Bradykinin-activated membrane-associated phospholipase C in Madin-Darby canine kidney cells

Targeting G protein-coupled receptor signalling by blocking G proteins

G protein-coupled receptors (GPCRs) are the largest class of drug targets, largely owing to their druggability, diversity and physiological efficacy. Many drugs selectively target specific subtypes of GPCRs, but high specificity for individual GPCRs may not be desirable in complex multifactorial disease states in which multiple receptors may be involved. One approach is to target G protein subunits rather than the GPCRs directly. This approach has the potential to achieve broad efficacy by blocking pathways shared by multiple GPCRs. Additionally, because many GPCRs couple to multiple G protein signalling pathways, blocking specific G protein subunits can 'bias' GPCR signals by inhibiting only a subset of these signals. Molecules that target G protein α or βγ-subunits have been developed and show strong efficacy in multiple preclinical disease models and biased inhibition of G protein signalling. In this Review, we discuss the development and characterization of G protein α and βγ-subunit ligands and the preclinical evidence that this exciting new approach has potential for therapeutic efficacy in a number of indications, such as pain, thrombosis, asthma and heart failure.

Cysteinyl leukotrienes mediate the response of submucosal ganglia from rat colon to bradykinin

The aim of the present study was to find out the mechanism by which the inflammatory mediator, bradykinin, induces an increase of the cytosolic Ca(2+) concentration ([Ca(2+)](i)) in enteric neurons. For this purpose, ganglia in the isolated submucosa from rat colon were loaded with the Ca(2+)-sensitive dye, fura-2, and were exposed to bradykinin (2·10(-8)mol/l). Under control conditions, the kinin evoked a transient increase in [Ca(2+)](i). Preincubation with quinacrine or arachidonyltrifluoromethylketone (AACOCF(3)), i.e. blockers of cytosolic phospholipase A(2), prevented the raise of [Ca(2+)](i). This inhibition was mimicked by 5,8,11,14-eicosatetrayonic acid (ETYA), an inhibitor of cyclooxygenases as well as lipoxygenases, and by BWA4C, a selective inhibitor of lipoxygenases, whereas indomethacin was ineffective, suggesting the mediation of the kinin response by a lipoxygenase metabolite. Indeed, a leukotriene, leukotriene D(4) (LTD(4)), mimicked the effect of bradykinin. The LTD(4) receptor blocker, MK-571, inhibited the increase in [Ca(2+)](i) evoked by LTD(4) and by bradykinin. Consequently, bradykinin receptors in submucosal ganglia from rat colon are coupled to a stimulation of phospholipase A(2), the release of arachidonic acid and the production of LTD(4), which seems to be finally responsible for the change in the cytosolic Ca(2+) concentration.

Mobilization of intracellular calcium in kidney epithelial cells is inhibited by lead

The effect of lead (Pb) on intracellular calcium (Cai) after stimulation with agonists was studied in Madin-Darby canine kidney (MDCK) cells. In response to the agonist ADP, the levels of Cai increased by approximately threefold in MDCK cells bathed in a buffer with calcium (Ca) or in a buffer with nominal Ca. Pb inhibited the response to ADP in MDCK cells bathed in either buffer. The inhibition by Pb was observed after a 5 and 20-min exposure to Pb, but not after 2-min. Very high concentrations of ADP did not reverse the effects of Pb. Concentrations of Pb of 1 microM or more inhibited the response to ADP. Similarly, the response to bradykinin was also inhibited by Pb. Protein kinase C did not play a role since the protein kinase C inhibitor GF 109203X did not reverse the effects of Pb. Interestingly, MDCK cells treated with Pb at concentrations above 1 microM, for periods of 5-20 min, displayed elevated levels of inositol 1,4,5-trisphosphate. In conclusion, Pb inhibits mobilization of Cai after agonist stimulation by a mechanism that is unrelated to the type of agonist used. Evidence is presented suggesting that the inhibition is due to increases in levels of inositol 1,4,5-trisphosphate, which possibly decreases the amount of Cai available for mobilization.

