Gap junction-dependent and -independent EDHF-type relaxations may involve smooth muscle cAMP accumulation

We have compared the mechanisms that contribute to endothelium-derived hyperpolarizing factor (EDHF)-type responses induced by ACh and the Ca(2+) ionophore A-23187 in the rabbit iliac artery. Relaxations to both agents were associated with ~1.5-fold elevations in smooth muscle cAMP levels and were attenuated by the adenylyl cyclase inhibitor 2',5'-dideoxyadenosine (DDA) and potentiated by the cAMP phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX). Mechanical responses were inhibited by coadministration of the Ca(2+)-activated K(+) channel blockers apamin and charybdotoxin, both in the absence and presence of IBMX, but were unaffected by blockade of ATP-sensitive K(+) channels with the sulphonylurea glibenclamide. Relaxations and elevations in cAMP evoked by ACh were abolished by 18alpha-glycyrrhetinic acid, which disrupts gap junction plaques, whereas the corresponding responses to A-23187 were unaffected by this agent. Consistently, in "sandwich" bioassay experiments, A-23187, but not ACh, elicited extracellular release of a factor that evoked relaxations that were inhibited by DDA and potentiated by IBMX. These findings provide evidence that EDHF-type relaxations of rabbit iliac arteries evoked by ACh and A-23187 depend on cAMP accumulation in smooth muscle, but involve signaling via myoendothelial gap junctions and the extracellular space, respectively.     

Alterations in EDHF-type relaxation and phosphodiesterase activity in mesenteric arteries from diabetic rats

In isolated superior mesenteric artery rings from age-matched control rats and streptozotocin (STZ)-induced diabetic rats, we investigated the role of cAMP in endothelium-derived hyperpolarizing factor (EDHF)-type relaxation. The ACh-induced EDHF-type relaxation was significantly weaker in STZ-induced diabetic rats than in control rats, and in both groups of rats it was attenuated by 18alpha-glycyrrhetinic acid (18alpha-GA), an inhibitor of gap junctions, and enhanced by IBMX, a cAMP-phosphodiesterase (PDE) inhibitor. These enhanced EDHF-type responses were very similar in magnitude between diabetic and age-matched control rats. The EDHF-type relaxation was enhanced by cilostamide, a PDE3-selective inhibitor, but not by Ro 20-1724, a PDE4-selective inhibitor. The expression levels of the mRNAs and proteins for two cAMP PDEs (PDE3A, PDE3B) were significantly increased in STZ-induced diabetic rats, but those for PDE4D were not. We conclude that the impairment of EDHF-type relaxations in STZ-induced diabetic rats may be attributed to a reduction in the action of cAMP via increased PDE activity.     

Catecholamine-induced desensitization in turkey erythrocytes: cAMP mediated impairment of high affinity agonist binding without alteration in receptor number

Desensitization of turkey erythrocyte adenylate cyclase by exposure of these cells to the beta-adrenergic agonist isoproterenol leads to a decrease in subsequent adenylate cyclase stimulation by isoproterenol, F-, or Gpp(NH)p without any apparent loss or down regulation of receptors (B.B. Hoffman et al. J. Cyclic Nucl. Res. 5: 363-366, 1979). We now report that the desensitization is associated with a functional "uncoupling" of the beta-adrenergic receptor. This is evidenced by an impaired ability of receptors to form a high affinity, guanine nucleotide sensitive complex with agonist as assessed by computer analysis of radioligand binding data. The changes in adenylate cyclase responsiveness as well as the alterations in receptor affinity for agonists are reproduced by incubation of turkey erythrocytes with the cAMP analog 8-Bromo-adenosine 3':5'- cyclic monophosphate. These findings suggest that one possible mechanism for the development of desensitization in adenylate cyclase systems may be a cAMP mediated alteration of a component(s) of the beta-adrenergic receptor-adenylate cyclase complex which results in impaired receptor-cyclase coupling.     

