Magnesium and bupivacaine-induced convulsions in awake pregnant rats.

BACKGROUND: Magnesium sulfate (MgSO(4)) is widely used for the treatment and prevention of convulsions associated with preeclampsia. The aim of this study was to determine whether it alters the dose of bupivacaine required to produce convulsions in awake pregnant rats. METHOD: Twelve pregnant rats were pretreated with an intravenous infusion of either MgSO(4) or saline. Following 2 h of the pretreatment, bupivacaine was concomitantly infused in all animals until the onset of convulsions. Mean arterial pressure (MAP) and heart rate (HR) were monitored throughout. Serial arterial samples were obtained during the infusion. At the onset of convulsions, fetuses were delivered and maternal and fetal blood, as well as various tissue samples, were obtained. All samples were assayed for bupivacaine and magnesium concentrations. RESULTS: Maternal MAP and HR decreased significantly shortly after the initiation of MgSO(4), while saline did not affect these measurements. Baseline concentrations of magnesium in plasma were similar in both MgSO(4) and saline groups; magnesium increased significantly during the infusion of MgSO(4). The dose (mean+/-SD) of bupivacaine required to produce convulsions in the animals receiving MgSO(4) was significantly larger (10.2+/-1.9 mg/kg) than that in the saline group (5.9+/-1.0 mg/kg) (P<0.05). As a consequence, bupivacaine concentrations in the brain and liver at the onset of convulsions were greater in animals receiving MgSO(4) (16.0+/-8.4 and 18.2+/-4.3 microg/g wet weight, respectively) than in those given saline (12.1+/-2.2 and 9.9+/-2.0 microg/g wet weight, respectively). Fetal bupivacaine concentrations at the onset of convulsions in the MgSO(4) group were also higher than those in saline group. However, the rate of placental transfer of this drug was similar between MgSO(4) and saline animals. CONCLUSION: This study demonstrates that the clinically used concentration of magnesium sulfate increased the threshold of bupivacaine-induced convulsions in awake rats.

Dexmedetomidine decreases extracellular dopamine concentrations in the rat nucleus accumbens.

The effects of dexmedetomidine, a highly selective alpha(2)-adrenoceptor agonist, on extracellular dopamine (DA) concentrations in the nucleus accumbens of awake rats were collected via in vivo cerebral microdialysis and measured using HPLC with electrochemical detection. The administration of dexmedetomidine (DEX) at a low dose (2 microg/kg bolus i.v. over 2 min followed by a continuous infusion of 0.1 microg/kg per min) and a high dose (20 microg/kg bolus i.v. over 2 min followed by a continuous infusion of 1 microg/kg per min), significantly decreased extracellular dopamine concentrations in the nucleus accumbens. The observed decrease was dose-dependent, occurring sooner and to a greater magnitude in the rats receiving a high dose of DEX. This inhibitory modulation of accumbal dopamine was receptor-specific, as the decrease in extracellular DA produced by DEX was no longer evident following pre-treatment and co-infusion with the highly selective alpha(2)-adrenoceptor antagonist, atipamezole (ATZ). Thus, these data suggest that adrenoceptor agonists and antagonists may modulate dopaminergic neurotransmission via mechanisms that are specific to the alpha(2)-adrenoceptor.

The effects of single and fractionated doses of mepivacaine on the extent of thoracic epidural block.

