuge tube containing 0.01% EDTA as antioxidant. Blood was centrifuged for 10 min at 4°C 13,000 revolutions/ min. An aliquot of 100 ul plasma was stored at -80°C for catecholamine analysis. An aliquot of 50 ul plasma was stored in a tube containing 5 ul of 6% sodium fluoride at -80°C for cocaine analysis. The rest of the plasma was stored at -20°C for glucose and lactate analysis. Biochemical analysis. Plasma lactate and glucose concentrations were determined using an ANALOX GM7 Micro-Stat analyzer (InterCon, Champaign, IL). Catecholamine and cocaine were separately determined by high-pressure liquid chromatography (HPLC) using a solvent system appropriate for each compound. The HPLC system consisted of a Bioanalytical System (BAS) (Lafayette, IN) 480 liquid chromatograph, CMA 200 refrigerated microsampler (Carnegie Medicine), and INJECT (BAS) data acquisition software. Catecholamines and standards were extracted from 100 ul plasma or standard dissolved in phosphate buffer (pH 7.0) as described by Smedes et al. (29) with some modification. Dihydroxybenzylamine (100 ul, 14.4 nM) in 80 mM acetic acid with 20% EDTA was used as an internal standard. An aliquot of 100 ul of extract was injected into a phase II ODS-3 column

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(BAS, C&, 3 urn, 100 X 3 mm). A BAS LC-4C electrochemical detector was used with the glassy carbon working electrode potential set at 650 mV with respect to the silver-silver chloride reference electrode. The mobile phase consisted of 0.1 M monochloroacetic acid, 0.07 M sodium octyl sulfate, 0.05 M EDTA (adjusted to pH 3.2 with 6 M NaOH), 2.5% acetonitrile, and 1.6% tetrahydrofuran at a flow rate of 1 mllmin. Plasma cocaine was determined using the method of Lau et al. (17) with some modification. Spiked standards or 50 ul plasma, 10 ul internal standard (lidocaine, 5 ug/ml), 100 ul borate buffer (1 M), and 1 ml chloroform were mixed in a 5-ml conical centrifuge tube. The mixture was vortexed for 2 min and then centrifuged for 5 min. The organic phase was separated from the aqueous phase using chloroform prewashed Phase Separator (Whatman, Reno, NV) filter paper. The organic phase was allowed to evaporate under a mild flow of nitrogen gas at 40°C and resuspended in 50 ul of the mobile phase. An aliquot of 20 ul reconstituted samples was injected into a uBondapak column (C 18, 10 urn, 300 X 2 mm; Milford, MA), and cocaine was detected using an ultraviolet (UV) detector (UV-116, BAS) working at 230 nm and 0.004 absorbance units full scale. The mobile phase consisted of 0.031 M acetic acid (adjusted to pH 5.1 with 6 M NaOH), 2 X lop4 M tetrabutylammonium phosphate, 15% methanol, and 20% acetonitrile at a flow rate of 0.4 ml/min. Plasma concentrations were calculated using a linear regression curve generated by the peak heights ratio of standard to internal standard vs. concentrations in molar concentrations. Statistical anaZyses. The data are expressed as means t SE except cocaine elimination half-life, which is presented as 95% confidence intervals. Multivariate analysis of variance on complete case was used to compare response patterns among treatments for NE, Epi, lactate, and glucose. Restricted maximum likelihood (15) analysis of the longitudinal data was used to compare response patterns among treatments for cocaine. The natural log of the cocaine concentrations was taken to linearize the depletion curve. Plasma cocaine elimination half-lives were determined from the fitted depletion curves. RESULTS

Plasma catecholamine. Basal levels (-40 and 0 min time points) of catecholamine were not different among groups (SR, SE, CR, CE). At -40 min, NE (Fig. 1) and Epi (Fig. 2) levels were 1.02 ? 0.07 and 0.70 2 0.07 nM, respectively. The 40-min rest interval after handling and transferring rats (SE and CE groups) from cage to treadmill allowed catecholamine levels (1.25 ? 0.12 and 0.82 L 0.08 nM; NE and Epi, respectively) to return to basal levels. Frequent blood samplings as well as transfusions did not change resting control (SR) catecholamine levels throughout the experiment (Figs. 1 and 2). Cocaine (CR, P < 0.005) alone caused plasma NE and Epi to rise to levels of 4.79 t 0.50 and 5.52 2 0.64 nM, respectively. These peaks occurred , cocaine vs. saline (P < O.OOl), interaction between treatments (P < 0.001). Each point represents data from 8-13 animals.

