G-protein beta gamma-subunits activate the cardiac muscarinic K+-channel via phospholipase A2

Interactions of bradykinin, calcium, G-protein and protein kinase in the activation of phospholipase A2 in bovine pulmonary artery endothelial cells

Rise in free cytosolic calcium concentrations [Ca2+]i in response to bradykinin and guanosine 5'-O-thiotriphosphate (GTP tau S) was related to the action of phospholipase A/ (arachidonic acid release). At 900 microM extracellular CaCl2, bradykinin induced a typical Ca2+ movement consisting of an initial [Ca2+]i peak at approximately 400 nM followed by a sustained increase in the steady-state cytosolic Ca2+ level at approximately 290 nM. As the extracellular CaCl2 concentration was reduced to 100 microM, the bradykinin induced initial spike was reduced followed by only a marginal increase in steady-state cytosolic Ca2+ levels. Treatment of endothelial cells with saponin (0.002% w/w) did not increase [Ca2+]i and saponin treated cells exhibited a very similar pattern of Ca2+ mobilization in response to bradykinin. However, with saponin treatment, GTP tau S (100 microM) increased [Ca2+]i at an almost identical tracing exhibited with 50 nM bradykinin stimulation (in either the presence or absence of 0.002% saponin). No additive increase in [Ca2+]i was observed in cells stimulated with both 100 microM GTP tau S and 50 nM bradykinin or in bradykinin stimulated cells subsequently exposed to GTP tau S. Pertussis toxin (PTX) did not affect the bradykinin induced Ca2+ mobilization. However, as we showed previously, PTX inhibited bradykinin stimulated arachidonic acid release. These results indicate transduction of the bradykinin signal by G-protein for both phospholipase A2 (PLA2) activation and Ca2+ mobilization but likely by different G alpha subunits, a PTX sensitive and an insensitive subunit. Furthermore, the bradykinin and GTP tau S stimulated release of arachidonic acid appears to be only partially dependent on [Ca2+]i.(ABSTRACT TRUNCATED AT 250 WORDS)

Whole-cell recordings of inwardly rectifying K+ currents activated by 5-HT1A receptors on dorsal raphe neurones of the adult rat

1. An inwardly rectifying K+ current activated by serotonin (5-HT) was recorded from acutely isolated adult dorsal raphe (DR) neurones using the whole-cell recording mode of the patch clamp technique. 2. The 5-HT-induced K+ current (I5-HT) was only visible at an [K+]0 > 5 mM and it was observed in 69% of the cells. 3. The reversal potential for I5-HT was close to the potassium equilibrium potential and was shifted by 51 mV per 10-fold change in [K+]0 indicating that I5-HT was carried predominantly by K+. The chord conductance of I5-HT at -90 mV was proportional to the external [K+] raised to a fractional power. 4. A dose-response relationship revealed that I5-HT was activated with an ED50 of 30 nM. Ba2+ (0.1 mM) blocked I5-HT completely. Spiperone reversibly antagonized the response to 5-HT and 8-OHDPAT (8-hydroxy-2-(di-n-propylamino)tetralin) mimicked the response indicating that the receptor activated was of the 5-HT1A subtype. 5. The response to 5-HT was largely prevented by in vitro pretreatment of the cells with pertussis toxin (PTX) indicating the involvement of a PTX-sensitive G-protein in the transduction mechanism. 6. cAMP and lipoxygenase metabolites, both implicated in the modulation of similar currents in other preparations, were found not to alter the effectiveness of 5-HT. 7. Glibenclamide and tolbutamide, blockers of the ATP-regulated K+ channel, did not reduce the effect of 5-HT in DR neurones. 8. These results show that in acutely isolated adult DR neurones 5-HT activates an inwardly rectifying K+ current and this involves a PTX-sensitive G-protein in the transduction pathway which may interact with the K+ channel directly.

Modulatory action of arachidonic acid on GABAA/chloride channel receptor function in adult and aged brain cortex membranes

Effect of arachidonic acid on binding parameters of two binding sites on the GABAA receptor and on GABA activated Cl- uptake was investigated in synaptic plasma membrane and in synaptoneurosomes from brain cortex of adult (4-months old) and aged (27-months old) rats. The ligands used were [3H]muscimol, a GABA agonist and [35S]-t-butylbicyclophosphorothionate ([35S]TBPS), a convulsant that binds to the site near the chloride channel. Arachidonic acid increases significantly GABAA agonist binding and concomitantly decreases [35S]TBPS binding in a concentration dependent manner. The analysis of binding parameters in adult brain showed a significant decrease by AA of KD value for low but not for high affinity of [3H]muscimol binding. Concomitantly, AA enhances Bmax values for high affinity binding and has no effect on Bmax value for low affinity binding in synaptic plasma membrane (SPM) from adult brain. In synaptic plasma membrane from aged brain AA increases low and high affinity binding of agonist to GABAA receptor, modified significantly KD value by about 30 and 66%, respectively. On the other hand, AA significantly decreases of [35S]TBPS binding to chloride channel recognition site. Scatchard's analysis indicates that this inhibition results from a decrease of total number of binding sites. Moreover, the affinity of [35S]TBPS binding was increased (KD = 37.0 nM for AA-treated membrane, as compared to 69.3 nM KD in control membrane). GABA-dependent chloride uptake into synaptoneurosomes is also inhibited by AA in a concentration dependent manner in adult brain. In aged brain synaptoneurosomes AA has similar inhibitory effect on GABA-activated chloride uptake.(ABSTRACT TRUNCATED AT 250 WORDS)

