Action of propofol on central sympathetic mechanisms controlling blood pressure

TRPA1 Polymorphisms Modify the Hypotensive Responses to Propofol with No Change in Nitrite or Nitrate Levels

Anesthesia with propofol is frequently associated with hypotension. The TRPA1 gene contributes to the vasodilator effect of propofol. Hypotension is crucial for anesthesiologists because it is deleterious in the perioperative period. We tested whether the TRPA1 gene polymorphisms or haplotypes interfere with the hypotensive responses to propofol. PCR-determined genotypes and haplotype frequencies were estimated. Nitrite, nitrates, and NOx levels were measured. Propofol induced a more expressive lowering of the blood pressure (BP) without changing nitrite or nitrate levels in patients carrying CG+GG genotypes for the rs16937976 TRPA1 polymorphism and AG+AA genotypes for the rs13218757 TRPA1 polymorphism. The CGA haplotype presented the most remarkable drop in BP. Heart rate values were not impacted. The present exploratory analysis suggests that TRPA1 genotypes and haplotypes influence the hypotensive responses to propofol. The mechanisms involved are probably other than those related to NO bioavailability. With better genetic knowledge, planning anesthesia with fewer side effects may be possible.

Associated Factors of High Sedative Requirements within Patients with Moderate to Severe COVID-19 ARDS

The coronavirus pandemic continues to challenge global healthcare. Severely affected patients are often in need of high doses of analgesics and sedatives. The latter was studied in critically ill coronavirus disease 2019 (COVID-19) patients in this prospective monocentric analysis. COVID-19 acute respiratory distress syndrome (ARDS) patients admitted between 1 April and 1 December 2020 were enrolled in the study. A statistical analysis of impeded sedation using mixed-effect linear regression models was performed. Overall, 114 patients were enrolled, requiring unusual high levels of sedatives. During 67.9% of the observation period, a combination of sedatives was required in addition to continuous analgesia. During ARDS therapy, 85.1% (n = 97) underwent prone positioning. Veno-venous extracorporeal membrane oxygenation (vv-ECMO) was required in 20.2% (n = 23) of all patients. vv-ECMO patients showed significantly higher sedation needs (p < 0.001). Patients with hepatic (p = 0.01) or renal (p = 0.01) dysfunction showed significantly lower sedation requirements. Except for patient age (p = 0.01), we could not find any significant influence of pre-existing conditions. Age, vv-ECMO therapy and additional organ failure could be demonstrated as factors influencing sedation needs. Young patients and those receiving vv-ECMO usually require increased sedation for intensive care therapy. However, further studies are needed to elucidate the causes and mechanisms of impeded sedation.

Is the Sympathetic System Detrimental in the Setting of Septic Shock, with Antihypertensive Agents as a Counterintuitive Approach? A Clinical Proposition

Mortality in the setting of septic shock varies between 20% and 100%. Refractory septic shock leads to early circulatory failure and carries the worst prognosis. The pathophysiology is poorly understood despite studies of the microcirculatory defects and the immuno-paralysis. The acute circulatory distress is treated with volume expansion, administration of vasopressors (usually noradrenaline: NA), and inotropes. Ventilation and anti-infectious strategy shall not be discussed here. When circulation is considered, the literature is segregated between interventions directed to the systemic circulation vs. interventions directed to the micro-circulation. Our thesis is that, after stabilization of the acute cardioventilatory distress, the prolonged sympathetic hyperactivity is detrimental in the setting of septic shock. Our hypothesis is that the sympathetic hyperactivity observed in septic shock being normalized towards baseline activity will improve the microcirculation by recoupling the capillaries and the systemic circulation. Therefore, counterintuitively, antihypertensive agents such as beta-blockers or alpha-2 adrenergic agonists (clonidine, dexmedetomidine) are useful. They would reduce the noradrenaline requirements. Adjuncts (vitamins, steroids, NO donors/inhibitors, etc.) proposed to normalize the sepsis-evoked vasodilation are not reviewed. This itemized approach (systemic vs. microcirculation) requires physiological and epidemiological studies to look for reduced mortality.

An Overview of Analytical Methods for the Estimation of Propofol in Pharmaceutical Formulations, Biological Matrices, and Hair Marker

Propofol (PFL) owing to its excellent inhibitory property of neurotransmitters in CNS by positive modulation of ligand gated ion channels to an integrated chloride channeled GABA_A thereby acts as a general anesthetic. It differs from other general anesthetics chemically and pharmacologically as it has lesser side effects compared to other general anesthetics and is most commonly used. The present review focuses on two aspects (a) various analytical methods used in quantification of Propofol in pharmaceutical formulations and (b) various analytical methods used to determine Propofol in biological matrices and some biological markers like hair and end tidal nasal air for forensic purpose to estimate drug concentration in suspected cases. Here the various analytical methods are developed using different parameters and validation of employed methods are discussed. Estimated parameters like the linearity, LOQ (Limit of quantification), % recovery, slope, intercept, validation are discussed for the individual method. The critical quality attributes like the wavelength of detection, columns, flow rate, gas flow, and the sample preparation methods for the determination of PFL by bioanalytical methods are also discussed. Type of electrode, mechanism involved and the potential voltage applied for a particular electrochemical method are also discussed.

Volatiles or TIVA: Which is the standard of care for pediatric airway procedures? A pro-con discussion

Anesthesia for pediatric airway procedures constitutes a true art form that requires training and experience. Communication between anesthetist and surgeon to establish procedure goals is essential in determining the most appropriate anesthetic management. But does the mode of anesthesia have an impact? Traditionally, inhalational anesthesia was the most common anesthesia technique used during airway surgery. Introduction of agents used for total intravenous anesthesia (TIVA) such as propofol, short-acting opioids, midazolam, and dexmedetomidine has driven change in practice. Ongoing debates abound as to the advantages and disadvantages of volatile-based anesthesia versus TIVA. This pro-con discussion examines both volatiles and TIVA, from the perspective of effectiveness, safety, cost, and environmental impact, in an endeavor to justify which technique is the best specifically for pediatric airway procedures.

