Endothelium-dependent relaxation of the pig aorta: relationship to stimulation of 86Rb efflux from isolated endothelial cells

Identification and Functional Analysis of Novel Bradykinin-Related Peptides (BRPs) from Skin Secretions of Five Asian Frogs

In recent decades, various types of bioactive substances have been identified from amphibian skin and its secretions. Bradykinin-related peptides (BRPs) are among these compounds that make up the host defence system of amphibians. In the present study, we identified six novel BRPs, amolopkinin-GN1, amolopkinin-RK1, amolopkinin-TR1, amolopkinin-LF1, ranakinin-MS1, and ranakinin-MS2, from five East Asian amphibians, Amolops granulosus, Amolops ricketti, Amolops torrentis, Amolops lifanensis, and Hylarana maosonensis. This is the first report on BRPs in the skin of these species. Physiological assays reveal that these peptides have a contractive effect on the smooth muscle of rat ileum.

The paracrine control of vascular motion. A historical perspective

During the last quarter of the past century, the leading role the endocrine and nervous systems had on the regulation of vasomotion, shifted towards a more paracrine-based regulation. This begun with the recognition of endothelial cells as active players of vascular control, when the vessel's intimal layer was identified as the main source of prostacyclin and was followed by the discovery of an endothelium-derived smooth muscle cell relaxing factor (EDRF). The new position acquired by endothelial cells prompted the discovery of other endothelium-derived regulatory products: vasoconstrictors, generally known as EDCFs, endothelin, and other vasodilators with hyperpolarizing properties (EDHFs). While this research was taking place, a quest for the discovery of the nature of EDRF carried back to a research line commenced a decade earlier: the recently found intracellular messenger cGMP and nitrovasodilators. Both were smooth muscle relaxants and appeared to interact in a hormonal fashion. Prejudice against an unconventional gaseous molecule delayed the acceptance that EDRF was nitric oxide (NO). When this happened, a new era of research that exceeded the vascular field commenced. The discovery of the pathway for NO synthesis from L-arginine involved the clever assembling of numerous unrelated observations of different areas of knowledge. The last ten years of research on the paracrine regulation of the vascular wall has shifted to perivascular fat (PVAT), which is beginning to be regarded as the fourth layer of the vascular wall. Starting with the discovery of an adipose-derived relaxing substance (ADRF), the role that different adipokines have on the paracrine control of vasomotion is now filling the research activity of many vascular pharmacology labs, and surprising interactions between the endothelium, PVAT and smooth muscle are being unveiled.

Purinergic signaling and blood vessels in health and disease

Purinergic signaling plays important roles in control of vascular tone and remodeling. There is dual control of vascular tone by ATP released as a cotransmitter with noradrenaline from perivascular sympathetic nerves to cause vasoconstriction via P2X1 receptors, whereas ATP released from endothelial cells in response to changes in blood flow (producing shear stress) or hypoxia acts on P2X and P2Y receptors on endothelial cells to produce nitric oxide and endothelium-derived hyperpolarizing factor, which dilates vessels. ATP is also released from sensory-motor nerves during antidromic reflex activity to produce relaxation of some blood vessels. In this review, we stress the differences in neural and endothelial factors in purinergic control of different blood vessels. The long-term (trophic) actions of purine and pyrimidine nucleosides and nucleotides in promoting migration and proliferation of both vascular smooth muscle and endothelial cells via P1 and P2Y receptors during angiogenesis and vessel remodeling during restenosis after angioplasty are described. The pathophysiology of blood vessels and therapeutic potential of purinergic agents in diseases, including hypertension, atherosclerosis, ischemia, thrombosis and stroke, diabetes, and migraine, is discussed.

Endothelium-dependent contractions in SHR: a tale of prostanoid TP and IP receptors

In the aorta of spontaneously hypertensive rats (SHR), the endothelial dysfunction is due to the release of endothelium-derived contracting factors (EDCFs) that counteract the vasodilator effect of nitric oxide, with no or minor alteration of its production. The endothelium-dependent contractions elicited by acetylcholine (ACh) involve an increase in endothelial [Ca(2+)](i), the production of reactive oxygen species, the activation of endothelial cyclooxygenase-1, the diffusion of EDCF and the subsequent stimulation of smooth muscle cell TP receptors. The EDCFs released by ACh have been identified as PGH(2) and paradoxically prostacyclin. Prostacyclin generally acts as an endothelium-derived vasodilator, which, by stimulating IP receptors, produces hyperpolarization and relaxation of the smooth muscle and inhibits platelet aggregation. In the aorta of SHR and Wistar-Kyoto rats, prostacyclin is the principal metabolite of arachidonic acid released by ACh. However, in SHR aorta, prostacyclin does not produce relaxations but activates the TP receptors on vascular smooth muscle cells and produces contraction. The IP receptor is not functional in the aortic smooth muscle cells of SHR as early as 12 weeks of age, but its activity is not reduced in platelets. Therefore, prostacyclin in the rule protects the vascular wall, but in the SHR aorta it can contribute to endothelial dysfunction. Whether or not prostacyclin plays a detrimental role as an EDCF in other animal models or in human remains to be demonstrated. Nevertheless, because EDCFs converge to activate TP receptors, selective antagonists of this receptor, by preventing endothelium-dependent contractions, curtail the endothelial dysfunction in diseases such as hypertension and diabetes.

Mechanisms underlying ATP-induced endothelium-dependent contractions in the SHR aorta

In mature spontaneously hypertensive (SHR) and Wistar-Kyoto (WKY) rats, acetylcholine, the calcium ionophore A 23187 and ATP release endothelium-derived contracting factor (EDCF), cyclooxygenase (COX) derivatives that activate thromboxane-endoperoxide (TP) receptors on vascular smooth muscle. The EDCFs released by acetylcholine have been identified as prostacyclin and prostaglandin (PG) H(2) while in response to A 23187 thromboxane A(2), along with the two other prostaglandins, contributes to the endothelium-dependent contractions. The purpose of the present study was to identify the EDCFs produced by ATP. Isometric tension and the release of prostaglandins were measured in isolated aortic rings of WKY rats and SHR. ATP produced the endothelium-dependent release of prostacyclin, thromboxane A(2) and PGE(2) (PGI(2)>>TXA(2)> or =PGE(2)>PGF(2alpha)) in a similar manner in aorta from WKY rats and SHR. In SHR aortas, the release of thromboxane A(2) was significantly larger in response to ATP than to acetylcholine while that to prostacyclin was significantly smaller. The inhibition of cyclooxygenase with indomethacin prevented the release of prostaglandins and the occurrence of endothelium-dependent contractions. The thromboxane synthase inhibitor dazoxiben selectively abolished the ATP-dependent production of thromboxane A(2) and partially inhibited the corresponding endothelium-dependent contractions. U 51605, a non-selective inhibitor of PGI-synthase, reduced the release of prostacyclin elicited by ATP but induced a parallel increase in the production of PGE(2) and PGF(2alpha), suggestive of a PGH(2)-spillover, which was associated with the enhancement of the endothelium-dependent contractions. Thus, in the aorta of SHR, endothelium-dependent contractions elicited by ATP involve the release of thromboxane A(2) and prostacyclin with a possible contribution of PGH(2).