BACKGROUND AND OBJECTIVES: The effects of single and fractionated doses of local anesthetic on the extent of thoracic epidural blockade has not yet been determined. This single blinded and randomized study was designed to examine the effects of the initial dose and timing of the additional dose of local anesthetic on the sensory block level of the thoracic epidural anesthesia. METHODS: Eighty-nine patients, who received thoracic epidural anesthesia followed by general anesthesia, were randomly divided into 4 groups: Group I received 5 mL of mepivacaine; Group II, 10 mL; Group III, 5 mL twice, with an interval of 5 minutes; and Group IV, 5 mL twice, with an interval of 10 minutes. After 15 minutes of either a single bolus or after the second bolus drug administration, the level of sensory block to coldness and pinprick were determined by an individual who was uninformed of the groups. RESULTS: The median (range) number of spinal segments with sensory block to coldness in Groups I, II, III, and IV were 8 (5 to 12), 12 (7 to 17)*, 11 (7 to 16)*, and 9 (6 to 17)# (*P < .05 v Group I, #P < .05 v Group II), respectively. The number of segments with sensory block to pinprick in the 4 groups were 7 (4 to 11), 11 (6 to 14)*, 10 (6 to 14)*, and 9 (4 to 16)*#, respectively. These differences were mainly due to the differences of the lower sensory block level. CONCLUSIONS: We concluded that the timing of the second administration of mepivacaine was one of the factors for the spread of the drug into thoracic epidural space. The more extensive sensory block level occurred by shorter time interval of the second drug administration.

The disposition of benzoylecgonine in maternal and fetal rats.

We tested our hypothesis that pregnancy alters the pharmacokinetic profile of benzoylecgonine, and that this metabolite accumulates in the fetus longer than in the mother. Chronically catheterized near-term pregnant and nonpregnant female Sprague-Dawley rats received an intravenous infusion of benzoylecgonine over a period of 30 min. Adult or fetal blood and tissue samples were obtained either at the end of the infusion or 6 h postinfusion for analysis of benzoylecgonine and other cocaine metabolite concentrations via gas chromatography/mass spectrometry (GC/MS). Pregnancy altered benzoylecgonine pharmacokinetics. At the end of the infusion, benzoylecgonine concentration in the fetal plasma was markedly lower than in the maternal plasma with a fetal/maternal ratio of 0.14+/-0.01. A significantly lower concentration of benzoylecgonine was found in both maternal and fetal brain at 0 h postinfusion, with tissue/plasma concentration ratios of 0.04 and 0.24, respectively, suggesting that benzoylecgonine does not readily penetrate into the brain. At 6 h, the fetal concentration of benzoylecgonine was significantly higher than in the corresponding maternal blood and tissues. Ecgonine methyl ester, a metabolite of benzoylecgonine was found in the maternal liver, but not in the fetus. In addition, the amniotic fluid concentration of benzoylecgonine became significantly higher in the 6-h postinfusion samples as compared to the end of infusion value, suggesting that repeated intrauterine exposure to cocaine may cause an accumulation of benzoylecgonine in the fetus.

Dose-related differences in the distribution of cocaine in the maternal-fetoplacental compartments in rats.

Our goals were to examine whether a high dose of cocaine to causing CNS toxic manifestations in the pregnant rats influences the delivery of cocaine to the fetus, and whether the non-placental compartments have a significant role in the distribution of cocaine to the fetal tissues. Either a low or high dose of cocaine was infused intravenously to near-term pregnant rats. Arterial blood pressure and heart rate were monitored. Cardiac output and uterine and placental blood flows were measured by using radiolabeled microspheres. Plasma and tissue samples were obtained from the mother, placenta, and fetus and analyzed for cocaine and its metabolites via capillary gas chromatography/mass spectrometry. A high dose of cocaine induced convulsions that were accompanied by increased arterial blood pressure and decreased uteroplacental blood flow. However, the distribution pattern of cocaine and metabolites in the mother and fetus were similar between the high and low dose groups. Considerable amounts of cocaine and its metabolites were in the placenta. Previously ignored non-placental tissues, such as the amnion and myometrium appear to be a significant source for cocaine accumulation in the fetus.

Disposition of bupivacaine and its metabolites in the maternal, placental, and fetal compartments in rats.

