Gurkeerat Singh

Distribution and Retention of Nicotine and Its Metabolite, Cotinine, in the Rat as a Function of Time

Nicotine is oxidized to its major metabolite, cotinine, which has a long biological half-life (19-24 h). The plasma concentration of cotinine has been used as an index of tobacco smoke exposure. Cotinine possibly increases the turnover rate of platelet-activating factor (PAF) because it is a potent activator of PAF hydrolase, and it may play a significant role in tobacco-induced arterial thrombosis. Therefore, we studied the distribution and retention of nicotine as it was metabolized to cotinine in the rat. Nicotine (1 mg/kg, 5 microCi/kg) was administered into the femoral vein of male Sprague-Dawley rats under nembutal anesthesia. At different times (5-60 min) after nicotine administration, nicotine and its metabolite, cotinine, were determined by HPLC in plasma, liver, kidney, heart and brain. Within 5-10 min after administration, nicotine concentrations reached peak values in plasma (2,160 pmol/ml) and the organs analyzed. The plasma level of nicotine decreased by 50% within 20 min (half-time) after its intravenous administration. The half-time of nicotine in the brain was about 50 min. The half-times of nicotine for the other organs were about 20-25 min. The major metabolite, cotinine, accumulated in plasma, and by about 30 min the concentrations of nicotine and cotinine in plasma were about equal (890-1,000 pmol/ml). While cotinine accumulated in plasma, nicotine was eliminated by the kidney. While the nicotine concentrations decreased with time in all organs, cotinine concentrations remained constant. These observations indicate that nicotine is renally eliminated or metabolized to cotinine while cotinine exhibits a long retention time and accumulates in plasma.(ABSTRACT TRUNCATED AT 250 WORDS)

Halothane, Isoflurane, Xenon, and Nitrous Oxide Inhibit Calcium ATPase Pump Activity in Rat Brain Synaptic Plasma Membranes

Background Perturbation of neuronal calcium homeostasis may alter neurotransmission in the brain, a phenomenon postulated to characterize the anesthetic state. Because of the central role of plasma membrane Calcium2+ ‐ATPase (PMCA) in maintaining Calcium2+ homeostasis, the authors examined the effect of several inhalational anesthetics on PMCA function in synaptic plasma membranes (SPM) prepared from rat brain. Methods Calcium2+ ‐ATPase pumping activity was assessed by measurement of ATP‐dependent uptake of Calcium2+ by SPM vesicles. ATPase hydrolytic activity was assessed by spectrophotometric measurement of inorganic phosphate (Pi) released from ATP. For studies of anesthetic effects on PMCA activity, Calcium2+ uptake or Pi release was measured in SPM exposed to halothane, isoflurane, xenon, and nitrous oxide at partial pressures ranging from 0 to 1.6 MAC equivalents. Halothane and isoflurane exposures were carried out under a gassing hood. For xenon and nitrous oxide exposures, samples were incubated in a pressure chamber at total pressures sufficient to provide anesthetizing partial pressures for each agent. Results Dose‐related inhibition of Calcium2+ ‐ATPase pumping activity was observed in SPM exposed to increasing concentrations of halothane and isoflurane, confirmed by ANOVA and multiple comparison testing (P < 0.05). Concentrations of halothane and isoflurane equivalent to one minimum effective dose (MED) depressed PMCA pumping approximately 30%. Xenon and nitrous oxide also inhibited Calcium2+ uptake by SPM vesicles. At partial pressures of these two gases equivalent to 1.3 MAC, PMCA was inhibited approximately 20%. Hydrolysis of ATP by SPM fractions was also inhibited in a dose‐related fashion. An additive effect occurred when 1 vol% of halothane was added to xenon or nitrous oxide at partial pressures equivalent to 0–1.6 MAC for the latter two agents. Conclusions Plasma membranes Calcium2+ ‐ATPase is significantly inhibited, in a dose‐related manner, by clinically relevant partial pressures of halothane, isoflurane, xenon, and nitrous oxide. Furthermore, these anesthetics inhibit PMCA activity in accordance with their known potencies, and an additive effect was observed. How inhalational anesthetics inhibit the PMCA pump is not known at this time. It is noteworthy that the only shared characteristic of this group of agents of widely different structure is anesthetic action. The relevance of this dual commonality, anesthetic action and PMCA inhibition, to actual production of the anesthetic state remains to be determined.