Evidence for the involvement of nitric oxide in A2B receptor-mediated vasorelaxation of mouse aorta

We have investigated the role of adenosine and its analogs on vasorelaxation of mouse aorta in intact endothelium with rank order of potency as follows: 5'-N-ethylcarboxamidoadenosine (NECA) > 2-chloroadenosine > adenosine >> CGS-21680, which is consistent with the profile of A(2B)-adenosine receptor (A(2B)AR). In endothelium-intact tissues, acetylcholine produced relaxation ranging from 65 to 80% in phenylephrine (PE, 10(-7) M)-precontracted mouse aorta, whereas no relaxation was observed in endothelium-denuded tissues. The A(2B)AR antagonist alloxazine (10(-5) M) shifted concentration-response curve for NECA (EC(50) = 0.005 x 10(-5) M) to the right with an EC(50) of 2.8 x 10(-5) M, demonstrating that this relaxation is partially dependent on functional endothelium mediated predominantly via A(2B)AR in this tissue. This conclusion was further supported by the following findings: 1) in the endothelium-intact mouse aorta, the EC(50) values for NECA and adenosine were found to be 0.05 and 1.99 x 10(-4) M, respectively; however, in denuded endothelium, these values were 0.098 and 3.55 x 10(-4) M, respectively; 2) NECA-induced relaxation was significantly blocked by N(G)-nitro-l-arginine methyl ester (l-NAME; 10(-4) M) in endothelium-intact tissues, which was reversed by pretreatment with l-arginine (10(-4) M), whereas no significant inhibition was found in endothelium-denuded tissues; 3) total nitrites and nitrates (NOx) in intact endothelium with l-NAME (10(-4) M) alone and in combination with l-arginine were 59% (P < 0.05) and 96%, respectively, in comparison with control (PE + NECA); and 4) endothelial nitric oxide synthase gene expression was found to be 67% (P < 0.05) less in endothelium-denuded as opposed to endothelium-intact mouse aorta. Thus these data demonstrate that adenosine-mediated vasorelaxation is partially dependent on A(2B)AR in mouse aorta.

Endothelium-dependent smooth muscle hyperpolarization: do gap junctions provide a unifying hypothesis?

An endothelium-derived hyperpolarizing factor (EDHF) that is distinct from nitric oxide (NO) and prostanoids has been widely hypothesized to hyperpolarize and relax vascular smooth muscle following stimulation of the endothelium by agonists. Candidates as diverse as K(+) ions, eicosanoids, hydrogen peroxide and C-type natriuretic peptide have been implicated as the putative mediator, but none has emerged as a 'universal EDHF'. An alternative explanation for the EDHF phenomenon is that direct intercellular communication via gap junctions allows passive spread of agonist-induced endothelial hyperpolarization through the vessel wall. In some arteries, eicosanoids and K(+) ions may themselves initiate a conducted endothelial hyperpolarization, thus suggesting that electrotonic signalling may represent a general mechanism through which the endothelium participates in the regulation of vascular tone.

Potassium Channels and Membrane Potential in the Modulation of Intracellular Calcium in Vascular Endothelial Cells

The endothelium plays a vital role in the control of vascular functions, including modulation of tone; permeability and barrier properties; platelet adhesion and aggregation; and secretion of paracrine factors. Critical signaling events in many of these functions involve an increase in intracellular free Ca(2+) concentration ([Ca(2+)](i)). This rise in [Ca(2+)](i) occurs via an interplay between several mechanisms, including release from intracellular stores, entry from the extracellular space through store depletion and second messenger-mediated processes, and the establishment of a favorable electrochemical gradient. The focus of this review centers on the role of potassium channels and membrane potential in the creation of a favorable electrochemical gradient for Ca(2+) entry. In addition, evidence is examined for the existence of various classes of potassium channels and the possible influence of regional variation in expression and experimental conditions.

K(Ca) channel blockers reveal hyperpolarization and relaxation to K+ in rat isolated mesenteric artery

Raising extracellular K+ concentration ([K+](o)) around mesenteric resistance arteries reverses depolarization and contraction to phenylephrine. As smooth muscle depolarizes and intracellular Ca(2+) and tension increase, this effect of K+ is suppressed, whereas efflux of cellular K+ through Ca(2+)-activated K+ (K(Ca)) channels is increased. We investigated whether K+ efflux through K(Ca) suppresses the action of exogenous K+ and whether it prestimulates smooth muscle Na(+)-K(+)-ATPase. Under isometric conditions, 10.8 mM [K+](o) had no effect on arteries contracted >10 mN, unless 100 nM iberiotoxin (IbTX), 100 nM charybdotoxin (ChTX), and/or 50 nM apamin were present. Simultaneous measurements of membrane potential and tension showed that phenylephrine depolarized and contracted arteries to -32.2 +/- 2.3 mV and 13.8 +/- 1.6 mN (n = 5) after blockade of K(Ca), but 10.8 mM K+ reversed fully the responses (107.6 +/- 8.6 and 98.8 +/- 0.6%, respectively). Under isobaric conditions and preconstriction with phenylephrine, 10.7 mM [K+](o) reversed contraction at both 50 mmHg (77.0 +/- 8.5%, n = 9) and 80 mmHg (83.7 +/- 5.5%, n = 5). However, in four additional vessels at 80 mmHg, raising K+ failed to reverse contraction unless ChTX was present. Increases in isometric and decreases in isobaric tension with phenylephrine were augmented by either ChTX or ouabain (100 microM), whereas neither inhibitor altered tension under resting conditions. Inhibition of cellular K+ efflux facilitates hyperpolarization and relaxation to exogenous K+, possibly by indirectly reducing the background activation of Na(+)-K(+)-ATPase.

