Doxapram only slightly reduces the shivering threshold in healthy volunteers

Bacterial lipopolysaccharide hyperpolarizes the membrane potential and is antagonized by the K2p channel blocker doxapram

Exposure of Drosophila skeletal muscle to bacterial lipopolysaccharides (LPS) rapidly and transiently hyperpolarizes membrane potential. However, the mechanism responsible for hyperpolarization remains unclear. The resting membrane potential of the cells is maintained through multiple mechanisms. This study investigated the possibility of LPS activating calcium-activated potassium channels (K_Ca) and/or K2p channels. 2-Aminoethyl diphenylborinate (2-APB), blocks uptake of Ca²⁺ into the endoplasmic reticulum (ER); thus, limiting release from ryanodine-sensitive internal stores to reduce the function of K_Ca channels. Exposure to 2-APB produces waves of hyperpolarization even during desensitization of the response to LPS and in the presence of doxapram. This finding in this study suggests that doxapram blocked the acid-sensitive K2p tandem-pore channel subtype known in mammals. Doxapram blocked LPS-induced hyperpolarization and depolarized the muscles as well as induced motor neurons to produce evoked excitatory junction potentials (EJPs). This was induced by depolarizing motor neurons, similar to the increase in extracellular K⁺ concentration. The hyperpolarizing effect of LPS was not blocked by decreased extracellular Ca²⁺or the presence of Cd²⁺. LPS appears to transiently activate doxapram sensitive K2p channels independently of K_Ca channels in hyperpolarizing the muscle. Septicemia induced by gram-negative bacteria results in an increase in inflammatory cytokines, primarily induced by bacterial LPS. Currently, blockers of LPS receptors in mammals are unknown; further research on doxapram and other K2p channels is warranted. (220 words).

The effects of doxapram (blocker of K2p channels) on resting membrane potential and synaptic transmission at the Drosophila neuromuscular junction

The resting membrane potential of most cells is maintained by potassium K2p channels. The pharmacological profile and distribution of various K2p channel subtypes in organisms are still being investigated. The Drosophila genome contains 11 subtypes; however, their function and expression profiles have not yet been determined. Doxapram is clinically used to enhance respiration in humans and blocks the acid-sensitive K2p TASK subtype in mammals. The resting membrane potential of larval Drosophila muscle and synaptic transmission at the neuromuscular junction are pH sensitive. The present study investigated the effects of doxapram on membrane potential and synaptic transmission using intracellular recordings of larval Drosophila muscles. Doxapram (1 mM and 10 mM) depolarizes the muscle and appears to depolarize motor neurons, causing an increase in the frequency of spontaneous quantal events and evoked excitatory junction potentials. Verapamil (1 and 10 mM) paralleled the action of doxapram. These changes were matched by an extracellular increase in KCl (50 mM) and blocked by Cd²⁺. It is assumed that the motor nerve depolarizes to open voltage-gated Ca²⁺ channels in presynaptic nerve terminals because of exposure to doxapram. These findings are significant for building models to better understand the function of pharmacological agents that affect K2p channels and how K2p channels contribute to the physiology of tissues. Drosophila offers a genetically amenable model that can alter the tissue-specific expression of K2p channel subtypes to simulate known human diseases related to this family of channels.

Hyperpolarization Induced by Lipopolysaccharides but Not by Chloroform Is Inhibited by Doxapram, an Inhibitor of Two-P-Domain K+ Channel (K2P)

Bacterial septicemia is commonly induced by Gram-negative bacteria. The immune response is triggered in part by the secretion of bacterial endotoxin lipopolysaccharide (LPS). LPS induces the subsequent release of inflammatory cytokines which can result in pathological conditions. There is no known blocker to the receptors of LPS. The Drosophila larval muscle is an amendable model to rapidly screen various compounds that affect membrane potential and synaptic transmission such as LPS. LPS induces a rapid hyperpolarization in the body wall muscles and depolarization of motor neurons. These actions are blocked by the compound doxapram (10 mM), which is known to inhibit a subtype of the two-P-domain K+ channel (K2P channels). However, the K2P channel blocker PK-THPP had no effect on the Drosophila larval muscle at 1 and 10 mM. These channels are activated by chloroform, which also induces a rapid hyperpolarization of these muscles, but the channels are not blocked by doxapram. Likewise, chloroform does not block the depolarization induced by doxapram. LPS blocks the postsynaptic glutamate receptors on Drosophila muscle. Pre-exposure to doxapram reduces the LPS block of these ionotropic glutamate receptors. Given that the larval Drosophila body wall muscles are depolarized by doxapram and hyperpolarized by chloroform, they offer a model to begin pharmacological profiling of the K2P subtype channels with the potential of identifying blockers for the receptors to mitigate the actions of the Gram-negative endotoxin LPS.

