5-Methyltryptamine stimulates phospholipase C-mediated breakdown of exogenous phosphoinositides by blowfly salivary gland membranes

Muscarinic regulation of cytosolic free calcium in canine tracheal smooth muscle cells: Ca2+ requirement for phospholipase C activation

1. The relationship between muscarinic receptor-mediated phosphatidylinositol 4,5-bisphosphate (PIP2) breakdown and the increase of intracellular Ca2+ ([Ca2+])i has been examined in canine cultured tracheal smooth muscle cells (TSMCs). 2. Addition of acetylcholine (ACh) and carbachol led to a 2-3 fold increase in [Ca2+]i over the resting level as determined by fura-2, with half-maximal stimulation (EC50) obtained at concentrations of 97 and 340 nM, respectively. Addition of the partial agonist, bethanechol, showed a smaller increase in PIP2 turnover and [Ca2+]i than did ACh or carbachol. 3. Addition of ACh or carbachol to TSMCs that had been prelabelled with [3H]-inositol led to the rapid (5-15 s) release of inositol mono, bis and trisphosphates IP1, IP2 and IP3. The time course of IP3 accumulation is correlated with the time course of the peak rise in [Ca2+]i. 4. Inclusion of EGTA lowered the resting [Ca2+]i and markedly reduced the extent of the agonist-induced rise in [Ca2+]i. When assayed under conditions similar to those used for the [Ca2+]i measurements, EGTA reduced the muscarinic agonist-stimulated inositol phosphates (IPs) accumulation. Conversely, ionomycin could stimulate IPs accumulation and elevate [Ca2+]i. The addition of Ca2+ (2.7-617 nM) to digitonin-permeabilized TSMCs directly stimulated IPs accumulation. 5. Both Ca2+ and guanosine-5'-O-(3-thiotriphosphate) (GTP gamma S) stimulated the formation of IPs in digitonin-permeabilized TSMCs prelabelled with [3H]-inositol. A further calcium-dependent increase in IPs accumulation was obtained by inclusion of either GTP gamma S or carbachol. The combined presence of carbachol and GTP gamma S elicited a synergistic effect on IPs accumulation, with half-maximal stimulation observed at approximately 8 nM free Ca2+.6. These results indicate that (i) the magnitude of the initial rise in [Ca2+], is directly related to the production of IPs and (ii) the phospholipase C-mediated PIP2 breakdown in TSMCs is sensitive to regulation by physiologically relevant concentrations of free Ca2+ ([Ca2+]f).

Regulation of inositol 1,4,5-trisphosphate metabolism by guanine nucleotides in membranes of cultured newborn rat cardiomyocytes

Membranes of cultured newborn rat cardiomyocytes contain enzymatic activities that regulate the formation and the breakdown of inositol 1,4,5-trisphosphate (1,4,5-IP3). GTP gamma S increased the rate of exogenous [3H]phosphatidyl 4,5-bisphosphate ([3H]PIP2) hydrolysis (EC50: 40 microM). This effect was dependent on the presence of deoxycholate and maximal at 2 mM deoxycholate. GTP gamma S increased the efficacy of phospholipase C (PLC) (by 2.3-fold), but did not alter the apparent affinity of the enzyme for PIP2. Other nucleotides, GDP beta S and ATP gamma S, and pyrophosphate also stimulated PIP2 hydrolysis, while AlF4- was ineffective. The effect of GTP gamma S was not inhibited by GDP beta S. The agonists norepinephrine and thrombin, which by themselves had no effect, did not potentiate the response to GTP gamma S. In contrast, 1,4,5-IP3 hydrolysis was decreased by GTP gamma S (EC50: 100 microM) as well as by other nucleotides and by pyrophosphate, but not by AlF4-. GDP beta S did not antagonize the GTP gamma S-induced inhibition of IP3 hydrolysis. These results suggest that GTP can stimulate the hydrolysis of exogenous PIP2 by an action on membrane-bound PLC at a site beyond the G protein activating PLC and inhibit the hydrolysis of 1,4,5-IP3 by a mechanism common to all nucleotides. Thus, GTP can regulate 1,4,5-IP3 metabolism by stimulating its formation and inhibiting its breakdown.