Regulation of Vascular Endothelial Growth Factor Expression by cAMP in Rat Aortic Smooth Muscle Cells

Vascular endothelial growth factor (VEGF) is an endothelial cell mitogen which stimulates angiogenesis. VEGF is regulated by multiple factors such as hypoxia, phorbol esters, and growth factors. However, data concerning the expression of VEGF in the different vascular cell types and its regulation by cAMP are not available. In the present study, we have investigated the effect of adenylate cyclase activation on VEGF mRNA expression in rat vascular cells in primary culture. Basal VEGF expression is greater in smooth muscle cells than in endothelial cells and fibroblasts. A 4-h treatment with forskolin (10⁻⁵M) induced a 2-fold stimulation of VEGF mRNA expression in smooth muscle cells and fibroblasts, but, in contrast, did not affect VEGF expression in endothelial cells. In smooth muscle cells, a pharmacologically induced increase in intracellular cAMP levels using iloprost or isoprenaline led to a rise in VEGF mRNA expression comparable to that induced by forskolin. Adenosine, which increases cAMP levels in smooth muscle cells, also increases VEGF expression. Moreover, the 2.2-fold stimulation of VEGF expression by adenosine was enhanced following a cotreatment with cobalt chloride (a hypoxia miming agent). The observed additive effect (4.3-fold increase) suggests that these two factors, hypoxia and adenosine, regulate VEGF mRNA expression in smooth muscle cells by independent mechanisms.     

Angiogenin depresses aortic smooth muscle cell cAMP by a pertussis toxin sensitive mechanism

Angiogenin transiently depresses the cAMP level of rat aortic smooth muscle cells. The dose response is similar to angiogenin activation of the inositol-specific phospholipase C in this cell line [Moore, F. & Riordan, J.F. (1989) Biochemistry. Submitted]. The time course showed a maximal depression (28%) in cAMP at 2 min, followed by a return to that of unstimulated cells by 3.5 min. Angiogenin also inhibited isoproterenol stimulated cAMP formation, but the percentage depression in cAMP (9%) was less than that in cells treated with angiogenin alone (28%). In contrast angiogenin enhanced forskolin stimulation of adenylate cyclase, an effect previously linked with agonist activation of protein kinase C. The effect of angiogenin on cellular cAMP was abolished by pre-incubation with pertussis toxin. Angiogenin had no effect on cellular cGMP. These results are consistent with activation of adenylate cyclase Gi following exposure of the cells to angiogenin and provide further evidence for interaction between cellular signalling pathways.     

Gap junction-dependent increases in smooth muscle cAMP underpin the EDHF phenomenon in rabbit arteries

We have investigated the role of cAMP in nitric oxide (NO)- and prostanoid-independent vascular relaxations evoked by acetylcholine (ACh) in isolated arteries and perfused ear preparations from the rabbit. These EDHF-type responses are shown to be associated with elevated cAMP levels specifically in smooth muscle and are attenuated by blocking adenylyl cyclase or protein kinase A (PKA). Relaxations are amplified by 3-isobutyl-1-methylxanthine, which prevents cAMP hydrolysis, while remaining susceptible to inhibition by the combination of two K(Ca) channel blockers, apamin and charybdotoxin. Analogous endothelium- and cAMP-dependent relaxations were evoked by cyclopiazonic acid (CPA) which stimulates Ca(2+) influx via channels linked to the depletion of Ca(2+) stores. Responses to ACh and CPA were both inhibited by interrupting cell-to-cell coupling via gap junctions with 18alpha-glycyrrhetinic acid and a connexin-specific Gap 27 peptide. The findings suggest that EDHF-type responses are initiated by capacitative Ca(2+) influx into the endothelium and propagated by direct intercellular communication to effect relaxation via cAMP/PKA-dependent phosphorylation events in smooth muscle.     