The Effect of Doxapram on Proprioceptive Neurons: Invertebrate Model

The resting membrane potential enables neurons to rapidly initiate and conduct electrical signals. K2p channels are key in maintaining this membrane potential and electrical excitability. They direct the resting membrane potential toward the K+ equilibrium potential. Doxapram is a known blocker for a subset of K2p channels that are pH sensitive. We assessed the effects of 0.1 and 5 mM doxapram on the neural activity within the propodite-dactylopodite (PD) proprioceptive sensory organ in the walking legs of blue crabs (Callinectes sapidus). Results indicate that 0.1 mM doxapram enhances excitation, while the higher concentration 5 mM may over-excite the neurons and promote a sustained absolute refractory period until the compound is removed. The effect of 5 mM doxapram mimics the effect of 40 mM K+ exposure. Verapamil, another known K2p channel blocker as well as an L-type Ca2+ channel blocker, reduces neural activity at both 0.1 and 5 mM. Verapamil may block stretch activated channels in sensory endings, in addition to reducing the amplitude of the compound action potential with whole nerve preparations. These findings are notable as they demonstrate that doxapram has acute effects on neurons of crustaceans, suggesting a targeted K2p channel. The actions of verapamil are complex due to the potential of affecting multiple ion channels in this preparation. Crustacean neurons can aid in understanding the mechanisms of action of various pharmacological agents as more information is gained.

Simultaneous Quantification and Pharmacokinetic Characterization of Doxapram and 2-Ketodoxapram in Porcine Plasma and Brain Tissue

Atrial fibrillation (AF) is an arrhythmia associated with an increased stroke risk and mortality rate. Current treatment options leave unmet needs in AF therapy. Recently, doxapram has been introduced as a possible new option for AF treatment in a porcine animal model. To better understand its pharmacokinetics, three German Landrace pigs were treated with intravenous doxapram (1 mg/kg). Plasma and brain tissue samples were collected. For the analysis of these samples, an ultra performance liquid chromatography tandem mass spectrometry (UPLC-MS/MS) assay for the simultaneous measurement of doxapram and its active metabolite 2-ketodoxapram was developed and validated. The assay had a lower limit of quantification (LLOQ) of 10 pg/mL for plasma and 1 pg/sample for brain tissue. In pigs, doxapram pharmacokinetics were biphasic with a terminal elimination half-life (t_1/2) of 1.38 ± 0.22 h and a maximal plasma concentration (c_max) of 1780 ± 275 ng/mL. Its active metabolite 2-ketodoxapram had a t_1/2 of 2.42 ± 0.04 h and c_max of 32.3 ± 5.5 h after administration of doxapram. Protein binding was 95.5 ± 0.9% for doxapram and 98.4 ± 0.3% for 2-ketodoxapram with a brain-to-plasma ratio of 0.58 ± 0.24 for doxapram and 0.12 ± 0.02 for 2-ketodoxapram. In conclusion, the developed assay was successfully applied to the creation of pharmacokinetic data for doxapram, possibly improving the safety of its usage.

Development and validation of an LC–MS/MS method for simultaneously determining doxapram and keto-doxapram in human serum

Aim: Doxapram, a respiratory stimulant, is used to treat apnea. A reliable method of determining doxapram in blood is required for monitoring purposes. Results: Doxapram, keto-doxapram (active metabolite) and propranolol (internal standard) were extracted from human serum by protein precipitation and plate filtration. Molecular ions were generated by electrospray ionization in positive ion mode, and the ions were analyzed using a triple-quadrupole mass spectrometer. The calibration curves were linear from 20 to 5000 ng/ml. The method was validated and the selectivity, reproducibility and stability met the acceptance criteria. Conclusion: An LC–MS/MS method was successfully developed for determining doxapram and keto-doxapram in human serum. The method can be used to monitor doxapram and keto-doxapram concentrations in blood.