Propofol Formulation Affects Myocardial Function in Newborn Infants

This study aimed to evaluate the effects of propofol in diluted and undiluted formulations on cardiac function in infants. Infants > 30 days received propofol sedation for central line insertion. Cases were divided into two groups: those who received undiluted 1% propofol (P1%); and those who received a diluted formulation (Pd) of equal volumes propofol 1% and 0.9% NaCl. Echocardiograms were performed pre (t₀)-, immediately post (t₁)-, and 1-h post (t₂) propofol administration. Myocardial deformation was assessed with tissue Doppler imaging (TDI) analysis and peak longitudinal strain (LS). 18 cases were included: nine (50%) P1% and nine (50%) Pd. In the P1% group, TDI velocities and LS were significantly reduced at t₁ and t₂. In the Pd Group, only TDI velocities in the left ventricle were reduced at t₁, but not at t₂. Dilution of propofol may minimize myocardial dysfunction while maintaining adequate sedation in infants. Further comparative studies are needed to investigate the safety and efficacy of this approach.

Effects of HCN Channels in the Rostral Ventrolateral Medulla Contribute to the Cardiovascular Effects of Propofol

Hyperpolarization-activated cyclic nucleotide-gated (HCN) channels were reported to express in the well-known vasomotor region, rostral ventrolateral medulla (RVLM), and can be inhibited by propofol. However, whether HCN channels in RVLM contribute to propofol-induced cardiovascular depression remains unclear. We recorded the hemodynamic changes when either continuous intravenous infusions or microinjections of propofol and ZD-7288 (4-ethylphenylamino-1,2-dimethyl-6-methylaminopyrimidinium chloride; HCN channel blocker) in RVLM. Expressions of HCN channels in RVLM neurons of mice of different ages were examined by quantitative real-time polymerase chain reaction and Western blotting. The effects of propofol and ZD-7288 on HCN channels and the excitability of RVLM neurons were examined by electrophysiological recording. Propofol (1.25, 2.5, 5, and 7.5 mg/kg per minute, i.v., 10 minutes) decreased mean arterial pressure (MAP) and heart rate (HR) in a concentration-dependent manner in wild-type mice that were markedly attenuated in HCN1 knockout mice. Bilateral microinjection of propofol (1%, 0.1 μl) in RVLM caused a sharp and pronounced drop in MAP and HR values, which were abated by pretreatment with ZD-7288. In electrophysiological recording, propofol (5, 10, and 20 μM) concentration-dependently inhibited HCN current, increased input resistance, decreased firing rate, and caused membrane hyperpolarization in RVLM neurons. These actions of propofol were attenuated by ZD-7288 pretreatment. The mRNA and protein level of HCN channels increased in an age-dependent manner, which may contribute to the age-dependent increase in the sensitivity to propofol. Our results indicated that the inhibition of HCN channels in RVLM neurons may contribute to propofol-induced cardiovascular inhibition.

Bispectral index to guide induction of anesthesia: a randomized controlled study

Background

It is unknown to what extent hypotension frequently observed following administration of propofol for induction of general anesthesia is caused by overdosing propofol. Unlike clinical signs, electroencephalon-based cerebral monitoring allows to detect and quantify an overdose of hypnotics. Therefore, we tested whether the use of an electroencephalon-based cerebral monitoring will cause less hypotension following induction with propofol.

Methods

Subjects were randomly assigned to a bispectral index (BIS)-guided (target range 40–60) or to a weight-related (2 mg.kg^− 1) manual administration of propofol for induction of general anesthesia. The primary endpoint was the incidence of hypotension following the administration of propofol. Secondary endpoints included the degree of hypotension and correlations between BIS and drop in mean arterial pressure (MAP). Incidences were analyzed with Fisher’s Exact-test.

Results

Of the 240 patients enrolled into this study, 235 predominantly non-geriatric (median 48 years, 25th – 75th percentile 35–61 years) patients without severe concomitant disease (88% American Society of Anesthesiology physical status 1–2) undergoing ear, nose and throat surgery, ophthalmic surgery, and dermatologic surgery were analyzed. Patients who were manually administered propofol guided by BIS (n = 120) compared to those who were given propofol by weight (n = 115) did not differ concerning the incidence of hypotension (44% vs. 45%; p = 0.87). Study groups were also similar regarding the maximal drop in MAP compared to baseline (33% vs. 30%) and the proportion of hypotensive events related to all measurements (17% vs. 19%). Final propofol induction doses in BIS group and NON-BIS group were similar (1.93 mg/kg vs. 2 mg/kg). There was no linear correlation between BIS and the drop in MAP at all times (r < 0.2 for all) except for a weak one at 6 min (r = 0.221).

Conclusion

Results of our study suggest that a BIS-guided compared to a weight-adjusted manual administration of propofol for induction of general anesthesia in non-geriatric patients will not lower the incidence and degree of arterial hypotension.

Trial registration

German Registry of Clinical Trials (DRKS00010544), retrospectively registered on August 4, 2016.

Electronic supplementary material

The online version of this article (10.1186/s12871-018-0522-8) contains supplementary material, which is available to authorized users.

Endothelial nitric oxide synthase polymorphisms affect the changes in blood pressure and nitric oxide bioavailability induced by propofol

Propofol anesthesia is usually accompanied by hypotension, which is at least in part related to enhanced endothelial nitric oxide synthase (NOS3)-derived NO bioavailability. We examined here whether NOS3 polymorphisms (rs2070744, 4b/4a VNTR, rs3918226 and rs1799983) and haplotypes affect the changes in blood pressure and NO bioavailability induced by propofol. Venous blood samples were collected from 168 patients at baseline and after 10 min of anesthesia with propofol 2 mg/kg administered intravenously by bolus injection. Genotypes were determined by polymerase chain reaction and haplotype frequencies were estimated. Nitrite concentrations were measured by using an ozone-based chemiluminescence assay, while NOx (nitrites + nitrates) levels were determined by using the Griess reaction. We found that CT + TT genotypes for the rs3918226 polymorphism, the ba + aa genotypes for the 4b/4a VNTR and the CTbT haplotype were associated with lower decreases in blood pressure and lower increases in nitrite levels after propofol anesthesia. On the other hand, the TCbT and CCbT haplotypes were associated with more intense decreases in blood pressure and higher increases in nitrite levels in response to propofol. Our results suggest that NOS3 polymorphisms and haplotypes influence the hypotensive responses to propofol, possibly by affecting NO bioavailability.

Total intravenous anesthesia will supercede inhalational anesthesia in pediatric anesthetic practice

Inhalational anesthesia has dominated the practice of pediatric anesthesia. However, as the introduction of agents such as propofol, short-acting opioids, midazolam, and dexmedetomidine a monumental change has occurred. With increasing use, the overwhelming advantages of total intravenous anesthesia (TIVA) have emerged and driven change in practice. These advantages, outlined in this review, will justify why TIVA will supercede inhalational anesthesia in future pediatric anesthetic practice.