The regulation of adenylate cyclase by guanine nucleotides in Dictyostelium discoideum membranes

Extracellular cAMP induces the activation of adenylate cyclase in Dictyostelium discoideum cells. Conditions for both stimulation and inhibition of adenylate cyclase by guanine nucleotides in membranes are reported. Stimulation and inhibition were induced by GTP and non-hydrolysable guanosine triphosphates. GDP and non-hydrolysable guanosine diphosphates were antagonists. Stimulation was maximally twofold, required a cytosolic factor and was observed only at temperatures below 10 degrees C. An agonist of the cAMP-receptor-activated basal and GTP-stimulated adenylate cyclase 1.3-fold. Adenylate cyclase in mutant N7 could not be activated by cAMP in vivo; in vitro adenylate cyclase was activated by guanine nucleotides in the presence of the cytosolic factor of wild-type but of not mutant cells. Preincubation of membranes under phosphorylation conditions has been shown to alter the interaction between cAMP receptor and G protein [Van Haastert (1986) J. Biol. Chem. in the press]. These phosphorylation conditions converted stimulation to inhibition of adenylate cyclase by guanine nucleotides. Inhibition was maximally 30% and was not affected by the cytosolic factor involved in stimulation. In membranes obtained from cells that were treated with pertussis toxin, adenylate cyclase stimulation by guanine nucleotides was as in control cells, whereas inhibition by guanine nucleotides was lost. When cells were desensitized by exposure to cAMP agonists for 15 min, and adenylate cyclase was measured in isolated membranes, stimulation by guanine nucleotides was lost while inhibition was retained. These results suggest that Dictyostelium discoideum adenylate cyclase may be regulated by Gs-like and Gi-like activities, and that the action of Gs but not Gi is lost during desensitization in vivo and by phosphorylation conditions in vitro.     

Stimulation of adenylate cyclase by adenosine and other agonists in mesenteric artery smooth muscle cells in culture

An adenosine-sensitive adenylate cyclase has been characterized in cultured mesenteric artery smooth muscle cells. N-Ethylcarboxamide-adenosine (NECA), N-Methylcarboxamide-adenosine (MECA), L-N6-phenylisopropyladenosine (PIA) and 2-chloroadenosine (2-cl-Ado) all stimulated adenylate cyclase in a concentration dependent manner. NECA was the most potent analog (EC50, 1 microM), whereas PIA (EC50, 15 microM), 2-Cl-Ado (EC50, 15 microM) and MECA (EC50, 24 microM), were less potent and had efficacies relative to NECA of 0.61, 0.61 and 0.65, respectively. Adenosine showed a biphasic effect: stimulation at lower concentrations and inhibition at higher concentrations, whereas 2' deoxyadenosine only inhibited adenylate cyclase activity. The stimulatory effect of NECA on adenylate cyclase was dependent on metal ion concentration and was blocked by 3-isobutyl-l-methylxanthine (IBMX) and 8-phenyltheophylline (8-PT). Adenylate cyclase from these cultured cells was also stimulated by other agonists such as epinephrine, norepinephrine, prostaglandins, dopamine, NaF and forskolin. The stimulation of adenylate cyclase by isoproterenol, epinephrine and norepinephrine was blocked by propranolol but not by phentolamine. On the other hand, phentolamine, propranolol and flupentixol all inhibited dopamine-stimulated adenylate cyclase activity. In addition, the stimulation by an optimal concentration of PIA was additive or almost additive with maximal stimulation caused by catecholamines and prostaglandins. These data indicate the presence of adenosine (Stimulatory "Ra"), catecholamine and prostaglandin receptors in mesenteric artery smooth muscle cells and suggest that these agents may exert their physiological actions through their interaction with their respective receptors coupled to adenylate cyclase.     

Endothelial control of vascular tone by nitric oxide and gap junctions: a haemodynamic perspective

Local haemodynamic forces acting on the endothelium modulate vascular tone through mechanisms that normalize intimal shear stress. This flow-dependent diameter response contributes to the optimization of circulatory function and is mediated via shear stress-induced release of NO, vasodilator prostanoids and a putative endothelium-derived hyperpolarizing factor or EDHF. There is growing evidence that NO/prostanoid independent relaxations involve direct heterocellular signalling between endothelial and smooth muscle cells via gap junctions.     