Fig. 3. Temporal response of plasma lactate to intravenous injection of cocaine (5 mg/kg). All response patterns are significantly different from each other as follows: rest vs. exercise (P < O.OOl>, cocaine vs. saline (P < O.OOl), interaction between treatments (P < 0.001). Each point represents data from 8-13 animals.

between 7 and 13 min after intravenous injection and tended to decrease even before the termination of exercise (Fig. 1). Plasma Epi continued to increase throughout the exercise period to a peak of 24.2 t 5.0 nM at 16 min and gradually decreased to levels three times the SR value at the end of the recovery period (Fig. 2). Plasma lactate. Plasma lactate levels followed a pattern similar to those of plasma catecholamine (Fig. 3). There were no changes in SR plasma lactate concentrations (0.95 t 0.06 mM) throughout the measurement period. In CR and SE groups, plasma lactate reached its peak in , interaction between treatments (P < 0.001). E ac h point represents data from 8-13 animals.

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Fig. 4. Temporal response of plasma glucose to intravenous of cocaine (5 mg/kg). Response patterns are significantly follows: rest vs. exercise (P < O.OOl), interaction between data from 8-13 animals. (P < 0.01). Ea c h point represents

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Fig. 5. Temporal response of plasma cocaine to intravenous injection of cocaine (5 mg/kg) during rest and exercise. Response patterns are significantly different between cocaine-exercise and cocaine-rest (P < 0.001). El imination half-lives were 11.6 + 1.8 min for cocainerest and 13.9 + 2.7 min for cocaine-exercise. Each point represents data from 11-13 animals.

period after injection. This difference disappeared when exercise was terminated. Elimination half-life was determined to be 13.9 tr. 2.7 min for CE and 11.6 tr 1.8 min for CR, and these values were not statistically different. DISCUSSION

This study was primarily designed to assess the temporal responses of plasma catecholamines, lactate, and glucose concentrations to cocaine treatment under rest and exercise conditions. In our previous work (l-3, 8, 9), we have shown an effect of cocaine on each of these parameters, but those results were only qualitative because they were based on single time point observations. The present results show a clear temporal response of each variable, and the quantitative peaks or nadirs, although similar in direction, are different from the qualitative results previously reported. In this discussion, we will attempt to explain the mechanisms responsible for the temporal responses observed. Cocaine is known to inhibit the neuronal reuptake of the sympathetic neurotransmitter, NE, resulting in an increased concentration at the synaptic junction (23). However, this blocking action cannot be the only reason for the increase in resting NE and Epi levels observed in this study (see Figs. 1 and 2). Chiueh and Kopin (6) showed that a complete block of the reuptake of NE by desipramine, a tricyclic antidepressant drug, failed to elevate plasma catecholamine in resting rats. It is likely that, in addition to reuptake blocking, cocaine may act centrally to stimulate sympathoadrenal discharge (6, 13, 33). This central stimulation can elevate NE concentration at the synaptic junction, and the eventual overflow of NE due to the blocking action at the sympathetic terminal will result in higher plasma levels. In addition, the increase of plasma Epi at rest is likely explained by the cocaine-induced centrally medi-