Skin blood perfusion and cellular response to insertion of insulin pen needles with different diameters

Today most research on pen needle design revolves around pain perception statements through clinical trials, but these are both costly, timely, and require high sample sizes. The purpose of this study was to test if tissue damage, caused by different types of needles, can be assessed by evaluating skin blood perfusion response around needle insertion sites. Three common sized pen needles of 28G, 30G, and 32G as well as hooked 32G needles, were inserted into the neck skin of pigs and then removed. Laser Speckle Contrast Analysis was used to measure skin blood perfusion for 20 minutes after the insertions. Seven pigs were included in the study and a total of 118 randomized needle insertions were conducted. Histology was made of tissue samples inserted with 18G, 28G, and 32G needles, and stained to quantify red and white blood cell response. Based on area under curve, calculated for each individual blood perfusion recording and grouped according to needle type, skin blood perfusion response relates to needle diameter. The response was significantly higher after insertions with 28G and hooked 32G needles than with 30G (P < .05) and 32G (P < .01) needles. Histology results were not significant, but there was a trend of an increased response with increasing needle diameter. Skin blood perfusion response to pen needle insertions rank according to needle diameter, and the tissue response caused by hooked 32G needles corresponds to that of 28G needles. The relation between needle diameter and trauma when analyzing histology was also suggested.

Severe bradycardia during suspension laryngoscopy performed after tracheal intubation using a direct laryngoscope with a curved blade -A case report-

There are a few reports about bradycardia or asystole caused by direct laryngoscopy. However, we encountered severe bradycardia in response to suspension laryngoscopy for laryngeal polypectomy after safely completing tracheal intubation using a direct laryngoscope with a curved blade. The tip of the curved blade of the direct laryngoscope is positioned at the vallecula (between the base of the tongue and the pharyngeal surface of the epiglottis) during tracheal intubation, while the blade tip of the suspension laryngoscope lifts the laryngeal surface of the epiglottis or supraglottic area during surgery. Therefore, suspension laryngoscopy can be said more vagotonic than curved-blade direct laryngoscopy. Because of the possibility of bradycardia induced by suspension laryngoscopy, clinicians must be careful about severe bradycardia even after safely completing intubation using direct laryngoscopy.

Anesthesia and the Microcirculation

There is increasing evidence that the microcirculation and its regulation are severely compromised during many pathological conditions, such as hemorrhage, sepsis, or trauma. The effects of anesthetic agents on macrohemodynamics were investigated intensively in the last several decades. Research regarding modern anesthetics and anesthesia techniques has increased knowledge regarding the nonanesthetic effects of anesthetic agents, including those on organ perfusion and the microcirculation. Alterations in microvascular reactivity, nitric oxide pathways, and cytokine release are presumably the main mechanisms of anesthetic-induced tissue perfusion changes. This review summarizes current methods of microcirculatory status assessment and current knowledge regarding the microcirculatory effects of intravenous and potent volatile anesthetics and anesthesia-related techniques under both normal and pathophysiological conditions.

Mechanism of differential cardiovascular response to propofol in Dahl salt-sensitive, Brown Norway, and chromosome 13-substituted consomic rat strains: role of large conductance Ca2+ and voltage-activated potassium channels

Cardiovascular sensitivity to general anesthetics is highly variable among individuals in both human and animal models, but little is known about the genetic determinants of drug response to anesthetics. Recently, we reported that propofol (2,6-diisopropylphenol) causes circulatory instability in Dahl salt-sensitive SS/JRHsdMcwi (SS) rats but not in Brown Norway BN/NHsdMcwi (BN) rats and that these effects are related to genes on chromosome 13. Based on the hypothesis that propofol does target mesenteric circulation, we investigated propofol modulation of mesenteric arterial smooth muscle cells (MASMC) in SS and BN rats. The role of chromosome 13 was tested using SS-13(BN)/Mcwi and BN-13(SS)/Mcwi consomic strains with chromosome 13 substitution. Propofol (5 microM) produced a greater in situ hyperpolarization of MASMC membrane potential in SS than BN rats, and this effect was abrogated by iberiotoxin, a voltage-activated potassium (BK) channel blocker. In inside-out patches, the BK channel number, P(o), and apparent Ca(2+) sensitivity, and propofol sensitivity all were significantly greater in MASMC of SS rats. The density of whole-cell BK current was increased by propofol more in SS than BN myocytes. Immunolabeling confirmed higher expression of BK alpha subunit in MASMC of SS rats. Furthermore, the hyperpolarization produced by propofol, the BK channel properties, and propofol sensitivity were modified in MASMC of SS-13(BN)/Mcwi and BN-13(SS)/Mcwi strains toward the values observed in the background SS and BN strains. We conclude that differential function and expression of BK channels, resulting from genetic variation within chromosome 13, contribute to the enhanced propofol sensitivity in SS and BN-13(SS)/Mcwi versus BN and SS-13(BN)/Mcwi strains.

Uso do propofol na indução anestésica de cutias (Dasyprocta sp.)

Avaliou-se o emprego do propofol, por via intravenosa, como agente indutor anestésico em cutias (Dasyprocta sp), utilizando-se 10 fêmeas adultas, com pesos entre 1,5 a 2,0kg, criadas em cativeiro. Avaliaram-se as freqüências cardíaca e respiratória, a temperatura retal e a pressão arterial sistólica, antes da administração do propofol e logo após a anestesia dos animais. Foram mensurados o período hábil e de recuperação da anestesia. As médias das freqüências cardíaca e respiratória, temperatura retal e a pressão arterial sistólica basais foram de 220bcm, 92mrm, 38,95ºC e 110mmHg, respectivamente. Após a indução, as médias obtidas para essas variáveis foram de 214bcm, 84mrm, 39,05ºC e 80mmHg. As médias dos períodos hábil e de recuperação da anestesia foram de 10min e 45seg e 15min e 40seg, respectivamente. O propofol, quando utilizado em dose única, mostrou-se seguro, não alterando significativamente os parâmetros fisiológicos, podendo ser considerado um fármaco de escolha para a indução anestésica de cutias.

Comparison of effects of propofol and midazolam at sedative concentrations on sympathetic tone generation in the isolated spinal cord of neonatal rats

BACKGROUND

Propofol and midazolam are common sedatives for critically ill patients. Little is known about the effects of propofol and midazolam on central sympathetic activity when drug concentrations in extracellular milieu are under precise control. Previous work using an in vitro neonatal rat splanchnic nerve-spinal cord preparation has demonstrated that tonic sympathetic activity is generated spontaneously in the thoracic spinal cord. The aim of this study was to investigate the concentration effects of propofol and midazolam on spinally generated sympathetic activity.