Modulation by islet-activating protein of adenylate cyclase activity in C6 glioma cells

The cAMP content of intact cells as well as adenylate cyclase of the membrane-rich particulate fractions was studied with C6 glioma cells that had been exposed to the culture medium supplemented with islet-activating protein (IAP), one of the pertussis toxins. Both the increase in the cellular cAMP content in response to a beta-adrenergic agonist and the stimulation of membrane adenylate cyclase by the beta-agonist and/or GTP were markedly enhanced by the IAP treatment of C6 cells, but no change was induced in affinities of the agonist (or an antagonist) or GTP for their respective sites of action (or binding). The concentration of IAP required for the half-maximal enhancement was as low as 1 pg/ml, when the time of cell exposure to the toxin was prolonged to 18 h. No enhancement was observed for the basal cAMP content or basal enzyme activity, nor was activation of adenylate cyclase by Gpp(NH)p (or NaF) affected by IAP treatment. The Vmax value of a specific and low Km GTPase was significantly smaller in the membranes of IAP-treated cells than in those of control cells. Cholera toxin treatment of cells activated adenylate cyclase without exerting any influence on these IAP actions. Thus, IAP would appear to enhance beta-receptor-coupled stimulation of adenylate cyclase, in a manner distinct from cholera toxin, by rendering more GTP available to the GTP sites on the regulatory subunit of the receptor-enzyme system.     

Presence of “Ra” and “P”-site receptors for adenosine coupled to adenylate cyclase in cultured vascular smooth muscle cells

The existence of adenosine receptors coupled to adenylate cyclase in cultured vascular smooth muscle cells from rat aorta is demonstrated in these studies. Adenosine, N⁶-phenylisopropyladenosine, adenosine N′-oxide and 2-chloroadenosine stimulated adenylate cyclase in a concentration dependent manner. The stimulation was dependent on the presence of guanine nucleotides and was blocked by 3-isobutyl-1-methylxanthine. In contrast, 2′ deoxyadenosine inhibited adenylate cyclase activity. Adenosine and 2-chloroadenosine showed a biphasic effect on adenylate cyclase, stimulation occurred at low concentrations. The activation of adenylate cyclase by N⁶-phenylisopropyladenosine was also dependent on the Mg²⁺ concentration. The data suggest that vascular smooth muscle cells have both “Ra” and “P” receptors for adenosine, and it can be postulated that the relaxant effect of adenosine on vascular smooth muscle may be mediated by its interaction with “Ra” receptors associated with adenylate cyclase.     

Preparation of hormone-sensitive airway smooth muscle adenylate cyclase from dissociated canine trachealis cells

Conventional homogenizing methods produced membrane preparations of canine trachealis airway smooth muscle which contained adenylate cyclase activity that was stimulated by fluoride but not by isoproterenol. We have devised methods using collagenase digestion of minced trachealis which destroy most of the tough connective tissues but leave dissociated canine trachealis cells in suspension. Gentle homogenization of these cells permitted preparation of a particulate fraction containing adenylate cyclase that was readily stimulated by beta-adrenergic agonist of prostaglandin E2. Isoproterenol stimulation was 2.34 +/- 0.58 (S.E.) times basal and 122 +/- 25% of the stimulation induced by NaF. The beta-adrenergic blocking agent propranolol prevented isoproterenol-induced stimulation of the cyclase but had no effect on prostaglandin E2 stimulation. Catecholamine order of potency was isoproterenol greater than epinephrine greater than norepinephrine. These methods enable demonstration of stimulatory effects of hormones in broken cell preparations of airway smooth muscle that are comparable to those when hormone-stimulated cyclic AMP formation is measured in intact muscle strips.     

