Cytotoxicity of Local Anesthetics in Mesenchymal Stem Cells.

Cell therapy based on the trophic, mitogenic, and immunomodulatory capacity of mesenchymal stem cells is a promising treatment modality for degenerative musculoskeletal conditions. Local anesthetics have been commonly used in interventional procedures for alleviating pain, but local anesthetics may have negative impact on MSC dosing because of cytotoxicity or other biological effects. Because previous studies have not reached consensus yet on the potential complications of local anesthetics in cell therapy, we reviewed 11 studies that involve in vitro experimentation with MSCs using aminoamide-type anesthetics including lidocaine, ropivacaine, mepivacaine, bupivacaine, articaine, and prilocaine. Three studies that compare the effects of different types of local anesthetic agents showed that ropivacaine has the least detrimental effects on mesenchymal stem cell populations, whereas lidocaine seems to have the most significant effects on stem cell viability. Concentration- and time-dependent effects on cell viability were reported with bupivacaine, ropivacaine, lidocaine, and mepivacaine. We conclude that local anesthetic agents have time- and concentration-dependent detrimental effects on MSCs. However, in vivo studies will be required to understand the interactions of these agents with MSCs, because in vitro studies cannot replicate the pharmacokinetics of anesthetics in vivo or the recovery of MSCs in a more physiological environment.

Lipid emulsion pretreatment has different effects on mepivacaine and bupivacaine cardiac toxicity in an isolated rat heart model.

BACKGROUND: The use of lipid emulsions to reduce cardiac toxicity of local anaesthetics (LAs) has shown success in experimental studies and some clinical cases, and thus has been implemented in clinical practice. However, lipid treatment is usually given after the occurrence of neurological or cardiovascular symptoms of systemic intoxication. The aim of this study was to determine if pretreatment with lipid emulsion reduces cardiac toxicity produced by bupivacaine or mepivacaine. METHODS: Isolated rat hearts were perfused with or without lipid emulsion (0.25 ml kg(-1) min(-1)) before administration of equipotent doses of bupivacaine (250 µM) or mepivacaine (1000 µM). Haemodynamic parameters and times from start of perfusion LA to a 1 min period of asystole and recovery were determined. RESULTS: Pretreatment with lipid emulsion extended the time until occurrence of asystole and decreased times to recovery in bupivacaine-induced cardiac toxicity but not in mepivacaine-induced cardiac toxicity compared with control. Lipid pretreatment impaired rate-pressure product recovery in mepivacaine-intoxicated hearts. CONCLUSIONS: This study confirms that pretreatment with a lipid emulsion reduces cardiac toxicity of LAs. The efficacy of pretreatment with lipid emulsion was LA-dependent, so pharmacokinetic properties, such as lipophilicity, might influence the effects of lipid emulsion pretreatment.

Cytotoxicity of local anesthetics on human mesenchymal stem cells in vitro.

PURPOSE: The purpose of this study was to investigate the cytotoxic potency of local anesthetics on human mesenchymal stem cells (MSCs) before and after chondrogenic differentiation. METHODS: MSCs were exposed to equal and equipotent concentrations of bupivacaine, ropivacaine, and mepivacaine for 1 hour. Cell viability, apoptosis, and necrosis were determined using flow cytometry and live/dead staining. After chondrogenic differentiation, MSC viability was determined in aggregates exposed to equipotent concentrations of the named agents, applying fluorescence microscopy. RESULTS: All local anesthetics showed detrimental cytotoxic effects on MSC monolayer cultures in a concentration- and time-specific manner. Minimum viability rates were found 96 hours after a 1-hour exposure. Bupivacaine 0.5% caused a reduction of vital MSCs to 5% ± 1%. Sixteen percent ± 2% viable cells were detected after treatment with 0.75% ropivacaine. Exposure to 2% mepivacaine decreased vitality rates to 1% ± 0%. Ropivacaine was significantly less cytotoxic than were bupivacaine and mepivacaine. Immediate cell death was mainly caused by necrosis followed by apoptosis afterward. Viability rates of MSCs embedded in cartilaginous tissue after chondrogenic differentiation were not reduced by local anesthetic treatment. CONCLUSIONS: Local anesthetics are cytotoxic to MSCs in a concentration-, time-, and agent-dependent manner in monolayer cultures but not in whole-tissue probes. CLINICAL RELEVANCE: MSCs are applied for treatment of cartilage defects. Intra-articular application of local anesthesia is a common procedure in pain management and has shown chondrotoxic effects. Therefore, it is crucial to evaluate the impact of local anesthetics on human MSCs and regenerative cartilage tissue engineering.