BACKGROUND: This study was designed to determine the disposition of bupivacaine and its metabolites in the maternal, placental, and fetal compartments, using multiple sampling time points in chronically prepared awake pregnant rats. METHODS: All animals received an intravenous infusion of bupivacaine at a rate of 0.33 mg. kg-1. min-1 over a period of 15 min. The fetuses were delivered either at the end of infusion or at 2 or 4 h after dosing. Maternal and fetal blood and tissue samples were obtained for the assays of bupivacaine and its metabolites using capillary gas chromatography-mass spectrometry. RESULTS: The elimination half-life of bupivacaine was 37.7 min. The major metabolite was 3'-hydroxybupivacaine. Bupivacaine and 3'-hydroxybupivacaine were present in all samples at the end of administration. The fetal to maternal concentration ratio of bupivacaine in plasma was 0.29, and in the placenta was 0.63. The amnion contained the highest bupivacaine concentration: threefold higher in the maternal and 11-fold higher than in the fetal plasma. At 4 h after dosing, bupivacaine was no longer detectable in any maternal and fetal samples, whereas 3'-hydroxybupivacaine was still present in all tissues except the fetal plasma and heart. CONCLUSIONS: These data indicate that a considerable amount of bupivacaine is taken up by both sides of the placenta, as well as the amnion and myometrium. 3'-Hydroxybupivacaine was present in all tissues except the fetal plasma and heart samples, even after the parent compound became no longer detectable. Whether this slow elimination of 3'-hydroxybupivacaine causes any adverse effects on the fetus-newborn needs to be explored.

Effect of standard diluted epinephrine infusion on epidural anesthesia in labor.

BACKGROUND AND OBJECTIVES: Epinephrine is used with local anesthetics to prolong the duration of epidural analgesia and decrease the peak plasma concentrations of local anesthetics. However, the duration of labor may be prolonged because epinephrine reduces uterine activity. We designed a prospective, randomized, and doubleblind study to examine the effects of epinephrine infusion on the quality of analgesia and plasma concentration of local anesthetic, as well as the effect on the uteroplacental circulation, duration of the first or second stage of labor, and fetal outcome. METHODS: Twenty-four parturients received continuous epidural bupivacaine 0.125% (8 mL/h) combined either with epinephrine (40 microg/h) (n = 12) or without epinephrine (n = 12) for analgesia during labor. If patients requested additional analgesia, a bolus of 1% or 1.5% lidocaine (6 to 10 mL) was given. RESULTS: Only the plain bupivacaine group required additional lidocaine. However, epinephrine infusion prolonged the median (range) duration of the second stages of labor: 69 (21 to 231) minutes with epinephrine group versus 31 (8 to 99) minutes without epinephrine group (P < .05), and decreased pH in umbilical artery at the time of delivery. Epinephrine infusion did not change the uterine and umbilical blood flow, which were determined as the resistance indices. Changes in the fetal heart rate and Apgar score were also comparable. Epinephrine significantly reduced the umbilical venous to maternal arterial bupivacaine concentration (P < .05). CONCLUSIONS: A standard diluted epinephrine infusion (40 microg/h) into epidural space decreased anesthetic requirements. The possibility of the prolonged duration of labor remains a problem.

Long-term intravenous perinatal cocaine exposure on the mortality of rat offspring.

To examine the effects of chronic perinatal cocaine exposure, cocaine was administered intravenously throughout pregnancy and the postpartum period to the rat. Pregnant rats were divided into five groups: nontreated (naive); normal saline control (saline); cocaine first generation (cocaine); saline in the first generation and cocaine in the second generation (Sal-2G); and cocaine in both first and second generations (Coc-2G). The rats receiving cocaine in the second generation (Sal-2G and Coc-2G) were offspring of the saline and cocaine group, respectively. All cocaine-treated groups received cocaine 2 mg/kg/day intravenously (IV), and the saline group received normal saline 0.2 ml/day IV from GD 2 to the 21st day postpartum. Mean perinatal mortality was greater in all pups exposed to cocaine in utero during gestation; Cocaine (6.4%); Sal-2G (5.6%); Coc-2G (11.4%) groups than in the noncocaine groups (3.2%, 1.3%). Weight gain, physical, and neurological developments of the offspring were not affected. It was concluded that perinatal cocaine exposure had an increased perinatal mortality even at doses approximately 10 times lower than those previously reported, which were administered by extravascular routes. These findings indicate the importance of the route of drug administration in perinatal cocaine research.