Muscarinic cholinergic stimulation of exogenous phosphatidylinositol hydrolysis is regulated by guanine nucleotides in rabbit brain cortical membranes

Rabbit brain cortical membranes, which have been extracted with 2 M KCl, hydrolyze exogenously added [3H]phosphatidylinositol [( 3H]PI) in a guanine nucleotide- and carbachol-dependent manner. Both oxotremorine-M and carbachol are full agonists with EC50 values of 8 and 73 microM, respectively. Pirenzepine and atropine inhibit carbachol-stimulated [3H]PI hydrolysis. The hydrolysis-resistant guanine nucleotide analog guanosine 5'-O-(3-thiotriphosphate) (GTP gamma S) is the most potent in supporting carbachol-stimulated hydrolysis of PI. There is no effect of carbachol in the absence of guanine nucleotides or in the presence of 100 microM adenosine 5'-O-(3-thiotriphosphate), adenosine-5'-(beta, gamma-imido)triphosphate, or sodium pyrophosphate. Guanylyl-5'-(beta,gamma-imido)triphosphate [Gpp(NH)p] in the presence of carbachol also stimulates PI hydrolysis although much less than that seen with GTP gamma S. GDP and Gpp(NH)p are potent antagonists of the GTP gamma S-dependent carbachol response. Optimal stimulation by carbachol and GTP gamma S was observed at 0.3-1 microM free Ca2+ and 6 mM MgCl2. Limited trypsinization resulted in loss of receptor-regulated PI breakdown and a slight decrease in basal activity. These results demonstrate that phospholipase C hydrolysis of exogenous PI by rabbit cortical membranes may be stimulated by carbachol in a guanine nucleotide-dependent manner.

Assay of a phosphatidylinositol bisphosphate phospholipase C activity in postmortem human brain

The activity of a phospholipase C which hydrolyses exogenous phosphatidylinositol-4,5-bisphosphate [( 3H]PtdIns(4,5)P2) in membranes prepared from frozen postmortem human brain and rat brain was investigated. Enzyme characteristics were essentially similar in membranes prepared from frozen postmortem brain and fresh or frozen rat brain. The [3H]PtdIns(4,5)P2 solubilization and assay procedure employed resulted in an efficient availability of the substrate for the enzyme. The non-hydrolysable guanosine triphosphate analogue guanosine 5'-[beta gamma-imido]diphosphate (Gpp[NH]p) stimulated hydrolysis rapidly with a half maximum activity of approximately 25 microM. This stimulation was not specific for guanine nucleotides as ATP, imidodiphosphate and pyrophosphate also caused enzyme activation. However these activation effects could be distinguished by the polyanion spermine. The non-hydrolysable guanine dinucleotide analogue guanosine 5'-[beta-thio]diphosphate acted as a partial agonist thereby inhibiting the stimulatory effect of Gpp[NH]p. Gpp[NH]p-stimulated enzyme activity showed a maximum response in the presence of 1 mM deoxycholate and displayed a pH optima in the range 7.0-7.5. PtdIns(4,5)P2 hydrolysis was observed in the absence of added calcium, but hydrolytic cleavage was inhibited in the presence of divalent ion chelators. Magnesium inhibited PtdIns(4,5)P2 hydrolysis in a concentration-dependent manner. Elucidation of these aspects of the phosphatidylinositol cycle in normal human postmortem brain will permit comparative studies in CNS disease states.

Evidence for the involvement of a pertussis toxin-insensitive G-protein in egg activation of the frog, Xenopus laevis