The cytotoxicity of bupivacaine, ropivacaine, and mepivacaine on human chondrocytes and cartilage.

BACKGROUND: Intraarticular injections of local anesthetics are frequently used as part of multimodal pain regimens. However, recent data suggest that local anesthetics affect chondrocyte viability. In this study, we assessed the chondrotoxic effects of mepivacaine, ropivacaine, and bupivacaine. We hypothesized that specific cytotoxic potencies directly correlate with analgesic potencies, and that cytotoxic effects in intact cartilage are different than in osteoarthritic tissue. METHODS: Human articular chondrocytes were exposed to equal and equipotent concentrations of bupivacaine, ropivacaine, and mepivacaine for 1 hour. Cell viability, apoptosis, and necrosis were determined at predefined time points using flow cytometry, live-dead staining, and caspase detection. Intact and osteoarthritic human cartilage explants were treated with equipotent concentrations of named drugs to determine cell viability applying fluorescence microscopy. RESULTS: Chondrotoxic effects increased from ropivacaine to mepivacaine to bupivacaine in a time-dependent and concentration-dependent manner. Compared with control, bupivacaine 0.5% decreased chondrocyte viability to 78% ± 9% (P = 0.0183) 1 hour and 16% ± 10% (P < 0.0001) 24 hours later, as determined by live-dead staining in monolayer cultures. Viability rates were reduced to 80% ± 7% (P = 0.0475) 1 hour and 80% ± 10% (P = 0.0095) 24 hours after treatment with ropivacaine 0.75%. After exposure to mepivacaine 2%, viable cells were scored 36% ± 6% (P < 0.0001) after 1 hour and 30% ± 11% (P < 0.0001) after 24 hours. Ropivacaine treatment was less chondrotoxic than bupivacaine (P = 0.0006) and mepivacaine exposure (P = 0.0059). Exposure to concentrations up to 0.25% of bupivacaine, 0.5% of ropivacaine, and 0.5% of mepivacaine did not reveal significant chondrotoxicity in flow cytometry. However, chondrotoxicity did not correlate with potency of local anesthetics. Immediate cell death was mainly due to necrosis followed by apoptosis. Cellular death rates were clearly higher in osteoarthritic compared with intact cartilage after bupivacaine, mepivacaine, and ropivacaine treatment in a decreasing order. CONCLUSION: Bupivacaine, ropivacaine, and mepivacaine are chondrotoxic in a time-dependent, concentration-dependent, and drug-dependent manner. Chondrotoxic and analgesic potencies do not directly correlate. Cellular death rates were higher in osteoarthritic compared with intact cartilage after local anesthetic treatment.

Intravenous lipid emulsion only minimally influences bupivacaine and mepivacaine distribution in plasma and does not enhance recovery from intoxication in pigs.

BACKGROUND: The reported successful use of IV lipid emulsions in local anesthetic intoxications is thought to be due to lipid sequestration of local anesthetics. However, controlled efficacy studies were lacking, and other mechanisms of action have also been suggested. We investigated the effect of lipid infusion on plasma concentrations and cardiovascular effects of 2 local anesthetics differing in lipophilicity, bupivacaine, and mepivacaine. METHODS: Bupivacaine (n = 20) or mepivacaine (n = 20) was infused into a central vein of anesthetized (isoflurane 1%, Fio(2) 0.21) pigs until mean arterial blood pressure decreased to 50% from baseline. Isoflurane was discontinued and Fio(2) was increased to 1.0. Ten pigs in each local anesthetic group were treated with 20% lipid emulsion (ClinOleic®), and 10 pigs with Ringer's solution: 1.5 mL/kg in 1 minute followed by an infusion of 0.25 mL · kg(-1) · min(-1) for 29 minutes. Five additional pigs were infused bupivacaine and Intralipid®. Total and nonlipid-bound local anesthetic concentrations were determined from repeated blood samples. RESULTS: There were no overall differences in total or nonlipid-bound plasma local anesthetic concentrations between the lipid and Ringer's groups. However, plasma median total bupivacaine concentration was 21% and 23% higher at 20 and 30 minutes, respectively, in the lipid group (P = 0.016 without Holm-Bonferroni correction). There was also no overall difference between lipid and Ringer's groups in the rate of recovery of hemodynamic and electrocardiographic variables. Median mean arterial blood pressure in the lipid group with bupivacaine intoxication was 16 mm Hg and 15 mm Hg higher than in the corresponding Ringer's group at 10 and 15 minutes, respectively (P = 0.016 and P = 0.021, respectively, without Holm-Bonferroni correction). Intralipid® also caused no difference between total plasma and nonlipid-bound concentrations of bupivacaine with no apparent enhancement of recovery. CONCLUSIONS: Lipid emulsion neither had any measurable effect on the disposition of the studied local anesthetics in plasma, nor did it improve the rate of recovery from intoxication by either local anesthetic as measured by hemodynamic variables.