Expression of an atrial G-protein-activated potassium channel in Xenopus oocytes

Injection of rat atrial RNA into Xenopus oocytes resulted in the expression of guanine nucleotide binding (G) protein-activated K+ channel. Current through the channel could be activated by acetylcholine or, if RNA encoding a neuronal 5HT1A receptor was coinjected with atrial RNA, by serotonin (5HT). A 5HT-evoked current (I5HT) was observed in oocytes injected with ventricle RNA fractions (of 2.5-5.5 kb) and 5HT1A receptor RNA. I5HT displayed strong inward rectification with very little conductance above the K+ equilibrium potential, was highly selective for K+ over Na+, and was blocked by 5-300 microM Ba2+. I5HT was suppressed by intracellular injection of the nonhydrolyzable analog of GDP, guanosine 5'-[beta-thio]diphosphate, but not by treatment with pertussis toxin (PTX), suggesting coupling of the receptor to the G-protein-activated K+ channel via a PTX-insensitive G protein, possibly endogenously present in the oocyte. Coexpression of the alpha subunit of a PTX-sensitive G protein, G(i2), rendered I5HT sensitive to PTX inhibition. Native oocytes displayed a constitutively active inwardly rectifying K+ current with a lower sensitivity to Ba2+ block; expression of a similar current was also directed by atrial or ventricle RNA of 1.5-3 kb. Xenopus oocytes may be employed for cloning of the G-protein-activated K+ channel cDNA and for studying the coupling between this channel and G proteins.

Regulation of the electrogenic H+ channel in the plasma membrane of neutrophils: possible role of phospholipase A2, internal and external protons

Possible factors regulating the opening of and the rate of H+ flux through a recently described, Cd(2+)-sensitive, phorbol ester- and arachidonic acid (AA)-activatable H(+)-conducting pathway in the plasma membrane of neutrophil granulocytes were investigated. (1) The phospholipase A2 blocker p-bromophenacyl bromide (BPB) inhibited the phorbol 12-myristate 13-acetate (PMA)-induced activation of this channel in a concentration-dependent manner (IC50, 4 microM). (2) Neither BPB nor the protein kinase C (PKC) inhibitor staurosporine influenced the AA-elicited stimulation of this route. (3) Intracellular acidification (cytoplasmic pH below 6.9) itself is capable of activating an electrogenic, Cd(2+)-sensitive H+ efflux indicating that protons can open up this route in the absence of any other stimulator. (4) PMA significantly decreases the intracellular H+ concentration ([H+]i) threshold for the opening of the channel, thus providing a conductive state at resting pH values, and elevates the rate of H+ efflux at any [H+]i. (5) Changes in external pH also modify the operation of the channel: above an extracellular pH (pH(o)) value of 7.4, the H(+)-flux/driving force relationship is approx. 5-fold greater than below this value. Our results suggest a multifactorial regulation of the electrogenic H+ channel: most probably PKC activates the channel indirectly, via stimulation of phospholipase A2 that subsequently liberates AA. In addition to this, the channel conductance seems to be promoted by internal H+ and inhibited by external H+.

Muscarinic receptors--characterization, coupling and function

At least five muscarinic receptor genes have been cloned and expressed. Muscarinic receptors act via activation of G proteins: m1, m3 and m5 muscarinic receptors couple to stimulate phospholipase C, while m2 and m4 muscarinic receptors inhibit adenylyl cyclase. This review describes the localization, pharmacology and function of the five muscarinic receptor subtypes. The actions of muscarinic receptors on the heart, smooth muscle, glands and on neurons (both presynaptic and postsynaptic) in the autonomic nervous system and the central nervous system are analyzed in terms of subtypes, biochemical mechanisms and effects on ion channels, including K+ channels and Ca2+ channels.

Oocyte maturation in starfish is mediated by the beta gamma-subunit complex of a G-protein

The stimulation of meiotic maturation of starfish oocytes by the hormone 1-methyladenine is mimicked by injection of beta gamma subunits of G-proteins from either retina or brain. Conversely, the hormone response is inhibited by injection of the GDP-bound forms of alpha i1 or alpha t subunits, or by injection of phosducin; all of these proteins should bind free beta gamma. alpha-subunit forms with reduced affinity for beta gamma (alpha i1 or alpha t bound to hydrolysis-resistant GTP analogs, or alpha i1-GMPPCP treated with trypsin to remove the amino terminus of the protein) are less effective inhibitors of 1-methyladenine action. These results indicate that the beta gamma subunit of a G-protein mediates 1-methyladenine stimulation of oocyte maturation.

Protein intake affects levels of G-protein subunits G alpha i2, G alpha i3, and G beta in rat glomerular membranes

Using toxin-catalyzed ADP-ribosylation and specific immunoblots we examined whether the mass of G-protein subunits, G alpha s, G alpha i (includes G alpha i2, G alpha i3, and G alpha 0) and G alpha beta, in glomerular membranes was altered by dietary protein intake. ADP-ribosylation catalyzed by cholera toxin (CT) or pertussis toxin (PT) detected significant amounts of G alpha s or G alpha i in glomerular membranes from rats fed a low (6% casein) or a high (40% casein) protein diet. There was no significant difference in G alpha s content between glomerular membranes from low or high protein-fed rats. However, the amounts of G alpha i were significantly lower in glomerular membranes from rats fed a high protein diet when compared to glomerular membranes from rats fed a low protein diet. Two isoforms of immunoreactive G alpha s, 45 and 52 kDa proteins, were detected in glomerular membranes. The predominant isoform of G alpha s was a 52 kDa protein. As with ADP-ribosylation, immunoblots showed no significant difference in G alpha s content between glomerular membranes obtained from the two diet groups of rats. Also, immunoreactive G alpha i2, G alpha i3 and G beta were present in glomerular membranes. The mass of G alpha i2 and G alpha i3 was significantly lower in glomerular membranes of rats fed a high protein diet than in those of rats fed a low-protein diet. The decreased mass of total G alpha i, that is G alpha i2 and G alpha i3, was comparable to that seen with PT-catalyzed ADP-ribosylation.(ABSTRACT TRUNCATED AT 250 WORDS)