Diminished brain synaptic plasma membrane Ca2+-ATPase activity in rats with streptozocin-induced diabetes: Association with reduced anesthetic requirements

Recent evidence suggests that chronic hyperglycemia may inhibit plasma membrane Ca(2+)-ATPase (PMCA) in cells from several tissues. Inhalational anesthetics (IA) can inhibit brain synaptic PMCA activity. We proposed that diabetic rats may manifest chronic inhibition of brain synaptic PMCA and thus provide a model for testing the hypothesis that synaptic PMCA plays a key role in IA pharmacodynamics. Ca2+ pumping activity of PMCA was measured in cerebral synaptic plasma membrane (SPM) vesicles prepared from rats with streptozocin (STZ)-induced diabetes and from control, normoglycemic rats. Dose requirements for halothane and xenon were estimated in treated and untreated rats. Brain PMCA activity in hyperglycemic rats was depressed by about 8.4%, compared to controls. In vitro glycation also caused a significant decrease in PMCA pumping activity. Halothane requirement for STZ-hyperglycemic rats was dramatically reduced to about 65% of control. Xenon requirement was also significantly reduced, to 88% of control. Correlation of IA dose with percent glycated hemoglobin for each rat revealed a strong association between reduced requirements for halothane or xenon and increased protein glycation. These results indicate that inhibition of brain synaptic PMCA in chronically hyperglycemic rats is associated with a significant reduction in IA requirement.

Stable inhibition of brain synaptic plasma membrane calcium ATPase in rats anesthetized with halothane.

Background The authors recently showed that plasma membrane Calcium2+ ‐ATPase (PMCA) activity in cerebral synaptic plasma membrane (SPM) is diminished in a dose‐related fashion during exposure in vitro to halothane, isoflurane, xenon, and nitrous oxide at clinically relevant partial pressures. They have now extended their work to in vivo studies, examining PMCA pumping in SPM obtained from control rats decapitated without anesthetic exposure, from rats decapitated during halothane anesthesia, and from rats decapitated after recovery from halothane anesthesia. Methods Three treatment groups were studied: 1) C, control rats that were decapitated without anesthetic exposure, 2) A, anesthetized rats exposed to 1 minimum effective dose (MED) for 20 min and then decapitated, and 3) R, rats exposed to 1 MED for 20 min and then decapitated after recovery from anesthesia, defined as beginning to groom. Plasma membrane Calcium2+ ‐ATPase pumping and Calcium2+ ‐dependent ATPase hydrolytic activity, as well as sodium‐calcium exchanger activity and Sodium sup + ‐Potassium sup + ‐ATPase hydrolytic activity, were assessed in cerebral SPM. In addition, halothane effect on smooth endoplasmic reticulum Calcium2+ ‐ATPase (SERCA) was examined. Results Plasma membrane Calcium2+ ‐ATPase transport of Calcium2+ into SPM vesicles from anesthetized rats was reduced to 71% of control (P < 0.01) compared with 113% of control for the recovered group (NS). No depression by halothane of SERCA activity, sodium‐calcium exchanger, or Sodium sup + ‐Potassium sup + ‐ATPase activity was noted among the CAR treatment groups. Conclusions Plasma membrane Calcium2+ ‐ATPase is selectively and stably inhibited in cerebral SPM from rats killed while anesthetized with halothane, compared with rats killed without anesthesia or after recovery from anesthesia. The studies described in this report, in conjunction with previously reported inhibition of PMCA activity in vitro by a wide range of anesthetic agents, indicate a relationship between inhibition of PMCA and action of inhalational anesthetics.

Reduced anesthetic requirements, diminished brain plasma membrane Ca2+-ATPase pumping, and enhanced brain synaptic plasma membrane phospholipid methylation in diabetic rats: Effects of insulin

We have recently reported that streptozocin (STZ)-induced diabetes in rats was associated with i) reduced Ca2+ pumping by rat brain synaptic plasma membrane Ca(2+)-ATPase (PMCA) and ii) a substantial reduction in the partial pressures of halothane and xenon required to prevent movement in response to stimulation (minimum effective dose or MED). MED for both agents correlated well with the degree of hemoglobin glycation and with PMCA activity. We now report that MEDs for isoflurane, enflurane, and desflurane were also substantially reduced in STZ-diabetic rats, compared with placebo-injected controls. In addition, we examined the effect of insulin treatment, begun 2 weeks after induction of diabetes and continued for 3 more weeks, on isoflurane MED and on brain synaptic PMCA and phospholipid-N-methyltransferase I (PLMT I), another enzyme altered by inhalation anesthetics (IA). Partial treatment of diabetes, as indicated by decreased glycated hemoglobin (GHb) compared to untreated diabetic rats, was associated with an isoflurane MED of 1.05 vol%, intermediate between a control mean of 1.57 vol% and an untreated diabetic mean of 0.82 vol% (p < 0.01), with a trend toward normalization of both PMCA and PLMT I activity. We also examined isoflurane MED and PMCA activity in the cerebrum and diencephalon-mesencephalon (D-M) of control and diabetic rats 2 and 12 weeks after induction of diabetes. Isoflurane MED was substantially reduced in diabetic rats from both treatment periods. Cerebral and D-M PMCA activities were each reduced to about 90% of control values 2 weeks after STZ induction. At 12 weeks, cerebral PMCA pumping in SPM from diabetic rats did not differ from control values, but PMCA pumping in SPM from the D-M was reduced to about 85% of control levels. Good correlation (r = 0.89, p < 0.01) was found between isoflurane MED and GHb in all treatment groups. These findings provide further evidence for an important role for PMCA in IA action. They also suggest that anesthetic effects on the calcium pump at specific anatomic sites may be of major importance in producing anesthesia.