The comparative toxicity of cocaine and its metabolites in conscious rats.

BACKGROUND: The metabolites of cocaine, benzoylecgonine and ecgonine methyl ester, have been considered pharmacologically inactive when administered systemically. However, recent in vitro studies suggest that this may not be true. The current study was designed to evaluate the systemic toxicity of cocaine and its metabolites when administered systemically to awake rats fitted with catheters for long-term monitoring. METHODS: Cocaine, norcocaine, cocaethylene, benzoylecgonine, and ecgonine methyl ester were infused intravenously to produce sequential behavioral alterations and central nervous system and cardiovascular toxic effects. Arterial blood pressure and heart rate were monitored continuously. Plasma and tissue samples were analyzed for all compounds by capillary gas chromatography-mass spectrometry. RESULTS: The dose of norcocaine necessary to produce toxic effects was smaller than that of cocaine and cocaethylene. Benzoylecgonine and ecgonine methyl ester did not produce toxic manifestations at infusion rates that produced toxicity in the cocaine, norcocaine, and cocaethylene groups. Furthermore, 30- and 60-fold higher doses of benzoylecgonine and ecgonine methyl ester, respectively, were necessary to produce only mild neurobehavioral changes. Benzoylecgonine was not lethal even at doses 100 times greater than cocaine. CONCLUSIONS: These results indicate that benzoylecgonine and ecgonine methyl ester are not as toxic as cocaine, norcocaine, or cocaethylene when administered intravenously to pharmacologically naive rats.

Role of gender in the toxicity of norcocaine.

Gender differences may significantly influence the toxicity of cocaine in mammals. In this study, the influence of gender on the toxicity of norcocaine, a pharmacologically active metabolite of cocaine, was compared with its parent compound in adult male and female rats. In addition, the plasma and tissue norcocaine concentrations were evaluated after the administration of norcocaine and cocaine. Norcocaine or cocaine was administered intravenously at a rate of 2 mg/kg/min until circulatory collapse. Arterial blood samples as well as heart, liver, and brain tissues were obtained at circulatory collapse for the measurement of concentrations of norcocaine as well as cocaine and its major metabolites. There were no gender-related differences in the doses of norcocaine required to produce circulatory collapse; however, there were significant gender-related differences in the norcocaine tissue-to-plasma concentration ratios (T:P ratios). After the administration of norcocaine, T:P ratios for heart, liver, and brain tissue were significantly greater in males. Furthermore, after cocaine administration, the hepatic norcocaine T:P ratio was approximately 3-fold greater in the male rats than in the female rats. In contrast, female rats had a greater percentage of norcocaine in the plasma at circulatory collapse after acute cocaine administration. Although no gender differences in the lethality of norcocaine were observed, it remains to be seen whether the gender differences in the distribution and uptake of norcocaine play a role in the hepatotoxicity of the drug, particularly after chronic exposure.

The disposition of cocaethylene in rat maternal, placental, and fetal compartments.

OBJECTIVE: The aim of this project was to examine the disposition of maternally administered cocaethylene in the fetus. STUDY DESIGN: Pregnant rats with long-term catheter placement received an intravenous infusion of cocaethylene during a period of 30 minutes. At either the completion of the infusion or 6 hours after the infusion the fetuses were delivered by hysterotomy. Maternal and fetal blood and major tissue samples were obtained for assays of cocaethylene and its metabolites. RESULTS: Cocaethylene was present in all samples obtained at the end of the infusion, but after 6 hours it was no longer detectable in the maternal and fetal systemic circulations. However, a substantial amount of cocaethylene was still present in the placenta on both the maternal and fetal sides, with the concentration on the maternal side being higher, indicating that the placenta stores cocaethylene. At the end of the infusion benzoylecgonine was found in all samples and the maternal concentrations were higher than the corresponding fetal concentrations. This order was reversed 6 hours after infusion. Extremely high concentrations of cocaethylene and benzoylecgonine were found in the amnion. CONCLUSIONS: These results suggest that the placenta limits the transfer of cocaethylene to the fetus. The high affinity of this compound for extraplacental sites cannot be ignored.

