Synergistic inhibition of lysophosphatidic acid signaling by charged and uncharged local anesthetics

We investigated the mechanism of benzocaine (permanently uncharged) and QX314 (permanently charged) inhibition of lysophosphatidic acid (LPA) signaling. To determine their site of action, we studied effects of these drugs, alone and in combination, on LPA-induced Ca2+-dependent Cl currents (I(Cl(Ca))) in Xenopus oocytes. After 10 min exposure to benzocaine, QX314 (10(-6)-10(-2) M), or both, we measured effects on I(Cl(Ca)) induced by LPA (with and without protein kinase [PKC] activation/inhibition) and on I(Cl(Ca)) induced by the intracellular injection of IP3 and GTPgammaS. LPA application to oocytes resulted in I(Cl(Ca)) (50% effective concentration approximately 10(-8) M). Both anesthetics inhibited LPA signaling concentration-dependently (50% inhibitory concentration [IC50] benzocaine 0.9 mM, QX314 0.66 mM). The combination acted synergistically (IC50 benzocaine 0.097 mM/QX314 0.048 mM). Intracellular signaling pathways were not affected. This study shows that benzocaine and QX314 inhibit LPA signaling and act synergistically, which is most easily explained by the existence of two different binding sites. Lack of inhibition of IP3 or GTPgammaS-induced I(Cl(Ca)) identifies the receptor as a target. Activation of PKC can be excluded as a potential mechanism.

IMPLICATIONS

Lysophosphatidic acid may play a role in wound healing, and its signaling is inhibited by local anesthetics. We identified the membrane receptor as the local anesthetic site of action and showed that charged (QX314) and uncharged (benzocaine) local anesthetics inhibit lysophosphatidic acid signaling synergistically, which can be explained by the presence of different binding sites.

Influence of anesthetics on endogenous and recombinantly expressed G protein-coupled receptors in the Xenopus oocyte

1. The oocyte of the African clawed toad (Xenopus laevis) offers a reliable, sensitive and disease resistant system to investigate recombinantly and endogenously expressed Ca2+ signaling G protein-coupled receptors and their intracellular signaling pathways. 2. To study receptor induced Ca2+ release, two-electrode voltage clamping can quantify a Ca2+-activated transmembrane Cl- current. Intracellular steps of the signaling pathway can be inhibited by injections of EDTA or heparin into the oocyte. Components of the intracellular pathway can be activated directly by GTPgammaS or IP3 injection. 3. We have investigated the effects of volatile, local and i.v. anesthetics on the signaling properties of the endogenous lysophosphatidate receptor and on mammalian receptors expressed recombinantly by intracellular injection of the encoding mRNA or cDNA. A number of receptors are sensitive to these anesthetics. Anesthetics interact with muscarinic, thromboxane A2 and lysophosphatidate signaling. 4. Investigations of the intracellular pathways revealed that the receptor or the receptor-G protein coupling is affected primarily and that mechanisms further downstream are not influenced by the various types of anesthetics.

Volatile anaesthetics have differential effects on recombinant m1 and m3 muscarinic acetylcholine receptor function

Muscarinic acetylcholine signalling plays major roles in regulation of consciousness, cognitive functioning, pain perception and circulatory homeostasis. Halothane has been shown to inhibit m1 muscarinic signalling. However, no comparative data are available for desflurane, sevoflurane or isoflurane, nor have the anaesthetic effects on the m3 subtype (which is also prominent in the brain) been studied. Therefore, we have investigated the effects of these compounds on isolated m1 and m3 muscarinic receptor function. Defolliculated Xenopus oocytes expressing recombinant m1 or m3 muscarinic or (for comparison) AT1A angiotensin II receptors were voltage clamped, and Ca(2+)-activated Cl- currents (ICl(Ca)) induced by acetyl-beta-methylcholine (Mch) or angiotensin II were measured in the presence of clinically relevant concentrations of halothane, sevoflurane, desflurane or isoflurane. To determine the site of action of the volatile anaesthetics we compared anaesthetic effects on m1, m3 and AT1A receptor function and studied the effects of volatile anaesthetics on signalling induced by intracellular injection of the second messenger IP3. Desflurane had a biphasic effect on m1 signalling, enhancing at a concentration of 0.46 mmol litre-1 but depressing at 0.92 mmol litre-1. A similar, although not significant, trend was observed with m3 signalling. Isoflurane had no effect on m1 signalling, but profoundly inhibited m3 signalling. Sevoflurane depressed the function of m1 and m3 signalling in a dose-dependent manner. Halothane, similar to its known effect on m1 signalling, dose-dependently depressed m3 function. ICl(Ca) induced by intracellular injections of IP3 were unaffected by all four anaesthetics. Similarly, none of the anaesthetics tested affected AT1A signalling. Absence of interference with AT1A signalling and intracellular pathways suggest that the effects of anaesthetics on muscarinic signalling most likely result from interactions with the m1 or m3 receptor molecule. Multiple interaction sites with different affinities may explain the biphasic response to desflurane. Anaesthetic-specific effects on closely related receptor subtypes suggest defined sites of action for volatile anaesthetics on the receptor protein.