Activation responses of the frog egg at fertilization include the release of calcium from intracellular stores and the opening of calcium-dependent chloride channels, which produce the fertilization potential. To investigate the presence of guanine nucleotide-binding proteins (G-proteins), and their role in initiation of these events in the egg of the frog Xenopus laevis, we assayed for pertussis and cholera toxin substrates, and applied activators and inhibitors of G-proteins. Pertussis toxin catalyzed the [32P]ADP ribosylation of a Mr 40,000 component, but no cholera toxin substrates were demonstrated. Injection of greater than or equal to 25 pmole of guanosine-5'-O-(3-thiotriphosphate) GTP-gamma-S), an activator of G-proteins, produced a change in membrane potential that mimicked the fertilization potential and also caused cortical granule exocytosis and cortical contraction. Injections of up to 600 pmole of guanosine 3':5'-cyclic monophosphate or 9 nmole of guanosine-5'-(beta-gamma-imido)triphosphate did not active eggs. The membrane potential response to GTP-gamma-S injection showed the same peak and chloride dependence as the fertilization potential, although the duration of the GTP-gamma-S response was somewhat greater. GTP-gamma-S did not activate eggs if the calcium rise was prevented by prior injection of the calcium chelator BAPTA. Injection of up to 200 ng of cholera toxin did not activate eggs. However, eggs were activated by applying 1 nM serotonin to eggs that had been injected with a specific mRNA for the serotonin 1c receptor, a member of the class of receptors that act by way of G-proteins. Egg activation in response to either sperm or serotonin was not inhibited by pertussis toxin, under experimental conditions where approximately 80-90% of the toxin substrate was ADP-ribosylated. These results support the hypothesis that sperm activate Xenopus eggs at fertilization by way of a pertussis and cholera toxin-insensitive G-protein.

myo-inositol metabolites as cellular signals

The discovery of the second-messenger functions of inositol 1,4,5-trisphosphate and diacylglycerol, the products of hormone-stimulated inositol phospholipid hydrolysis, marked a turning point in studies of hormone function. This review focuses on the myo-inositol moiety which is involved in an increasingly complex network of metabolic interconversions, myo-Inositol metabolites identified in eukaryotic cells include at least six glycerophospholipid isomers and some 25 distinct inositol phosphates which differ in the number and distribution of phosphate groups around the inositol ring. This apparent complexity can be simplified by assigning groups of myo-inositol metabolites to distinct functional compartments. For example, the phosphatidylinositol 4-kinase pathway functions to generate inositol phospholipids that are substrates for hormone-sensitive forms of inositol-phospholipid phospholipase C, whilst the newly discovered phosphatidylinositol 3-kinase pathway generates lipids that are resistant to such enzymes and may function directly as novel mitogenic signals. Inositol phosphate metabolism functions to terminate the second-messenger activity of inositol 1,4,5-trisphosphate, to recycle the latter's myo-inositol moiety and, perhaps, to generate additional signal molecules such as inositol 1,3,4,5-tetrakisphosphate, inositol pentakisphosphate and inositol hexakisphosphate. In addition to providing a more complete picture of the pathways of myo-inositol metabolism, recent studies have made rapid progress in understanding the molecular basis underlying hormonal stimulation of inositol-phospholipid-specific phospholipase C and inositol 1,4,5-trisphosphate-mediated Ca2+ mobilisation.

Regulation of phosphoinositide-specific phospholipase C

The receptors involved in the regulation of phospholipase C by hormones, neurotransmitters and other ligands have seven transmembrane-spanning hydrophobic regions (seven-helix motif) and no known enzymatic activity. Furthermore these receptors can be isolated as complexes with guanine nucleotide binding (G) proteins. Guanine nucleotides affect the binding of hormones that stimulate phospholipase C and it has been possible to see activation of GTPase activity in membranes upon addition of these ligands. Further indirect evidence for a Gp (p stands for phospholipase C activation) protein is the finding that in membranes agonist activation of phospholipase C requires the presence of GTP gamma S a non-hydrolyzable analog of GTP. Furthermore, fluoride is able to activate phospholipase C but its inhibition of phosphatidylinositol-4' kinase (PI-4' kinase) can interfere with efforts to demonstrate this in intact cells. There are four major isozymes of phospholipase C that have been cloned and sequenced. Recently it was found that phospholipase C-gamma as well as PI-3'-kinase are substrates for phosphorylation on tyrosine residues by the EGF and PDGF receptors. The PI-3' kinase is able to convert phosphatidylinositol 4,5-bisphosphate (PIP2) to phosphatidylinositol 3,4,5-trisphosphate (PIP3) but the function of this lipid is unknown since it is not a substrate for any known phospholipase C. While much has been learned about the structure and regulation of the phosphoinositide specific kinases and phosphodiesterase enzymes this is a relatively new field in which we can expect many advances during the next few years.