METHODS

Using an in vitro neonatal rat splanchnic nerve-spinal cord preparation that allows the precise control of drug concentrations, the central sympathetic effects elicited by the application of propofol (10-640 microM) and midazolam (10-640 microM) were compared.

RESULTS

There was a prompt decrease in sympathetic activity on application of propofol or midazolam in a concentration-dependent manner. A significant decrease in sympathetic activity was observed on application of propofol at 80-640 microM; however, the application of propofol at 10-40 microM caused only a slight alteration in activity. The sympathetic activity was not altered significantly by 10 microM of midazolam, but the application of midazolam at 20-640 microM caused a significant decrease in activity. Thus, in these experimental conditions, the minimum concentration of propofol causing a significant decrease in sympathetic activity was 80 microM and that of midazolam was 20 microM.

CONCLUSIONS

The current findings suggest that the administration of 9-19 microM of propofol or 0.7-0.9 microM of midazolam, the clinically relevant concentrations for sedation, does not alter central sympathetic outflow at the spinal cord level. However, propofol at a concentration of 86 microM, which could be achieved by a single-bolus loading dose to induce sedation, depresses central sympathetic activity.

Propofol ensures a more stable A-line ARX index than thiopental during intubation

BACKGROUND

The A-line autoregressive modelling with exogenous input index (AAI) is a new method of assessing depth of anesthesia. We examined the effects of tracheal intubation on the AAI and hemodynamics during induction of anesthesia with propofol compared with thiopental in patients aged over 50 yr.

METHODS

40 patients scheduled for a laminectomy, posterior spinal fusion, vertebroplasty, or total hip replacement, ASA physical status I or II and aged over 50 yr, were randomly divided into two groups. Thiopental 5 mg.kg(-1) iv, fentanyl 2.5 microg.kg(-1) iv, and rocuronium 0.7 mg.kg(-1) iv were used in the thiopental group (n = 20) for anesthetic induction; the same protocol was used in the propofol group (n = 20) except that 2 mg.kg(-1) propofol iv was given instead of thiopental. The AAI, non-invasive blood pressure, and heart rate were measured every minute before induction for three minutes, at 1.5 min post-induction, and then each minute post-intubation for eight minutes.

RESULTS

The AAI increased significantly at one and two minutes after intubation in the thiopental group (to 56.5 +/- 18.6 at 1 min and 44.7 +/- 18.7 at 2 min after intubation vs 19.9 +/- 7.5 at 1.5 min after induction; P < 0.05). Thereafter, AAI values gradually decreased three minutes after intubation. The AAI was inhibited continuously after intubation in the propofol group, and no significant elevation was seen.

CONCLUSION

Our results, using the AAI to monitor anesthetic depth during induction and tracheal intubation, suggest that at equipotent doses propofol provided a more stable level of anesthesia than did thiopental.

Distinct molecular targets for the central respiratory and cardiac actions of the general anesthetics etomidate and propofol

General anesthetics are among the most widely used and important therapeutic agents. The molecular targets mediating different endpoints of the anesthetic state in vivo are currently largely unknown. The analysis of mice carrying point mutations in neurotransmitter receptor subunits is a powerful tool to assess the contribution of the respective receptor subtype to the pharmacological actions of clinically used general anesthetics. We examined the involvement of beta3-containing GABA(A) receptors in the respiratory, cardiovascular, hypothermic, and sedative actions of etomidate and propofol using beta3(N265M) knock-in mice carrying etomidate- and propofol-insensitive beta3-containing GABA(A) receptors. Although the respiratory depressant action of etomidate and propofol, as determined by blood gas analysis, was almost absent in beta3(N265M) mice, the cardiac depressant and hypothermic effects, as determined by radiotelemetry, and the sedative effect, as determined by decrease of motor activity, were still present. Taken together with previous findings, our results show that both immobilization and respiratory depression are mediated by beta3-containing GABA(A) receptors, hypnosis by both beta3- and beta2-containing GABA(A) receptors, while the hypothermic, cardiac depressant, and sedative actions are largely independent of beta3-containing GABA(A) receptors.

Neuropeptide specificity of prostaglandin E2-induced activation of splenic and renal sympathetic nerves in the rat

Sympathetic activation occurs rapidly following intracerebroventricular (icv) injection of prostaglandin E2(PGE2). This study examined whether neuropeptides mediate PGE2-induced sympathetic nerve activation in urethane/chloralose-anesthetized Sprague-Dawley rats. Animals were pretreated (20.0 microg, icv) with the following receptor antagonists; CRF ([D-Phe12,Nle21,38,Calpha-MeLeu37]CRF12-41), AVP-V1 (Des-Gly-[Phaa1, D-Tyr(Et)2,Lys6,Arg8]-vasopressin), or OT (OT+V1, [d(CH2)5,Tyr(Me)2,Orn8]-vasotocin) followed 20 min later by PGE2 (2.0 microg, icv). Pretreatment with the CRF antagonist attenuated the increase in renal nerve activity induced by PGE2 when measured 10 and 30 min post-injection. PGE2-induced renal nerve activity was also inhibited at both time points by the AVP antagonist and, to a similar extent, the OT antagonist. The AVP antagonist did not effect splenic nerve responses to PGE2 whereas the CRF antagonist produced an incomplete and transient reduction in PGE2-induced activation of the splenic nerve. However, the OT antagonist completely blocked the activation of the splenic nerve after central injection of PGE2. ICV injections of AVP and OT produced immediate changes in splenic and renal nerve activity whereas CRF failed to alter the activity of either nerve in anesthetized or conscious animals. Thus, PGE2 acts through neuropeptide-specific pathways to initiate sympathetic outflow and OT is a specific component of the sympathetic pathway innervating the spleen.

Effects of propofol on bronchoconstriction and bradycardia induced by vagal nerve stimulation

BACKGROUND

Vagolysis has been considered as a mechanism by which propofol produces bronchodilation. However, it has also been suggested that propofol-induced bradycardia may result from increased vagal tone. In this study, we have determined whether propofol has vagolytic effects on both the airway and cardiovascular system.