Lipid emulsion improves recovery from bupivacaine-induced cardiac arrest, but not from ropivacaine- or mepivacaine-induced cardiac arrest.

BACKGROUND: Cardiac toxicity significantly correlates with the lipophilicity of local anesthetics (LAs). Recently, the infusion of lipid emulsions has been shown to be a promising approach to treat LA-induced cardiac arrest. As the postulated mechanism of action, the so-called "lipid sink" effect may depend on the lipophilicity of LAs. In this study, we investigated whether lipid effects differ with regard to the administered LAs. METHODS: In the isolated rat heart, cardiac arrest was induced by administration of equipotent doses of bupivacaine, ropivacaine, and mepivacaine, respectively, followed by cardiac perfusion with or without lipid emulsion (0.25 mL x kg(-1) x min(-1)). Subsequently, the times from the start of perfusion to return of first heart activity and to recovery of heart rate and rate-pressure product (to 90% of baseline values) were assessed. RESULTS: In all groups, lipid infusion had no effects on the time to the return of any cardiac activity. However, recovery times of heart rate and rate-pressure product (to 90% of baseline values) were significantly shorter with the administration of lipids in bupivacaine-induced cardiac toxicity, but not in ropivacaine- or mepivacaine-induced cardiac toxicity. CONCLUSIONS: These data show that the effects of lipid infusion on LA-induced cardiac arrest are strongly dependent on the administered LAs itself. We conclude that lipophilicity of LAs has a marked impact on the efficacy of lipid infusions to treat cardiac arrest induced by these drugs.

Spinal procaine is less neurotoxic than mepivacaine, prilocaine and bupivacaine in rats.

BACKGROUND AND OBJECTIVES: Lidocaine has been reported to be more neurotoxic than other local anesthetics. Alternatives to lidocaine with lower toxicity and shorter duration of action are desirable. Therefore, we compared the histologic and functional changes induced by intrathecal injection of prilocaine, mepivacaine, procaine, and bupivacaine in rats. METHODS: Rats (n = 184) randomly received via an intrathecal catheter 0.12 microL/g body weight of 2%, 10%, 16%, or 20% prilocaine, mepivacaine, or procaine; 0%, 0.5%, 2.5%, 4%, or 5% bupivacaine in distilled water; or distilled water or 15% glucose solution alone as a control. We evaluated neurofunction by analyzing walking behavior and sensory threshold and examined the L3 spinal cord, posterior and anterior roots, and cauda equina by light and electron microscopy. RESULTS: The recovery time to normal ambulation after intrathecal injection was significantly faster with procaine than with the other 3 drugs at all concentrations. There were no significant differences in the sensory threshold among the 4 anesthetics. Histologic damage was observed only in rats treated with greater than 16% prilocaine or mepivacaine or with greater than 4% bupivacaine. Histologic damage occurred at the posterior root and posterior white matter and was characterized by axonal degeneration. Rats treated with procaine, even at 20%, showed no histologic abnormalities. CONCLUSION: In this animal model, the neurotoxicity of intrathecal procaine was the mildest, and the recovery time to ambulation with procaine was the fastest among the 4 tested anesthetics.

The effects of general anesthesia on the central nervous and cardiovascular system toxicity of local anesthetics.