Pregnancy enhances cocaine-induced stimulation of uterine contractions in the chronically instrumented rat.

OBJECTIVE: Our purpose was to test whether cocaine stimulates uterine activity in nonpregnant and pregnant rats. STUDY DESIGN: The carotid artery and jugular vein were chronically catheterized, and a microballoon probe was inserted into the uterine cavity of 15 pregnant and 14 nonpregnant female rats. Conscious animals received a bolus dose of either cocaine or saline solution intravenously. Cardiovascular and uterine contractile responses were studied. RESULTS: Cocaine (2.5 mg/kg) induced a marked increase in uterine activity and arterial blood pressure in both pregnant and nonpregnant animals without producing systemic toxicity. The maximum change in uterine contractions was greater in the pregnant group than in the nonpregnant group, and blood pressure responses were transient in both. CONCLUSION: This study is the first demonstration that cocaine stimulates the rat uterus in vivo, with a greater increase in contractions in pregnant compared with nonpregnant animals. These differences are not related to the hemodynamic response or pharmacokinetic profile of cocaine.

Comparative systemic toxicity of ropivacaine and bupivacaine in nonpregnant and pregnant ewes.

BACKGROUND: Ropivacaine is a new amide local anesthetic, having therapeutic properties similar to those of bupivacaine but with a wider margin of safety. Bupivacaine is probably the most commonly used drug in obstetric epidural analgesia, even though laboratory studies have suggested that pregnancy increases the cardiotoxicity of bupivacaine but not of other local anesthetics. The current study was designed to reevaluate, in a random and blinded fashion, the systemic toxicity of bupivacaine and ropivacaine in nonpregnant and pregnant sheep. METHODS: Chronically prepared nonpregnant and pregnant ewes were randomized to receive an intravenous infusion of ropivacaine or bupivacaine at a constant rate of 0.5 mg.kg-1.min-1 until circulatory collapse. The investigators were blinded to the identity of local anesthetic. Heart rate, arterial blood pressure, and cardiac rhythm were monitored throughout the study. Arterial blood samples were obtained before infusion and at the onset of toxic manifestations, which appeared in the following sequence: convulsions, hypotension, apnea, and circulatory collapse. Serum drug concentrations and protein binding were determined. Blood pH and gas tensions were measured. RESULTS: There were no significant differences between non-pregnant and pregnant animals in the doses or serum concentrations of either drug required to elicit toxic manifestations. In nonpregnant animals, similar doses and serum concentrations of ropivacaine and bupivacaine were associated with the onset of convulsions and circulatory collapse. In pregnant ewes, greater doses of ropivacaine as compared to bupivacaine were required to produce convulsions (7.5 +/- 0.5 vs. 5.0 +/- 0.6 mg.kg-1) and circulatory collapse (12.9 +/- 0.8 vs. 8.5 +/- 1.2 mg.kg-1). The corresponding serum concentrations of ropivacaine were similar to those of bupivacaine. Pregnancy did not affect the serum protein binding of either drug. The proportion of animals manifesting a malignant ventricular arrhythmia as the terminal event was similar among all groups. CONCLUSIONS: The systemic toxicity of ropivacaine or bupivacaine is not enhanced by gestation in sheep. This is in contrast to an earlier study in which the cardiotoxicity of bupivacaine was enhanced during ovine pregnancy. Greater doses of ropivacaine, as compared to bupivacaine, are needed to produce toxic manifestations in pregnant animals.

Sensitive assay for cocaine and benzoylecgonine using solid-phase extraction and gas chromatography.