METHODS

Mongrel dogs were anesthetized with pentobarbital. Bronchoconstriction was assessed by measuring changes in a bronchial cross-sectional area (BCA) using a bronchoscopic method. Heart rate (HR) and direct arterial blood pressure were also monitored. Vagal nerve stimulation (VNS) was performed for 60 s to produce both bronchoconstriction and bradycardia. To determine the effect of propofol on VNS-induced bronchoconstriction and bradycardia (n = 7), 0 (saline), 2.0 and 20 mg/kg propofol were administered intravenously at 20-min intervals with VNS commenced 5 min later. In addition, to determine if propofol-induced bradycardia is due to a vagomimetic action, two groups of six dogs were given 20 mg/kg propofol with or without 0.2 mg/kg atropine pre-treatment. HR was measured before and 5 min after propofol.

RESULTS

Propofol 20 mg/kg significantly inhibited VNS-induced bronchoconstriction. Although propofol per se significantly reduced HR (24%) and blood pressure (37%), the reduction in HR produced by VNS after 20 mg/kg propofol did not differ from that after saline or the lower dose of propofol (2 mg/kg). As atropine pre-treatment did not attenuate propofol-induced bradycardia, this response is unlikely to be simply due to vagomimetic actions.

CONCLUSION

Propofol has vagolytic effects on the airway but does not worsen bradycardia produced by parasympathetic stimulation.

Effects of General Anesthetics on Regulation of the Peripheral Vasculature

The heart is a passively filling pump in a circulatory system that is connected in series with distensible blood vessels. Therefore, systemic blood pressure and tissue perfusion depend upon adequate peripheral vascular tone as well as myocardial function. Likewise, pharmacologic agents that alter circulatory stability can affect one or both of these components. The generalized depressor effects of general anesthetics have been well known clinically for over 50 years. Moreover, there are many similarities in basic cellular regulatory mechanisms among the different tissue types, and general anesthetics are well known to distribute freely among the perfusion-rich tissues (eg, central nervous system, cardiovascular system, and renal system). Therefore, it is likely that the hemodynamic depression resulting from the systemic administration of anesthetics results from actions on regulatory mechanisms of the peripheral vasculature as well as on the heart. The peripheral vasculature is regulated by extrinsic neural, endothelial, and humoral mechanisms, which interact with each other as well as with intrinsic membrane and intracellular systems within the vascular smooth muscle cell. Different general anesthetics have been found to act on specific mechanisms at each of these levels. However, the large number and complexity of these known mechanisms, as well as the many anesthetic agents, has made it extremely difficult to determine which are significant in terms of the meaningful mechanisms that are responsible for anesthetic action, major side effects, or both. Current knowledge about the effects of general anesthetics on both the extrinsic intrinsic regulatory mechanisms of peripheral vascular control is reviewed.

Response of the rat cremaster microcirculation to hemorrhage in vivo: differential effects of intravenous anesthetic agents

Anesthetic agents are known to have differential effects on both the systemic circulation and the microcirculation. The aim of this study was to compare the effects of several intravenous (i.v.) agents on the microcirculatory response to hemorrhage. Male Wistar rats (n = 52) were anesthetized i.v. either with propofol and fentanyl (propofol fentanyl), ketamine, or thiopental. Cardiovascular variables were monitored. The cremaster muscle was observed by using fluorescent intravital microscopy. FITC-BSA was administered (0.25 mL/100 g, i.a.) to determine macromolecular leak, an index of vessel integrity. Animals were further allocated into control (C), 10% hemorrhage (H), or hemorrhage re-infusion (H-R, removal of 10% blood volume and then re-infusion of saline and blood) groups. When systolic arterial pressure (SAP) was maintained after hemorrhage, constriction of A3 and A4 arterioles (5-30 microm) was accompanied by no change in the diameter of A1 (80-130 microm): most frequent with ketamine (A1: -1.7 +/- 1.2; A4: -13.9 +/- 2.7%; H and H-R: n = 9/11). With lower SAP, dilation of the A3 and A4 was accompanied by constriction of the A1: most frequent with propofol/fentanyl (A1: -8.0 +/- 2.5; A4; 35.1 +/- 9.4%; H and H-R: n = 6/11). No increases in macromolecular leak occurred with any anesthetic agent or in H or H-R groups. The response of cremaster muscle microcirculation to hemorrhage differs with different i.v. anesthetic agents. Dilation of small arterioles is the predominant response with propofol/fentanyl and constriction of small arterioles with ketamine.

Propofol for electrical storm; a case report of cardioversion and suppression of ventricular tachycardia by propofol

PURPOSE

To present a case report where propofol abolished recurrent ventricular tachycardia (VT) and to suggest a mechanism by which this may have occurred.

CLINICAL FEATURES

A 65-yr-old male was admitted to the intensive care unit (ICU) with electrical storm. Recurrent episodes of VT persisted despite maximal anti-arrhythmic therapy and resulted in a prolonged ICU course and the need for intra-aortic balloon pump support. This was complicated by an ischemic limb, necessitating an anesthetic for femoral thrombectomy. On several occasions while in the ICU, episodes of VT had resolved with boluses of propofol prior to planned cardioversion. In the operating room, episodes of non-sustained VT resolved after a bolus of propofol and remained suppressed for the duration of the case with the use of a propofol infusion.

CONCLUSION

The effects of propofol on cardiac conduction and on the autonomic nervous system have been studied but its effects on arrhythmias are not well documented. In this case report, propofol was associated with the resolution and suppression of VT. Recent evidence suggests that sympathetic blockade may be an effective treatment for electrical storm. This may be the mechanism by which propofol can abolish this arrhythmia intraoperatively.

Differential effects of intravenous anaesthetic agents on the response of rat mesenteric microcirculation in vivo after haemorrhage

BACKGROUND

The differential effects of i.v. anaesthesia on the response of the mesenteric microcirculation after haemorrhage in vivo are previously unexplored.

METHODS

Male Wistar rats (n=56) were anaesthetized intravenously either with propofol and fentanyl (propofol/fentanyl), ketamine or thiopental. A tracheostomy and carotid cannulation were performed and the mesentery surgically prepared for observation of the microcirculation using fluorescent in vivo microscopy. Animals were allocated to one of three groups: control, haemorrhage or haemorrhage re-infusion.

RESULTS

After haemorrhage, the response of the microcirculation differed during propofol/fentanyl, ketamine and thiopental anaesthesia. During propofol/fentanyl anaesthesia there was constriction of arterioles (-16.7 (3.9)%), venules (-5.9 (1.7)) and capillaries (-16.3 (2.8)) (n=12). During ketamine and thiopental anaesthesia both constriction and dilation was observed. After haemorrhage and re-infusion, macromolecular leak occurred from venules during propofol/fentanyl and thiopental anaesthesia (P<0.05), but not during ketamine anaesthesia.