Perioperative thermoregulation and heat balance

Core body temperature is normally tightly regulated to within a few tenths of a degree. The major thermoregulatory defences in humans are sweating, arteriovenous shunt vasoconstriction, and shivering. The core temperature triggering each response defines its activation threshold. General anaesthetics greatly impair thermoregulation, synchronously reducing the thresholds for vasoconstriction and shivering. Neuraxial anaesthesia also impairs central thermoregulatory control, and prevents vasoconstriction and shivering in blocked areas. Consequently, unwarmed anaesthetised patients become hypothermic, typically by 1-2°C. Hypothermia results initially from an internal redistribution of body heat from the core to the periphery, followed by heat loss exceeding metabolic heat production. Complications of perioperative hypothermia include coagulopathy and increased transfusion requirement, surgical site infection, delayed drug metabolism, prolonged recovery, shivering, and thermal discomfort. Body temperature can be reliably measured in the oesophagus, nasopharynx, mouth, and bladder. The standard-of-care is to monitor core temperature and to maintain normothermia during general and neuraxial anaesthesia.

Prophylactic Effects of Doxapram, Ketamine and Meperidine in Postoperative Shivering

Background:

Postanaesthetic shivering is one of the most common unpleasant complications in patients.

Objectives:

The aim of this study was to compare the efficacy of doxapram, ketamine and meperidine in prevention of shivering after anaesthesia.

Patients and Methods:

In this randomized, double-blind clinical trial, 120 patients aged between 20 - 45 years old under general anaesthesia were enrolled. The patients were randomly allocated into one of three groups: group M received 20 mg meperidine (n = 40), group K received 0.25 mg/kg ketamine (n = 40) and group D received 0.25 mg/kg doxapram (n = 40). All of the drugs were administered intravenously. The core temperature, shivering, time of the first postoperative analgesic requirement, and some of the other side effects were recorded. Obtained data from the three groups were compared using one-way ANOVA and chi-square test.

Results:

Three patients (7.5%) of group K, four patients (10%) of group D and one patient (2.5%) of group M experienced shivering (P = 0.39). The interval to the first analgesic requirement significantly prolonged in the groups K and M compared to the group D (P < 0.001). No significant differences were identified in nausea and vomiting among the groups. No significant pharmaceutical adverse effects were observed in our study.

Conclusions:

The results of this study showed that ketamine, doxapram and meperidine are equally effective in the prevention of postoperative shivering.

Optimal management of shivering during therapeutic hypothermia after cardiac arrest

Both pharmacological and nonpharmacological methods are used to control shivering in therapeutic hypothermia. An evidence-based protocol based on the most current research has been developed for the management of shivering during therapeutic hypothermia. Meperidine is the drug of choice and provides the greatest reduction in the shivering threshold. Other effective pharmacological agents recommended for reducing the threshold include dexmedetomidine, midazolam, fentanyl, and magnesium sulfate. In addition, skin counterwarming techniques, such as use of an air-circulating blanket, are effective nonpharmacological methods for reducing shivering when used in conjunction with medication. As a last resort, neuromuscular blocking agents are considered appropriate therapy for management of refractory shivering.

Pharmacologic options for reducing the shivering response to therapeutic hypothermia

Recent literature has demonstrated significant improvements in neurologic outcomes in patients who have received induced hypothermia in the setting of out-of-hospital cardiac arrest. Through multiple metabolic mechanisms, the induction of hypothermia slows the progression and devastation of transient cerebral hypoxia. Despite these benefits, the desired reduction in core temperature is often a challenging venture as the body attempts to maintain homeostasis through the induction of thermoregulatory processes aimed at elevating body temperature. Shivering is an involuntary muscular activity that enhances heat production in an attempt to restore homeostasis. For successful induction and maintenance of induced hypothermia, shivering, as well as other thermoregulatory responses, must be overcome. Several pharmacologic options are available, either used alone or in combination, that safely and effectively prevent or treat shivering after the induction of hypothermia. We conducted a PubMed search (1966-March 2009) to identify all human investigations published in English that discussed pharmacologic mechanisms for the control of shivering. Among these options, clonidine, dexmedetomidine, and meperidine have demonstrated the greatest and most clinically relevant impact on depression of the shivering threshold. More research in this area is needed, however, and the role of the clinical pharmacist in the development and implementation of this therapy needs to be defined.