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ated stimulation of the adrenal medulla (6, l3), although Chiueh and Kopin (6) did demonstrate a direct stimulation of cocaine on the adrenal gland. The major contribution of the present study is the magnitude of the catecholamine response to the combined effect of cocaine and exercise, which is also illustrated in the temporal response pattern of Figs. 1 and 2. The peak NE value during CE occurred within 3 min after the beginning of treatment and was 27 times higher than rest, 5.5 times higher than cocaine alone, and 4 times higher than exercise alone. The peak Epi concentration did not occur until the end of the exercise period and was 35 times higher than resting levels, 4.4 times higher than cocaine alone, and 3.9 times higher than exercise alone. It is possible that, had the exercise continued for a longer period, Epi would have continued to rise to even higher levels. Nevertheless, these peaks in catecholamine levels in response to CE are considerably higher than our previous results, which were measured in anesthetized rats at a single time point at the end of 20-50 min of rest or exercise (3,8,9). Winder and Yang (34) have shown that arterial Epi concentration fell from a value of 3.9 rig/ml obtained during the last 30 s of exercise to a value of 0.9 rig/ml 43 s after pentobarbital sodium anesthesia. Given this rapid rate of decline in catecholamine, it is clear that, in our previous studies, considerable degradation of the catecholamines had occurred. Why the combined treatments of cocaine and exercise produce an exaggerated catecholamine response is not clear. It has been reported that, when combined with electrical stimulation of sympathetic nerves, cocaine increases the sympathetic outflow (16, 32). Because exercise is also a potent sympathoadrenal stimulant (7), the str’k1 ing increase in catecholamine response as illustrated in Figs. 1 and 2 presumably resulted from cocaine’s ability to exaggerate sympathetic outflow when external adrenergic stimulation was present (16, 32). A rapid decrease in catecholamine concentration after termination of exercise supports this point (Figs. 1 and 2). Another possibility is that the decline in blood glucose during this treatment (Fig. 4) may have contributed to the simultaneous elevation in Epi. The levels of this glucoregulatory hormone are known to rise when blood glucose levels fall (35), but this explanation seems unlikely given the very high Epi levels shown in Fig. 2. The only other study that followed the temporal response of plasma catecholamines after cocaine treatment in awake animals was that of Kiritsy-Roy et al. (13). Using the same dose of drug in resting rats as we used in the present study (5 mg/kg), they reported that Epi rose to -3 nM 5 min after and NE rose to 3.5 nM at 10 min after the intra-arterial injection. Our peak values at rest shown in Figs. 1 and 2 and obtained after intravenous injection are slightly higher (i.e., 5 and 4 nM, respectively) than theirs. Their temporal response, however, contained an artifact because they did not replace blood after each withdrawal, which caused their catecholamine control values to rise significantly over time due to loss of total blood volume (13). By

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replacing blood after each withdrawal, we eliminated this problem, as evidenced by the lack of increase in catecholamines in our control (SR) group (Figs. 1 and 2). Regarding the blood replacement protocol, one could question whether there were any cumulative dilution effects on hormonal, metabolic, or ionic factors that might have influenced the data or whether the citrate (6.25 pmol/replacement volume) in the replacement blood may have produced some artifact. The total blood exchange that occurred as a result of the 13 samplings was 5.2 ml. This volume represents -20-25s of the total blood volume of the average rat. However, because most of the significant effects observed in Figs. l-5 occurred almost immediately (O-3 min) after the simultaneous treatments began, there is little chance that they would have been due to some adverse effects of the blood exchange process. Only -0.4-2.0 ml blood had been exchanged during that time, and that time was too brief for any dramatic ionic, metabolic, or hormonal changes to have occurred that would have been diluted out (or chelated out as a result of citrate) by such small replacement volumes. That there was no effect of sampling on any of the variables across time in the SR group substantiates this position. We cannot deny a potential dilution effect near the end of the experiment as a result of the 20-25s replacement, but, again, we have no evidence to suggest any adverse effects from the control data, and the pattern of the recovery responses of each of the variables in the other experimental conditions appears in agreement with what we would predict from our previous results (1,2,&g). The abnormally high catecholamine values presented here give further credence to our previously stated concern regarding the risk of exercising, or engaging in any sympathostimulatory activity, under the influence of cocaine. This concern stems from the purported causal relationship between cocaine-induced elevations in catecholamine and myocardial infarctions attributed to cocaine use (14). It is quite clear that exercise, as a representative sympathostimulatory activity, exacerbates the risk. This supposition is reinforced by the recent clinical report of Sloan and Mattioni (28). They described a case in which a chronic cocaine user suffered both a coronary and cerebral vascular event 30 min after the subject used cocaine and exercised. In addition to the effect on catecholamine levels, CR also caused an increase in plasma lactate concentration above that of SR (Fig. 3). Our earlier studies failed to demonstrate an effect of cocaine on resting plasma lactate levels (1, 2, 8) because of the methodological limitations of those studies noted in the introduction. The elevation of plasma lactate under the CR condition may result from an Epi stimulation (Fig. 2) of glycogenolysis in skeletal muscle. This hypothesis is supported by our previous work which showed that cocaine caused a reduction in soleus muscle glycogen concentration at rest (1,2,S). Epi is known to enhance glycogenolysis by activating glycogen phosphorylase, the rate-limiting enzyme (11). The accelerated glycolysis as a result of glycogen breakdown can cause lactate accumulation.