Guanosine 5'-O-thiotriphosphate and sodium fluoride activate polyphosphoinositide hydrolysis in rat cortical membranes by distinct mechanisms

NaF and guanosine 5'-O-thiotriphosphate [GTP(S)] stimulated the accumulation of [3H]inositol monophosphate ([3H]InsP) in rat brain cortical membranes, with half-maximal stimulation at 2 mM and 1 microM, respectively. Calcium also increased basal [3H]InsP formation over a range of concentrations from 10(-7) to 10(-4) M. The stimulatory effect of GTP(S) (30 microM) on [3H]InsP production was insensitive to Ca2+, whereas NaF-evoked [3H]InsP formation was dependent on Ca2+ concentrations. Guanosine 5'-O-thiodiphosphate significantly attenuated GTP(S)- but not NaF-stimulated [3H]InsP production. Coincubation of GTP(S) (30 microM) and submaximal concentrations of NaF (1 or 3 mM) stimulated [3H]InsP formation to a degree that was nearly additive with that produced by either drug alone. However, the resultant accumulation of [3H]InsP in the presence of maximally effective concentrations of GTP(S) and NaF was not different from that produced by NaF alone. Incubation of cortical membranes with GTP(S) and NaF for 1 min stimulated the accumulation of [3H]inositol bisphosphate (InsP2) but not [3H]InsP. [3H]InsP2 production elicited by GTP(S) was markedly enhanced by the muscarinic cholinergic agonist carbachol. In contrast, NaF-stimulated [3H]InsP2 formation was not potentiated by carbachol. Our findings of different characteristics of GTP(S) and fluoride activation of polyphosphoinositide (PPI) hydrolysis suggest that separate regulatory mechanisms are involved in these two modes of stimulation in brain membranes. Activation of PPI hydrolysis by fluoride may be mediated by a direct stimulation of PPI phosphodiesterase or by activating a putative guanine nucleotide regulatory protein at a location distinct from the GTP-binding site.

Serotonin-stimulated biochemical events in the procerebrum of Limax

1. The procerebrum (PC) of the terrestrial slug Limax maximus is of interest as a potential site of olfactory information processing (Gelperin et al., 1989). The neuromodulator serotonin is present in the procerebrum and can elicit action potentials from cultured procerebral neurons. We have investigated the effects of serotonin on second-messenger signaling systems and protein phosphorylation as a prelude to studies on long-term synaptic plasticity in the Limax procerebral lobe. 2. We found that several biochemical changes are triggered within 20 min of adding serotonin to the isolated procerebral lobe: adenylate cyclase is activated, protein phosphorylation and synthesis are modulated, and phosphatidylinositol-metabolism is stimulated. 3. Serotonin causes a rapid synthesis of cAMP, reaching a 20- to 30-fold increase within 1 min. Serotonin affects the rate of phosphorylation of several proteins, detected after a brief (20-min) incubation of the procerebral lobe in [32P]phosphate-containing medium. The level of synthesis of several proteins is altered by serotonin, as determined by alterations in [35S]methionine incorporation during a 20-min incubation. Serotonin also causes a slow accumulation of inositoltrisphosphate. 4. Our study shows that within a short time (less than 20 min) serotonin can influence several second-messenger signaling systems and the functional state and abundance of proteins in the procerebral lobe. These serotonin-stimulated events should have direct consequences for intercellular communication in the odor-processing network of the procerebral lobe.

Receptors that inhibit phosphoinositide breakdown

Calcium-mobilizing receptors are believed to activate phospholipase C. Joel Linden and Thérèse Mary Delahunty summarize recent reports which indicate that activation of some receptors that inhibit the accumulation of Ca2+ within cells - notably receptors for adenosine, dopamine and several other neurotransmitters - can inhibit phosphoinositide metabolism. Two types of mechanism may be involved in these responses. Many instances of receptor-mediated inhibition of phosphoinositide breakdown can be detected only after a period of several minutes and may be secondary to receptor-mediated events that lower [Ca2+]i or activate certain protein kinases. In other instances the activation of receptors rapidly (within seconds) inhibits phosphoinositide breakdown, possibly via the activation of guanine nucleotide binding proteins that either directly, or by a rapid indirect action, inhibit phospholipase C. Putative mechanisms for direct and indirect regulation of phospholipase C are discussed.