BACKGROUND: Local anesthetic toxicity is often studied experimentally in acutely prepared, anesthetized laboratory animals. We determined the influence of halothane/O(2) anesthesia on cardiovascular and central nervous system (CNS) toxic responses to six amide-type local anesthetics administered i.v.. METHODS: Behavioral, cardiovascular, and pharmacokinetic responses were determined in previously instrumented ewes (approximately 45-50 kg, n = 18), on separate occasions when conscious and anesthetized, to bupivacaine (100 mg), levobupivacaine (125 mg), ropivacaine (150 mg), lidocaine (350 mg), mepivacaine (350 mg), prilocaine (350 mg), and saline (control) infused i.v. over 3 min. RESULTS: The local anesthetics caused convulsions in conscious sheep, but no overt CNS effects in anesthetized sheep. Negative inotropy and slight bradycardia without changes in arterial blood pressure occurred initially in conscious sheep, followed by positive inotropy, tachycardia, and hypertension at the abrupt onset of CNS excitotoxicity, along with widening of QRS complexes. Fatal cardiac arrhythmias occurred in, respectively, 3 of 11, 2 of 12, and 2 of 13 conscious sheep infused with bupivacaine, levobupivacaine, and ropivacaine; in 1 of 9 with prilocaine, electromechanical dissociation (followed by polymorphic ventricular tachycardia) caused death. In anesthetized sheep, cardiovascular depression, preexisting from the general anesthesia, was exacerbated by all local anesthetics, and increased QRS width was prolonged; concurrent blood local anesthetic concentrations were doubled. Nevertheless, all anesthetized animals survived. CONCLUSIONS: General anesthesia produced physiological perturbations, exacerbated local anesthetic-induced cardiovascular depression, and changed the pharmacokinetics of toxic doses of local anesthetics. However, cardiovascular fatalities from local anesthetics occurred only in conscious animals.

Intrathecal mepivacaine and prilocaine are less neurotoxic than lidocaine in a rat intrathecal model.

BACKGROUND AND OBJECTIVES: Histologic evidence of the comparative neurotoxicity of lidocaine, mepivacaine, and prilocaine is incomplete. We compared the intrathecal neurotoxicity in rats among these 3 drugs based on morphologic and neurofunctional findings. METHODS: Rats (n=169) randomly received 0.12 microL/g of 0%, 2%, 5%, 7.5%, 10%, or 20% lidocaine, mepivacaine, or prilocaine or 25% glucose dissolved in distilled water via a chronically implanted intrathecal catheter. The effect of the agents on neurofunction was evaluated by movement of the hind limb (behavior test) and by sensory threshold (paw-stimulation test). The L1 spinal cord, the posterior and anterior roots, and the cauda equina were removed en bloc 5 days later and examined by light and electron microscopy. RESULTS: A significant decrease in sensory threshold or irreversible hind-limb limitation was observed only in rats receiving 20% lidocaine. Morphologic abnormalities characterized by axonal degeneration were observed in rats receiving > or =7.5% lidocaine, 20% mepivacaine, and 20% prilocaine, at the posterior white matter and the proximal portion of the posterior root just at the entrance into the spinal cord. The incidence of lesions was significantly higher in rats receiving lidocaine than mepivacaine and prilocaine. CONCLUSION: It is suggested that intrathecal mepivacaine and prilocaine are less neurotoxic than highly concentrated lidocaine in a rat intrathecal model.

Serum mepivacaine concentrations after intraoral injection in young children.

The authors measured plasma concentrations of mepivacaine in 36 children from the ages of 2 to 5 years who received dental care under light general anesthesia. The subjects were randomly assigned to receive either 2 percent mepivacaine hydrochloride with 1:20,000 levonordefrin or 3 percent mepivacaine hydrochloride without vasoconstrictor. The volume of anesthetic injected depended on the planned procedures for each patient. Blood samples (3 mL) were drawn from an intravenous line before and 5, 10, 20, 30, 45, and 60 minutes after mepivacaine injection. The serum was collected and analyzed by gas-liquid chromatography. Mean serum concentrations, normalized to a dose of 1 mg/kg body weight, reached a peak of 0.67 +/- 0.42 microgram/mL (mean +/- SD) after 3 percent mepivacaine and 0.63 +/- 0.21 microgram/mL after 2 percent mepivacaine with levonordefrin. Levonordefrin had no significant effect on the plasma concentrations. However, because of the higher concentration of mepivacaine in the 3 percent formulation, it was potentially 1.5 times as toxic (P < 0.002) on a volume basis. Statistical analysis also suggested that the maximum recommended dose of 3 mg/lb could result in potentially toxic blood concentrations in a small percentage of pediatric patients. The authors conclude that 3 percent mepivacaine should not be used when relatively large volumes of local anesthetic must be administered to small children and recommend that the maximum dose of mepivacaine not exceed 5 mg/kg.

Neurotrophic factors can partially reverse morphological changes induced by mepivacaine and bupivacaine in developing sensory neurons.