CONCLUSION

In summary, i.v. anaesthetic agents differentially alter the response of the mesenteric microcirculation to haemorrhage.

Propofol and methohexital have no significant effect on mucus secretion or clearance in the anesthetized dog

OBJECTIVE

Anecdotal reports suggest that propofol (Diprivan) may stimulate mucus hypersecretion in patients without pulmonary disease. The purpose of this study was to evaluate the effect of methohexital or propofol anesthesia on the physical and transport properties of airway mucus in spontaneously breathing dogs.

DESIGN

Randomized, controlled, crossover laboratory study.

SETTING

University laboratory.

SUBJECTS

Four healthy, purpose-bred female beagle dogs.

INTERVENTIONS

Dogs were anesthetized with 5 mg/kg of propofol by intravenous bolus followed by a maintenance infusion at 0.4 mg x kg(-1) x min(-1) or 4 mg/kg of methohexital followed by an infusion at 0.3 mg x kg(-1) x min(-1). Premedication with 0.05 mg/kg of acepromazine was given, and either atropine (0.2 mg) or saline was given by intravenous bolus at the time of induction. Dogs were intubated but spontaneously breathing throughout the experiment. Tracheal secretions were collected after induction and again after 40 mins.

MEASUREMENTS AND MAIN RESULTS

The volume of secretions collected on the endotracheal tube during the 1.5-hr experiment and on a bronchoscopy brush over 10 mins during the experiment was measured. Tracheal epithelial potential difference was measured bronchoscopically by saturated agar bridges, and tracheal mucus transport velocity was determined by timing particle transport. The dynamic viscoelasticity of collected mucus was assessed by microrheometry, and secretion mucociliary transportability was measured in vitro. There were no differences in any physical or transport properties of airway secretions that could be attributed to the anesthetic agent. Secretion volume was significantly lower (p < .05) and epithelial potential difference was significantly higher (p = .03) with atropine premedication. Despite this, there were no differences in tracheal mucus transport velocity, viscoelasticity, or secretion mucociliary transportability with either anesthetic agent or with atropine.

CONCLUSIONS

This study suggests that neither methohexital nor propofol significantly affects mucus secretion or clearance in healthy dogs.

Intracerebroventricular propofol is neuroprotective against transient global ischemia in rats: extracellular glutamate level is not a major determinant

Excessive glutamate accumulation in extracellular space due to ischemia in the central nervous system (CNS) is believed to initiate the cascade toward irreversible neuronal damage. An intravenous general anesthetic, propofol (2,6-diisopropylphenol) has been implicated to be neuroprotective against cerebral ischemia. The purpose of this study was to test the hypothesis that intracerebroventricular propofol produced a reduction in extracellular glutamate level during global ischemia and the resultant neuroprotection. Adult male Wistar rats were anesthetized with halothane in nitrous oxide/oxygen and mechanically ventilated. Propofol (3 or 10 mg/kg), Intralipid((R)) as a vehicle for propofol, or artificial cerebrospinal fluid (aCSF) was administered into the cerebral ventricles 15 min prior to a 10-min forebrain ischemia elicited by the four-vessel occlusion. Extracellular glutamate concentration in the hippocampal CA1 was continuously monitored during the peri-ischemic period with a microdialysis biosensor. Neuronal cell loss in the hippocampal CA1 was evaluated by cresyl-violet staining of sections 7 days later. Propofol (3 and 10 mg/kg) and Intralipid, compared with aCSF, similarly reduced the extracellular glutamate accumulation during the peri-ischemic period (P<0.05), indicating that the extracellular glutamate reduction that was seen primarily reflects the effect of Intralipid. The number of intact neurons in the hippocampal CA1 in propofol 10 mg/kg-treated rats was significantly higher than that in rats treated with propofol 3 mg/kg, Intralipid, or aCSF (P<0.05). We conclude that intracerebroventricular propofol exhibits neuroprotection against transient global forebrain ischemia; however, the extracellular glutamate level during ischemia is not a major determinant of this neuroprotection.

Autonomic circulatory and cerebrocortical responses during increasing depth of propofol sedation/hypnosis in humans

PURPOSE

To describe the relative effects of graded central nervous system (CNS) depression, using increasing propofol infusion rates, on neurovegetative brainstem-mediated circulatory control mechanisms and higher cortical activity in healthy humans.

METHODS

Propofol was administered using an infusion scheme designed to achieve three target blood concentrations in ten healthy volunteers. Blood propofol concentrations and sedation scores were determined at baseline, during the three propofol infusion levels, and 30 min into the recovery period. Electroencephalographic (EEG) power was measured in three frequency bands to quantify cortical activity, and autonomic heart rate control was quantified using spontaneous baroreflex assessment and power spectral analysis of pulse interval.

RESULTS

Sedation scores closely paralleled propofol blood concentrations (0, 0.53 +/- 0.34, 1.24 +/- 0.21, 3.11 +/- 0.80, and 0.96 +/- 0.42 microg x mL(-1) at baseline, three infusion levels and recovery respectively), and all subjects were unconscious at the deepest level. Indices of autonomic heart rate control were decreased only at the deepest levels of CNS depression, while EEG effects were apparent at all propofol infusion rates. These EEG effects were frequency specific, with power in the beta band being affected at light levels of sedation, and alpha and delta power altered at deeper levels.

CONCLUSIONS

The results of this study support a relative preservation of neurovegetative circulatory control mechanisms during the early stages of CNS depression using gradually increasing rates of infusion of propofol. Indices of circulatory control did not reliably reflect depth of sedation.