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An alternate explanation for the rise in blood lactate after cocaine at rest is a reduction in the rate of removal of lactate from the plasma. If blood flow were diminished to the liver as a result of cocaine-induced vasoconstriction (4), then lactate removal from the blood could be retarded. The combination of exercise and cocaine exaggerates the plasma lactate response above that of rest or exercise alone (Fig. 3), and the peak occurs very early after treatment. In addition to the Epi effect on muscle glycogen, several other mechanisms affecting lactate production can be postulated to explain this response. First, Leon-Velarde et al. (18) reported that cocaine significantly reduced mitochondrial respiratory function. If this was the case during exercise, then muscle would have to rely heavily on anaerobic processes to supplement ATP production, resulting in an exaggerated production and accumulation of lactate. Second, it has been suggested that cocaine can increase the release of calcium from sarcoplasmic reticulum (21). The increased free calcium in combination with calmodulin can accelerate glycogenolysis by activating phosphorylase (5) and, again, can result in lactate accumulation. Finally, it could be argued that the rise in lactate during CE may result from a NE-induced vasoconstriction (24) of the vasculature serving the working muscle. Under the circumstance of reduced blood flow, the working muscle would become hypoxic, which would lead to a shift in energy production from aerobic to anaerobic and a concomitant rise in muscle and blood lactate. The results of Braiden et al. (3) would seem to indirectly support this hypothesis. They reported significant increases in muscle and blood lactate concentrations under the combined conditions of cocaine and exercise, although they did not measure blood flow. In apparent contradiction to this theory are the recent results of Branch and Knuepfer (4) who showed that, whereas cocaine caused vasoconstriction of the mesenteric vasculature through an al-adrenoceptor-mediated mechanism that is triggered by NE, the drug actually caused a vasodilation of the hindquarter vasculature of the rat at rest, presumably through a l&adrenoceptor. These latter findings may not be applicable to the present discussion, however, because they were obtained at rest, whereas we are attempting to explain the CE response. In the present study, the NE values in CE are 5.5 times higher than those elicited by cocaine at rest. NE spillover has been postulated as one mechanism to explain reduced blood flow to working muscle that occurs under some intense work conditions (22). Furthermore, sympathetically induced vasoconstriction through the al-receptor is the primary mechanism by which blood flow to resting skeletal muscle is reduced to redirect blood flow to working muscle (22). With regard to this NE-blood flow-lactate hypothesis, it is interesting to note the close similarity between the rise in NE shown in Fig. 1 and the rise in plasma lactate shown in Fig. 3. They are nearly simultaneous. On the other hand, instead of increasing lactate production as argued above, the NE-induced vasocon-