UNLABELLED: Both bupivacaine and mepivacaine induce morphological changes in growing neurons. We designed this study to investigate the role of some neurotrophic factors (NTFs) in supporting developing neurons exposed to the deleterious effects of these drugs. Dorsal root ganglia were isolated from chick embryos and exposed to either bupivacaine or mepivacaine. After 60 min of exposure, the culture media were replaced with fresh culture media free from local anesthetics. NTFs-brain-derived NTF, glial-derived NTF, or neurotrophin-3-were added to the replacement media, and the cells were examined up to 48 h after the washout. The growth cone collapse assay was applied by a quantitative method of assessment. When the replacement media were not supported by any NTF, the growth cone collapse values were significantly larger than the control values at 20 h after the washout of mepivacaine and 48 h after the washout of either bupivacaine or mepivacaine (P < 0.05). However, when any of the NTFs were used, the collapsing activity was significantly attenuated, and growth cone collapse values showed no statistically significant differences in comparison with the control values at these time points (P > 0.05). We conclude that several NTFs support the recovery of neurons after exposure to local anesthetics. The supporting effects of NTFs on the reversibility of mepivacaine-induced collapse tended to be more obvious than those seen after the bupivacaine washout. IMPLICATIONS: Three neurotrophic factors (NTFs) can partially support the reversibility of mepivacaine- and bupivacaine-induced growth cone collapse in growing primary cultured sensory neurons. The effect of NTFs is more apparent after mepivacaine than after bupivacaine washout.

Procaine and mepivacaine have less toxicity in vitro than other clinically used local anesthetics.

UNLABELLED: The neurotoxicity of local anesthetics can be demonstrated in vitro by the collapse of growth cones and neurites in cultured neurons. We compared the neurotoxicity of procaine, mepivacaine, ropivacaine, bupivacaine, lidocaine, tetracaine, and dibucaine by using cultured neurons from the freshwater snail Lymnaea stagnalis. A solution of local anesthetics was added to the culture dish to make final concentrations ranging from 1 x 10(-6) to 2 x 10(-2) M. Morphological changes in the growth cones and neurites were observed and graded 1 (moderate) or 2 (severe). The median concentrations yielding a score of 1 were 5 x 10(-4) M for procaine, 5 x 10(-4) M for mepivacaine, 2 x 10(-4) M for ropivacaine, 2 x 10(-4) M for bupivacaine, 1 x 10(-4) M for lidocaine, 5 x 10(-5) M for tetracaine, and 2 x 10(-5) M for dibucaine. Statistically significant differences (P < 0.05) were observed between mepivacaine and ropivacaine, bupivacaine and lidocaine, lidocaine and tetracaine, and tetracaine and dibucaine. The order of neurotoxicity was procaine = mepivacaine < ropivacaine = bupivacaine < lidocaine < tetracaine < dibucaine. Although lidocaine is more toxic than bupivacaine and ropivacaine, mepivacaine, which has a similar pharmacological effect to lidocaine, has the least-adverse effects on cone growth among clinically used local anesthetics. IMPLICATIONS: Systematic comparison was assessed morphologically in growth cones and neurites exposed to seven local anesthetics. The order of neurotoxicity was procaine = mepivacaine < ropivacaine = bupivacaine < lidocaine < tetracaine < dibucaine. Although lidocaine is more toxic than bupivacaine and ropivacaine, mepivacaine, which has a similar pharmacological effect to lidocaine, is the safest among clinically used local anesthetics.

The neurotoxicity of local anesthetics on growing neurons: a comparative study of lidocaine, bupivacaine, mepivacaine, and ropivacaine.

UNLABELLED: Local anesthetics can be neurotoxic. To test the hypothesis that exposure to local anesthetics produces morphological changes in growing neurons and to compare this neurotoxic potential between different local anesthetics, we performed in vitro cell biological experiments with isolated dorsal root ganglion neurons from chick embryos. The effects of lidocaine, bupivacaine, mepivacaine, and ropivacaine were examined microscopically and quantitatively assessed using the growth cone collapse assay. We observed that all local anesthetics produced growth cone collapse and neurite degeneration. However, they showed significant differences in the dose response. The IC(50) values were approximately, 10(-2.8) M for lidocaine, 10(-2.6) M for bupivacaine, 10(-1.6) M for mepivacaine, and 10(-2.5) M for ropivacaine at 15 min exposure. Some reversibility was observed after replacement of the media. At 20 h after washout, bupivacaine and ropivacaine showed insignificant percentage growth cone collapse in comparison to their control values whereas those for lidocaine and mepivacaine were significantly higher than the control values. Larger concentrations of the nerve growth factor (NGF) did not improve this reversibility. In conclusion, local anesthetics produced morphological changes in growing neurons with significantly different IC(50). The reversibility of these changes differed among the four drugs and was not influenced by the NGF concentration. IMPLICATIONS: Local anesthetics induce growth cone collapse and neurite degeneration in the growing neurons. Mepivacaine was safer than lidocaine, bupivacaine, and ropivacaine for the primary cultured chick neurons.