Dose-dependent effects of propofol on renal sympathetic nerve activity, blood pressure and heart rate in urethane-anesthetized rabbits

To evaluate the role of the autonomic nervous system in hemodynamic changes after propofol bolus injection, we used direct recordings of renal sympathetic nerve activity to examine the dose-dependent effects of propofol (2.5, 5, 10, and 20 mg/kg) on heart rate, mean blood pressure and renal sympathetic nerve activity in urethane-anesthetized rabbits. The animals were divided into four groups: animals with an intact neuraxis (intact group), cervical vagal nerve-sectioned animals (vagotomy group), carotid sinus and aortic-nerve sectioned animals (SAD group), and animals with SAD plus vagotomy (SADV group). Heart rate did not change significantly even after administration of 2.5 and 5 mg/kg but decreased markedly on 20 mg/kg injection in all groups. The intact and vagotomy groups had augmented renal sympathetic nerve activity with insignificant changes in mean blood pressure after 5 mg/kg injection of the agent. Insignificant changes of renal sympathetic nerve activity but a remarkable decrease of mean blood pressure appeared after 10 mg/kg propofol. Sustained hypotension in parallel with a profound depression of renal sympathetic nerve activity developed at the dose of 20 mg/kg. In SAD and SADV groups, however, dose-dependent depressions of renal sympathetic nerve activity were accompanied by decreases of mean blood pressure. These results suggest the following: (1) propofol-induced hypotensive effects are probably produced by the central-mediated sympathetic depression. (2) The baroreceptor reflex may be preserved at the lower dose of the agent. (3) Heart rate does not change significantly unless a large dose of propofol is used. The difference in effects on heart rate and on mean blood pressure may denote a greater inhibition of sympathetic vascular outflow than of the cardiac sympathetic outflow regulating cardiac rate and contractility. This hypothesis needs further clarification.

Propofol anesthesia enhances pressor response to ephedrine in patients given clonidine

We studied the hemodynamic effects of ephedrine in patients with or without clonidine premedication during either isoflurane or propofol anesthesia. Forty adult patients were randomly assigned to one of two groups: 20 patients received famotidine 20 mg orally (control group) and 20 received clonidine 3 microg/kg and famotidine 20 mg orally (clonidine group). Within each group, 10 patients were then anesthetized with isoflurane and 10 with propofol. Hemodynamic measurements were taken at 1-min intervals for 10 min after a bolus injection of ephedrine 0.1 mg/kg. The magnitude of the maximal pressor response to ephedrine was no different whether patients without clonidine were anesthetized with isoflurane (increase 5+/-7 mm Hg) or propofol (3+/-9 mm Hg); however, this response was greater (P<0.05) with propofol (17+/-6 mm Hg) versus isoflurane (6+/-5 mm Hg) in patients given clonidine. The arterial blood pressure increase in clonidine-premedicated patients with propofol anesthesia was the largest among the four subgroups. The heart rate response to ephedrine was not significant in patients anesthetized with isoflurane and was small but significant in those anesthetized with propofol. The present results, together with previous studies on the effect of ephedrine in patients medicated with clonidine, suggest that the interaction between clonidine and ephedrine is modulated by the anesthetic used.

IMPLICATIONS

We evaluated the pressor response to ephedrine during isoflurane or propofol anesthesia with or without clonidine premedication. Our study suggests that, in anesthetized patients premedicated with clonidine, decreases in blood pressure may be easier to reverse with ephedrine with some types of anesthesia (e.g., propofol) than with others (e.g., isoflurane).

The effect of rate of administration on brain concentrations of propofol in sheep

A marked reduction in the dose of propofol required to achieve the onset of anesthesia with slower administration rates has previously been reported, but the mechanism of this phenomenon is unclear. We used a chronically instrumented sheep preparation to examine the effects of different administration rates of propofol on its distribution in the brain using mass balance principles to calculate brain concentrations. The administration of 100 mg of propofol i.v. at rates of 200, 50, and 20 mg/min had minimal effect on both the peak brain concentrations of propofol and the total amount of drug entering the brain. The more rapid administration rates increased the rate of uptake into the brain but resulted in large increases in peak arterial blood propofol concentrations. These faster administration rates have previously been associated with high arterial propofol concentrations and an increased risk of hypotension. Simulation of titration to an end point revealed that the dose sparing previously reported at induction with slow administration rates relates only to improved titration to effect, and does not result in more anesthesia for a given dose. Therefore, we conclude that the administration of propofol over 2 min provides a reasonable rate of induction and improved titration to effect, yet avoids excessively high arterial concentrations.

IMPLICATIONS

Alterations in the rate of administration of propofol in sheep have been shown to have little effect on the quantity of propofol delivered to the brain. At induction of anesthesia, administration rates of approximately 50 mg/min seem likely to provide improved titration to effect without excessively prolonging induction.

Propofol inhibits medullary pressor mechanisms in cats

PURPOSE

Propofol may cause hypotension and the mechanism is complex. The present study was designed to determine the direct actions of propofol in medulla of cats.

METHODS

Mean systematic arterial pressure (MSAP), heart rate (HR) and cardiac contractility (dp/dt) were compared before and after administration of propofol the femoral vein (2, 3, or 4 mg.kg-1), vertebral artery (1 mg.kg-1) or the lateral cerebral ventricle (0.5 mg.kg-1) in eight anaesthetized cats. To study the direct effect of propofol in medulla, pressor areas of the dorsomedial medulla (DM) and rostral ventrolateral medulla (RVLM), or the depressor area of the caudal ventrolateral medulla (CVLM) were first identified with electrical stimuli and then confirmed by pressure microinjection of glutamate (Glu, 0.25M, 30 nl) via a multibarrel-micropipette in 28 cats. One hour later, propofol (0.001%, 50 nl) was microinjected at the same site. Electrical stimulation and Glu were applied again to compare changes of SAP, HR and dp/dt with that of the control.

RESULTS

Propofol dose-dependently decreased SAP, HR and cardiac contractility. The percent increase of MSAP induced by Glu were reduced by propofol in DM (59 +/- 3% vs 13 +/- 2%, n = 11, P < 0.01) or in RVLM (56 +/- 4% vs 18 +/- 2%, n = 9, P < 0.01). In CVLM, propofol slightly but not significantly increased depressor responses elicited by Glu (-27 +/- 2% vs -33 +/- 3%, n = 5, P > 0.05).

CONCLUSION

Our results show that propofol principally inhibits the vasomotor mechanism in the dorsomedial and ventrolateral medulla to effect its hypotensive actions.