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striction of the mesenteric circulation (4) may have reduced blood flow to the liver, which could have diminished lactate removal. If lactate removal were inhibited, then blood lactate would rise in the absence of any change in production. Concomitantly, a reduction in lactate delivery to the liver due to reduced blood flow would reduce gluconeogenesis and result in a fall in blood glucose concentration (35). The fall in blood glucose as a result of CE shown in Fig. 4 supports such a possibility. Whether this explanation accounts for the dramatic rise in blood lactate shown in Fig. 3 and the decrease in blood glucose shown in Fig. 4 remains to be determined. Although the results for NE, Epi, and lactate followed a similar pattern, the blood glucose response was peculiar. In resting rats, cocaine induced a continuous elevation in plasma glucose concentration up to 16 min after injection followed by a gradual decrease to normal (see Fig. 4). An opposite trend was observed in the CE group. Plasma glucose levels dropped dramatically during the 16-min exercise period and gradually returned to normal. We can only speculate regarding the mechanism responsible for the pattern illustrated in Fig. 4. Blood glucose levels at any given time represent a balance between hepatic glucose production (i.e., glycogenolysis and gluconeogenesis) and peripheral glucose uptake. At rest, the rise in blood glucose is likely due to gluconeogenic processes. Hepatic gluconeogenesis is mainly determined by the supply of gluconeogenie precursors, lactate and glycerol, to the liver (26). Our results (see Fig. 3) demonstrate an elevation in lactate in the CR animals. Although we did not determine plasma glycerol concentration, increases in plasma free fatty acid under a similar CR condition (10) may suggest a similar rise in glycerol, which would also promote gluconeogenesis. Under the present experimental condition, liver glycogenolysis is an unlikely contributor to the rise in plasma glucose level because Bracken et al. (1,2) have previously shown that cocaine treatment had no effect on liver glycogen content at rest. The decrease in blood glucose during CE is more difficult to explain. We would have expected glucose to rise for the same reasons given for the resting conditions, that is, elevated plasma lactate (see Fig. 3) and elevated glycerol accompanying elevated free fatty acid (10). We would suppose that such conditions would enhance gluconeogenic processes and elevate blood glucose levels, but our results indicate an overall decrease in blood glucose levels (see Fig. 4). On the other hand, if blood flow were decreased to the liver due to cocaine-induced vasoconstriction, then we could explain the fall in blood glucose as a result of a decrease in gluconeogenesis resulting from the retarded delivery of the precursor, lactate. Again, the blood flow hypothesis requires further experimentation. An additional purpose of this study was to evaluate plasma cocaine kinetics at rest and exercise in unanesthetized rats. The intravenous route of cocaine administration was used because this route gives instantaneous absorption as well as full bioavailability of the

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drug when it reaches the arterial side of the circulation (31). This was evident from the data of Fig. 5, which show that the peak values of plasma cocaine occur within l-2 min after injection. These data compare favorably with observations made in human subjects (27) but represent a contrast to the known data for the rat model (20). In a recent review of the disposition of cocaine, Shuster (27) noted that the standard time to peak concentration after intravenous injection was 15 min in the rat model. That value was reported by Nayak et al. (20) in 1976 and has persisted as the standard ever since (27). Unfortunately, those authors obtained their first blood sample in that study 15 min after injection of the drug. Our results clearly show that those early authors missed the true peak, as evidenced in Fig. 5. It is interesting that exercise would cause higher concentrations. This may reflect some reduction in the distribution space as a consequence of the vasoconstrictive actions of the drug. Cocaine is known to quickly diffuse through membranes and to accumulate in a variety of tissues (27). If access to those tissues were inhibited by vasoconstriction of the vasculature serving them, that could explain why higher concentrations of the drug remain in the blood during CE. In spite of the difference in peak values, the plasma cocaine elimination half-lives were not different. The range of 12-14 min is faster than the 18 min reported earlier (19, 20). The latter value was obtained from anesthetized animals, which probably explains the discrepancy. Therefore, we feel the values reported here are more reflective of the true half-lives. In summary, we have found that the combination of the two sympathoadrenal stimulants, exercise and cocaine, amplifies the catecholamine responses to levels far greater than when each stimulant is imposed alone. The peak response of the various variables occurs early on in the combined treatment period and falls off rapidly at the cessation of exercise. In addition, plasma cocaine levels peak rapidly (within 1-2 min) and disappear at an elimination half-life of 12-14 min. We are grateful for the technical assistance of Kimberly Carlson, Bryant Martin, Jared Jacobson, Philip Newman, James McGee, and Scott Smith on this project. This research was supported by National Institute on Drug Abuse Grant DA-04382. Present address of D. H. Han: Dept. of Internal Medicine, School of Medicine, Washington University, St. Louis, MO 63110. Address for reprint requests: R. K. Conlee, 212 RB, BrighamYoung University, Provo, UT 84602. Received

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