Functional studies of bradykinin receptors in Chinese hamster ovary cells stably expressing the human B2 bradykinin receptor

Bradykinin B1 and B2 receptors, members of the G-protein coupled receptor superfamily, are involved in inflammation and pain. Chinese hamster ovary (CHO) cells stably expressing the human B2 bradykinin receptor (CHO-B2) were used to characterize the signal transduction pathways associated with this receptor and its regulation. The selective B2 antagonist [3H]NPC17731 but not the selective B1 antagonist [3,4-prolyl-3,4-(3)H(N)]-[des-Arg10,Leu9]kallidin ([3H]DALKD) bound to CHO-B2 cell membranes with a Kd of 0.77 nM and a Bmax of 1087 fmol/mg protein. [3H]NPC17731 binding was inhibited by bradykinin ligands in the order: NPC17731 > bradykinin > kallidin >> DALKD > [des-Arg10] kallidin (DAKD), consistent with the pharmacological profile of B2 bradykinin receptors. The B2 agonist bradykinin and the B1/B2 agonist kallidin, but not the B1 agonist DAKD, increased [35S]GTP gamma S binding to the CHO-B2 cell membranes. The B2 bradykinin receptors were co-immunoprecipitated with G alpha q/11. In response to bradykinin stimulation, coupling of the B2 receptors to G alpha q/11 was increased by 10-fold. Bradykinin and kallidin, but not DAKD, induced intracellular calcium release in CHO-B2 cells, which was blocked by NPC17731 but not by DALKD. These results demonstrate that B2 bradykinin receptors directly coupled to G alpha q/11 to regulate intracellular calcium release. CHO-B2 cell is a useful system that can be applied to study the effect of potential agents that may influence the B2 receptor function.

Stable expression of the human kinin B1 receptor in Chinese hamster ovary cells. Characterization of ligand binding and effector pathways

To delineate ligand binding and functional characteristics of the human B1 kinin receptor, a stable clone of Chinese hamster ovary cells expressing a single class of binding sites for [3H]des-Arg10-lysylbradykinin with a Kd of 0.3 nM and a Bmax of 38 fmol/mg protein ( approximately 40,000 receptors/cell) was isolated. Studies with peptide analogs showed that a lysine residue at position 1 (based on the lysylbradykinin sequence) of ligands was essential for high affinity binding to the human B1 receptor. In marked contrast to cloned Chinese hamster ovary cells expressing the human kinin B2 receptor, which internalized approximately 80% of the ligand within 5 min upon exposure to 2 nM [3H]bradykinin, exposure of cells expressing the B1 receptor to 1 nM [3H]des-Arg10-lysylbradykinin resulted in minimal ligand internalization. Stimulation of the B1 receptor led to inositol phosphate generation and transient increases in intracellular calcium, confirming coupling to phospholipase C, while immunoprecipitation of photoaffinity-labeled G-proteins from membranes indicated specific coupling of the receptor to Galphaq/11 and Galphai1,2. The B1, unlike the B2, receptor does not desensitize (as demonstrated by continuous phosphoinositide hydrolysis), enhancing the potential role of this receptor during inflammatory events.

Regulation of eicosanoid synthesis in fibroblasts from inflamed gallbladders

Gallbladder cell cultures obtained from rabbits subjected to sham or 72 h of bile duct ligation (72 h BDL, cholecystitis model) were incubated with calcium ionophore (A23187), dibutyryl cAMP (cAMP), and phorbol 12,13-diacetate (phorbol) to determine the intracellular signal transduction mechanisms responsible for increased inflamed gallbladder eicosanoid synthesis. Incubation of sham and 72 h BDL cell cultures with A23187 or phorbol significantly increased, whereas cAMP decreased, release of 6-keto-PGF1 alpha, PGE2, thromboxane B2 (measured by enzyme immunoassay) in a dose-related manner. Seventy-two-hour BDL cell cultures contained a specific 2-fold increased level of prostacyclin synthase compared to sham cell cultures which was not altered by preincubation with A23187, phorbol or cAMP. These findings suggest that increased PGI2 release in the sham and inflamed cell cultures following A23187 and phorbol stimulation was mediated in part via the inositol triphosphate pathway and protein kinase C activation and was not associated with altered cyclooxygenase or prostacyclin synthase content.