Endothelial G protein beta-subunits trigger nitric oxide-but not endothelium-derived hyperpolarizing factor-dependent dilation in rabbit resistance arteries

A single subtype of heterotrimeric G protein-coupled receptor controls both nitric oxide (NO) (sensitive to L-arginine analogues) and endothelium-derived hyperpolarizing factor (EDHF) (sensitive to high-external K(+) and apamine) production by the vascular endothelium leading to dilation. We hypothesized that alpha- and betagamma-subunits of the G protein serve as distinct intermediates to produce NO and EDHF. In pressurized resistance arteries, selective pinocytotic endothelial incorporation of specific antibodies (Abs) directed against alpha(q/11)-subunits abolished acetylcholine (Ach)-mediated dilation but failed to influence oxymetazoline (Oxy, alpha(2)-adrenergic receptor agonist)-induced dilation. In contrast, alpha(i1-2)-subunit Abs prevented Oxy- but not Ach-induced dilation. Thus, as expected, endothelial muscarinic and alpha(2)-adrenoceptors couple to G(q) protein and G(i) proteins, respectively. beta-subunit Abs reduced both Ach- and Oxy-induced dilation. The beta-subunit Abs abolished the nitro-L-arginine (L-NNA)-sensitive component but did not impair the high-external K(+)-sensitive component of the dilation induced by Ach and Oxy. Thus, G protein beta-subunits primarily accounted for NO production. Neutralization of Hsp90 and inhibition of the phospholipase C by U73122 (1 micromol/L) or intracellular Ca(2+) buffering with BAPTA-AM (10 micromol/L) sharply reduced NO-dependent but not K(+)-sensitive dilation. In conclusion, mobilization of the G protein beta-subunit is pivotal to NO-dependent dilation triggered through muscarinic and alpha(2)-adrenergic receptors. In contrast, receptor-operated EDHF-dependent dilation was insensitive to beta-subunit Abs. Although not directly activating the NO pathway, alpha-subunit activation is an absolute prerequisite for receptor-operated endothelium-dependent dilation of resistance arteries.

Adenosine linking reduced O2 to arteriolar NO release in intestine is not formed from extracellular ATP

We have previously reported that adenosine formed in response to reduced arteriolar and/or tissue PO(2) preserves endothelial nitric oxide (NO) synthesis during sympathetic vasoconstriction in the rat intestine. To more precisely identify the site and mechanism of adenosine formation under these conditions, we tested the hypothesis that ATP released in response to reduced O(2) levels serves as a source of adenosine. Direct application of ATP to the wall of first-order arterioles elicited dose-dependent dilations of 15-33% above resting diameter that were reduced by 71-80% by the 5'-ectonucleotidase inhibitor alpha,beta-methyleneadenosine 5'-diphosphate (AOPCP, 4.5 x 10(-5) M) and completely abolished by N(G)-monomethyl-L-arginine (L-NMMA, 10(-4) M). Under control conditions, sympathetic nerve stimulation at 3 and 8 Hz induced arteriolar constrictions of 11 +/- 1 and 19 +/- 1 microm, respectively. These responses were enhanced by 58-69% in the presence of L-NMMA or when local PO(2) was maintained at resting levels. However, in the presence of AOPCP, the enhancing effects of L-NMMA and the high O(2) superfusate on sympathetic constriction were preserved. These results suggest that, although exogenously applied ATP can stimulate arteriolar NO release in the intestine largely through its sequential extracellular hydrolysis to adenosine, this process does not contribute to adenosine formation and sustained NO release during sympathetic constriction in this vascular bed.

Age-related changes in adenosine-mediated relaxation of coronary and aortic smooth muscle

We tested whether adenosine mediates nitric oxide (NO)-dependent and NO-independent dilation in coronary and aortic smooth muscle and whether age selectively impairs NO-dependent adenosine relaxation. Responses to adenosine and the relatively nonselective analog 5'-N-ethylcarboxamidoadenosine (NECA) were studied in coronary vessels and aortas from immature (1-2 mo), mature (3-4 mo), and moderately aged (12-18 mo) Wistar and Sprague-Dawley rats. Adenosine and NECA induced biphasic concentration-dependent coronary vasodilation, with data supporting high-sensitivity (pEC(50) = 5.2-5.8) and low-sensitivity (pEC(50) = 2.3-2.4) adenosine sites. Although sensitivity to adenosine and NECA was unaltered by age, response magnitude declined significantly. Treatment with 50 microM N(G)-nitro-L-arginine methyl ester (L-NAME) markedly inhibited the high-sensitivity site, although response magnitude still declined with age. Aortic sensitivity to adenosine declined with age (pEC(50) = 4.7 +/- 0.2, 3.5 +/- 0.2, and 2.9 +/- 0.1 in immature, mature, and moderately aged aortas, respectively), and the adenosine receptor transduction maximum also decreased (16.1 +/- 0.8, 12.9 +/- 0.7, and 9.6 +/- 0.7 mN/mm(2) in immature, mature, and moderately aged aortas, respectively). L-NAME decreased aortic sensitivity to adenosine in immature and mature tissues but was ineffective in the moderately aged aorta. Data collectively indicate that 1) adenosine mediates NO-dependent and NO-independent coronary and aortic relaxation, 2) maturation and aging reduce NO-independent and NO-dependent adenosine responses, and 3) the age-related decline in aortic response also involves a reduction in the adenosine receptor transduction maximum.