Role of ouabain-like compound in the rostral ventrolateral medulla in rats

To determine whether ouabain-like compound (OLC) exerts modulatory influences on the activity of vasomotor neurons in the rostral ventrolateral medulla (RVLM), we examined the effects of microinjecting ouabain, digoxin-specific antibody Fab fragments, and mAb against ouabain on the rat RVLM. Microinjection of ouabain into the unilateral RVLM of anesthetized normotensive rats elicited dose-dependent increases in mean arterial pressure (MAP) and renal sympathetic nerve activity (RSNA). The pressor and sympathoexcitatory effects of ouabain in the RVLM were reversed by microinjections of an M2 muscarinic antagonist, gallamine, or digoxin-specific antibody Fab fragments. Furthermore, a prior microinjection in the RVLM of gallamine, digoxinspecific antibody Fab fragments, or kainic acid or intravenous injection of hexamethonium all prevented the pressor and sympathoexcitatory effects induced by a subsequent microinjection of ouabain. Microinjections of either digoxinspecific antibody Fab fragments or gallamine per se significantly decreased baseline MAP and RSNA. Injection of digoxin-specific antibody Fab fragments attenuated the effects of a subsequent injection of gallamine. Microinjection of mAb against ouabain, but not nonspecific IgG, also significantly decreased baseline MAP and RSNA. These results suggest that OLC in the RVLM contributes to the tonic activity of vasomotor neurons in anesthetized normotensive rats, and the action of OLC in the RVLM is at least partly mediated by M2 muscarinic mechanisms.

Propofol differentially attenuates the responses to exogenous and endogenous norepinephrine in the isolated rat femoral artery in vitro

Propofol causes a decrease in vascular resistance mediated in part by a decrease in sympathetic output. To determine whether attenuation of norepinephrine release from sympathetic perivascular nerve terminals could contribute to decreased vascular resistance, we examined the effects of propofol on the contractile responses to exogenous and endogenous norepinephrine in the rat femoral artery. Endogenous norepinephrine was released from sympathetic nerve terminals using electrical field stimulation. The responses to both exogenous norepinephrine and neurally released norepinephrine were attenuated by propofol in concentrations from 1.0 to 10.0 micrograms/mL. At 50% of maximal and at maximal contractile responses to norepinephrine and electrical field stimulation, the response to electrical field stimulation was inhibited to a greater extent than the response to exogenous norepinephrine. This suggests that, in addition to direct postsynaptic vasodilation, propofol has the presynaptic effect of inhibiting norepinephrine release from perivascular nerves.

Actions of propofol on pontine neurons controlling arterial pressure in rats

Tonic firing of pontine neurons provides excitatory input to the vasomotor centre in the ventrolateral medulla. To increase our understanding of the actions of propofol on CNS neurons controlling the cardiovascular system, we evaluated the effects of propofol on this tonic firing of pontine neurons. The actions of propofol (doses 1-4, respectively: 24 +/- 2, 40 +/- 4, 65 +/- 3 and 104 +/- 3 mg.kg-1.hr-1) on the pontine neurons were studied using eight atropinized Wistar rats. Electrical activity of renal sympathetic nerves, systemic arterial blood pressure and heart rate were recorded. Propofol decreased renal nerve activity by 3 +/- 2%, 23 +/- 3%, 33 +/- 3% and 52 +/- 4% at the four doses. Arterial pressure and heart rate decreased similarly in a dose-dependent manner. Sympathetic and cardiovascular responses to blocking neurons in the pontine reticular formation by microinjection of glycine were depressed by propofol. Renal nerve activity was decreased by 44 +/- 5% 41 +/- 4%, 28 +/- 3% and 13 +/- 2% after pontine blockade during infusion of doses 1 to 4, respectively. Similarly, arterial pressure was decreased by 25 +/- 3, 15 +/- 2, 12 +/- 1 and 5 +/- 2 mmHg. Finally, heart rate decreased by 27 +/- 6, 20 +/- 4, 18 +/- 4 and 13 +/- 5 heats per min as the propofol dose increased. The tonic firing of pontine neurons was minimally depressed by the lower two doses of propofol but higher doses did appear to depress their firing, demonstrating dose-dependence of actions of this anaesthetic.(ABSTRACT TRUNCATED AT 250 WORDS)

Effect of chromogranin A on central autonomic control of blood pressure

We investigated the effect of exogenous chromogranin A (CgA) on central autonomic structures involved in the control of blood pressure. Actions of CgA were assessed on neurons in the rostral ventrolateral medulla, the most important brain area for cardiovascular control and for generation of sympathetic activity. Changes in renal sympathetic nerve activity, arterial pressure, heart rate, and somato-sympathetic reflexes were measured after microinjections of chromogranin A into the rostral ventrolateral medulla of anesthetized rats. The sites of microinjection of chromogranin A were determined from well known sympathetic and cardiovascular responses to microinjection of the inhibitory amino acid glycine. Significant decreases in renal nerve activity and arterial blood pressure began 10 min after microinjection of chromogranin A into the rostral ventrolateral medulla, and the maximum effect was observed at 20 min. Moreover, the bulbar component of somato-sympathetic reflexes was depressed 20 min after microinjections of chromogranin A. Our results indicate that chromogranin A can modify the activity of neurons within rostral ventrolateral medulla involved in regulation of cardiovascular autonomic control.

Central sympathetic mechanisms of blood pressure control in hamsters

The goal of this study was to investigate central vasomotor control of blood pressure in golden hamsters. Electrophysiological experiments demonstrated that tonic and reflex firing of renal nerves was controlled by brainstem and spinal circuits in manner similar to control of these nerves in rats, rabbits and cats. These findings confirmed that autonomic neural circuits for vasomotor control in hamsters are functionally similar to those of other well-studied species.

Role of dorsal medullary reticular formation in maintenance of vasomotor tone in rats

Recent investigations of central nervous system structures involved in the generation of vasomotor tone have focused on the rostral ventrolateral portion of the medulla, whereas other medullary areas have not been considered to have primary importance. The present study was designed to determine the role of the medullary reticular formation, dorsal to the ventrolateral medulla, in the maintenance of vasomotor tone. The effects of microinjections of the inhibitory amino acid glycine into this medullary region on systemic arterial pressure, heart rate and renal nerve activity were determined in male rats anaesthetized with propofol and treated with atropine to block vagal influences on the heart. Unilateral injections (24 of 57 sites) of glycine into the dorsal medulla caused significant decreases in arterial pressure (-10 +/- 1.0 mmHg) and renal sympathetic nerve activity (-12 +/- 1%) whereas heart rate was not significantly affected. Bilateral injections (12 of 20) of glycine caused significantly greater decreases in heart rate (-10 +/- 1 bpm) and renal nerve activity (-25 +/- 4%) than were elicited by unilateral injections. In contrast, although arterial pressure decreased significantly from control values (-10 +/- 1 mmHg), these responses were not different from the arterial pressure changes caused by unilateral injections. These results demonstrate that, in the propofol anaesthetized rat, neurons in the dorsal medullary reticular formation contribute to the maintenance of vasomotor tone.