Differential effects of prostaglandin E2 on contractions of airway smooth muscle

In an in vitro muscle bath, the active tension generated by strips of canine tracheal smooth muscle responding to cumulative additions of either histamine (10(-8) to 10(-3) M) or acetylcholine (10(-9) to 10(-3) M) was measured in the absence and presence of prostaglandin E2 (PGE2) (10(-6) to 10(-5) M). When contractile responses of equal magnitude were compared, the contractions elicited by acetylcholine were resistant to the inhibitory effects of PGE2, relative to comparable contractions elicited by histamine. To assess the role of adenylate cyclase in determining the different responses to histamine and acetylcholine in the presence of PGE2, we assayed adenylate cyclase activity in membranes prepared from canine tracheal smooth muscle and found that acetylcholine, but not histamine, decreased PGE2-stimulated adenylate cyclase activity by 48 +/- 2% (mean +/- SE; n = 5). However, in other experiments, we found that even large pharmacological increases in tissue adenosine 3',5'-cyclic monophosphate (cAMP) content only partially inhibited muscarinic tone. Also, exogenously applied analogues of cyclic AMP inhibited contractions induced by histamine more effectively than comparable contractions induced by acetylcholine. We concluded that acetylcholine decreased adenylate cyclase activity in membranes prepared from canine tracheal smooth muscle and that this effect may have contributed to, but did not completely account for, the relative resistance of muscarinic contractions to the inhibitory effects of PGE2.     

Cyclic 3',5'-adenosine monophosphate modulates vascular endothelial cell migration in vitro

Using a modified Boyden chamber assay, we have examined the effect of cyclic nucleotides on bovine aortic endothelial cell migration in vitro. Dibutyrl cyclic 3',5'-adenosine monophosphate (5 mM) inhibited endothelial cell random migration by 67% and inhibited fibronectin-induced chemotaxis by 75%. Agents which significantly stimulated adenylate cyclase activity in endothelial cell membranes were also effective inhibitors of endothelial cell migration. Timolol blocked both the isoproterenol-induced stimulation of adenylate cyclase and the ability of isoproterenol to inhibit endothelial cell migration. Caffeine and isoproterenol together had a greater inhibitory effect on endothelial cell motility than either alone. These data suggest that cAMP may modulate vascular endothelial cell migration in an inhibitory fashion.     

EDHF is not K+ but may be due to spread of current from the endothelium in guinea pig arterioles

Endothelium-derived hyperpolarizing factor (EDHF)-attributed hyperpolarizations and relaxations were recorded simultaneously from submucosal arterioles of guinea pigs with the use of intracellular microelectrodes and a video-based system, respectively. Membrane currents were recorded from electrically short segments of arterioles under single-electrode voltage clamp. Substance P evoked an outward current with a current-voltage relationship that was well described by the Goldman-Hodgkin-Katz equation for a K+ current, consistent with the involvement of intermediate- and small-conductance Ca2+-activated K+ channels. 1-Ethyl-2-benzimidazolinone relaxed the arterioles and evoked hyperpolarizations that were blocked by charybdotoxin, but not by iberiotoxin. Application of K+ induced depolarization under conditions in which EDHF evoked hyperpolarization. The Ba2+-sensitive component of the K+-induced current was inwardly rectifying, in contrast to the outwardly rectifying current evoked by substance P. EDHF-attributed hyperpolarizations in dye-identified smooth muscle cells were indistinguishable from those recorded from dye-identified endothelial cells in the same arterioles. These results provide evidence that EDHF is not K+ but may involve electrotonic spread of hyperpolarization from the endothelial cells to the smooth muscle cells.     

Inhibition of basal and hormone-stimulated adenylate cyclase in adipocyte ghosts by the prostaglandin endoperoxide prostaglandin H2.