Role of gap junctions in endothelium-derived hyperpolarizing factor responses and mechanisms of K(+)-relaxation

We have examined the effects of ouabain (1 mM), the gap junction inhibitors, 18 alpha-glycyrrhetinic acid (100 microM), N-(piperidin-1-yl)-5-(4-chlorophenyl)-1-(2, 4-dichlorophenyl)-4-methyl-1H-pyrazole-3-carboxamide hydrochloride (SR141716A; 10 microM) and palmitoleic acid (50 microM), and clotrimazole (10 microM) against endothelium-derived hyperpolarizing factor (EDHF)-mediated and K(+)-induced vasorelaxations in the rat mesentery. In the presence of indomethacin (10 microM) and 300-microM N(G)nitro-L-arginine methyl ester (L-NAME), carbachol caused EDHF-mediated relaxations (R(max)=85.3+/-4.0%). In the presence of ouabain, these responses were substantially reduced (R(max)=11.0+/-2.3%). 18 alpha-glycyrrhetinic acid, SR141716A, palmitoleic acid and clotrimazole also significantly inhibited these EDHF-mediated responses. K(+) caused vasorelaxation of preparations perfused with K(+)-free buffer (R(max)=73.7+/-2.4%), which were reduced by 10-microM indomethacin (R(max)=56.4+/-6.2%). K(+) vasorelaxation was essentially abolished by endothelial denudation. Both ouabain and 18 alpha-glycyrrhetinic acid opposed K(+) relaxations, however, neither SR141716A, clotrimazole nor palmitoleic acid had any effect. Direct cell-cell coupling via gap junctions was attenuated by ouabain, clotrimazole and palmitoleic acid. We conclude that: (i) that gap junctional communication plays a major role in EDHF-mediated relaxations, (ii) that K(+)-vasorelaxation is endothelium-dependent (thus, K(+) is unlikely to represent an EDHF), and (iii) that the inhibitory actions of ouabain and clotrimazole on gap junctions might contribute towards their effects against EDHF.

Salivary glands of the sand fly Phlebotomus papatasi contain pharmacologically active amounts of adenosine and 5′-AMP

Salivary gland homogenates of the sand fly Phlebotomus papatasi contain large amounts of adenosine and 5′-AMP, of the order of 1 nmol per pair of glands, as demonstrated by liquid chromatography, ultraviolet spectrometry, mass spectrometry and bioassays. These purines, 75–80 % of which are secreted from the glands following a blood meal, have vasodilatory and anti-platelet activities and probably help the fly to obtain a blood meal. Salivary 5′-AMP is also responsible for the previously reported protein phosphatase inhibitor in the salivary glands of P. papatasi, which is shown to be artifactual in nature as a result of allosteric modification by AMP of the phosphatase substrate used (phosphorylase a).

Metabotropic receptors for ATP and UTP: exploring the correspondence between native and recombinant nucleotide receptors

In the past five years, an extended series (P2Y1-n) of metabotropic nucleotide (P2) receptors has been cloned from vertebrate tissues; these receptors are activated by either ATP or UTP, or both nucleotides. While certain cloned P2Y receptors appear to correspond functionally to particular native P2 receptor phenotypes, such pharmacological phenotypes could be explained by either a combination of several members of the P2Y1-n series being coexpressed in the same tissue or the existence of novel, uncloned P2Y subtypes. Here, Brian King, Andrea Townsend-Nicholson and Geoffrey Burnstock review recent findings on the matter of pharmacological relationships between native P2 and cloned P2Y receptors.

K+ is an endothelium-derived hyperpolarizing factor in rat arteries

In arteries, muscarinic agonists such as acetylcholine release an unidentified, endothelium-derived hyperpolarizing factor (EDHF) which is neither prostacyclin nor nitric oxide. Here we show that EDHF-induced hyperpolarization of smooth muscle and relaxation of small resistance arteries are inhibited by ouabain plus Ba2+; ouabain is a blocker of Na+/K+ ATPase and Ba2+ blocks inwardly rectifying K+ channels. Small increases in the amount of extracellular K+ mimic these effects of EDHF in a ouabain- and Ba2+-sensitive, but endothelium-independent, manner. Acetylcholine hyperpolarizes endothelial cells and increases the K+ concentration in the myoendothelial space; these effects are abolished by charbdotoxin plus apamin. Hyperpolarization of smooth muscle by EDHF is also abolished by this toxin combination, but these toxins do not affect the hyperpolarizaiton of smooth muscle by added K+. These data show that EDHF is K+ that effluxes through charybdotoxin- and apamin-sensitive K+ channels on endothelial cells. The resulting increase in myoendothelial K+ concentration hyperpolarizes and relaxes adjacent smooth-muscle cells by activating Ba2+-sensitive K+ channels and Na+/K+ ATPase. These results show that fluctuations in K+ levels originating within the blood vessel itself are important in regulating mammalian blood pressure and flow.

Acetylcholine is a vasodilator of porcine skeletal muscle arteries

The purpose of this study was to test the hypothesis that porcine skeletal muscle arteries exhibit concentration-dependent vasodilation in response to acetylcholine (ACH) as observed in other mammals. We conducted three experiments. First, vasorelaxation responses to ACH were examined in isolated segments of femoral and brachial arteries, mounted on myographs and studied in vitro. Second, we determined whether resistance arteries from porcine skeletal muscle exhibit vasodilation in response to ACH by isolating second order arterioles (2-A) from the medial (MHT), deep-long (LOH) and lateral (LAT) heads of the triceps brachii muscles of four pigs. The rationale for selection of arterioles from these muscles was that these muscles represent muscles composed primarily of slow-oxidative, fast-oxidative-glycolytic, and fast-glycolytic muscle fiber types, respectively. 2-As were isolated and cannulated with micropipettes and intraluminal pressure set at 60 cm H2O. In both sets of in vitro experiments, we determined responses to an endothelium-independent dilator, sodium nitroprusside (10(-10)-10(-4) M), and to endothelium-dependent agents ACH (10(-10)-10(-4) M), and bradykinin (BK; 10(-11)-10(-6) M). Third, we used transcutaneous ultrasound imaging to measure changes in artery diameters and Doppler-principle measurements of blood flow velocities to estimate changes in total blood flow in the femoral vascular bed. Results reveal that ACH and BK produced similar vasorelaxation responses in femoral and brachial arteries and vasodilation of skeletal muscle 2-As. Also, ACH produced increases in blood flow and decreases in vascular resistance in the femoral vascular bed. These results indicate that the arterial tree of porcine skeletal muscle exhibits ACH-induced, endothelium-dependent vasodilation.