Bradykinin induces translocation of the protein kinase C isoforms alpha, epsilon, and zeta

Bradykinin exerts a broad spectrum of cellular effects on different tissues. It is believed that these effects are predominantly mediated by the recently cloned B2 receptor. The mechanism of post-receptor signal transduction is not known in detail. Involvement of protein kinase C (PKC) was suggested and activation of the classical PKC isoforms alpha and beta was recently demonstrated. The aim of the present study was to investigate whether the B2 receptor also activates new (delta, epsilon) and atypical (zeta) PKC isoforms. To investigate this, chinese hamster ovary (CHO) cells, stably transfected with human B2 receptor, were used. In these cells the PKC isoforms alpha, delta, epsilon and zeta were detected by immunoblotting with specific antibodies. To monitor hormone-induced PKC translocation plasma membranes were prepared. Stimulation of the cells with bradykinin resulted in a rapid (30-60 s) translocation of the PKC isoforms alpha, epsilon, and zeta. Translocation of PKC delta was not detected. The effect of bradykinin was reduced by simultaneous addition of the receptor antagonist HOE 140, a bradykinin-related decapeptide. The data show that the B2 receptor in this cell model is able to activate, in addition to the classical PKC isoform alpha, the new PKC isoform epsilon and the atypical PKC isoform zeta. To test whether these effects are as well observed in a non-transfected cell, the experiments were repeated in human foreskin fibroblasts which naturally express high levels of B2 receptors. In this cell system similar results on PKC alpha, epsilon, and zeta were observed, suggesting that all three PKC isoforms are involved in signal transduction of the B2 receptor.

15-HETE: selective incorporation into inositol phospholipids of MDCK cells

The interaction of 15-hydroxyeicosatetraenoic acid (15-HETE) and cultured MDCK renal tubular epithelial cells was investigated to determine whether incorporation of this lipoxygenase product will affect polyphosphoinositide formation. MDCK cells were incubated with 1 microM [3H]-15-HETE for 15 to 120 minutes. Maximum uptake occurred between 15 and 30 minutes, and after 60 minutes, 70% of the incorporated 15-HETE was present in the phosphatidylinositol (PI) fraction. Some 15-HETE was also incorporated into phosphatidylinositol-4-monophosphate (PIP) and phosphatidylinositol-4,5-bisphosphate (PIP2). However, even though more 15-HETE than arachidonic acid was incorporated into PI, the fractional amount of 15-HETE present in the polyphosphoinositides was smaller than arachidonic acid. Therefore, although 15-HETE is selectively channeled into PI, conversion of PI species containing 15-HETE to PIP and PIP2 is relatively impaired. This suggests that either PI containing 15-HETE is a less effective substrate for phosphorylation, or PI containing arachidonic acid is a preferred substrate. MDCK cells converted 15-HETE to polar metabolites that were released into the extracellular fluid. This process may constitute a renal tubular mechanism for the clearance of 15-HETE and related lipoxygenase products.

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)

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)

Inositol-lipid-specific phospholipase C isoenzymes and their differential regulation by receptors

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Stimulation of adenosine A1 receptors and bradykinin receptors, which act via different G proteins, synergistically raises inositol 1,4,5-trisphosphate and intracellular free calcium in DDT1 MF-2 smooth muscle cells