Intercellular signaling by lysophosphatidate

Lysophosphatidate (LPA) is an intercellular phospholipid messenger with a wide range of biologic effects. The first discovered source of LPA in the human body were activated platelets, but several other sites of LPA generation are now known. The number of cellular interactions is also growing steadily and responses to the compound range wide, from induction of mitogenesis to neurite retraction. LPA acts via a specific G protein-coupled receptor, of which one or more subtypes may exist. Intracellularly, this receptor activates several heterotrimeric G proteins. LPA induces cell proliferation via the small GTP-binding proteins ras, and triggers actin-based cytoskeletal events through rho. This review describes the most relevant recent developments in our understanding of LPA signaling.

Influence of volatile anesthetics on thromboxane A2 signaling

BACKGROUND

Thromboxane A2 (TXA2) is a member of the prostaglandin family; activation of its receptor induces several important effects, including platelet aggregation and smooth muscle contraction. Because volatile anesthetics interfere with aggregation and contraction, the authors investigated effects of halothane, isoflurane, and sevoflurane on TXA2 signaling in an isolated receptor model.

METHODS

mRNA encoding TXA2 receptors was prepared in vitro and expressed in Xenopus oocytes. The effects of halothane, isoflurane, and sevoflurane on Ca2+-activated Cl- currents induced by the TXA2 agonist U-46619 and on those induced by intracellular injection of inositol 1-4-5 trisphosphate or guanosine 5'-O-(2-thiodiphosphate) were measured using the voltage-clamp technique.

RESULTS

Expressed TXA2 receptors were functional (half maximal effect concentration [EC50], 3.2 x 10(-7) +/- 1.1 x 10(-7) M; Hill coefficient (h), 0.8 +/- 0.2). Halothane and isoflurane inhibition of TXA2 signaling was reversible and concentration dependent (halothane half maximal inhibitory concentration [IC50], 0.46 +/- 0.04 mM; h, 1.6 +/- 0.21; isoflurane IC50, 0.69 +/- 0.12 mM; h, 1.3 +/- 0.27). 0.56 mM halothane (1%) right-shifted the U-46619 concentration-response relationship by two orders of magnitude (EC50, 1 x 10[-5] M). That h and maximal effect (Emax) were unchanged indicates that halothane acts in a competitive manner. In contrast, isoflurane acted noncompetitively, decreasing Emax by 30% (h and EC50 were unchanged). Both halothane and isoflurane had no effect on intracellular signaling pathways. Sevoflurane (0-1.3 mM) did not affect TXA2 signaling.

CONCLUSIONS

Both halothane and isoflurane inhibit TXA2 signaling at the membrane receptor, but by different mechanisms. This suggests that the effects of these anesthetics on TXA2 signaling are evoked at different locations of the receptor protein: halothane probably acts at the ligand binding site and isoflurane at an allosteric site.

Characterization of a receptor subtype-selective lysophosphatidic acid mimetic

Despite an intriguing cell biology and the suggestion of a role in pathophysiological responses, the mechanism of action of such lipid phosphoric acid mediators as lysophosphatidic acid (LPA) remains obscure, in part because of an underdeveloped medicinal chemistry. We report now the agonist activity of a synthetic phospholipid in which the glycerol backbone of LPA is replaced by L-serine. Like LPA, the L-serine-based lipid mobilizes calcium and inhibits activation of adenylyl cyclase in the human breast cancer cell line MDA MB231. Treatment with LPA desensitizes MDA MB231 cells to subsequent application of the L-serine compound; when the order of application is reversed, however, the L-serine compound does not prevent calcium mobilization by LPA, which might indicate the existence of two LPA receptors in these cells. The analogous D-serine-based phospholipid was distinctly less potent than the L-isomer in those assays; this finding demonstrates stereoselectivity by an LPA receptor. Unlike LPA, the L-serine-based lipid does not evoke a chloride conductance in Xenopus laevis oocytes, but injection of poly(A)+ RNA from HEK 293 cells confers this phenotype on the oocyte. The latter result has practical importance in that it allows use of the frog oocyte for expression cloning of an LPA receptor DNA, an assay system made problematic by the oocyte's strong endogenous response to LPA.