Leukotriene D4 is a mediator of proteinuria and glomerular hemodynamic abnormalities in passive Heymann nephritis

We assessed the role of leukotrienes (LTs) in Munich-Wistar rats with passive Heymann nephritis (PHN), an animal model of human membranous nephropathy. 10 d after injection of anti-Fx1A antibody, urinary protein excretion rate (Upr) in PHN was significantly higher than that of control. Micropuncture studies demonstrated reduced single nephron plasma flow and glomerular filtration rates, increased transcapillary hydraulic pressure difference, pre- and postglomerular resistances, and decreased ultrafiltration coefficient in PHN rats. Glomerular LTB4 generation from PHN rats was increased. Administration of the 5-LO activating protein inhibitor MK886 for 10 d markedly blunted proteinuria and normalized glomerular hemodynamic abnormalities in PHN rats. An LTD4 receptor antagonist SK&F 104353 led to an immediate reduction in Upr and to reversal of glomerular hemodynamic impairment. Ia(+) cells/glomerulus were increased in PHN rats. In x-irradiated PHN rats, which developed glomerular macrophage depletion, augmented glomerular LT synthesis was abolished. Thus, in the autologous phase of PHN, LTD4 mediates glomerular hemodynamic abnormalities and a hemodynamic component of the accompanying proteinuria. The synthesis of LTD4 likely occurs directly from macrophages or from macrophage-derived LTA4, through LTC4 synthase in glomerular cells.

Mechanical stimulation of skeletal muscle generates lipid-related second messengers by phospholipase activation

Repetitive mechanical stimulation of cultured avian skeletal muscle increases the synthesis of prostaglandins (PG) E2 and F2 alpha which regulate protein turnover rates and muscle cell growth. These stretch-induced PG increases are reduced in low extracellular calcium medium and by specific phospholipase inhibitors. Mechanical stimulation increases the breakdown rate of 3H-arachidonic acid labelled phospholipids, releasing free 3H-arachidonic acid, the rate-limiting precursor of PG synthesis. Mechanical stimulation also increases 3H-arachidonic acid labelled diacylglycerol formation and intracellular levels of inositol phosphates from myo-[2-3H]inositol labelled phospholipids. Phospholipase A2 (PLA2), phosphatidylinositol-specific phospholipase C (PLC), and phospholipase D (PLD) are all activated by stretch. The stretch-induced increases in PG production, 3H-arachidonic acid labelled phospholipid breakdown, and 3H-arachidonic acid labelled diacylglycerol formation occur independently of cellular electrical activity (tetrodotoxin insensitive) whereas the formation of inositol phosphates from myo-[2-3H]inositol labelled phospholipids is dependent on cellular electrical activity. These results indicate that mechanical stimulation increases the lipid-related second messengers arachidonic acid, diacylglycerol, and PG through activation of specific phospholipases such as PLA2 and PLD, but not by activation of phosphatidylinositol-specific PLC.

Bilateral ureteral obstruction alters levels of the G-protein subunits G alpha s and G alpha q/11

To evaluate the effects of bilateral ureteral obstruction (BUO) on the levels of G-protein subunits in glomeruli, we examined the types and amounts of G-protein subunits in glomerular membranes from sham-operated control (SOC) rats and rats with BUO of 24 hours duration utilizing bacterial toxin-catalyzed ADP-ribosylation and specific antibodies. ADP-ribosylation catalyzed by cholera or pertussis toxin demonstrated the presence of Gs and Gi proteins in glomerular membranes. Immunoblots further revealed the existence of two types of G alpha s (45 and 52 kDa), as well as G alpha i2 (40 kDa), G alpha i3 (41 kDa), G alpha q/11 (42 kDa) and G beta (35 to 36 kDa) in glomerular membranes. The predominant subspecies of G alpha s was the 52 kDa protein. Detectable amounts of G alpha o were not found in glomerular membranes. Moreover, G-protein subunits were not detected in cytosolic extracts of glomeruli. Both forms of G alpha s and G alpha q/11 were significantly reduced in glomerular membranes from rats with BUO when compared to SOC rats. No significant difference in total G alpha i, G alpha i2 and G alpha i3 and G beta content was observed between the two groups of rats. In vivo pretreatment of rats with simultaneous administration of the angiotensin-converting enzyme inhibitor, enalaprilat, and the thromboxane synthase inhibitor, OKY-046, maintained the amount of G alpha s and G alpha q/11 in rats with BUO at the levels seen in SOC rats. The two drugs did not affect the amounts of G-protein subunits in glomerular membranes of SOC rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Synthesis and characterization of fluorescently labeled bovine brain G protein subunits

G proteins play an important role in transmitting hormonal signals, and fluorescence techniques would be useful to study their cellular distribution and mechanisms. To prepare active fluorescent G protein Go/Gi or beta gamma subunits were reacted with fluorescein isothiocyanate (FITC) to label the alpha (F-alpha) and gamma (F-gamma/beta) subunits or with (iodoacetamido)tetramethylrhodamine (TMR-IAA) to label the beta subunit (TMR-beta gamma). Unreacted dye was removed from the labeled proteins by ultrafiltration, followed by further purification using HPLC gel filtration. The molar ratios of dye to protein were 0.96 +/- 0.15, 0.59 +/- 0.07, and 1.37 +/- 0.09 for labeled alpha,beta, and gamma subunits, respectively. GTP gamma S binding to F-alpha and ADP-ribosylation by pertussis toxin of F-alpha were reduced to 63% and 78% of control, respectively. F-alpha was a heterogeneous population of alpha subunits. Active F-alpha containing less than one (0.7) label/subunit (F-alpha-Mono Q) was separated from unlabeled and multiply labeled F-alpha by Mono Q anion-exchange chromatography. F-alpha-Mono Q displayed reduced GTPase activity (turnover number was 46% of control), while GTP gamma S binding and ADP-ribosylation by pertussis toxin were only decreased to 78% and 82% of control, respectively. TMR-beta gamma and F-gamma/beta retain full function compared to native beta gamma, as measured by three methods: (1) TMR-beta gamma and F-gamma/beta are able to form heterotrimers with alpha o subunits, (2) TMR-beta gamma and F-gamma/beta support the ADP ribosylation of alpha o subunits by pertussis toxin, and (3) TMR-beta gamma and F-gamma/beta inhibit forskolin-stimulated adenylyl cyclase activity. The fluorescent G protein subunits will be valuable tools to study G protein mechanisms in reconstituted membranes and intact cells.

