Intraneural lidocaine uptake compared with analgesic differences between pregnant and nonpregnant rats.

BACKGROUND AND OBJECTIVES: Pregnant patients need less local anesthetic in order to obtain the same quality of functional block as nonpregnant patients. Our goal was to demonstrate a similarly increased functional susceptibility to local anesthetics in the awake pregnant rat during peripheral nerve block and to investigate the pharmacokinetic and/or pharmacodynamic mechanisms responsible for this phenomenon. METHODS: Radiolabeled lidocaine uptake was determined in vivo during sciatic nerve block with 0.1 ml of 1% lidocaine in the nerves of nine pregnant and five nonpregnant female rats and six male rats at the return of deep pain sensation, assessed by withdrawal of the hindlimb from a brief squeeze of a digit with serrated forceps. During recovery from complete functional block, the time at which deep pain returned and the amount of lidocaine in the nerve at that time were compared among the three groups of rats. Lidocaine content was also determined in vitro after exposure of ensheathed sciatic nerves from pregnant and nonpregnant rats to a 0.2% lidocaine bath for specified times. RESULTS: Full block of function developed in all groups within 6 minutes of the lidocaine injection and lasted significantly longer in pregnant rats than in nonpregnant and male rats (49.0 +/- 3.3 vs 34.0 +/- 3.1 and 32.0 +/- 1.3 minutes mean +/- SEMI, respectively. At the time of deep pain return, the intraneural lidocaine content of pregnant rats was significantly lower than that of nonpregnant and male rats (2.2 +/- 0.25 vs 3.9 +/- 0.7 and 3.7 +/- 0.6 nmoles/mg of wet nerve, respectively). No difference in lidocaine uptake kinetics between P and NP nerves was observed in vitro. CONCLUSIONS: Block of peripheral neural function is prolonged in pregnant rats, and lidocaine content in the nerve is lower at a specific stage of neural block. These results are consistent with a pharmacodynamic mechanism for increased susceptibility to lidocaine neural block during pregnancy.

Pharmacokinetic nature of tachyphylaxis to lidocaine: peripheral nerve blocks and infiltration anesthesia in rats.

Tachyphylaxis to peripheral neural blockade was determined with repeated injections of a constant dose of lidocaine in three experimental models: sciatic nerve block, produced by intraneural or extraneural injections, and infiltration anesthesia. A decrease in the duration of the subsequent blocks was used as the index of tachyphylaxis development. The anesthetic content in the nerve or skin was determined using radiolabeled lidocaine. Repeated injections of a constant dose of lidocaine resulted in a marked decrease in the duration of the blocks. Accelerated decline in lidocaine content of nerve or skin was observed with repeated blocks. Our data show that tachyphylaxis rapidly develops with both sciatic nerve blocks and infiltration anesthesia. The data also suggest that the mechanism is largely pharmacokinetic in nature.

Relation between functional deficit and intraneural local anesthetic during peripheral nerve block. A study in the rat sciatic nerve.

BACKGROUND: During peripheral nerve block, local anesthetic (LA) penetrates within and along the nerve to produce the observed functional deficits. Although much is known about the kinetics and steady-state relation for LA inhibition of impulse activity in vitro in isolated nerve, little is known about the relation between functional loss and intraneural LA content in vivo. This study was undertaken to investigate the relation of functional change to intraneural LA. METHODS: A sciatic nerve block was performed in rats with 0.1 ml 1% lidocaine radiolabeled with 14C. The total intraneural uptake of LA was determined at different times after injection, and the distribution of lidocaine along the nerve was assayed at different stages of functional block. Drug content was also compared with equilibrium lidocaine uptake in the isolated rat sciatic nerve. RESULTS: Total intraneural lidocaine in vivo increased to near steady-state in about 3 min, stabilizing at approximately 14.3 nmol/mg wet tissue for about 12 min before decreasing to zero at 70 min after injection. Although intraneural lidocaine was 1.6% of the injected dose during full block, only 0.3% was left when deep pain sensation returned and 0.065% was still detected when functions fully recovered. Despite these large differences in total lidocaine content, the longitudinal distribution remained constant. Intraneural lidocaine concentrations obtained at full block and partial recovery could be achieved in vitro by equilibration in 0.7-0.9 and 0.2-0.3 mM lidocaine, respectively. CONCLUSIONS: During peripheral nerve block only a small amount of injected LA penetrates into the nerve. The intraneural content of LA correlates with the depth of functional block.

Modality-dependent modulation of conduction by impulse activity in functionally characterized single cutaneous afferents in the rat.

Cutaneous afferents exhibit changes in excitability after impulse activity that are correlated with functional modality but are independent of axonal diameter, as studied in 39 cold fibers and 51 nociceptors of the rat. Latency of conducted impulses was used to indicate changes in axonal excitability caused by electrical stimulation. Stimuli were applied both at fixed frequencies and at the time intervals of impulses previously recorded during response to natural stimulation. Latency increased following both these forms of electrical stimulation, as well as after natural stimulation of the receptive fields. The latency increase was correlated with the number of impulses and the frequency of the preceding discharge in all of 4 nociceptors and 13 cold fibers studied for this feature. Increase of latency by electrical or natural stimulation led to reduced responsiveness to natural stimulation. The magnitude and time course of latency changes were correlated with fiber modality. In 32 nociceptors the latency increased continuously with time during a stimulus train, whereas in 21 cold fibers there was only an initial increase in latency over the first few seconds, after which the latency remained at a plateau even as the firing response continued. Paralleling this slowing, impulse failure occurred more frequently during repetitive stimulation in both A delta and C nociceptors than in velocity-matched cold fibers of either class. Based on the magnitude of latency increases during stimulus trains at different frequencies, two distinct patterns were discerned in A nociceptors: "Type II" fibers slowed significantly more than "Type I" or cold fibers. The results support the hypotheses (1) that the pattern of latency changes during activity are signatures for the modality in a given fiber; and (2) that endogenous, activity-dependent processes of the axon contribute to adaptation and encoding in cutaneous sensory afferents.