Synergistic inhibition of muscarinic signaling by ketamine stereoisomers and the preservative benzethonium chloride

BACKGROUND

Ketamine (Ketalar; Parke-Davis, Morris Plains, NJ) has been shown to inhibit muscarinic signaling with a median inhibitory concentration (IC50) of 5.7 microM. Whereas Ketalar is a racemic mixture, recent interest has focused on clinical use of the S(+) ketamine isomer, which is three times as potent an analgesic as the R(-) isomer yet seems to be associated with fewer psychoactive side effects. Therefore, the authors studied the effects of S(+) and R(-) ketamine and the preservative benzethonium chloride on muscarinic signaling.

METHODS

Rat ml muscarinic acetylcholine receptors were expressed recombinantly in Xenopus laevis oocytes. Ca2(+)-activated Cl- currents in response to 10(-7) M acetyl-beta-methylcholine were determined by two-electrode voltage clamping in the presence of various concentrations of ketamine and benzethonium. Concentration-inhibition curves were constructed and used for algebraic and isobolographic analysis.

RESULTS

The IC50. was 125 +/- 33 microM for S(+) ketamine, and 91 +/- 19 microM for R(-) ketamine. This difference was not statistically significant, indicating that muscarinic inhibition by ketamine is not stereoselective. The R(-)/S(+) mixture had an IC50 of 48 +/- 1 microM, and thus the stereoisomers interact synergistically. When appropriate concentrations of benzethonium were added, an IC50 of 15 +/- 2 microM resulted.

CONCLUSIONS

The muscarinic inhibitory action of ketamine isomers is not stereoselective. Because S(+) ketamine is a significantly more potent analgesic, it should have less muscarinic inhibitory action than R(-) ketamine when used in clinically equivalent doses. A significant fraction of the muscarinic inhibitory action of Ketalar is due to the preservative benzethonium. If reconstituted with a different preservative, Ketalar might be a less potent muscarinic antagonist.

Inhibition of lysophosphatidate signaling by lidocaine and bupivacaine

BACKGROUND

Lidocaine and bupivacaine impair wound healing, but the mechanism of this side effect has not been determined. The phospholipid messenger lysophosphatidate is released from activated platelets and induces fibroblast and smooth muscle proliferation. Because it may play a role in wound healing, the authors studied the effects of local anesthetics on lysophosphatidate signaling in Xenopus oocytes.

METHODS

Defolliculated Xenopus oocytes expressing endogenous G protein-coupled lysophosphatidate receptors were voltage clamped and studied in the presence or absence of lidocaine or bupivacaine. Lysophosphatidate-induced Ca(2+)-activated Cl- currents (IC1(Ca)) were measured. To determine the site of action of the local anesthetics on the signaling pathway, the authors studied 1) the effects of local anesthetics on signaling induced by intracellular injection of the second messenger inositoltrisphosphate, and 2) the effects of local anesthetics on functioning of recombinantly expressed angiotensin II receptor signaling through the same pathways as the lysophosphatidate receptor.

RESULTS

Lysophosphatidate signaling was inhibited in the presence of local anesthetics. The half maximal inhibitory concentration (IC50S) for lidocaine and bupivacaine were 29.6 mM and 4.7 mM, respectively. Neither responses induced by inositoltrisphosphate injection nor angiotensin signaling were influenced by local anesthetics.

CONCLUSIONS

Lysophosphatidate signaling is inhibited by the extracellular application of lidocaine or bupivacaine. In contrast, inositoltrisphosphate or angiotensin signaling was not affected by local anesthetics. Therefore local anesthetics have a specific, extracellular effect on lysophosphatidate receptor functioning. As the local anesthetic concentrations used were similar to those observed after injection around surgical wounds, LP inhibition may play a role in the observed detrimental effects of local anesthetics on wound healing.

Differential inhibition of lysophosphatidate signaling by volatile anesthetics

BACKGROUND

Volatile anesthetics have been found to interfere with the functioning of several G protein-coupled receptors, effects that may be relevant to the mechanism of anesthetic action. Lysophosphatidate (1-acyl-2-sn-glycero-3-phosphate; LP) is the simplest natural phospholipid. It has pronounced biological effects and signals through a specific G protein-coupled receptor. Because of its lipophilicity, the LP receptor is a feasible site of anesthetic interaction. Therefore, the authors investigated the effects of halothane and isoflurane on LP signaling using Xenopus oocytes.