Evidence that endogenous generation of leukotrienes does not regulate proliferation of malignant hemopoietic cell lines

The proliferation of malignant hemopoietic cell lines is inhibited by antagonists of 5-lipoxygenase, suggesting that the endogenous generation of leukotrienes via the action of this enzyme may play some role in the proliferation of these cells (Snyder D. S., Castro R. & Desforges J. F. (1989), Expl Hemat. 17, 6). Here we have confirmed that the lipoxygenase inhibitors piriprost, nordihydroguiaretic acid and BW755C decreased DNA synthesis and proliferation of leukemic cell lines. However, the concentrations of these drugs required for half-maximal inhibition of proliferation were significantly greater than their IC50 values for 5-lipoxygenase inhibition. We therefore studied the actions of two novel, potent lipoxygenase inhibitors, BWA4C and MK886, on proliferation (as measured by estimating the number of viable, trypan blue-excluding cells) and DNA synthesis (measured by the incorporation of radiolabeled thymidine) in the leukemia cell lines HL60, K562 and Jurkat. Neither parameter was affected by concentrations of these drugs which were shown in parallel studies to substantially inhibit leukotriene generation in whole blood. The data show that endogenous leukotriene generation does not play a significant role in the regulation of proliferation of these leukemic cell lines and suggest that conclusions about leukotriene involvement in the control of cellular metabolic pathways based on the use of lipoxygenase inhibitors should be re-assessed.

Guanine nucleotide binding proteins mediate D2 dopamine receptor activation of a potassium channel in rat lactotrophs

1. The involvement of guanine nucleotide binding proteins in the coupling of D2 dopamine (DA) receptors to single potassium channels was examined in rat pituitary lactotrophs. 2. Lactotrophs were unambiguously identified by the reverse haemolytic plaque assay (RHPA) and membrane potentials, whole-cell and single channel currents recorded using patch electrode methods. 3. DA or the D2 selective agonist, quinpirole, induced the opening of single K+ channels in cell-attached patches underlying robust hyperpolarizations of membrane potential in single cells. 4. Both whole-cell and single channel responses were independent of Ca2+ or cAMP concentrations. 5. Pertussis toxin (PTX) pretreatment (50-250 ng/ml, 6-12 h) blocked the action of DA on lactotroph membrane potential and uncoupled D2 receptors from single K+ channels in cell-attached patches. 6. Internal dialysis with GDP beta S (guanosine 5'-O-(2-thiodiphosphate) greatly reduced whole-cell responses to DA in a dose-dependent manner. 7. Internal dialysis of lactotrophs with GTP gamma S (guanosine 5'-O-(3-thiotriphosphate) potentiated DA responses in a dose-dependent manner while rendering the responses irreversible at higher doses. 8. DA (100 nM) or quinpirole (10 microM) activated K+ channels in excised outside-out membrane patches that were identical to those identified in cell-attached patches in terms of conductance and gating kinetics. 9. It is proposed that D2 receptors are coupled to non-voltage-dependent K+ channels by G proteins of the Gi/Go class and that this coupling is via a direct, membrane delimited pathway.

Selectivity in signal transduction determined by gamma subunits of heterotrimeric G proteins

Various heterotrimeric guanine nucleotide-binding proteins have been identified on the basis of the individual subtypes of their alpha subunits. The beta gamma complexes, composed of beta and gamma subunits, remain tightly associated under physiological conditions and have been assumed to constitute a common pool shared among various guanosine triphosphate (GTP)-binding (G) protein heterotrimers. Particular alpha and beta subunit subtypes participate in the signal transduction processes between somatostatin or muscarinic receptors and the voltage-sensitive L-type calcium channel in rat pituitary GH3 cells. Among gamma subunits the gamma 3 subtype was found to be required for coupling of the somatostatin receptor to voltage-sensitive calcium channels, whereas the gamma 4 subtype was found to be required for coupling of the muscarinic receptor to those channels.

Activation of muscarinic K+ current in guinea-pig atrial myocytes by a serum factor

1. Atrial myocytes obtained by enzymatic perfusion of hearts from adult guinea-pigs and cultured for 0-14 days were studied using the whole-cell voltage-clamp technique. 2. Superfusion of the myocytes with diluted sera (1:100 to 1:10,000) from different species (human, horse, guinea-pig) evoked an inward rectifying K+ current. The voltage-dependent properties of this current were identical to those of the K+ current activated by acetylcholine (IK(ACh)). Current density in the presence of horse serum (1:100) approximately corresponded to the non-desensitizing fraction of IK(ACh) during superfusion with 1-2 x 10(-6) M ACh. 3. During a maximal serum-evoked current, application of ACh (10(-6) M) failed to evoke additional K+ current. After switching superfusion from serum-containing to serum-free solution, the K+ current decayed 1-2 orders of magnitude slower than ACh-activated IK(ACh). During the decay of the serum-evoked current, a proportional increase in responsiveness to ACh was recorded. During submaximal activation of K+ current by serum, a saturating concentration of ACh resulted in a total current that was identical to the current evoked by ACh alone minus the desensitizing component. Thus, activation of K+ current by serum caused desensitization of IK(ACh). From these results it is concluded that sera contain a factor that activates the same population of K+ channels as ACh. 4. Irreversible activation of IK(ACh) by ACh in myocytes dialysed with the GTP-analogue GTP-gamma-S abolished sensitivity to serum and vice versa. 5. The effect of serum was not modified by atropine (10(-6) M) which completely blocked the response to 2 x 10(-6) M ACh. Furthermore, theophylline (1 mM), which completely inhibited IK(ACh) activation by adenosine (100 microM), failed to inhibit the effect of serum. Thus, neither muscarinic nor purinergic (A1) receptors are involved. 6. The peptide somatostatin (10(-6) M) and the alpha 1-agonist phenylephrine (1 microM) which previously have been shown to cause activation of IK(ACh) channels, in the present study failed to evoke any measurable current, which excludes the involvement of the corresponding receptors. 7. Pre-incubation of the cells with pertussis toxin completely abolished IK(ACh) evoked by ACh, adenosine and serum, suggesting that the activating factor, like the classical agonists, causes opening of IK(ACh) channels via a G protein (Gi, GK). 8. The potency of serum to activate IK(ACh) was not reduced by dialysis, suggesting the molecular mass of the unknown factor to be > or = 5 kDa. No activating potency was found in the dialysing solutions.(ABSTRACT TRUNCATED AT 400 WORDS)