Diffusion into the patch-clamp recording pipette of a factor necessary for muscarinic current response

Ca-dependent K and Cl currents were measured in isolated cells from rat lacrimal glands using the tight-seal whole-cell recording method. Upon application of acetylcholine (ACh), both K and Cl-selective currents were activated. The size of the ACh-activated currents declined after a few minutes of whole-cell recording. The rundown curve was composed of an initial stable period followed by a rather rapid decline. Both the length of the initial plateau and the speed of the falling phase were dependent on cell size and recording pipette resistance. The results suggest that the rundown was due to washout of an unknown cytosolic substance. Another manifestation of washout was an increase in the delay of the response. Plots of the inverse of the delay as a function of time in whole-cell recording showed again an initial plateau and a falling phase, but the stable period lasted less than in amplitude plots. Analysis of the washout time course suggested that the cytosolic substance has a diffusion coefficient of 5.4 x 10(-6) cm2/s, corresponding to a molecular weight of 350. Washed-out cells were insensitive to GTP-gamma-S, but responded normally to an internal application of inositol-trisphosphate (InsP3), introduced through the pipette. Thus, the step of the response which is sensitive to washout is closely related to the production of InsP3. Addition of various exogenous water soluble substances failed to halt washout. Among the inactive substances were GTP (or a combination of Mg and GTP) and small water soluble precursors of InsP3. The results imply that the production of InsP3 by muscarinic agonists in exocrine glands requires the presence of a small molecular weight, water soluble substance. It is suggested that this substance is an unknown co-factor of phospholipase C or of Gp, the GTP binding protein governing the production of InsP3.

GTP gamma S-stimulated hydrolysis of phosphatidylinositol-4,5-bisphosphate soluble phospholipase C from human platelets requires soluble GTP-binding protein

GTP-binding activity was fractionated into two peaks (GI and GII) by chromatography on heparin-agarose. GTP-dependent PLC activity eluted as a single peak, which co-chromatographed with GTP-binding peak GII. Rechromatography of peak GII on heparin-agarose, in the presence of 0.5% sodium cholate, resulted in separation of PLC and GTP-binding activities, and loss of GTP-dependent PLC activity. Recombining fractions containing PLC and GTP-binding activities restored GTP-dependent PLC activity. A specific GTP-binding protein of 29,000 daltons was identified in peak GII by Western blotting of column fractions with [alpha-32P]GTP. These results demonstrate that the soluble phospholipase C from human platelets is regulated by GTP S-binding protein (G29).

The evolutionary origin of eukaryotic transmembrane signal transduction

1. A comparison was made of transmembrane signal transduction mechanisms in different eukaryotes and prokaryotes. 2. Much attention was given to eukaryotic microbes and their signal transduction mechanisms, since these organisms are intermediate in complexity between animals, plants and bacteria. 3. Signal transduction mechanisms in eukaryotic microbes, however, do not appear to be intermediate between those in animals, plants and bacteria, but show features characteristic of the higher eukaryotes. 4. These similarities include the regulation of receptor function, adenylate cyclase activity, the presence of a phosphatidylinositol cycle and of GTP-binding regulatory proteins. 5. It is proposed that the signal transduction systems known to operate in present-day eukaryotes evolved in the earliest eukaryotic cells.

Guanine nucleotide and pyrophosphate activate exogenous phosphatidylinositol 4,5-bisphosphate hydrolysis in rat liver plasma membranes

5'-guanylylimidodiphosphate (GppNHp) in the presence of deoxycholate, stimulated the phospholipase C-mediated hydrolysis of exogenous [3H]phosphatidylinositol 4,5-bisphosphate ([3H]PIP2) to myo-[3H]inositol 1,4,5-trisphosphate in rat liver plasma membranes. Activation was not specific for guanine nucleotides as 5'-adenylylimidodiphosphate, imidodiphosphate and pyrophosphate stimulated the enzyme with similar efficacies and potencies. Enzyme activation by GppNHp was most pronounced when [3H]PIP2 was used as substrate. No added Ca++ was required for [3H]PIP2 breakdown but hydrolysis was inhibited by divalent ion chelators. GppNHp stimulation was apparent in the presence of Ca++ or Mg++ as well as chelator concentrations that partially inhibited the enzyme, indicating that this effect was not attributed to changes in affinity of these divalent cations for the enzyme or substrate. These results suggest that guanine nucleotides can stimulate the hydrolysis of exogenous [3H]PIP2 in rat liver membranes by a non-specific effect probably due to the interaction of the diphosphate moiety with the enzyme or substrate.

