Analysis of the variation in use-dependent inactivation of high-threshold tetrodotoxin-resistant sodium currents recorded from rat sensory neurons.

This study addressed variation in the use-dependent inactivation (UDI) of high-threshold tetrodotoxin-resistant Na+ currents (TTX-R currents) and action potential firing behavior among acutely isolated rat dorsal root ganglion (DRG) cells. UDI was quantified as the percent decrease in current amplitude caused by increasing the current activation rate from 0.1-1.0 Hz for 20 s. TTX-R current UDI varied from 6% to 66% among 122 DRG cells examined, suggesting the existence of two or more levels of UDI. The voltage-dependency of the TTX-R currents was consistent with Na(V)1.8, regardless of UDI. However, TTX-R currents with more UDI had a more negative voltage-dependency of inactivation, a greater tendency to enter slow inactivation, and a slower recovery rate from slow inactivation, compared with those with less UDI. TTX-R currents with more UDI ran down faster than those with less UDI. However, UDI itself changed little over time, regardless of the initial UDI level observed in a particular DRG cell. Together, these two observations suggest that individual DRG cells did not express mixtures of TTX-R channels that varied regarding UDI. TTX-R current UDI was correlated with expression of a low-threshold A-current and whole-cell capacitance, suggesting that it varied among different nociceptor types. Whole-cell inward currents (WCI-currents), recorded without channel blockers, also exhibited UDI. WCI-current UDI varied similarly to TTX-R current UDI in magnitude, and relative to whole-cell capacitance and A-current expression, suggesting that the WCI-currents were carried predominantly by TTX-R channels. DRG cells with more WCI-current UDI exhibited a greater decrease in action potential amplitude and number, and a greater increase in action potential threshold over seven ramp depolarizations, compared with DRG cells with less WCI-current UDI. Variation in UDI of Na(V)1.8 channels expressed by different nociceptor types could contribute to shaping their individual firing patterns in response to noxious stimuli.

Comparison of two immobilization techniques using portal film and digitally reconstructed radiographs for pediatric patients with brain tumors.

PURPOSE: To compare the accuracy of two immobilization techniques for pediatric brain tumor patients. METHODS AND MATERIALS: We analyzed data from 128 treatments involving 22 patients. Patients were immobilized with either a relocatable head frame (12 patients) or a vacuum bag (10 patients). Orthogonal portal films were used as verification images. Errors in patient positioning were measured by comparing verification images with digitally reconstructed radiographs generated by a three-dimensional treatment-planning system. RESULTS: With the head frame, systematic errors ranged from 1.4 mm to 2.1 mm; random errors, from 1.7 mm to 2.1 mm. With the vacuum bag, systematic errors ranged from 2.1 mm to 2.5 mm; random errors, from 2.0 mm to 2.6 mm. For the head frame, the mean length of the radial displacement was 4.4 mm; 90% of the total three-dimensional deviation was less than 6.8 mm. The corresponding values for the vacuum bag were 5.0 and 6.6 mm, respectively. CONCLUSIONS: The head frame and vacuum bag techniques limit the random and systematic errors in each of the three directions to within +/- 5 mm. We have used these results to determine the margin used to create the planning target volume for conformal radiation therapy.

Safety of lumbar puncture for children with acute lymphoblastic leukemia and thrombocytopenia.

CONTEXT: Patients with thrombocytopenia are at risk for spontaneous or procedure-related hemorrhage. Whether such patients can safely undergo lumbar puncture (LP) without prophylactic platelet transfusion is unknown. OBJECTIVE: To determine whether an association exists between thrombocytopenia and LP complications among children with acute lymphoblastic leukemia. DESIGN, SETTING, AND PATIENTS: Retrospective review of the records of 958 consecutive children (median age, 5.5 years) with newly diagnosed acute lymphoblastic leukemia who were treated at a pediatric cancer center between February 1984 and July 1998. INTERVENTIONS: All patients underwent a diagnostic LP followed by a median of 4 LPs to instill intrathecal chemotherapy. MAIN OUTCOME MEASURE: Serious complications of LP occurring during the remission induction and consolidation treatment periods (when thrombocytopenia is likely to occur), defined as any neurologic, infectious, or hemorrhagic problems related to the procedure, reported by platelet count at the time of the procedure. RESULTS: Of the 5223 LPs evaluated, 29 were performed at platelet counts of 10 x 10(9)/L or less, 170 at platelet counts of 11 to 20 x 10(9)/L, and 742 at platelet counts of 21 to 50 x 10(9)/L. No serious complications were encountered, regardless of the platelet count. The 95% confidence interval for the proportion of serious complications in the 199 patients with platelet counts of 20 x 10(9)/L or less was 0% to 1.75% and that for the 941 patients with platelet counts of 50 x 10(9)/L or less was 0% to 0.37%. CONCLUSIONS: In our study of children undergoing remission induction or consolidation therapy for acute lymphoblastic leukemia, serious complications of LP were not observed, regardless of platelet count. Prophylactic platelet transfusion is not necessary in children with platelet counts higher than 10 x 10(9)/L. Due to the small number of patients in our study with platelet counts of 10 x 10(9)/L or less, conclusions cannot yet be drawn for such patients. JAMA. 2000;284:2222-2224.