Modulation of Ca2+ cycling in cardiac myocytes by arachidonic acid

It is believed that inotropic agents exert their effects in cardiac muscle via a modulation of Ca2+ cycling; however, the involvement of phospholipase activation and the biochemical pathways participating in inotropic responsiveness remain unclear. The aim of the current study was to determine whether arachidonic acid and/or eicosanoids participate in inotropic responses by modulating Ca2+ cycling in cardiac myocytes. Experiments were performed with populations of freshly isolated, fura-2-loaded adult rat ventricular myocytes. Arachidonic acid stimulated a transient increase in cytosolic free Ca2+, which was still present after addition of EGTA but was significantly reduced by pretreatment with caffeine. Addition of arachidonic acid to either electrically stimulated or quiescent myocytes enhanced the amplitude of the ATP-induced Ca2+ transient. This effect was still observed in the presence of inhibitors of cyclooxygenase, lipoxygenase, and epoxygenase pathways but was significantly diminished after pretreatment with inhibitors of protein kinase C. In contrast, arachidonic acid attenuated the amplitude of electrically induced Ca2+ transients. This effect was mimicked by eicosatetraynoic acid and by the K+ channel agonist pinacidil. The inhibitory effect of eicosatetraynoic acid and arachidonic acid was reversed by addition of fatty acid-free bovine serum albumin. Together, these results suggest that arachidonic acid may play a physiological role in cardiac muscle excitation-contraction coupling as a modulator of sarcolemmal ion channels and/or Ca2+ release from the sarcoplasmic reticulum.

G protein activation inhibits amiloride-blockable highly selective sodium channels in A6 cells

Single-channel methods were used to examine the regulation of amiloride-blockable highly selective sodium channels by guanine nucleotide-binding proteins (G proteins). A6 cells (a renal cell line derived from Xenopus laevis kidney) were cultured on permeable collagen films, and patch recordings were made from the apical membranes of confluent cells. The predominant channel in the apical membranes is a highly selective, 4-pS, amiloride-blockable sodium channel (the Na(+)-to-K+ permeability ratio is > 30). In inside-out patches, application to the cytosolic surface of guanosine-5'-O-(2-thiodiphosphate) (GDP beta S), which deactivates G proteins, increased sodium channel activity. GDP beta S produced a sevenfold increase in channel activity. In contrast, GTP and guanosine-5'-O-(3-thiotriphosphate) (GTP gamma S) decreased sodium channel activity to about one-twentieth of the untreated value. The effect of GTP (but not GTP gamma S) was reversible. In cell-attached patches, a 3- to 4-h exposure of the apical membrane to pertussis toxin (PTX) increased the mean open time of sodium channels approximately 2.7 times and the open probability approximately 1.6-fold, but pretreatment of apical membranes with cholera toxin (250 ng/ml) for 3-4 h had no effect on open probability or mean open time. These results imply that a PTX-sensitive G protein regulates amiloride-blockable highly selective sodium channels in the apical membranes of A6 cells and that the G protein in a GTP-bound, activated state inhibits sodium channel activity.

Agonist receptors and G proteins as mediators of platelet activation

Recent studies have helped to define the earliest events of signal transduction in platelets, particularly those involved in the generation of second messengers. The best-understood of these events are those which involve guanine nucleotide binding regulatory proteins. G proteins are heterotrimers comprised of alpha, beta and gamma subunits, each of which can exist in multiple forms. Some, but not all, of the known variants of G alpha are substrates for ADP-ribosylation by pertussis toxin, a modification which disrupts the flow of information from receptor to effector. The G proteins that have been identified in platelets to date are Gs, Gi1, Gi2, Gi3, Gz and Gq. Gs and one or more of the Gi family members regulate cAMP formation by adenylylcyclase. Gi may also be responsible for the pertussis toxin-sensitive activation of phospholipase C which occurs when platelets are activated by thrombin. Gq is thought to be responsible for the pertussis toxin-resistant activation of phospholipase C by TxA2. Gz does not have an established role, but has the unique property of being phosphorylated by protein kinase C during platelet activation. Recent efforts to clone the receptors that interact with G proteins in platelets have been successful for epinephrine, thrombin, TxA2 and platelet activating factor. Each of these resembles other G protein-coupled receptors, being comprised of a single polypeptide with 7 transmembrane domains. In the case of thrombin, receptor activation is thought to involve a unique mechanism in which thrombin cleaves its receptor, creating a new N-terminus that can serve as a tethered ligand. Peptides corresponding to the tethered ligand can mimic the effects of thrombin, while antibodies to the same domain inhibit platelet activation. Shortly after activation, thrombin receptors become resistant to re-activation by thrombin. This desensitization, which appears to be due to a combination of proteolysis, phosphorylation and internalization, provides a potential mechanism for limiting the duration of thrombin-initiated signals in platelets.