The prostaglandin endoperoxide PGH2 (15-hydroxy-9alpha, 11alpha-peroxidoprosta-5,13-dienoic acid), at a concentration of 2.8 x 10(-5) M inhibited basal adenylate cyclase activity 11% and epinephrine-stimulated activity 30 to 35%. PGH2 inhibited epinephrine-stimulated enzyme activity in the presence of 10 mM theophylline, 2.5 mM adenosine 3':5'-monophosphate (cAMP), or in the absence of inhibitors or substrates of the cAMP phosphodiesterase. When the cAMP phosphodiesterase was assayed directly using 62 nM and 1.1 muM cAMP, PGH2 did not affect the 100,000 x g particulate cAMP phosphodiesterase from fat cells. The inhibition of adenylate cyclase by PGH2 was readily reversible. A 6-min preincubation of ghost membranes with PGH2, followed by washing, did not alter subsequent epinephrine-stimulated adenylate cyclase activity. During epinephrine stimulation, the PGH2 inhibition was apparent on initial rates of cAMP synthesis, and the addition of PGH2 to the enzyme system at any point during an assay markedly reduced the rate of cAMP synthesis. Between 2.8 x 10(-7) M and 2.8 x 10(-5) M, PGH2 inhibited epinephrine-stimulated enzyme activity in a concentration-dependent manner. The stimulation of adenylate cyclase by thyroid-stimulating hormone, glucagon, and adrenocorticotropic hormone as well as by epinephrine was antagonized by PGH2, suggesting that PGH2 may be an endogenous feedback regulator of hormone-stimulated lipolysis in adipose tissue.     

Ring-deleted analogs of atrial natriuretic factor inhibit adenylate cyclase/cAMP system. Possible coupling of clearance atrial natriuretic factor receptors to adenylate cyclase/cAMP signal transduction system

We have recently shown that atrial natriuretic factor (ANF) inhibits adenylate cyclase activity in rat platelets where only one population of ANF receptors (ANF-R2) is present, indicating that ANF-R2 receptors may be coupled to the adenylate cyclase/cAMP system. In the present studies, we have used ring-deleted peptides which have been reported to interact with ANF-R2 receptors also called clearance receptors (C-ANF) without affecting the guanylate cyclase/cGMP system, to examine if these peptides can also inhibit the adenylate cyclase/cAMP system. Ring-deleted analog C-ANF4-23 like ANF99-126 inhibited the adenylate cyclase activity in a concentration-dependent manner in rat aorta, brain striatum, anterior pituitary, and adrenal cortical membranes. The maximal inhibition was about 50-60% with an apparent Ki between 0.1 and 1 nM. In addition, C-ANF4-23 also decreased the cAMP levels in vascular smooth muscle cells in a concentration-dependent manner without affecting the cGMP levels. The maximal decrease observed was about 60% with an apparent Ki of about 1 nM. Furthermore, C-ANF4-23 was also able to inhibit cAMP levels and progesterone secretion stimulated by luteinizing hormone in MA-10 cell line. Other smaller fragments of ANF with ring deletions were also able to inhibit the adenylate cyclase activity as well as cAMP levels. Furthermore, the stimulatory effects of various agonists such as 5'-(N-ethyl)carboxamidoadenosine, dopamine, and forskolin on adenylate cyclase activity and cAMP levels were also significantly inhibited by C-ANF4-23. The inhibitory effect of C-ANF4-23 on adenylate cyclase was dependent on the presence of GTP and was attenuated by pertussis toxin treatment. These results indicate that ANF-R2 receptors or so-called C-ANF receptors are coupled to the adenylate cyclase/cAMP signal transduction system through inhibitory guanine nucleotide regulatory protein.     

Adenosine receptors in smooth muscle: structure-activity studies and the question of adenylate cyclase involvement in control of relaxation

Structure-activity studies with a number of adenosine derivatives and analogs, measuring their relaxant effects in a variety of smooth muscle systems, were conducted in the hope of obtaining indications of the possible involvement of adenylate cyclase in their mechanism of action. While it was confirmed that a C6 aminofunction is of importance for agonist activity, several compounds, in particular the relatively potent N6-hydroxylaminopurine ribonucleoside, were not antagonized by 8-p-sulfophenyltheophylline, indicating that some nucleosides cause smooth muscle relaxation by a mechanism other than adenosine receptor stimulation. Nucleosides not bearing a C6 aminofunction were essentially inactive in rabbit intestine but showed weak relaxant effects in bovine coronary artery; this may indicate a difference between the adenosine receptor systems in these tissues and the intracellular mechanisms of relaxation. Comparing the relative potencies of compounds such as adenosine, 2-chloroadenosine, 5'-(N-ethylcarboxamido)adenosine, and (-)N6-(R-phenylisopropyl)adenosine, which have been used widely to classify adenylate cyclase-coupled adenosine receptors, no uniform pattern became apparent among different smooth muscle systems used in this study and reported in the recent literature. Thus, we conclude that a classification of smooth muscle adenosine receptors according to criteria established for cyclase-coupled receptors may be inappropriate or misleading, particularly with respect to implications of adenylate cyclase involvement in the relaxant effects of adenosine and related nucleosides.     