METHODS

Mature oocytes were harvested from Xenopus frogs, isolated, and defolliculated manually. Lysophosphatidate receptors are endogenously present in these cells. Angiotensin receptors were expressed recombinantly to study anesthetic effects on intracellular signaling. Oocytes were studied individually with a two-electrode voltage clamp at room temperature. Integrated Ca(2+)-activated Cl- currents (ICl(Ca)) were used to evaluate the effects of anesthetics on changes in intracellular Ca2+ concentration in response to receptor agonists (10(-7) M LP or 10(-7) M angiotensin II) or intracellular inositoltrisphosphate (IP3) injection.

RESULTS

Halothane depressed LP signaling in a concentration-dependent manner, with half-maximal inhibition at 0.23 mM and virtually complete inhibition at 0.34 mM. Responses could be recovered after an anesthetic-free wash. Oocyte injection with heparin, an IP3 receptor antagonist, completely blocked LP and angiotensin signaling, indicating similar IP3- dependent pathways. However, ICl(Ca) induced by angiotensin receptor activation or intracellular IP3 injection were not inhibited by halothane. Isoflurane, at comparable concentrations, did not depress LP responses in oocytes significantly.

CONCLUSIONS

Lipid-mediator signaling can be affected profoundly by volatile anesthetics. At clinically relevant concentrations, halothane and isoflurane have different effects on LP signaling. The inhibitory effects of halothane on the LP signaling pathway occur before the IP3 receptor.

Local anesthetic administration for awake direct laryngoscopy. Are glossopharyngeal nerve blocks superior?

BACKGROUND

Glossopharyngeal nerve (GPN) blocks may provide reliable analgesia for awake direct laryngoscopy, although this has not been evaluated prospectively. This study was designed to determine if GPN blocks provide a superior route of local anesthetic administration for awake direct laryngoscopy as measured by hemodynamic, gag, and subjective pain responses.

METHODS

A prospective, randomized, single-blinded, crossover design was used. All participants (n = 11) were anesthesiologists. Three routes of local anesthetic administration were evaluated: 2 min of 2% viscous lidocaine swish and gargle (S&G); S&G combined with 10% lidocaine spray (S&G/spray); and S&G combined with 1% lidocaine bilateral GPN blocks (S&G/block; anterior tonsillar pillar method). Five minutes after the local anesthetic was administered, laryngoscopy was performed and sustained for 20 s. Noninvasive hemodynamic measurements and serum lidocaine concentrations were determined. Visual analogue scale scores and a poststudy questionnaire were used to assess participants' ability to tolerate local anesthetic administration and laryngoscopy and their choice for use in clinical practice.

RESULTS

No significant hemodynamic changes were observed, although there was a modest increase (< 15%) in heart rate in the S&G/block group in the first minute after laryngoscopy. Serum lidocaine concentrations were higher (P < 0.05) in the S&G/block group at 5 and 10 min (0.5 +/- 0.1 and 1.0 +/- 0.2 microgram/ml) compared with the S&G group. Participants' visual analogue scale scores, which assessed their ability to tolerate laryngoscopy, showed that S&G (5.4 +/- 0.9) resulted in more discomfort (P < 0.05) than either S&G/spray (3.5 +/- 0.9) or S&G/block (3.3 +/- 0.7). The laryngoscopist's visual analogue scale scores, which assessed the ease of visualization, revealed a trend (P < 0.08) toward less coughing and gagging with S&G/spray (1.8 +/- 0.9) compared with S&G (4.0 +/- 1.3) and S&G/block (3.7 +/- 1.1). Oropharyngeal discomfort lasting 24 h or more was reported by 91% of participants after S&G/block, whereas no participant reported oropharyngeal discomfort after S&G or S&G/spray. Significantly more participants (73%) indicated their preference for using S&G/spray in future clinical practice compared with S&G (P < 0.01) and S&G/block (P < 0.05).

CONCLUSIONS

Glossopharyngeal nerve blocks do not provide a superior route of local anesthetic administration for awake direct laryngoscopy. Two minutes of 2% viscous lidocaine S&G followed by 10% lidocaine spray was the anesthetic route preferred by participants and laryngoscopists.