Muscarinic acetylcholine receptors in invertebrates: comparisons with homologous receptors from vertebrates

The pharmacology, physiology and molecular biology of invertebrate muscarinic acetylcholine receptors are compared with current knowledge concerning vertebrate muscarinic acetylcholine receptors. Evidence for the existence of multiple receptor subtypes in invertebrates is examined, emphasizing what is presently known about the sensitivity of invertebrate preparations to subtype selective ligands previously defined in vertebrate studies. Other evidence for muscarinic receptor subtypes which is examined includes: heterogeneous responses to classical muscarinic ligands and evidence for coupling of invertebrate muscarinic receptors to several different classes of second messenger systems. Clues regarding possible functions for invertebrate muscarinic receptors are discussed, including evidence from both physiological studies and in situ localization studies which reveal patterns of receptor protein and mRNA expression. A detailed analysis of the structural similarities between a cloned Drosophila muscarinic receptor and vertebrate muscarinic receptors is also presented. Regions of the receptors that may be involved in ligand binding, effector coupling and receptor regulation are identified in this comparison. Future directions for invertebrate muscarinic receptor research are considered including: methods for cloning other receptor subtypes, methods for cloning homologous receptors from other species and genetic approaches for determining the physiological roles of muscarinic receptors.

Arachidonic acid and other unsaturated fatty acids alter membrane potential in PC12 and bovine adrenal chromaffin cells

The action of arachidonic acid and other fatty acids on membrane potential in PC12 and bovine chromaffin cells was investigated using a membrane potential-sensitive fluorescent dye. Arachidonic acid (1-40 microM) provoked dose-dependent membrane hyperpolarization, thereby reducing hyperpolarization induced by the K(+)-selective ionophore valinomycin. Other cis-unsaturated fatty acids, but not lipoxygenase products or the saturated fatty acid palmitic acid, also affected membrane potential. Tetraethylammonium blocked the arachidonic acid-induced hyperpolarization. These data suggest that cis-unsaturated fatty acids alter membrane potential in PC12 and bovine chromaffin cells by modulating K+ conductances. Valinomycin-generated hyperpolarization had no effect on agonist-induced Ca2+ influx into bovine chromaffin cells, whereas preincubation with arachidonic acid and other cis-unsaturated fatty acids blocked Ca2+ influx and secretion. We propose a model where internally generated fatty acids act as a feedback to desensitize the stimulated cell via inhibition of receptor-dependent Ca2+ influx and induction of membrane hyperpolarization.

Mechanisms of UV-induced inflammation

The inflammation produced by exposure to ultraviolet (UV) light has been well documented clinically and histologically. However, the mechanisms by which mediators induce this clinical response remain poorly defined. It is clear that photochemistry occurring after UV absorption must be responsible for initiating these events. Some of these underlying mechanisms have been defined. After exposure to UV light, the formation of prostaglandins and the release of histamine are increased. In addition to an increase in the quantity of these mediators, an increase in sensitivity of irradiated tissue to agonist stimulation also occurs. This increased sensitivity may cause tissue to respond to agonist levels previously present. Phospholipase activity also increases, making more substrate available for prostaglandin formation. Oxygen radical-induced peroxidation of membrane lipids caused by irradiation may contribute to increased phospholipase activity. Oxygen-free radicals also participate in sunburn cell formation and in UV-induced decreases in Langerhans cell numbers. Several enzymatic and non-enzymatic mechanisms are present in skin for reducing these highly reactive oxygen species.

Muscarinic receptor subtypes: modulation of ion channels

The discovery of five muscarinic receptor subtypes by molecular genetic techniques has resulted in new approaches to understanding their function. This involves the expression of the individual genes encoding each receptor subtype in isolation, such that their effects and mechanisms of action can be studied. The coupling of the receptors with G-proteins and ion channels is the subject of this review and emphasis is placed upon the assignment of genetically defined receptor subtypes with a given physiological function. Activation of inwardly rectifying potassium conductances by m2 and m4 and inhibition by m1, as well as stimulation of calcium-dependent conductances by m1, m3 and m5 are discussed.

Membrane-delimited cell signaling complexes: direct ion channel regulation by G proteins

Ion channels are signaling molecules and by themselves perform no work. In this regard they are unlike the usual membrane enzyme effectors for G proteins. The pathways of G protein receptor, G protein and ion channels are, therefore, purely informational in function. Because a single G protein may have several ion channels as effectors, the effects should be coordinated and this seems to be the case. Inhibition of Ca2+ current and stimulation of K+ currents would have a greater impact than either alone. Additional flexibility is provided by spontaneous noise in the complexes of G protein receptor, G protein, and ion channel. By having a non-zero setpoint, the range of control is extended and the responses become bi-directional.

Three types of Gi alpha protein of the guinea-pig lung: cDNA cloning and analysis of their tissue distribution

cDNA clones encoding three types of Gi alpha, the alpha subunit of GTP-binding protein (Gi1 alpha, Gi2 alpha, and Gi3 alpha), were isolated from a cDNA library of the guinea-pig lung. Nucleotide sequence analysis revealed a high degree of homology with other mammalian Gi alpha cDNAs. By RNA blot analysis, the expression pattern of Gi1 alpha was more tissue-specific than those of other types of Gi alphas in the guinea-pig tissues examined. While Gi2 alpha and Gi3 alpha mRNAs were ubiquitously expressed in all tissues examined, Gi1 alpha mRNA was mainly expressed in the brain, lung and kidney. These results suggest that each Gi alpha protein may have a different role.