We have examined the cross talk between adenosine and bradykinin receptors in DDT1 MF-2 smooth muscle cells. Both adenosine and bradykinin mobilized intracellular free calcium via the formation of inositol 1,4,5-trisphosphate in a time- and dose-dependent manner. Adenosine exerted its actions via adenosine A1 receptors as demonstrated by the observations that N6-cyclopentyladenosine, a selective A1 receptor agonist, had an EC50 in the low nanomolar range and that a selective adenosine A1 receptor antagonist, 8-cyclopentyl-1,3-dipropylxanthine, counteracted adenosine-mediated responses at concentrations typical for signaling via adenosine A1 receptors. Adenosine A1 receptors were coupled to phospholipase C via pertussis toxin-sensitive guanine nucleotide-binding regulatory protein(s) [G protein(s)], whereas bradykinin responses were unaffected by pertussis toxin. When adenosine or N6-cyclopentyladenosine was combined with bradykinin, the resulting formation of inositol 1,4,5-triphosphate was more than additive, and the EC50 value for adenosine and N6-cyclopentyladenosine was shifted to the left by bradykinin, the affinity of which was unaltered. Combining N6-cyclopentyladenosine and bradykinin also synergistically raised intracellular free calcium both at subthreshold levels and at maximal concentrations of the two agonists. The interaction was not dependent upon cAMP. In conclusion, stimulation of adenosine A1 receptors coupled to pertussis toxin-sensitive G protein(s) and bradykinin receptors coupled to pertussis toxin-insensitive G protein(s) synergistically mobilizes intracellular free calcium and inositol 1,4,5-trisphosphate formation.

Effect of bradykinin, ATP and adrenaline on cell membrane resistances of Madin-Darby canine kidney cells

1. Previous studies have shown that bradykinin, ATP and adrenaline hyperpolarize the cell membrane of Madin-Darby canine kidney (MDCK) cells by activation of calcium-sensitive K+ channels. The present study has been performed to determine the effect of these hormones on the resistance of the cell membrane and the cellular coupling. To this end, cellular cable analysis has been performed. 2. As a result, all three hormones lead to the expected, marked decrease of cell membrane resistance. 3. However, the bradykinin-induced reduction of cell membrane resistance was sustained, contrasting with only transient hyperpolarization induced by bradykinin and only transient activation of the K+ channels. Thus, the cable analysis reveals the sustained activation of an additional conductance. 4. ATP, but not the other two hormones, leads to a delayed increase of the intercellular coupling resistances. 5. Prolonged exposure of the cells to adrenaline leads to oscillations of the cell membrane potential, apparently by oscillatory activation of the K+ channels.

Action of bradykinin at the cyclooxygenase step in prostanoid synthesis through the arachidonic acid cascade

Bradykinin enhances prostanoid synthesis in aorta smooth muscle cells. Free arachidonic acid also enhances prostanoid synthesis and bradykinin, unlike fatty acid releasing agents, has a synergistic effect with free arachidonic acid. Bradykinin promotes metabolite release from cells prelabeled with [14C]-arachidonic acid and this effect is blocked completely by indomethacin. High performance liquid chromatography shows increase amounts of labeled 6-keto-prostaglandin F1 alpha, prostaglandin E2 and three additional cyclooxygenase-dependent metabolites but no increase in free arachidonic acid or other metabolites either in the absence or presence of indomethacin. Fatty acid releasing agents such as A23187 and cyclosporine A have very different effects on cells. These agents enhance levels of prostanoids, a number of other cyclooxygenase-independent metabolites, and free arachidonic acid which is even more elevated with added indomethacin. Bradykinin behaves in all respects like another agent, bacterial lipopolysaccharide, and the action of both agents is consistent with a mechanism involving cyclooxygenase rather than fatty release in the arachidonic acid cascade.

Further analysis of ATP-mediated activation of K+ channels in renal epithelioid Madin Darby canine kidney (MDCK) cells

ATP activates K+ channels by increasing intracellular calcium activity in Madin Darby canine kidney (MDCK) cells. The present study has been performed to test for the involvement of G-proteins and of protein kinase C in the intracellular transmission of these effects. To this end, the effect of ATP on intracellular calcium and K+ channel activity has been studied in cells pretreated with the phorbol ester 12-O-tetradecanoyl-phorbol 13-acetate (TPA) and/or pertussis toxin. The ATP-induced increase of intracellular calcium is not significantly affected by pretreatment with pertussis toxin, is significantly blunted by pretreatment with TPA and is abolished by pretreatment with both pertussis toxin and the phorbol ester. The ATP activation of K+ channels is similarly blunted by pretreatment with TPA, but is not abolished by pretreatment with both the phorbol ester and pertussis toxin. Furthermore, the ATP-induced hyperpolarization is not abolished in cells pretreated with both pertussis toxin and TPA. In those cells, ATP may activate K+ channels by calcium-dependent mechanisms or lead to localized increases of intracellular calcium sufficient to activate the K+ channels but escaping detection with fura-2 fluorescence.