Multiple mechanisms of serotonergic signal transduction

In this article we review serotonergic signal transduction mechanisms in the central and peripheral nervous systems and in a variety of target organs. The various classes of pharmacologically defined serotonergic receptors are coupled to three major effector systems: (1) adenylate cyclase; (2) phospholipase C mediated phosphoinositide (PI) hydrolysis and (3) ion channels (K+ and Ca++). Long term occupancy of serotonergic receptors also appears to induce alterations in mRNA and protein synthesis. For all three types of signal transduction there is evidence accumulating which suggests the involvement of guanine nucleotide regulatory proteins. Recent findings suggest that the distinct types of pharmacologically defined serotonergic receptors (5HT1A, 5HT1B, 5HT1c, 5HT2) may be coupled to one or more signal transduction systems. Thus, 5HT1 receptors may both activate and inhibit adenylate cyclase and increase K+-ion conductance in the hippocampus. 5HT2 receptors which activate PI hydrolysis in the brain, both open voltage-gated calcium channels and activate PI metabolism in certain smooth muscle preparations. Thus, each class of serotonergic receptor may be linked to one or more distinct biochemical transduction mechanisms. The possibility is raised that selective agonists and antagonists might be developed which have specific effects on a particular receptor-linked effector system.

Modulation of phosphatidylinositol-4,5-bisphosphate hydrolysis in rat aorta by guanine nucleotides, calcium and magnesium

Rat aortic smooth muscle homogenates and membrane preparations contain a phospholipase C which hydrolyzes phosphatidylinositol 4,5-biphosphate (PIP2). We discovered that guanyl-5'yl-imidodiphosphate (Gpp(NH)p) activated the hydrolysis of exogenous PIP2 but not of phosphatidylinositol (PI) in rat aortic membranes. Further, maximal Gpp(NH)p-dependent hydrolysis was dependent on physiological levels of calcium. Also, magnesium inhibited PIP2 hydrolysis and catalyzed the dephosphorylation of PIP2 to phosphatidylinositol-4-phosphate (PIP). The results imply that PIP2 is the primary substrate of the nucleotide-regulated phospholipase C in rat aorta and that calcium and magnesium are physiological regulators of this activity.

In vitro synthesis of 32P-labelled phosphatidylinositol 4,5-bisphosphate and its hydrolysis by smooth muscle membrane-bound phospholipase C

For studies of phospholipase C (PLC) activity in cell-free systems, 32P-labelled phosphatidylinositol 4,5-bisphosphate (PIP2) was prepared enzymatically by phosphorylating phosphatidylinositol 4-phosphate (PIP) in the presence of [gamma-32P]ATP using a PIP kinase partially purified from bovine retinae. PLC activity was determined by incubating membranes of DDT1 MF-2 cells with 32P-PIP2 and measuring remaining non-hydrolyzed substrate as well as accumulation of the hydrolysis product, inositol trisphosphate (IP3). Guanine nucleotides stimulated PIP2 hydrolysis and IP3 release. Additional increase in IP3 accumulation was observed with adrenaline plus guanine nucleotides.

GDP beta S enhances the activation of phospholipase C caused by thrombin in human platelets: evidence for involvement of an inhibitory GTP-binding protein

Guanosine 5'-O-thiotriphosphate (GTP gamma S) and thrombin stimulate the activity of phospholipase C in platelets that have been permeabilized with saponin and whose inositol phospholipids have been prelabeled with [3H]inositol. Ca2+ has opposite effects on the formation of [3H]inositol phosphates induced by thrombin or GTP gamma S. While the action of GTP gamma S on the formation of [3H]inositol phosphates is inhibited by Ca2+, action of thrombin is stimulated by Ca2+. Guanosine 5'-O-(2-thiodiphosphate) (GDP beta S), which inhibits the function of GTP-binding proteins, also inhibits the effect of GTP gamma S on phospholipase C stimulation but, surprisingly, increases the effect of thrombin. Ca2+ increases the inhibitory effect of GDP beta S on GTP gamma S activation of phospholipase C, but Ca2+ further enhances the stimulatory effect of GDP beta S on the thrombin activation of phospholipase C. This indicates that two mechanisms are responsible for the activation of phospholipase C in platelets. A GTP-binding protein is responsible for regulation of phospholipase C induced by GTP gamma S, while the effect of thrombin on the stimulation of phospholipase C is independent of GTP-binding proteins. However, the effect of thrombin may be modulated by the action of an inhibitory GTP-binding protein.