Multiple phospholipase A2 activities in canine vascular smooth muscle

Three phospholipase A2 activities from canine vascular smooth muscle were identified and characterized including: (1) a cytosolic calcium-independent phospholipase A2 which is activated by nucleotide di- and triphosphates; (2) a cytosolic calcium-dependent phospholipase A2 which is activated by physiologic increments in calcium ion concentration; and (3) a microsomal calcium-independent phospholipase A2 which was highly selective for plasmenylcholine substrate. Vascular smooth muscle cytosolic calcium-independent phospholipase A2 was activated 338% +/- 11 (X+S.E.; n = 15) by physiologic concentrations of ATP. Similar amounts of activation were also present utilizing other nucleotide di- and triphosphates (e.g., ADP, CTP, GDP and GTP) as well as non-hydrolyzable nucleotide triphosphate analogs (e.g., ATP-gamma-S, AMP-PNP and GTP-gamma-S). Vascular smooth muscle cytosolic calcium-dependent phospholipase A2 was purified 455-fold by sequential DEAE-Sephacel, Phenyl-Sepharose, Mono Q, hydroxyapatite and Superose 12 chromatographies. The partially purified calcium-dependent phospholipase A2 was activated by physiologic increments in calcium ion concentration (e.g., 1 microM) and possessed an apparent native molecular weight of 95 kDa, an acidic isoelectric point (pI = 4.8) and a neutral pH optimum (pH 7.0). Vascular smooth muscle microsomal phospholipase A2 activity was predominantly calcium-independent and was over six-fold selective for hydrolysis of plasmenylcholine substrate. Taken together, these results demonstrate the existence of three separate and distinct phospholipase A2 activities in vascular smooth muscle and identify ATP and calcium ion as independent modulators of discrete phospholipase A2 activities in vascular smooth muscle cells.

Retinal rods and cones have distinct G protein beta and gamma subunits

Guanine nucleotide-binding proteins (G proteins) involved in transmembrane signal-transduction processes are heterotrimers composed of alpha, beta, and gamma subunits. The alpha subunit shows great diversity and is thought to confer functional specificity to a particular G protein. By contrast, the beta and gamma subunits appear much less diverse; in particular, the beta subunit is believed to have no role in G protein specificity. Using immunocytochemistry, we found distinct distribution patterns for different beta and gamma subunits in the retina. In particular, rod and cone photoreceptors, which both subserve phototransduction but differ in light-response properties, have different beta and gamma subunits in their outer segments. Thus, the G protein mediating phototransduction shows cell-specific forms of the beta and gamma subunits in addition to the alpha subunit. This surprising finding supports the hypothesis that these subunits may also contribute to functional specificity of a G protein.

The gamma-subunit of the principal G-protein from squid (Loligo forbesi) photoreceptors contains a novel N-terminal sequence

The squid (Loligo forbesi) visual system presents as accessible a system for study of G-protein mediated signal transduction as the vertebrate rod outer segment with the added advantage that the major G-protein is a member of the Gq-class. Here the cDNA clone encoding the gamma-subunit of this G-protein is reported, thereby completing the molecular cloning of the heterotrimeric G-protein. The deduced protein structure of G-gamma has relatively little sequence identity with known mammalian counterparts particularly in comparison with the relatively high degree found for both the alpha- and beta-subunits of this protein. In particular, the N-terminus of the squid visual G-gamma contains a repetitive, highly charged region, rich in lysine and glutamate, that has no parallel in other G-proteins. The amino acid sequence of a number of peptides derived by chemical cleavage of G-gamma accounted for much of the protein sequence predicted from the cDNA, including the unusual N-terminal region.

The effect of arachidonic acid on the M current of NG108-15 neuroblastoma x glioma hybrid cells

The M current, IM, a voltage-dependent non-inactivating K+ current, was recorded in NG108-15 neuroblastoma x glioma hybrid cells, using the whole-cell mode of the patch-clamp technique. We studied the effect of arachidonic acid, other fatty acids and inhibitors of the arachidonic acid metabolism. In relatively high concentrations (25-50 microM) arachidonic acid first increased and later decreased the current, Ih, which holds the membrane potential at -30 mV and mainly flows through open M channels. It shifted the midpoint potential, Vo, of the relation between M conductance, gM, and membrane potential, V, to more negative values and decreased the maximum conductance gM and the time constant tau M. In smaller concentrations (5-10 microM) arachidonic acid merely decreased Ih and gM with little effect on Vo and tau M. Eicosatetraynoic acid and docosahexaenoic acid acted similarly to arachidonic acid whereas stearic acid had no effect. Of the three enzyme inhibitors studied, nordihydroguaiaretic acid acted similarly to arachidonic acid. i.e. caused a biphasic change in Ih. Indomethacin and quinacrine caused, respectively, a pure increase and a pure decrease of Ih and gM. Possible explanations are build-up of internally produced arachidonic acid, depletion of eicosanoid products or an inhibitory effect unrelated to arachidonic acid metabolism.

Cell killing and induction of manganous superoxide dismutase by tumor necrosis factor-alpha is mediated by lipoxygenase metabolites of arachidonic acid

The signalling pathways utilized by tumor necrosis factor-a (TNF) to elicit its actions have been examined in TA1 adipogenic cells. A role for lipoxygenase metabolites of arachidonic acid as mediators of TNF action in the induction of c-fos has been described. In this paper we report that acute cytotoxicity elicited by TNF, in the presence of cycloheximide (CHX), also utilizes this pathway since inhibitors of lipoxygenase action fully prevent TNF/CHX killing of several cell lines. Our data reveal that TNF induction of manganous superoxide dismutase (MnSOD) is also dependent upon lipoxygenase activity. Radical scavengers such as NAC and PDTC prevent TNF/CHX-induced cell killing and reduce MnSOD induction by TNF. Therefore, cell death by TNF/CHX treatment occurs via a pathway in which lipoxygenase products directly or indirectly operate via the generation of superoxide anions.