Interactions between propofol and lipid mediator receptors: inhibition of lysophosphatidate signaling

As a highly lipophilic drug, propofol may interact with lipophilic domains in addition to its likely primary site of action on the gamma-aminobutyrateA) (GABA(A)) receptor. Likely candidates for such interaction are the G protein-coupled membrane receptors for lipid intercellular mediators. The phospholipid lysophosphatidate (LP) has attracted attention as such a signaling molecule. It has a variety of biological actions, including vasoconstriction. We therefore studied the interaction between propofol and the LP receptor. Intracellular Ca2+ release in response to LP was assessed by measuring C1- flux through Ca(2+)-activated C1- channels in Xenopus oocytes. The average charge movement in response to LP 10(-7)M was 2.0 +/- 0.2 microCoulombs. Propofol in Intralipid (0.01%) dose-dependently inhibited LP signaling (50% inhibitory concentration [IC50] 5.38 microM). Propofol 28 microM inhibited LP signaling by 81%. Intralipid (0.01%) was without effect. To ascertain that intracellular signaling pathways and the Ca(2+)-activated C1- channel were not affected by propofol, we tested the effects of propofol (5.6 microM) on currents induced by methylcholine (10(-7)M) in oocytes expressing the m1 muscarinic acetylcholine receptor. No inhibition was observed. As both receptors share the same intracellular signaling pathway, we conclude that clinically relevant concentrations of propofol most likely inhibit the LP receptor or its G protein. Inhibition of LP signaling may explain some of propofol's vasodilating actions.

Muscarinic signaling in the central nervous system. Recent developments and anesthetic implications

During the last decade, major advances have been made in our understanding of the physiology and pharmacology of CNS muscarinic signaling. It is time to emphasize that the well-known peripheral parasympathetic and cardiovascular actions represent only one component of muscarinic signaling. Interestingly, many new findings have the potential to influence the practice of anesthesiology. Inhibition of muscarinic signaling may explain some of the anesthetic state, and subtype-selective drugs may allow wider perioperative manipulation of CNS muscarinic systems. The next years will doubtlessly see progress in this area, and our specialty may well reap the benefits.

Inhibition by ketamine of muscarinic acetylcholine receptor function

Although ketamine's primary site of action appears to be the phencyclidine receptor on the N-methyl-D-aspartate (NMDA) receptor complex, additional activity on opiate and quisqualate receptors is suggested. Some phencyclidines have been shown to interact with muscarinic receptors, but this has not been determined for ketamine. We studied the interaction between ketamine and the m1 muscarinic receptor, the most prominent subtype in cortex and hippocampus. Receptors were expressed recombinantly in Xenopus oocytes, and intracellular Ca2+ release in response to the agonist acetyl-beta-methylcholine (MCh, 10(-6)M) was assessed by measuring charge movement through Ca(2+)-activated Cl- channels. Average responses to MCh were 4.1 +/- 0.7 microC. Ketamine inhibited responses to MCh, with complete inhibition at approximately 200 microM ketamine. The IC50 was 5.7 microM, (1.56 micrograms/mL), well within the clinically relevant concentration range. To demonstrate that intracellular signaling pathways and the Ca2+ activated Cl- channel were not affected by ketamine, we tested the effect of ketamine (365 microM) on currents induced by angiotensin II (10(-6) M) in oocytes expressing the AT1A angiotensin receptor. No inhibitory effect was noted. In summary, ketamine profoundly inhibits muscarinic signaling. This effect might explain some of the anticholinergic clinical effects of ketamine, both central (effects on memory and consciousness) and peripheral (prominent sympathetic tone, bronchodilation, mydriasis).

Halothane inhibits signaling through m1 muscarinic receptors expressed in Xenopus oocytes

BACKGROUND

Interactions between volatile anesthetics and muscarinic acetylcholine receptors have been studied primarily in binding assays or in functional systems derived from tissues or cells, often containing multiple receptor subtypes. Because interactions with muscarinic signaling systems may explain some effects and side effects of anesthetics and form a model for anesthetic-protein interactions in general, the author studied anesthetic inhibition of muscarinic signaling in an isolated system.

METHODS

mRNA encoding the m1 muscarinic receptor subtype was prepared in vitro and expressed in Xenopus oocytes. Effects of halothane on methylcholine-induced intracellular Ca2+ release was measured. Angiotensin II receptors were expressed to evaluate anesthetic effects on intracellular signaling.