An improved method for the simultaneous determination of cocaine and benzoylecgonine by means of gas chromatography with nitrogen-phosphorus detection is described. Following a solid-phase extraction and derivatization of benzoylecgonine to its butyl ester, chromatography was performed using a capillary column. The n-propylester of benzoylecgonine served as the internal standard. The assay was linear from 4 to 2000 ng/ml for both cocaine and benzoylecgonine, with a good precision over this concentration range. This method may be particularly useful for small volume samples since it requires only 250 microliters of plasma for analysis of both cocaine and benzoylecgonine.

[The effect of cocaine on uterine contractions in the rat].

The abuse of cocaine during pregnancy has become a major problem in substance abuse in the U.S.A. Although cocaine often induces preterm labor, little is known about the effect of this drug on uterine contractions. In 23 pregnant (P) and 23 nonpregnant (NP) rats, a carotid artery and jugular vein were catheterized, and a balloon catheter was inserted into the uterine cavity 24 hrs prior to the study. Arterial blood pressure, heart rate, intrauterine pressure were recorded throughout the study. Uterine contractions were expressed in Montevideo Units (M.V.U.). Cocaine (1.25-20 mg/kg) or saline was administered intravenously to test its toxicity and the effect on uterine contractions. A 1.25 or 2.5 mg/kg dose of cocaine did not produce toxic symptoms, but 2.5 mg/kg resulted in a marked increase in M.V.U. in both P and NP. In P, the maximum change in M.V.U. was as high as 250% from the baseline value, twice as much as that observed in NP. Mean arterial blood pressure increased 15% on the average and heart rate decreased 10%. This study indicates that a non-toxic dosage of cocaine stimulates uterine contractions, and pregnancy enhances this effect.

Pregnancy decreases the threshold for cocaine-induced convulsions in the rat.

The objective of this study was to test our hypothesis that pregnancy modifies the central nervous and cardiovascular toxicity of cocaine. Ten chronically catheterized term pregnant rats and 13 chronically catheterized nonpregnant female rats were infused with cocaine (2 mg/kg/min) intravenously to observe the sequential toxic manifestation of cocaine from mild central nervous stimulation (hyper-locomotor activities) to fatal cardiovascular collapse. Arterial blood samples were withdrawn at the onset of major toxic signs or symptoms--namely convulsion, hypotension, and circulatory collapse--for determination of cocaine concentrations and plasma cholinesterase activity. The dosage and plasma concentrations of cocaine associated with the onset of convulsions and cardiovascular depression were significantly lower in pregnant rats when compared with the nonpregnant animals. The mean time required to develop convulsions in the pregnant rat was significantly shorter (21 minutes) than that in the nonpregnant animal (33 minutes). However, once convulsive activity had developed, the time interval to achieve circulatory collapse was similar in both groups. Although the baseline plasma cholinesterase activity was higher in the pregnant rats than in the nonpregnant ones, the values in the samples obtained from the pregnant group at the onset of circulatory collapse were similar to the baseline values for the nonpregnant group. These findings suggest that a higher enzyme activity does not protect the development of toxic manifestations in the pregnant rat as compared to the nonpregnant animal when cocaine was administered at the same infusion rate.

Gender-related differences in cocaine toxicity in the rat.

To test our hypotheses that gender-related differences in cocaine toxicity exist in the rat, cocaine (2 mg/kg/min) was infused intravenously in chronically catheterized male and ovariectomized or intact female rats until the onset of circulatory collapse. Sequential manifestations of cocaine toxicity from mild central nervous stimulation to fatal cardiovascular collapse were observed. Arterial blood was withdrawn at the onset of the toxic signs or symptoms for determination of cocaine concentrations. Dosages and plasma concentrations of cocaine required to produce cardiovascular toxic manifestations were significantly lower in male and ovariectomized rats than in intact females. Plasma cholinesterase activity was lowest in the male animals and highest in intact females. These data suggest that systemic toxicity of cocaine is enhanced in male rats, because lower doses and plasma concentrations are required to induce toxic signs and symptoms.