Assembly and sealing of tight junctions: possible participation of G-proteins, phospholipase C, protein kinase C and calmodulin

The making and sealing of a tight junction (TJ) requires cell-cell contacts and Ca2+, and can be gauged through the development of transepithelial electrical resistance (TER) and the accumulation of ZO-1 peptide at the cell borders. We observe that pertussis toxin increases TER, while AIF3 and carbamil choline (carbachol) inhibit it, and 5-guanylylimidodiphosphate (GTPTs) blocks the development of a cell border pattern of ZO-1, suggesting that G-proteins are involved. Phospholipase C (PLC) and protein kinase C (PKC) probably participate in these processes since (i) activation of PLC by thyrotropin-1 releasing hormone increases TER, and its inhibition by neomycin blocks the development of this resistance; (ii) 1,2-dioctanoylglycerol, an activator of PKC, stimulates TER development, while polymyxin B and 1-(5-isoquinoline sulfonyl)-2-methyl-piperazine dihydrochloride (H7), which inhibit this enzyme, abolish TER. Addition of 3-isobutyl-1-methyl-xanthine, dB-cAMP or forskolin do not enhance the value of TER, but have just the opposite effect. Trifluoperazine and calmidazoline inhibit TER development, suggesting that calmodulin (CaM) also plays a role in junction formation. These results indicate that junction formation may be controlled by a network of reactions where G-proteins, phospholipase C, adenylate cyclase, protein kinase C and CaM are involved.

Cellular mechanisms of bradykinin-induced hyperpolarization in renal epitheloid MDCK-cells

Previous studies have demonstrated that bradykinin hyperpolarizes the cell membrane of subconfluent MDCK cells by increase of the potassium conductance. The present study has been performed to elucidate the intracellular mechanisms involved. To this end, the effects of bradykinin on the potential difference across the cell membrane (PD), on formation of inositol phosphates, and on intracellular calcium concentration (Cai) have been analyzed in cells without or with pretreatment with pertussis toxin or 12-O-tetradecanoylphorbol 13-acetate diester (TPA). In untreated cells, bradykinin leads to a transient increase of inositol 1,4,5-trisphosphate and inositol 1,3,4,5-tetrakisphosphate, increase of Cai, activation of potassium channels and hyperpolarization of the cell membrane. The effects of bradykinin on PD and Cai are still present in the absence of extracellular calcium. In cells pretreated with pertussis toxin the effect of bradykinin on inositol trisphosphate formation is almost abolished but bradykinin still leads to a transient increase of Cai and PD in the presence and absence of extracellular calcium. In cells pretreated with TPA the bradykinin-induced increase of inositol trisphosphate formation is blunted, the bradykinin-induced increase of Cai abolished, but the bradykinin-induced hyperpolarization still present. The observations indicate that bradykinin increases Cai in part by phorbol ester and pertussis toxin sensitive activation of phospholipase C. In addition, bradykinin is capable of enhancing Cai by utilizing pertussis toxin insensitive mechanisms. Furthermore, bradykinin is able to transiently enhance the potassium conductance without a general increase of intracellular calcium.