Channel inhibition by alkanols occurs at a binding site on the nicotinic acetylcholine receptor.

The mechanism by which normal alkanols longer than ethanol inhibit cation flux through the transient open state of the nicotinic acetylcholine receptor (nAcChoR) is unknown. They might act nonspecifically either by perturbing the lipid bilayer or by binding to many low affinity sites. Alternatively, they might act in a mutually exclusive manner at a well defined site on the protein. To address this problem, a rapid assay of agonist-induced 86Rb+ efflux from nAcChoR-rich Torpedo membrane vesicles was used that enabled the anesthetic-induced inhibition to be measured on a millisecond time scale, under conditions where the concentration of all ligands was raised in < 1 msec, thereby avoiding complications due to desensitization. By measuring the inhibition constant of one agent as a function of the fixed concentration of a second agent, it is possible to distinguish between nonspecific action and mutually exclusive action. Our data are inconsistent with the hypothesis that 1-octanol and 1-heptanol act in a nonspecific manner, but they are consistent with the hypothesis that these two alkanols act in a mutually exclusive manner at a well defined site. The data suggest that the alkanols sterically compete for the site, but experimental limitations prevented a less plausible model, in which there is a strong negative allosteric interaction between separate octanol and heptanol sites, from being ruled out. Should the latter interaction occur, the data indicate that occupation of one alkanol site would decrease the affinity of the other by about 50-fold. The local anesthetic procaine is known to act in a mutually exclusive manner with the agonist self-inhibition site. We found that octanol and procaine acted as separate sites, which exhibited a negative heterotrophic interaction such that octanol reduced the affinity of procaine 6-fold. We conclude that octanol and heptanol inhibit cation flux through the channel of the nAcChoR by binding to a site (or a set of sites of equal affinity) whose location is distinct from, but allosterically coupled to, the agonist self-inhibition site.

Barbiturate action is dependent on the conformational state of the acetylcholine receptor.

BACKGROUND: Barbiturates act on many neuronal ion channels by poorly understood mechanisms. The authors investigated the hypothesis that barbiturates inhibit the transient open-channel conformation of the nicotinic acetylcholine receptor (nAchR) by binding to a discrete site. METHODS: Inhibition curves of the agonist-stimulated efflux of 8bRb+ from nAchR-rich membrane vesicles prepared from the electric tissue of Torpedo nobiliana were obtained for 14 barbiturates using a filter assay. RESULTS: When added simultaneously with agonist, all agents inhibited the ion efflux with half-inhibitory concentrations (IC50), varying from 23 microM for pentobarbital to 880 microM for barbital, and with Hill coefficients of one. The effect of several barbiturates on the agonist concentration-response curve for carbachol-stimulated efflux indicated that this inhibitory action was not competitive. CONCLUSIONS: The IC50s of these agents did not correlate with their octanol/water partition coefficients, nor with general anesthetic potency, although a degree of channel inhibition occurred with many agents at general anesthetic concentrations. The existence of a barbiturate-inhibitory site of action was indicated by the structural specificity. This conclusion was supported by the Hill coefficient of one, and by the high inhibitory potencies, which ruled out membrane perturbations as a mechanism. This site on the transient open-channel conformation exhibits different structure-activity relationships than an allosteric site established by equilibrium barbiturate binding on the resting conformation of the AchR. Thus, barbiturate action depends on the nAchR's conformational state.

[A new case of bronchospasm under extracorporeal circulation]. / Un nouveau cas de bronchospasme sous circulation extracorporelle.

A case of bronchospasm occurring after the termination of cardiopulmonary bypass is reported. The complement fractions C3a, C4a and C5a were measured before and right after CPB. Complement activation is not specific to CPB but may occur in any thoracotomy. The statistically significant increase in complement C3a without any pulmonary symptomatology has been reported by several authors. Complement activation cannot therefore be considered as the explanation of this bronchospasm.