RESULTS

m1 Receptors expressed in oocytes were functional, and could be inhibited by atropine and pirenzepine. Halothane depressed m1 muscarinic signaling in a dose-dependent manner: half-maximal inhibition of 10(-7) M methylcholine was obtained with 0.3 mM halothane. The effect was reversible and could be overcome by high concentrations of muscarinic agonist. Angiotensin II signaling was unaffected by 0.34 mM halothane.

CONCLUSIONS

m1 Muscarinic signaling is inhibited by halothane, and lack of halothane effect on angiotensin signaling indicates that the intracellular signaling systems of Xenopus oocytes are unaffected. Therefore, the most likely site of halothane action is the receptor and/or G protein. Oocytes provide a versatile system for detailed investigation into the molecular mechanism of anesthetic-protein interactions.

Trypsin induces Ca(2+)-activated Cl- currents in X. laevis oocytes

The protease trypsin induces Ca(2+)-activated Cl- currents when applied in concentrations as low as 0.1 mg/ml to defolliculated, voltage clamped X. laevis oocytes. The response is dose-dependent and specific, as other proteases (chymotrypsin, Lys-C and Arg-C), or trypsin pretreated with soybean trypsin inhibitor, did not induce currents. Intracellular trypsin injection did not induce responses. The current does not appear to result from proteolytic activation of the endogenous receptor for lysophosphatidic acid, the only known Ca(2+)-mobilizing receptor consistently present in oocytes. These results suggest the presence on the oocyte membrane of a specific receptor for trypsin.

OoClamp: an IBM-compatible software system for electrophysiologic receptor studies in Xenopus oocytes

A software system for IBM-compatible microcomputers running MS-DOS or Microsoft Windows 3.1 is described which facilitates the acquisition, analysis and storage of data from electrophysiologic studies of receptors expressed in Xenopus laevis oocytes. The system is designed to provide standardization of test conditions, automation of all routine functions, rapid, on-line analysis of data, and self-documentation and compact storage of data files. All system settings are optimized for use with the Xenopus expression system, but can be adapted to other large cells. An example application, expression of muscarinic acetylcholine receptors in Xenopus oocytes, is described.

Signalling properties of lysophosphatidic acid

Lysophosphatidic acid (LPA) is the simplest natural phospholipid, primarily known as a membrane component and metabolic intermediate. However, a remarkable variety of biological effects of this compound have come to light, seemingly pointing to an additional role for LPA as a signalling molecule. In this review, Marcel Durieux and Kevin Lynch integrate the recent information that indicates that LPA could be an intercellular messenger, possibly acting through a G protein-coupled receptor, and with a role in cell growth and motility.

Responses to sphingosine-1-phosphate in X. laevis oocytes: similarities with lysophosphatidic acid signaling

Sphingosine-1-phosphate (S1P, 50 microM) induces inward currents in Xenopus laevis oocytes voltage clamped at -70 mV. The currents are Ca(2+)-activated Cl- currents, as shown by a reversal potential of -20 mV and absence of the response after intracellular injection of ethylene glycolbis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA; 10 mM). The response is nearly indistinguishable from that to the related compound lysophosphatidic acid (LPA), and complete cross-desensitization occurs between LPA and S1P responses. Both the LPA and S1P responses are inhibited by suramin (2 mM) and dithiothreitol (5 mM). These responses appear mediated by a specific membrane receptor, since intracellular injection of S1P (5 microM) does not induce currents, and sphingosine and the related compounds sphingosylphosphorylcholine and N,N-dimethylsphingosine, all at 100 microM, neither induce currents nor block the response to S1P. HEK-293 and COS-1 cells respond with intracellular Ca2+ release to both 50 microM S1P and 10 microM LPA; K-562 cells do not. No cross-desensitization was noted in the responsive cells. Our findings indicate that S1P and LPA might act through the same mechanism, probably a membrane receptor.

The influence of hemorrhage on organ perfusion during deliberate hypotension in rats