Changes in axonal impulse conduction correlate with sensory modality in primary afferent fibers in the rat.

Conduction velocity was measured in vivo in single cutaneous afferent fibers of rat sciatic nerve that were characterized by natural stimulation. During sustained electrical stimulation, impulses slowed less and propagated more reliably in cold fibers (both A delta and C) than in nociceptive fibers of similar conduction velocity. Velocity in cold fibers tended to stabilize after an initial decrease rather than decrease throughout the stimulation as for nociceptive fibers. The slowing correlated with axon modality and hence with natural firing pattern, raising the possibility that impulse activity can determine conduction properties of axons.

Bone marrow transplantation in miniature swine. III. Graft-versus-host disease and the effect of T cell depletion of marrow.

Graft-versus-host disease (GVHD) has been evaluated in partially inbred miniature swine in order to study this complication of allogeneic bone marrow transplantation (BMT) in a major histocompatibility complex (MHC) genetically defined large animal model. Bone marrow from MHC homozygous ("parental") swine was injected into irradiated (900 rads total-body irradiation) MHC heterozygous ("F1") swine that shared one haplotype with the donor. All 18 animals successfully engrafted with donor bone marrow, and 17 of these developed skin rash of varying intensity depending on the extent of T cell depletion of infused marrow. Of 18 animals, 8 received undepleted bone marrow from exsanguinated donors and 2 also received additional peripheral blood lymphocytes (PBL) as a source of mature T cells. All 8 showed a moderate-to-severe rash, and the 2 pigs that received additional donor PBL developed the most severe rash. The cutaneous eruption seen in this model clinically, histologically, and immunologically resembled human GVHD. Two protocols of T cell depletion of donor bone marrow by antiporcine T cell monoclonal antibodies plus complement were tested for their effect on development of GVHD. The combination of two monoclonal antibodies, 74-12-4 (PT4) and 76-2-11 (PT8), had a marginal effect on the subsequent development of cutaneous manifestations of GVHD. However, treatment of the donor marrow by a combination of three monoclonal antibodies--PT4, PT8, and MSA4 (PT11)--effectively decreased the severity of the GVHD skin rash. These results indicate that (1) the GVHD associated with allogeneic bone marrow transplantation in swine is dependent on T cells in the marrow; (2) effective T cell depletion of donor marrow by monoclonal antibodies and complement does not prevent engraftment; and (3) this swine GVHD model, which allows study with F1 and homozygous parental combinations in an MHC genetically defined large animal, is particularly useful for the understanding of GVHD pathogenesis, prevention, and treatment.

Bone marrow transplantation in miniature swine. I. Development of the model.

Procedures for successful autologous and MHC-matched allogeneic bone marrow transplantation in partially inbred, MHC-defined miniature swine have been established. All marrow recipients were conditioned with single-dose total-body irradiation at the upper level of tolerance, and supported with antibiotics and irradiated blood products during aplasia. Surgical harvest of autologous and allogeneic marrow yielded sufficient numbers of cells to successfully reconstitute recipients. Radiation control animals that received no marrow failed to show recovery of marrow function. Pigs transplanted with autologous marrow at doses greater than 10(8) cells/kg routinely engrafted and recovered normal marrow function. The major clinical complications were acute and chronic infections and hemorrhage. T cell-depleted autologous marrow also engrafted, and there was no observed increase in clinical complications. In bone marrow transplantation across non-MHC allogeneic differences, engraftment and survival were similar to that observed for autologous transplants. The T cell depletion of marrow in such MHC-matched allogeneic recipients was associated in one animal, however, with early reconstitution by cells of autologous origin.

Bone marrow transplantation in miniature swine. II. Effect of selective genetic differences on marrow engraftment and recipient survival.

In order to study the effect of defined genetic differences on bone marrow transplantation in miniature swine, five different combinations of major histocompatibility complex (MHC)-matched and mismatched bone marrow transplants were performed. Eight of nine fully MHC-mismatched allogeneic bone marrow transplants failed to reconstitute, and one animal reconstituted briefly but then died quickly thereafter. Five of six class I-matched/class II-mismatched (g----c) bone marrow transplants engrafted, showed a skin rash typical of graft-versus-host (GVH) reaction, and died 3 weeks after the marrow transplantation. None of five class II-matched/class I-mismatched (g----d) transplants engrafted. Parental marrow transplants into F1 hosts engrafted and caused GVH skin rash, with survivals from 1 to 9 months (n = 5). Serologic typing of the F1 recipients of parental marrow showed only donor-type peripheral blood lymphocytes (PBL), suggesting complete marrow replacement. Conversely, F1 into parental marrow transplants showed no engraftment (n = 6). These results indicate that resistance to MHC-mismatched allogeneic bone marrow engraftment in swine represents a host response recognizing donor class I MHC differences. This response appears to interfere with engraftment of donor bone marrow cells despite host preparation with 900-1100 rads total-body irradiation. In the absence of donor MHC class I differences, engraftment was seen despite the existence of multiple non-MHC differences, and even in the presence of class II differences. Such engraftment also led to GVH, varying in intensity according to the strength of genetic disparity (i.e., worst in parent----F1 combination). These results suggest that miniature swine should provide an effective model for study of both GVH elimination (in the parent----F1 combination) and problems of engraftment (in the F1----parent combination), the two most important obstacles to clinical allogeneic transplantation.