Stabilization of acetylcholine receptors by exogenous ATP and its reversal by cAMP and calcium

Innervation of the neuromuscular junction (nmj) affects the stability of acetylcholine receptors (AChRs). A neural factor that could affect AChR stabilization was studied using cultured muscle cells since they express two distinct populations of AChRs similar to those seen at the nmjs of denervated muscle. These two AChR populations are (in a ratio of 9 to 1) a rapidly degrading population (Rr) with a degradation half-life of approximately 1 d and a slowly degrading population (Rs) that can alternate between an accelerated form (half-life approximately 3-5 d) and a stabilized form (half-life approximately 10 d), depending upon the state of innervation of the muscle. Previous studies have shown that elevation of intracellular cAMP can stabilize the Rs, but not the Rr. We report here that in cultured rat muscle cells, exogenous ATP stabilized the degradation half-life of Rr and possibly also the Rs. Furthermore, pretreatment with ATP caused more stable AChRs to be inserted into the muscle membrane. Thus, in the presence of ATP, the degradation rates of the Rr and Rs overlap. This suggests that ATP released from the nerve may play an important role in the regulation of AChR degradation. Treatment with either the cAMP analogue dibutyryl-cAMP (dB-cAMP) or the calcium mobilizer ryanodine caused the ATP-stabilized Rr to accelerate back to a half-life of 1 d. Thus, at least three signaling systems (intracellular cAMP, Ca2+, and extracellular ATP) have the potential to interact with each other in the building of an adult neuromuscular junction.

Impairment of endothelium-dependent but not of endothelium-independent dilatation in guinea-pig aorta rings incubated in the presence of elevated glucose

1. Purine compounds such as ATP and adenosine, respectively endothelium-dependent and- independent vasodilators, are largely involved in the control of vascular tone and vascular reactivity to contracting stimuli. We investigated the relaxing activity of ATP and adenosine in guinea-pig aorta rings exposed for 6 h to elevated glucose concentration (50 mM), in order to mimic hyperglycaemic conditions. Guinea-pigs were reserpine-treated (2 mg kg-1, i.p., 48 and 24 h before death). 2. Rings of aortae incubated in 50 mM glucose, contracted submaximally by 1 microM noradrenaline, lost endothelium-dependent relaxation in response to acetylcholine (10 nM to 10 microM). Aortae incubated with 50 mM mannose, as a hyperosmotic control, relaxed to acetylcholine normally. Rings of aortae incubated in 50 mM glucose, contracted submaximally by 3 mM 4-aminopyridine, lost endothelium-dependent relaxation in response to ATP (30 microM) whereas endothelium-independent relaxation in response to adenosine (0.3 mM) was well preserved. 4. The relaxation induced by A23187 or sodium nitroprusside (10 nM to 0.1 microM) did not differ between rings exposed to control (5.5 mM) or elevated glucose (50 mM) and contracted submaximally by 3 mM 4-aminopyridine. 5. When incubated with aortic tissue in the presence of elevated glucose, the cyclo-oxygenase inhibitors, indomethacin (10 microM) and mefenamic acid (30 microM), or the scavenger of superoxide anions, superoxide dismutase (150 u ml-1), prevented the impairment of ATP-mediated relaxation. 6. The present results indicate that endothelium-dependent, receptor-induced relaxation in response to acetylcholine and ATP is impaired in guinea-pig aorta rings exposed to elevated glucose. The endothelial dysfunction caused by glucose might be located at a step between receptor activation and intracellular calcium increase, and might be related to an increased metabolism of arachidonic acid coupled to an increased production, or to a reduced inactivation of superoxide anions.

17 beta-Estradiol increases intracellular free calcium concentrations of human vascular endothelial cells and modulates its responses to acetylcholine

In this study, we have investigated the effect of 17 beta-estradiol (E2) on intracellular free calcium concentrations ([Ca2+]i) in human umbilical vein endothelial cells (HUVEC) using fura-2 fluorescence. E2 at concentrations of 1nM -1 microM was added subsequently to HUVEC cultures which were either deprived of estrogens or preincubated with E2 (100 nM) for 24 hours. In both groups of cultures, E2 stimulated significant increases in [Ca2+]i in a dose-dependent manner. The effects were more prominent in E2-deprived cells. Preincubation of cells with tamoxifen or the presence of it in the buffer during the experiments did not inhibit the response of the cells to E2. Experiments performed in Ca2+ free/EGTA buffer yielded transient increases in [Ca2+]i suggesting release of Ca2+ from intracellular stores was responsible for the initial peak, while sustained elevations were supported by Ca2+ influx from the extracellular space. Addition of La3+ abolished the sustained [Ca2+]i elevations. Carbachol (CCh) (1nM, 100 nM) did not induce changes in [Ca2+]i of estrogen-deprived cells but produced significant increases in [Ca2+]i of the same cells after incubation with E2 for 30 minutes. The cultures which were preincubated with E2 for 24 hours responded to carbachol directly. The results of our study indicate that E2 may modulate the functions of endothelial cells after only a brief exposure and also may be necessary for the response to acetylcholine especially at low concentrations.

Nucleotide receptor P2u partially mediates ATP-induced cell cycle progression of aortic smooth muscle cells

mRNA of the P2u purinoceptor (or nucleotide receptor) is detected both by polymerase chain reaction or Northern blot analyses in cultured aortic smooth muscle cells. When added to the culture medium of these cells, UTP, a specific ligand of the P2u receptor, induces an increased expression of both immediate-early and delayed-early cell cycle-dependent genes. This induction demonstrates similar features (kinetics, concentration dependence) to those obtained after stimulation of aortic smooth cells by exogenous ATP, a common ligand for most P2 purinoceptors. In contrast, 2-methylthioATP, a preferential ligand for P2y purinoceptors, induces only a significant increase of immediate-early genes but not of delayed-early genes. Moreover, the 2-methylthioATP-induced responses (c-fos mRNA increase, free intracellular calcium transient) are lower than those induced by ATP or UTP and are complementary to those of UTP. These results demonstrate that functional P2u receptors are present on cultured aortic smooth muscle cells and suggest that the bulk of responses induced by extracellular ATP on cell cycle progression are mediated via P2u purinoceptors, a hypothesis confirmed by cytofluorometric studies. Since some ATP- or UTP-induced genes code for chemotactic proteins (monocyte chemoattractant protein-1 and osteopontin), this study suggests that these nucleotides may contribute to vascular or blood cell migration and proliferation and consequently to the genesis of arterial diseases.