There is general concern that major blood loss during deliberate hypotension could produce severe organ ischemia, but documentation of the magnitude of this response remains obscure. To examine this response, we studied 43 male Sprague-Dawley rats that were divided into seven groups: the control animals received 1 MAC (1.4%) isoflurane only; the hypotensive animals received a 1.4% isoflurane baseline anesthetic and were then rendered hypotensive by either increasing the isoflurane concentration (dISO), or by adding sodium nitroprusside (SNP), or 2-chloroadenosine (2AD) to the baseline anesthetic, decreasing the MAP to 51 mmHg; hemorrhaged animals had hypotension produced in the same manner as for the hypotensive animals, but additionally were bled 20% of estimated blood volume during deliberate hypotension produced with either deep isoflurane (dISOH), sodium nitroprusside (SNPH), or 2-chloroadenosine (2ADH). After a 25-min period of hypotension, or hypotension plus hemorrhage, cardiac output and blood flow to brain, heart, gastrointestinal tract, kidney, and liver were measured with 141Ce-labelled 15-microns microspheres. Hypotension was associated with decreased blood flow to the kidneys in all groups and to the liver in the 2AD group and an increased blood flow to the heart in the SNP and 2AD groups. Hemorrhage decreased blood flow during deliberate hypotension to the brain and the gastrointestinal tract in the dISOH and 2ADH groups and to the liver in the dISOH group. Our results suggest that hemorrhage during deliberate hypotension with dISO or isoflurane plus 2AD may be associated with compromised organ blood flow, whereas blood flow to vital organs is maintained after 20% hemorrhage during isoflurane and superimposed SNP-induced hypotension.

Lysophosphatidic acid induces a pertussis toxin-sensitive Ca(2+)-activated Cl- current in Xenopus laevis oocytes

Lysophosphatidic acid (LPA) induces a Ca(2+)-activated Cl- current in defolliculated Xenopus laevis oocytes. The response appears mediated by a specific membrane receptor, because no current is induced when related compounds [phosphatidic acid (PA), lysophosphatidylcholine (LPC), and lysophosphatidylserine (LPS)] are applied extracellularly or when LPA is injected intracellularly. Incubation in pertussis toxin prevents the response. The response is mediated by a Ca(2+)-activated Cl- current because 1) it is abolished by intracellular ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid (EGTA; 5 mM) but not affected by changes in extracellular Ca2+ concentration and 2) the reversal potential becomes more positive at lower Cl- concentrations. Suramin (2 mM) blocks the LPA-induced current, but PA, LPS, LPC, and the platelet-activating factor antagonist WEB-2086 do not. The response is dose dependent for LPA concentrations from 10(-8) to 10(-3) M. Incubation of oocytes in LPA does not induce germinal vesicle breakdown. These findings suggest that this novel oocyte response to LPA is mediated by a specific membrane receptor linked to a pertussis toxin-sensitive G protein.

Molecular cloning and expression of a cDNA encoding endothelial cell nitric oxide synthase

Endothelium-derived relaxing factor (EDRF), identified as nitric oxide (NO), is derived from a guanidino nitrogen of L-arginine via its metabolism by nitric oxide synthase (NOS). Herein, we report the molecular cloning of a cDNA encoding the constitutive calcium-calmodulin (Ca2+/CaM)-regulated nitric oxide synthase (ECNOS). A full-length ECNOS clone was isolated by screening a bovine aortic endothelial cell cDNA library using a fragment of rat brain NOS (bNOS) cDNA. This cDNA has an open reading frame of 3615 nucleotides encoding a 1205-amino acid protein. Membranes prepared from COS cells transfected with the ECNOS cDNA demonstrated NADPH- and Ca2+/CaM- dependent conversion of L-, but not D-, arginine to NO and citrulline that was inhibited by NG-nitro-L-arginine methyl ester. Comparison of the deduced amino acid sequence of ECNOS to the bNOS and macrophage NOS (Mac-NOS) sequences revealed 57 and 50% identity, respectively. In addition, ECNOS contains a unique N-myristylation consensus sequence (not shared by bNOS or Mac-NOS) that may explain its membrane localization.

Phospholemman expression induces a hyperpolarization-activated chloride current in Xenopus oocytes

A new type of chloride channel has been identified by functional expression of phospholemman, a 72-amino acid cardiac sarcolemmal protein with a single transmembrane domain. Xenopus oocytes injected with phospholemman RNA developed a chloride-selective current, which was activated by hyperpolarizing pulses. The current activated very slowly with a pronounced sigmoidal delay, did not inactivate, and increased in amplitude with trains of pulses, depolarized holding potentials, and low extracellular pH. Point mutations within the single transmembrane region abolished the sigmoidal delay of expressed currents. Phospholemman appears to be the smallest plasma membrane channel protein yet known. The structure is dissimilar to any chloride channel described thus far.