Changes in satellite cell population associated with regenerating muscle fibers in rats.

Qualitative and quantitative analysis of satellite cells in regenerating muscles was performed at 4, 7 and 30 days after necrosis induced by mepivacaine injection. At 4 days, the small regenerating fibers were accompanied by a high number of satellite cells showing signs of activation; at 7 days, the number of satellite cells decreased and two morphological types of these cells were observed; at 30 days, satellite cells were similar in both number and characteristics to those of the control group. These results would indicate that satellite cells may vary both in number and morphological and morphometric features, according to the degree of maturity of the regenerating muscle fiber.

Circadian phase-dependent protein and tissular binding of three local anesthetics (bupivacaine, etidocaine, and mepivacaine) in mice: a possible mechanism of their chronotoxicokinetics?

The aim of this study was to investigate the possible influence of the time of administration on bupivacaine (B), etidocaine (E), and mepivacaine (M) protein and tissue (brain and heart) binding. For each anesthetic agent, a single dose of B (20 mg/kg), E (40 mg/kg), or M (60 mg/kg) was administered intraperitoneally at 10:00, 16:00, 22:00, and 04:00 h. Blood and tissue samples were collected 15 min after drug administration. This study documents significant circadian variations in protein and tissue binding of the three local anesthetic agents. We did not demonstrate a temporal relationship between the respective free and tissue levels. Thus, the temporal variations of free plasma, brain, and heart levels do not seem to be involved in the temporal changes of induced mortality.

Local anesthetic-induced toxicity may be modified by low doses of flumazenil.

The purpose of this study was to investigate the influence of flumazenil on local anesthetic-induced acute toxicity. For each of the three tested anesthetics (etidocaine, mepivacaine and lidocaine) 6 groups of mice were treated by a single dose of flumazenil (0.125, 0.25, 0.5, 1 and 2 mg/kg), or an equal volume of saline, 15 minutes before the injection of the anesthetic (etidocaine: 50 mg/kg, mepivacaine: 110 mg/kg and lidocaine: 115 mg/kg). The convulsant activity, the time of latency to convulse and the mortality rate were assessed in each group. The local anesthetic-induced mortality was not significantly modified by flumazenil. The convulsant activity of lidocaine and mepivacaine was significantly increased by flumazenil but not for etidocaine. Also, increasing doses of flumazenil decreased the time of latency to obtain lidocaine-induced convulsions. This effect was not obtained with etidocaine or mepivacaine.

Pregnancy does not alter lidocaine toxicity.

Pregnant sheep are more vulnerable to the toxic effects of bupivacaine, a potent local anesthetic, than are nonpregnant sheep. In contrast, ovine pregnancy does not enhance the toxicity of mepivacaine, a drug with properties similar to lidocaine. We studied the central nervous and cardiovascular toxicity of lidocaine in pregnant sheep receiving a continuous intravenous drug infusion at the rate of 2 mg/kg/min and compared our results with data previously obtained in nonpregnant ewes. In all animals, toxic manifestations occurred in the following sequence: convulsions, hypotension, respiratory arrest, and circulatory collapse. The doses of lidocaine required to produce these symptoms in pregnant and nonpregnant ewes were similar. Convulsions occurred at 5.9 +/- 0.6 mg/kg (mean +/- SE) in the pregnant ewe and 5.8 +/- 1.8 mg/kg in the nonpregnant ewe, whereas circulatory collapse occurred at 40.7 +/- 2.6 and 36.7 +/- 3.3 mg/kg in the pregnant and nonpregnant animals, respectively. Lidocaine plasma concentrations associated with the onset of convulsions in both pregnant and nonpregnant ewes were almost identical (12.1 +/- 0.7 and 11.7 +/- 2.0 micrograms/ml, respectively). At circulatory collapse, these concentrations were 35.1 +/- 3.2 and 41.2 +/- 6.7 micrograms/ml, respectively. It appears that pregnancy does not enhance the toxic effects of lidocaine. These findings are similar to those for mepivacaine but not for bupivacaine, and may be related in part to differences in the way pregnancy affects serum protein binding of these drugs.