Cryptolepine-induced vasodilation in the isolated perfused kidney of the rat: role of G-proteins, K+ and Ca2+ channels

The isolated perfused kidney of the rat was used to examine the contribution by guanosine triphosphate (GTP)-binding (G-) proteins, K+ and Ca2+ channels to the vasodilator actions of cryptolepine (5-methylquindoline). In normal Krebs-Henseleit buffer (4.7 mM KCl), cryptolepine elicited dose-dependent reductions in perfusion pressure of phenylephrine-preconstricted kidneys. The reductions in perfusion pressure by cryptolepine at bolus doses of 2.5, 5, and 10 micrograms were -18.0 +/- 3.4, -30.6 +/- 5.3, and -38.3 +/- 6.8 mm Hg, respectively (n = 19). In K(+)-free (0 mM KCl) Krebs-Henseleit solution, the vasodilator response to cryptolepine was reduced by 44.7 +/- 5.7% (n = 5; P < 0.01). The addition of ouabain (10(-4) M) further reduced cryptolepine-induced vasodilation to 63.0 +/- 7.2% (n = 11: P < 0.01) of the control. A combination of both conditions did not abolish the vasodilator responses to cryptolepine, suggesting the involvement of additional mechanisms. In 80, as opposed to 20 mM KCl, the reductions in perfusion pressure by cryptolepine, 2.5, 5, and 10 micrograms were markedly reduced to -0.8 +/- 0.8, -2.3 +/- 1.4, and -4.0 +/- 2.1 mm Hg, respectively (P < 0.01; n = 6). Responses to acetylcholine and diazoxide, an adenosine triphosphate (ATP)-dependent K+ channel activator, were also markedly reduced, suggesting the involvement of K+ channels for these agents. Furthermore, tetraethylammonium (5 and 10 mM), a non-specific K+ channel blocker, inhibited the vasodilator responses to cryptolepine (n = 5; P < 0.01) and to diazoxide and acetylcholine in a dose-related manner. However, glibenclamide (5 and 10 microM), an ATP-sensitive K+ channel blocker, inhibited the vasodilator responses to diazoxide and acetylcholine but was without effect on cryptolepine-induced vasodilation. This suggests that cryptolepine activates K+ channels which are tetraethyl ammonium- but not glibenclamide-sensitive. In pertussis toxin-treated rats, the vasodilator response to cryptolepine was not affected while that to acetylcholine and especially diazoxide was markedly inhibited. This suggests that, unlike diazoxide and acetylcholine, the K+ channels activated by cryptolepine are not coupled to pertussis toxin-sensitive G-proteins. In the presence of verapamil (5 microM) and cobalt chloride (1 mM), Ca2+ channel blockers, the vasodilator response to cryptolepine was inhibited (n = 5; P < 0.01), suggesting that Ca2+ flux across membranes is also involved in cryptolepine-induced vasodilation in the rat kidney.

Vasodilatation produced by adenosine in isolated rat perfused mesenteric artery: a role for endothelium

Adenosine and adenosine triphosphate (ATP) induced vasodilatation was studied in isolated rat perfused mesenteric artery at constant flow. Decrease in perfusion pressure was measured after induction of tone by continuous infusion with phenylephrine (5-7 microM). Adenosine and ATP caused dose-dependent vasodilation. Following infusion with selective A2 adenosine receptor antagonist, 3,7-dimethyl-1-propargylxanthine (DMPX) (10 microM), or non-selective adenosine receptor antagonist, theophylline (30 microM), vasodilation produced by adenosine were significantly reduced at lower doses. Responses to adenosine were not affected by pretreatment of tissues with either the P2-purinoceptor desensitizing agent, alpha, beta methylene ATP (30 microM), or the P2-purinoceptor antagonist, suramin (10 microM). In contrast, both alpha, beta methylene ATP and suramin significantly attenuate relaxation produced by ATP. Further, it was found that relaxation elicited by either adenosine or ATP was not significantly affected by the presence of glibenclamide (30 microM). Vasodilatation induced by adenosine and ATP was greatly reduced in denuded arteries but more so for ATP than adenosine. It is concluded that adenosine-mediated vasodilatation may hardly be due to the stimulation of A2 adenosine receptors and is strongly dependent on the presence of functional endothelium whereas ATP-mediated vasodilator responses were mediated via the activation of P2-purinoceptors and appeared to be entirely dependent upon the presence of functional endothelium. Further, vasodilator responses to neither adenosine nor ATP were sensitive to inhibition by the potassium channel blocker glibenclamide, in isolated mesenteric perfused bed. This would imply that ATP-sensitive potassium channels were not involved in adenosine and ATP mediated vasodilatation.

Characterization of adenosine receptors in the rat isolated aorta

1. Adenosine and its analogues relaxed the isolated rat aorta by an endothelium-dependent mechanism with an order of potency of 5'-N-ethylcarboxamidoadenosine (NECA) > 2-(p-(2-carboxy-ethyl)phenethylamino)-5'-N-ethylcarboxamidoadenosi ne (CGS 21680) > adenosine = N6-(2-(4-amino-phenyl)ethyl)adenosine (APNEA) = N6-cyclopentyladenosine (CPA) > 5'-methylthioadenosine (MTA), although the maximal response achieved by CGS 21680 was less than that achieved by NECA. 2. Both 8-sulphophenyltheophylline (8-SPT) and MTA antagonized responses to the adenosine analogues, but there were some anomolous features of this antagonism and NECA was inhibited more powerfully than the other agonists. This suggests that as well as A2a receptors mediating relaxation, the rat aorta may relax to adenosine analogues by other mechanisms.

Adenosine modulation of neurotransmission in penile erection

1. Adenosine inhibited the noradrenaline-induced contraction of rabbit corpus cavernosum in a dose-dependent manner. The effect of adenosine was greater in intact corpus cavernosa than in endothelium-denuded preparations. This finding indicates that the relaxing effect of adenosine is partially endothelium-dependent and involved in the release of endothelium-derived relaxing factors. 2. Adenosine and its analogues relaxed the noradrenaline-induced contractile response as well as inhibited the transmural nerve induced contraction with the potency order: NECA > R-PIA > adenosine. These data indicate that adenosine can modulate both the non-adrenergic non-cholinergic and adrenergic neurotransmission. DMPX, an adenosine antagonist selective for the A2 receptors, abolished the electrically elicited relaxation. However, CGS 21680, selective for A2a receptor, had no effect on relaxation. Therefore, adenosine receptors involved in the modulation of neurotransmission in rabbit corpus cavernosum appear to be A2b subtype. 3. Adenosine also induced an increase in human cavernosal arterial velocity and resistive index measured by colour duplex sonography. The combination of adenosine and 10 micrograms prostaglandin E1 was more effective in resistive index and erection grade than 20 micrograms prostaglandin E1 alone. Our results suggest that adenosine seems to be an important neuromodulator for penile erection and can be an effective and alternative combination in the treatment of impotence.