Properties of phospholipase C in isolated olfactory cilia from the channel catfish (Ictalurus punctatus)

1. Cilia were isolated from the olfactory epithelium of the channel catfish (Ictalurus punctatus) with improved yield. The isolated preparations were enriched in cilia as indicated by electron microscopy, tubulin immunoblotting and identification of a ciliary-specific glycoprotein. 2. The isolated cilia preparations exhibited phospholipase C (EC 3.1.4.11) activity. The enzyme was maximally active at pH 6.7. 3. Analysis of inositol phosphates resulting from the hydrolysis of exogenous radiolabeled phosphatidylinositol-4,5-bisphosphate in isolated cilia, indicated that inositol triphosphate was the major (90%) inositol phosphate produced. 4. Three molecular forms of the enzyme, Mr greater than or equal to 100,000, 82,000 and 60,000 were resolved by gel filtration chromatography from a cytosolic fraction from the olfactory epithelium.

Central serotonin receptors: effector systems, physiological roles and regulation

Radioligand binding studies have revealed four distinct serotonin (5HT) binding sites in rat brain that are thought to function as 5HT receptors. These include the 5HT-1a, 5HT-1b, 5HT-1c, and 5HT-2 binding sites. Studies have shown that the 5HT-2 binding site mediates a number of effects of 5HT agonists and serves as a 5HT receptor in neuronal and non-neuronal tissues. The 5HT-2 site employs phosphoinositide hydrolysis for signal transduction. The 5HT-1c binding site is also a functional receptor that is linked to phosphoinositide hydrolysis. However, the physiological role of the 5HT-1c receptor is not yet known. Lack of appropriate pharmacological tools for probing the 5HT-1a and 5HT-1b binding sites has made it difficult to definitively determine whether these binding sites are coupled to biochemical effector systems or mediate any of the physiological responses to 5HT agonists. However, there is some evidence that the 5HT-1a site is coupled to adenylate cyclase, and a number of functional roles for the 5HT-1a and 5HT-1b sites have been proposed.

GTP and GDP will stimulate platelet cytosolic phospholipase C independently of Ca2+

The hydrolysis of [3H]phosphatidylinositol 4,5-bisphosphate (PIP2) by cytosolic phospholipase C from human platelets was determined. Cytosolic fractions were prepared from platelets that had or had not been preactivated with thrombin. Thrombin pretreatment did not affect cytosolic phospholipase C activity. In both cytosolic fractions, phospholipase C was activated by GTP and GTP gamma S. This action is observed in the presence of 2 mM EGTA. GDP was as effective as GTP in stimulating cytosolic phospholipase C in the presence of Ca2+ or EGTA. Partially purified phospholipase C obtained from platelet cytosol is activated by GTP, but not by GTP gamma S, in the presence of 2 mM EGTA. However, in the presence of 6 microM Ca2+, both GTP and GTP gamma S stimulated the partially purified phospholipase C. Our present information indicates that GTP and GDP have a direct effect on the cytosolic phospholipase C.

Regulation of phosphoinositide breakdown by guanine nucleotides

Phosphoinositide hydrolysis is coupled to receptor systems involved in the elevation of cytosolic Ca2+ and activation of protein kinase C. In cell-free systems, guanine nucleotides are required to transduce the effects of receptor activation to phosphoinositide breakdown. Non-hydrolyzable guanine nucleotides stimulate phosphoinositide breakdown in permeabilized cells as well as membranes prepared from salivary glands, GH3 cells, neutrophils, hepatocytes and cerebral cortical tissue. In blowfly salivary gland membranes, 5-hydroxytryptamine stimulates a guanine-nucleotide dependent breakdown of both endogenous and exogenous phosphoinositide substrate through activation of phospholipase C. These data suggest that a GTP-binding protein modulates phospholipase C activity. The identity of this GTP-binding protein has not been established but may resemble other regulatory GTP-binding proteins which have been identified as transducing proteins in a variety of receptor systems.