Cellular mechanisms of ATP-induced hyperpolarization in renal epitheloid MDCK-cells

Previous studies have shown that ATP enhances intracellular calcium concentration and activates potassium channels in Madin Darby canine kidney (MDCK)-cells, thus leading to hyperpolarization of the cell membrane. The present study has been performed to elucidate the intracellular mechanisms involved. To this end, the effects of ATP on the potential difference across the cell membrane (PD), on formation of inositol phosphates, and on intracellular calcium concentration (Cai) have been analyzed in cells without or with pretreatment with pertussis toxin or 12-O-tetradecanoyl phorbol 13-acetate diester (TPA). In untreated cells, ATP leads to a sustained hyperpolarization and an increase of inositol 1,4,5-trisphosphate (IP3), inositol 1,3,4,5-tetrakisphosphate (IP4), and Cai. In the absence of extracellular calcium, the effect of ATP on PD and Cai is only transient. In cells pretreated with pertussis toxin, the effect of ATP on inositol trisphosphate is almost abolished, but ATP still leads to an increase of PD and Cai, which is sustained in the presence, and transient in the absence, of extracellular calcium. In cells pretreated with TPA, the effect of ATP on inositol trisphosphate is reduced and the effect on Cai blunted; but ATP still leads to a hyperpolarization of the cell membrane, which is sustained in the presence, and transient in the absence, of extracellular calcium. The observations indicate that ATP activates phospholipase C by a phorbol ester and pertussis toxin sensitive mechanism. In addition, ATP enhances Cai by pertussis toxin insensitive mechanisms allowing recruitment of calcium from both, extracellular fluid and intracellular stores. Calcium then activates the potassium channels and thus leads to the hyperpolarization of the cell membrane.

Proximal tubular epithelial cells possess a novel 42-kilodalton guanine nucleotide-binding regulatory protein

The proximal tubule of the kidney represents an important location where adenylate cyclase regulates salt and water transport; yet a detailed characterization of the distribution and classification of guanine nucleotide-binding protein (G protein) and adenylate cyclase is lacking. We used purified brush border (20-fold) and basolateral membranes (14-fold) to characterize parathyroid hormone- and G protein-regulated adenylate cyclase and G-protein distribution. Adenylate cyclase was predominantly localized to basolateral membranes, while the 46-kDa alpha subunit of the stimulatory G protein (Gs) was 2-fold higher in brush border membranes than in basolateral membranes. The alpha subunit of the inhibitory G protein (Gi; 41 kDa) was equally distributed on immunoblotting but was 2-fold higher in brush border membranes than in basolateral membranes on radiolabeling with pertussis toxin. A 42-kDa cholera toxin substrate that cross-reacted with antisera to the common alpha subunit of G proteins and to Gs on immunoblotting and that was not immunoprecipitated with two Gi antisera was the most abundant alpha subunit and comprised approximately 1% of the total membrane proteins. These observations suggest that G proteins are important regulators of proximal tubular transport independent of adenylate cyclase.

Peptide-dependent regulation of epithelial nephron functions

It has become evident that the nephron is an important target organ of many of the regulatory peptides; this brief overview will not attempt to consider the vast amount of work on peptide-dependent kidney functions; instead, it will emphasize recent work directed towards understanding intracellular signal pathways between peptide ligand-receptor interaction and expression of physiological transport responses in renal epithelial cells. The awareness that peptide hormones of differing origin, e.g., intestinal and cardiac, share at least some of the signal steps in nephron cells, has stimulated work on nephron segmental analysis of receptor binding, of second messengers, of membrane G proteins, of protein phosphorylation, and of final membrane transport responses, such as peptide-dependent ion channel regulation. Peptides involved in cell growth and differentiation, e.g., growth factors, appear to act through part of the signal pathway shared by other peptides. The peptides selected for the purpose of this review, then, are those that have been linked, by experimental evidence, to intracellular messenger systems in nephron epithelia.