Effects of P1 and P2Y purinoceptor antagonists on endothelium-dependent and -independent relaxations of rat mesenteric artery to GTP and guanosine

1. Guanosine 5'-triphosphate (GTP) and guanosine can relax both endothelium-intact and -denuded arterial preparations. In the present work the P1 and P2Y purinoceptor antagonists, 8-phenyltheophylline and reactive blue 2, respectively, were used to study the mechanisms of relaxation responses induced by GTP, guanosine, adenosine 5'-triphosphate (ATP) and adenosine in noradrenaline-precontracted rat mesenteric artery rings. 2. GTP (10 microM-1mM) dose-dependently relaxed endothelium-intact mesenteric artery rings and also induced moderate relaxation responses in endothelium-denuded preparations. Pretreatment of the rings with 8-phenyltheophylline (10 microM) or reactive blue 2 (10 microM) did not attenuate the relaxant effect of GTP. 3. Guanosine (10 microM-1mM) relaxed both endothelium-intact and -denuded artery rings in a dose-dependent manner. The presence of 8-phenyltheophylline or reactive blue 2 had no effects on guanosine-induced relaxations. 4. ATP-induced (0.1 microM-0.1 mM) relaxation of endothelium-intact artery rings was attenuated by reactive blue 2 while 8-phenyltheophylline was ineffective. ATP also relaxed endothelium-denuded artery rings and this relaxation was inhibited by 8-phenyltheophylline, but not by reactive blue 2. 5. Adenosine-induced (10 microM-1 mM) relaxation of endothelium-intact and -denuded artery rings was attenuated by the presence of 8-phenyltheophylline, but not of reactive blue 2. 6. In conclusion, the endothelium-dependent and -independent relaxations of rat mesenteric arteries to GTP and guanosine are not mediated via P1 and P2Y purinoceptors. Therefore, these results support our previous suggestion on the presence of a novel guanine nucleotide-specific receptor, a putative PG receptor, on both endothelial and smooth muscle cells, which may participate in the regulation of arterial tone.

Oxidant stress inhibits the store-dependent Ca(2+)-influx pathway of vascular endothelial cells

Oxidant stress induced by t-butyl hydroperoxide (t-BuOOH) inhibits bradykinin-stimulated Ca2+ signalling in vascular endothelial cells. The effect of t-BuOOH on intracellular Ca2+ pools was determined by addition of Ca(2+)-releasing agents to fura-2-loaded cells suspended in Ca(2+)-free/EGTA buffer. In control cells, sequential additions of bradykinin and ionomycin produced similar increases in cytosolic free [Ca2+] ([Ca2+]i). By contrast, incubation with t-BuOOH progressively decreased the response of [Ca2+]i to bradykinin and increased that to ionomycin, suggesting that the total (ionomycin-releasable) Ca2+ pool remains replete during oxidant stress. The effect of t-BuOOH on the InsP3-sensitive Ca2+ pool was measured by the increase in [Ca2+]i or efflux of 45Ca2+ stimulated by 2,5-di-t-butylhydroquinone (BHQ). Incubation with t-BuOOH did not inhibit BHQ-stimulated increases in [Ca2+]i or 45Ca2+ efflux, suggesting that the InsP3-sensitive Ca2+ pool remains replete and releasable. Activity of the Ca(2+)-influx pathway stimulated by release of internal Ca2+ stores was determined via re-addition of Ca2+ to BHQ-stimulated cells suspended in Ca(2+)-free/EGTA buffer and via BHQ-stimulated 45Ca2+ uptake. Incubation of cells with t-BuOOH for 1 h significantly inhibited the influx pathway. At later time points, t-BuOOH increased basal [Ca2+]i and potentiated the response of [Ca2+]i to BHQ. Similar results were demonstrated with thapsigargin. Together, these findings suggest that (1) the inhibitory effect of t-BuOOH on bradykinin-stimulated release of Ca2+ from internal stores is not related to depletion of these stores, and (2) inhibition of the store-dependent Ca(2+)-influx pathway occurs by a direct effect of the influx pathway or by inhibition of the mechanism which links the internal Ca2+ store to plasmalemmal Ca2+ influx.

Role of Na-K-Cl cotransport in vascular endothelial cell volume regulation

Vascular endothelial cells have been shown previously to possess a highly active Na-K-Cl cotransport system that mediates the major portion of total K influx and is regulated by a variety of vasoactive hormones and neurotransmitters. These observations suggest that the cotransporter may be an important component of endothelial cell function. The present study was conducted to investigate the role of Na-K-Cl cotransport in regulation of endothelial cell volume. Cultured bovine aortic endothelial cells were exposed to media of varying tonicities and Na-K-Cl cotransport activity assessed as bumetanide-sensitive K influx. Increasing the extracellular tonicity by increments as small as 10 mosM was found to cause significant stimulation of cotransport activity, and lowering tonicity reduced activity of the transporter. Exposure of endothelial cells to hypertonic medium was also found to increase bumetanide-sensitive net uptake of Na and K and total cellular Na and K content. Endothelial cell volume was evaluated by [14C]urea determination of intracellular water space in endothelial monolayers and by electronic cell sizing of suspended cells. Treatment of the cells with agents that stimulate Na-K-Cl cotransport activity was found to increase cell volume, whereas cotransport-inhibiting agents decreased cell volume. Exposure of the cells to hypertonic medium caused a rapid decrease in cell volume, followed by a regulatory volume increase that was greatly attenuated by bumetanide. The volume recovery was partially inhibited by the Na-H exchange inhibitor amiloride and was nearly abolished by bumetanide and amiloride in combination. Endothelial cells of pulmonary artery and cerebral microvessels were also found to exhibit increased Na-K-Cl cotransport activity on exposure to hypertonic media. These findings suggest that Na-K-Cl cotransport is of major importance in endothelial cell volume regulation.