Production, processing and partial purification of functional G protein beta gamma subunits in baculovirus-infected insect cells

As a result of the inability to resolve the heterogeneous mixture of G protein beta gamma subunits present in tissues, it has not been possible to compare different beta gamma subunits of the G proteins in terms of their proposed roles in receptor-effector coupling. This study was undertaken to establish the utility of the baculovirus expression system in producing homogeneous beta gamma subunits of defined composition for the comparative analysis of these subunits in reconstitution systems. In this study we report the expression, and appropriate post-translational processing, of recombinant beta 2, gamma 2 and gamma 3 subunits. In addition, we show that the recombinant beta gamma subunits can be readily purified, and can functionally interact with the alpha subunits of the G proteins.

Chloride channels in cultured human skeletal muscle are regulated by G proteins

The regulation of Cl- channels in human myoballs by G proteins was studied using whole-cell and inside-out patch recordings. After perfusion of the cell with 0.1 mM GTP[gamma S], the specific Cl- conductance, GCl, at standard resting potential (-85 mV) was increased from 5.9 microS/cm2 to 103 microS/cm2, and the kinetics upon stepping the potential to positive values was changed from an activating current with very slow inactivation to a fast inactivating current with no potential-dependent activation. These effects were not affected by the simultaneous blockade of several signal cascades involving G proteins. Addition of the protein kinase blockers PKI (25 microM), H8 (10 microM), or of the phospholipase-A2-blocking agent quinacrine (10 microM), had not much influence on these GTP[gamma S] effects. Buffering of the intracellular Ca2+ concentration (0.1 microM) or addition of the Ca2+/calmodulin antagonist trifluoperazine (50 microM) was also without effect. Pre-incubation of the cells with pertussis toxin or with cholera toxin did not change GCl. In excised inside-out patches voltage-clamped at -85 mV, application of GTP[gamma S] influenced the "intermediate" Cl- channel, the Cl- channel type having the highest density in these cells, by increasing the number of transitions in a half-conductance state. The probability of the channel being in one of the two conducting states rose from 0.015 to 0.67, and the kinetics of the single-channel currents was changed so that, on average, it was similar to the whole-cell current kinetics seen after application of GTP[gamma S]. It is concluded that a G protein is directly interacting with these channels.

Reciprocal regulation of fatty acid release in the brain by GABA and glutamate

Several model systems have been used to test the hypothesis that the release of FFA in the brain is regulated by depolarization of neurons. This FFA release is likely the result of the activation of phospholipase A2. The increased neuronal activity that occurs due to synchronous depolarization during seizures causes activation of phospholipase A2. Decreasing neuronal activity by administering the anxiolytic, diazepam, appears to decrease the activity of phospholipase A2. The GABA antagonist, bicuculline, which causes depolarization by negating the hyperpolarizing tone imposed on neurons by GABA, causes FFA release in synaptosomes and in neurons in tissue culture. Likewise, the glutamate agonist, kainic acid, which depolarizes neurons by opening sodium channels, increases the activity of phospholipase A2. PC-specific phospholipase C, another enzyme important in the generation of the second messenger, DG, is also activated by depolarization. Several important questions remain to be answered. The site of FFA release, in terms of the pre-vs. postsynaptic membrane, is not clear, although the experiments with synaptosomes support the hypothesis that activation of phospholipase A2 may be an important regulator of presynaptic events. This idea has also been suggested by studies on the phenomenon of long-term potentiation, where free 20:4 or its metabolites may be involved in presynaptic facilitation of neurotransmitter release (Freeman et al., 1990; Massicotte et al., 1990; Williams et al., 1989; also see Dorman, this volume). The activation of the PI cycle and subsequent stimulation of protein kinase C may be a postsynaptic event important in the integration of inputs at the dendrite and soma or a presynaptic event involved in the modulation of neurotransmitter release (Taniyama et al., 1990; El-Fakahany et al., 1990; also see Nishizuka, this volume). Therefore the stimulation of a PC-specific phospholipase C, which is capable of generating large amounts of DG over a prolonged period of time (Exton, 1990; Martinson et al., 1990; Diaz-Laviada et al., 1990), could occur at either site. Another important question is the role of FFA and DG in affecting cell-cell signaling events, particularly with regard to ion fluxes. Modulation of an acetylcholine-linked K+ channel in the heart by FFA and their oxygenation products has been reported (Kim and Clapham, 1989). The cardiac muscarinic receptor is linked to a hyperpolarizing K+ channel via a G protein.(ABSTRACT TRUNCATED AT 400 WORDS)

Different beta-subunits determine G-protein interaction with transmembrane receptors

Regulatory GTP-binding proteins (G proteins) are membrane-attached heterotrimers (alpha, beta, gamma) that mediate cellular responses to a wide variety of extracellular stimuli. They undergo a cycle of guanine-nucleotide exchange and GTP hydrolysis, during which they dissociate into alpha-subunit and beta gamma complex. The roles of G-protein alpha-subunits in these processes and for the specificity of signal transduction are largely established; the beta- and gamma-subunits are essential for receptor-induced G-protein activation and seem to be less diverse and less specific. Although the complementary DNAs for several beta-subunits have been cloned, isolated subunits have only been studied as beta gamma complexes. Functional differences have been ascribed to the gamma-subunit on the basis of extensive sequence similarity among beta-subunits and apparent heterogeneity in gamma-subunit sequences. Beta gamma complexes can interact directly or indirectly with different effectors. They seem to be interchangeable in their interaction with pertussis toxin-sensitive alpha-subunits, so we tested this by microinjecting antisense oligonucleotides into nuclei of a rat pituitary cell line to suppress the synthesis of individual beta-subunits selectively. Here we show that two out of four subtypes of beta-subunits tested (beta 1 and beta 3) are selectively involved in the signal transduction cascades from muscarinic M4 (ref. 4) and somatostatin receptors, respectively, to voltage-dependent Ca2+ channels.