CD3-dependent increase in cyclic AMP in human T-cells following stimulation of the CD2 receptor.

We have previously shown that stimulation of the Ti/CD3 receptor complex on human T-cells potentiates adenylate cyclase activation by adenosine or forskolin. Anti-CD2 receptor antibodies shared with anti-CD3 antibodies the ability to potentiate dose dependently the adenosine- and forskolin-stimulated cyclic adenosine monophosphate (cAMP) accumulation, whereas stimulation of the CD45 receptor had no effect on cyclase activity. Modulation of the CD3 complex with anti-CD3 antibodies was found to decrease the CD2 receptor effect on adenylate cyclase activity greatly. The possible involvement of CD3-stimulated phospholipase C (PLC) activation on the cAMP potentiation was examined using HPB-ALL cells that express a CD3 complex with a defect coupling to PLC. Stimulation of the CD3 complex on HPB-ALL cells had only slight effects on adenosine-stimulated cAMP formation, whereas the effect on forskolin-stimulated cAMP was virtually unchanged. The CD3 effect was further analyzed in Jurkat cell membranes. In contrast to the results obtained after stimulation of intact cells, it was found that OKT3 stimulation of membranes did not potentiate the forskolin response. Finally, we tested whether inhibition of endogenous adenylate cyclase agonist production affected the CD3 effect. Inhibition of adenosine production or adenosine breakdown with 8-p-sulphophenyl theophylline (8-PST) or adenosine deaminase (ADA), respectively, did not alter the CD3 effects. Indometacin, which inhibits prostaglandin production, also had no effect. Together, these data show that stimulation of the CD2 receptor potentiates adenylate cyclase responses by a mechanism that is dependent on CD3 expression. Furthermore, the CD3 effect on cAMP appears to be mediated by two different mechanisms, one which is, and one which is not dependent on PLC. Finally, this effect is not due to an endogenous production of adenylate cyclase agonists.     

Mechanisms of endothelial dysfunction in resistance arteries from patients with end-stage renal disease

The study focuses on the mechanisms of endothelial dysfunction in the uremic milieu. Subcutaneous resistance arteries from 35 end-stage renal disease (ESRD) patients and 28 matched controls were studied ex-vivo. Basal and receptor-dependent effects of endothelium-derived factors, expression of endothelial NO synthase (eNOS), prerequisites for myoendothelial gap junctions (MEGJ), and associations between endothelium-dependent responses and plasma levels of endothelial dysfunction markers were assessed. The contribution of endothelium-derived hyperpolarizing factor (EDHF) to endothelium-dependent relaxation was impaired in uremic arteries after stimulation with bradykinin, but not acetylcholine, reflecting the agonist-specific differences. Diminished vasodilator influences of the endothelium on basal tone and enhanced plasma levels of asymmetrical dimethyl L-arginine (ADMA) suggest impairment in NO-mediated regulation of uremic arteries. eNOS expression and contribution of MEGJs to EDHF type responses were unaltered. Plasma levels of ADMA were negatively associated with endothelium-dependent responses in uremic arteries. Preserved responses of smooth muscle to pinacidil and NO-donor indicate alterations within the endothelium and tolerance of vasodilator mechanisms to the uremic retention products at the level of smooth muscle. We conclude that both EDHF and NO pathways that control resistance artery tone are impaired in the uremic milieu. For the first time, we validate the alterations in EDHF type responses linked to kinin receptors in ESRD patients. The association between plasma ADMA concentrations and endothelial function in uremic resistance vasculature may have diagnostic and future therapeutic implications.