Thermoregulatory defense mechanisms

Core body temperature is normally tightly regulated by an effective thermoregulatory system. Thermoregulatory control is sometimes impaired by serious illness, but more typically remains intact. The primary autonomic defenses against heat are sweating and active precapillary vasodilation; the primary autonomic defenses against cold are arteriovenous shunt vasoconstriction and shivering. The core temperature triggering each response defines its activation threshold. Temperatures between the sweating and vasoconstriction thresholds define the inter-threshold range. The shivering threshold is usually a full 1 degrees C below the vasoconstriction threshold and is therefore a "last resort" response. Both vasoconstriction and shivering are associated with autonomic and hemodynamic activation; and each response is effective, thus impeding induction of therapeutic hypothermia. It is thus helpful to accompany core cooling with drugs that pharmacologically induce a degree of thermal tolerance. No perfect drug or drug combination has been identified. Anesthetics, for example, induce considerable tolerance, but are rarely suitable. Meperidine-especially in combination with buspirone-is especially effective while provoking only modest toxicity. The combination of buspirone and dexmedetomidine is comparably effective while avoiding the respiratory depression association with opioid administration.

A new look at the respiratory stimulant doxapram

A number of life-threatening clinical disorders may be amenable to treatment with a drug that can stimulate respiratory drive. These include acute respiratory failure secondary to chronic obstructive pulmonary disease, post-anesthetic respiratory depression, and apnea of prematurity. Doxapram has been available for over forty years for the treatment of these conditions and it has a low side effect profile compared to other available agents. Generally though, the use of doxapram has been limited to these clinical niches involving patients in the intensive care, post-anesthesia care and neonatal intensive care units. Recent basic science studies have made considerable progress in understanding the molecular mechanism of doxapram's respiratory stimulant action. Although it is unlikely that doxapram will undergo a clinical renaissance based on this new understanding, it represents a significant advance in our knowledge of the control of breathing.

DI. Ondansetron does not reduce the shivering threshold in healthy volunteers

BACKGROUND

Ondansetron, a serotonin-3 receptor antagonist, reduces postoperative shivering. Drugs that reduce shivering usually impair central thermoregulatory control, and may thus be useful for preventing shivering during induction of therapeutic hypothermia. We determined, therefore, whether ondansetron reduces the major autonomic thermoregulatory response thresholds (triggering core temperatures) in humans.

METHODS

Control (placebo) and ondansetron infusions at the target plasma concentration of 250 ng ml(-1) were studied in healthy volunteers on two different days. Each day, skin and core temperatures were increased to provoke sweating; then reduced to elicit peripheral vasoconstriction and shivering. We determined the core-temperature sweating, vasoconstriction and shivering thresholds after compensating for changes in mean-skin temperature. Data were analysed using t-tests and presented as means (sds); P<0.05 was taken as significant.

RESULTS

Ondensetron plasma concentrations were 278 (57), 234 (55) and 243 (58) ng ml(-1) at the sweating, vasoconstriction and shivering thresholds, respectively; these corresponded to approximately 50 mg of ondansetron which is approximately 10 times the dose used for postoperative nausea and vomiting. Ondansetron did not change the sweating (control 37.4 (0.4) degrees C, ondansetron 37.6 (0.3) degrees C, P=0.16), vasoconstriction (37.0 (0.5) degrees C vs 37.1 (0.3) degrees C; P=0.70), or shivering threshold (36.3 (0.5) degrees C vs 36.3 (0.6) degrees C; P=0.76). No sedation was observed on either study day.

CONCLUSIONS

/b>. Ondansetron appears to have little potential for facilitating induction of therapeutic hypothermia.