Effects of hypoxemia on regional blood flows during anesthesia with halothane, enflurane, or isoflurane

Hypoxemia during anesthesia can cause severe morbidity and mortality. To determine how the volatile anesthetics alter the normal hemodynamic compensation for hypoxemia, we investigated the effects of various anesthetics on regional blood flows during normoxemia and during normocapnic hypoxemia (FIO2 0.12 for 20 min) in rats. Using the radioactive microsphere method, organ blood flows were determined in animals anesthetized with 1 MAC of halothane, enflurane, or isoflurane and in awake animals. Brain blood flow increased significantly with hypoxemia in awake animals. However, brain blood flow decreased in all anesthetized animals that were hypoxemic. Coronary blood flow also increased significantly with hypoxemia in awake animals. In the presence of volatile anesthetics, coronary blood flow decreased, a decrease that was unchanged with hypoxemia. Thus, there was a large difference in brain and coronary blood flows between awake hypoxemic and anesthetized hypoxemic animals. Hypoxemia did not alter the magnitude of renal, gastrointestinal tract, or total hepatic blood flows in awake animals. However, all three blood flows decreased significantly in anesthetized hypoxemic animals. We conclude that volatile anesthetics modify the compensatory responses to hypoxemia that occur in awake animals, resulting in decreased blood flow to vital organs.

The hemodynamic response to isoflurane is altered in genetically hypertensive (SHR), as compared with normotensive (WKY), rats

The authors compared the hemodynamic effects of isoflurane anesthesia in normotensive (WKY) and genetically hypertensive (SHR) rats. Eighteen male SHR and 18 WKY rats were subdivided into conscious animals and those anesthetized with isoflurane, 1.2 vol% inspired. During brief isoflurane anesthesia, cannulae were placed in the left cardiac ventricle, the femoral artery, and the femoral vein. Central and regional hemodynamics were determined with 85Sr-labeled microspheres (15 +/- 1 micron) using the reference sample technique in both conscious and anesthetized animals. Isoflurane anesthesia caused similar reductions in mean arterial blood pressure (MAP) in all rats. This was due to a significant decrease in systemic vascular resistance in WKY rats, whereas MAP declined due to a significant decrease in cardiac output in SHR rats. In the anesthetized WKY rat, the decrease in total systemic vascular resistance resulted from significant decreases in vascular resistance of the brain and nonrespiratory skeletal muscles. In the anesthetized SHR rat, both decreases (cerebellum, hepatic artery) and increases (GI tract, skin, diaphragm) in regional vascular resistances occurred, resulting in no net change in total systemic vascular resistance. In both SHR and WKY rats, isoflurane redistributed blood flow in favor of the brain at the expense of blood flow to the GI tract, diaphragm, and skin. Blood flows to the liver, GI tract, and skin were significantly less in the anesthetized SHR as compared with WKY rats. It is concluded that isoflurane influences central and regional hemodynamics differently in hypertensive, as compared with normotensive, rats.

Saralasin dilates arterioles in SHR but not WKY rats

Microvascular responses to topical or intravascular saralasin were determined in the cremaster muscle arterioles of adult spontaneously hypertensive rats (SHR, n = 19) and Wistar-Kyoto (WKY, n = 16) normotensive rats. Animals were anesthetized with chloralose and urethane, and they breathed room air spontaneously. Mean arterial pressure was obtained from a catheter in a carotid artery, and microvascular diameters were determined by video microscopy. Plasma renin activity was measured in animals that were treated identically except that saralasin was not administered. For all animals, mean arterial pressure averaged 126 +/- 4 mm Hg in SHR and 82 +/- 4 mm Hg (p less than 0.001) in WKY rats. Topical saralasin, 10(-6)M, was applied to the cremaster muscles of SHR (n = 9) or WKY (n = 8) rats while internal diameters of first-through fourth-order arterioles (A1, A2, A3, A4) were measured. Topical saralasin did not alter arteriolar diameters (A1 through A4) in WKY rats, but A3 and A4 vessels dilated significantly (29% +/- 5% and 38% +/- 7% respectively; p less than 0.01) in SHR. Fourth-order diameters were measured in other SHR (n = 10) and WKY (n = 8) rats while saralasin was administered intraarterially (300 micrograms bolus into the hypogastric artery) or intravenously (10 micrograms/kg/min for 30 minutes). Intraarterial or intravenous saralasin caused significant dilation (32% +/- 12% and 20% +/- 4%, respectively; p less than 0.01) of A4 arterioles in SHR, but no dilation occurred in the arterioles of WKY rats. Arteriolar responses were significantly different (p less than 0.001) in SHR vs WKY rats for both the topical and the intravascular administration of saralasin.(ABSTRACT TRUNCATED AT 250 WORDS)