Inhibition of plasma membrane Ca2+-atpase pump activity in cultured c6 glioma cells by halothane and xenon

We have compared the effect of two inhalational anesthetics, halothane and xenon, on Ca(2+)-ATPase (PMCA) pumping activity in plasma membrane vesicles prepared from cultured rat C6 glioma cells. Halothane, at concentrations ranging from 0.5 to 1.75 vol% (equivalent to 0.5 to 1.6 MAC), significantly inhibited Ca2+ uptake (transport) by plasma membrane vesicles in a dose-related fashion. Xenon, at partial pressures ranging from 0.5 to 1.5 atm (equivalent to 0.5 to 1.6 MAC), similarly inhibited PMCA pumping activity. Additive effects on suppression of PMCA pump activity were observed when C6 cell plasma membrane vesicles were exposed to increasing partial pressures of xenon in the presence of halothane (1 vol%). Halothane also inhibited PMCA pumping in cells from two other lines of neural origin, B104 (rat neuroblastoma) and PC12 (rat pheochromocytoma). Studies described in this report support the thesis that PMCA in cells of neural origin is inhibited by quite different inhalational anesthetics at clinically relevant concentrations.

Reduced anesthetic requirements in aged rats: Association with altered brain synaptic plasma membrane Ca2+-ATPase pump and phospholipid methyltransferase I activities

Aging is associated with a decrease in anesthetic requirements. Animal models of aging manifest alteration of brain Ca2+ homeostasis and increased methyltransferase I (PLMTI) activity. In this study we evaluated concurrently anesthetic requirements and brain plasma membrane Ca(2+)-ATPase (PMCA) and PLMTI activities in young and aged rats. Halothane, desflurane, isoflurane and xenon MEDs (lowest partial pressures that suppress a pain response) were measured in 2 and 25 month old, male Fisher-344 rats. Halothane MED was also measured in 2 and 30 month old F344/BNF1 rats, a strain that undergoes aging with less debilitation. PMCA pumping and PLMTI activities were measured in synaptic plasma membranes (SPM) prepared from the cortex and diencephalon-mesencephalon (DM). For aged Fisher-344 rats, MEDs for halothane, desflurane, isoflurane and xenon were reduced to 81%, 82%, 67% and 86%, respectively, of young controls; PMCA activity was diminished to 91% in cortical SPM and 82% in DM SPM; and cortical and DM PLMTI activities were increased to 131% and 114% of young control. For F344/BNF1 rats, MED for halothane was reduced to 87%, PMCA activity was diminished to 90% in cortical SPM and 72% DM SPM, and PLMTI activity was increased to 133% in cortical SPM and 112% in DM SPM. The strong association between age and reduced anesthetic requirements for inhalational agents on the one hand and altered PMCA and PLMTI activity on the other lends support to the underlying hypothesis that PMCA and PLMTI may be involved in the production of the anesthetic state.

Increased anesthetic requirements for isoflurane, halothane, enflurane and desflurane in obese zucker rats are associated with insulin-induced stimulation of plasma membrane Ca2+-ATPase

A wide spectrum of structurally disparate inhalational anesthetics reduce brain synaptic plasma membrane Ca(2+)-ATPase (PMCA) activity, whereas phospholipid methyltransferase I (PLMTI) is enhanced by anesthetics. Several rat models with incidental or disease-induced reduction of PMCA and enhancement of PLMTI activities manifest increased sensitivity to inhalational anesthetics. Because insulin is known to stimulate PMCA, anesthetic requirements in hyperinsulinemic obese Zucker rats (fa/fa) and in normoinsulinemic lean Zucker heterozygotes (fa/+) were examined, and brain synaptic PMCA and PLMTI activities were determined in both genotypes. Significantly higher partial pressures of halothane, enflurane, isoflurane, and desflurane were required to inhibit the pain response in obese rats compared to lean Zucker rats. Dose dependent stimulation of PMCA pumping was observed in synaptic membranes from both types, but insulin concentrations in extracts of diencephalon-mesencephalon, cerebellum, and medulla (but not cortex) were higher in obese than in lean Zucker rats. Microdialysis of three subcortical regions showed marked increases in insulin levels with halothane exposure in obese rats, compared to lean controls. These observations in an anesthetic resistant rat model lend further support to the hypothesis that the calcium pump plays a functional role in production of the anesthetic state.