Interaction of cerebral-cortical membranes with exogenously added phosphatidylinositol 4,5-bisphosphate. Effects on measured phospholipase C activity

Exogenously added phosphatidylinositol 4,5-bisphosphate (PtdInsP2) is rapidly associated with cerebral-cortical membranes. Substrate association with membranes was promoted by Mg2+, but inhibited by bivalent chelators. Once associated with the membrane, the PtdInsP2 was resistant to displacement by EDTA. The apparent phospholipase C activity was dependent on the degree of association of substrate with membranes. After preincubation of membranes with substrate, PtdInsP2 hydrolysis was independent of the incubation volume, indicating that substrate and membrane-associated phospholipase C were not independently diluted. Hydrolysis of the membrane-associated substrate was stimulated by Ca2+, guanosine 5'-[beta gamma-imido]triphosphate (p[NH]ppG), guanosine 5'[gamma-thio]triphosphate and carbachol in the presence of p[NH]ppG. Carbachol in the absence of guanine nucleotides, GDP, GTP, ATP and pyrophosphate was ineffective. These results demonstrate that exogenously added PtdInsP2 substrate is rapidly associated with membranes and hydrolysed by a phospholipase C whose activity is regulated by guanine nucleotides and agonist in the presence of guanine nucleotides. Use of exogenously added substrate for studies on the regulation of membrane phospholipase C requires consideration as to possible effects of incubation conditions on the partitioning of substrate into membranes.

Bradykinin receptors: characterization, distribution and mechanisms of signal transduction

Bradykinin is a peptide consisting of nine amino acids. It is a member of the kinin family, a class of molecules sometimes considered to be locally acting hormones. Bradykinin acts through cell surface receptors to elicit a series of biological responses, many of which have been well characterized at the whole organ or body level. However, little is known about the bradykinin receptor itself or its mechanisms of signal transduction, its function and its tissue distribution. Increasing evidence suggests that bradykinin is a member of a group of locally produced peptides which may act in a paracrine fashion as microenvironmental modulators of cell proliferation. Evidence for this derives from studies of the interaction between bradykinin and its receptor, receptor-effector coupling systems and in vitro studies of the biological effects of bradykinin. These areas, together with questions concerning the nature and number of different types of bradykinin receptors, form the main bulk of current interest in bradykinin research and are the subject of this review. The ability of bradykinin to synergize with other growth regulating ligands will also be considered.

Bradykinin-induced generation of inositol 1,4,5-trisphosphate in fibroblasts and neuroblastoma cells: effect of pertussis toxin, extracellular calcium, and down-regulation of protein kinase C

The net content of inositol 1,4,5-trisphosphate [Ins(1,4,5)P3] was measured in bradykinin (BK)-stimulated NIH3T3 fibroblasts and neuroblastoma-glioma hybrid cells (NG108-15). BK-mediated production of Ins(1,4,5)P3 was not affected by replacing the medium with Ca2+-free medium, but addition of EGTA (1mM) to Ca2+-free medium markedly prevented production of Ins(1,4,5)P3. Although pertussis toxin (PT) treatment caused ADP-ribosylation in both NIH3T3 cells and NG108-15 cells, the BK-induced Ins(1,4,5)P3 formation was considerably reduced in the former cells but not in the latter cells, suggesting that PT-sensitive and PT-insensitive GTP-binding proteins are involved in phosphoinositide phospholipase C (PI-PLC) activation in fibroblasts and neuroblastoma cells, respectively. In NG108-15 cells down-regulated in protein kinase C (PKC) by long-term exposure to phorbol 12-myristate 13-acetate (PMA), BK-stimulated Ins(1,4,5)P3 accumulation was significantly enhanced compared to control cells.

Defective formation of inositol 1,4,5-trisphosphate in bradykinin-stimulated fibroblasts from progressive systemic sclerotic patients

The effect of bradykinin on inositol 1,4,5-trisphosphate (1,4,5-IP3) formation was investigated in fibroblasts from normal subjects and patients with progressive systemic sclerosis (PSS). 1,4,5-IP3 in both PSS and normal fibroblasts reached peak levels at 15 sec after stimulation with bradykinin, though this level was significantly lower in PSS fibroblasts than in normal cells. There was no difference in 1,4,5-IP3 content between islet-activating protein (IAP)-treated and untreated cells in either PSS or normal fibroblasts. These findings suggest that bradykinin stimulates phosphoinositide hydrolysis by phospholipase C in human fibroblasts via IAP-insensitive pathway, and that PSS fibroblasts appear to be defective in the pathway.