Effect of shear stress on 86Rb+ efflux from calf pulmonary artery endothelial cells

The effect of flow-induced shear stress on membrane K+ permeability was investigated by measuring 86Rb+ efflux in cultured calf pulmonary artery endothelial cells. Cells were subjected to step changes in shear stress from 1 dyn/cm2 to 2.4, 4.8, or 10 dyn/cm2 in a parallel-plate flow chamber. Increasing shear stress produced a graded, transient increase in 86Rb+ efflux which peaked within 1 min and subsequently declined rapidly toward pre-stimulus levels. Upon returning shear stress to 1 dyn/cm2, 86Rb+ efflux initially decreased, but returned slowly to basal values. In contrast, application of bradykinin at a constant shear stress of 1 dyn/cm2 produced a transient increase in 86Rb+ efflux that was followed by a sustained elevated phase during which time efflux gradually returned to pre-stimulus levels. In order to exclude the possibility that the transient increase in 86Rb+ efflux with shear stress simply reflects a flow-dependent change in the washout of radiotracer, the transient convection-diffusion equation was solved using finite element simulation. When the flux of 86Rb+ from the cell monolayer was assumed to be constant with time, the mathematical model predicted an increase in efflux rate coefficients upon step increases in flow that were only 7-19% of that observed experimentally. The numerical predictions correlated well with the experimentally obtained peaks when the flux of 86Rb+ from the cell monolayer was simultaneously increased with flow to a new steady value. These simulations however, could not predict the transient nature of the response to increased shear stress.(ABSTRACT TRUNCATED AT 250 WORDS)

Pharmacological characterization of a new purinergic receptor site in rabbit aorta

1. The pharmacological properties of a vasodilating purine-activated receptor that is not a P1 or P2-purinoceptor were investigated. 2. In rabbit isolated thoracic aorta precontracted with noradrenaline, ATP induced a 50% relaxation at 0.25 mM (EC 50%); in the absence of endothelium, EC 50% was 2.5 mM. 3. Adenosine induced a relaxation that was not different in both the presence and absence of endothelium, being EC 50% 0.48 and 0.37 mM, respectively. 4. The potent and selective P2y-purinoceptor agonist 2-methylthio-ATP (0.03-10 microM) induced a relaxation only in the presence of endothelium. 5. In de-endothelialized aorta, 8-phenyltheophilline (8-PT: P1 antagonist) and 3,7-dimethyl-1-propargylxanthine (DMPX: A2 antagonist) did not antagonize ATP- and adenosine-induced relaxation. 6. The present data support the presence of a new site of action for purines in rabbit isolated thoracic aorta. 7. A P3 subtype of purinoceptor, that may be identified in the hypothesized "nucleotide" receptor, is proposed.

A possible mechanism of endothelium-dependent relaxation induced by pirarubicin and carbachol in rat isolated aorta

The mechanism of endothelium-dependent relaxation induced by pirarubicin, (2''R)-4'-O-tetrahydropyranyladriamycin, THP, or carbachol was investigated in the rat isolated aorta. The relaxant effect of THP (1.5 x 10(-6)-4.5 x 10(-5) M) or carbachol (10(-8)-10(-4) M) on the aorta with endothelium was decreased by lowering Ca2+ in the medium. The relaxation induced by THP was not inhibited by pretreatment with verapamil (10(-6)-10(-5) M), and that induced by carbachol was only partially inhibited. However, on replacement of all but 20 mM Na+ with either Li+ or choline, the THP- or carbachol-induced relaxation was inhibited. Furthermore, the relaxing effect of THP or carbachol was inhibited by pretreatment with amiloride (10(-4)-3 x 10(-4) M), with ouabain (10(-4)-10(-3) M), or with K(+)-depletion. These results suggest that the THP- or carbachol-induced relaxation depending on endothelium was affected by modifying the calcium ion concentration, and that a Na(+)-Ca2+ exchange process is involved.

Calcium-activated potassium channels in native endothelial cells from rabbit aorta: conductance, Ca2+ sensitivity and block

1. Isolated native endothelial cells, obtained by treatment of rabbit aortic endothelium with papain and dithiothreitol, were voltage clamped, and single channel (unitary) and spontaneous transient outward currents (STOCs) were recorded from both whole cells and excised membrane patches. 2. In inside-out patches, the reversal potential of unitary currents was dependent on the extracellular K+ concentration and had a single-channel slope conductance of 220 pS in symmetrical 140 mM-K+ solutions. The open-state probability (Po) of the unitary K+ currents was sensitive to the intracellular Ca2+ concentration with half-maximal activation at approximately 1 microM at +20 mV. The ionic selectivity and Ca2+ sensitivity indicate that a large conductance, Ca(2+)-activated K+ channel is present in freshly dissociated rabbit aortic endothelial cells. 3. The frequency and amplitude of whole-cell unitary currents and amplitude of spontaneous transient outward currents were voltage-dependent. Whole-cell outward K+ currents evoked by depolarizing voltage ramps had amplitudes often corresponding to the simultaneous opening of more than five single Ca(2+)-activated K+ channels. Lowering the intracellular EGTA concentration tenfold, and hence the Ca2+ buffering capacity of the cell, increased unitary K+ current activity and shifted the relationship between Po and membrane potential by approximately -20 mV. 4. Bradykinin (1 microM), adenosine 5'-triphosphate (3 microM) and acetylcholine (3 microM) applied extracellularly evoked a biphasic increase in N Po (where N is number of channels activated) of the Ca(2+)-activated K+ channel studied in the whole-cell recording configuration. The development of a biphasic response to agonist stimulation requires a source of extracellular Ca2+. The sustained increase in N Po of the Ca(2+)-activated K+ channel was attenuated upon the removal of external Ca2+ (Mg2+ replacement) or in the presence of the Ca2+ entry blocker, Ni2+, and the potassium channel blockers tetrabutylammonium (TBA) or tetraethylammonium (TEA). 5. Unitary and spontaneous transient outward currents were inhibited by extracellularly applied TEA (0.5 mM), TBA (0.5-5 mM) and charybdotoxin (100 nM). Ca(2+)-activated K+ currents were blocked completely by 5 mM-TEA, whereas 3,4-diaminopyridine (1 mM), Ba2+ (10 mM) and apamin (0.1-1 microM) did not abolish these K+ currents. 6. The K+ channel opener cromakalim (10 microM) evoked a sustained increase in N Po of the Ca(2+)-activated K+ channels which was not potentiated by the addition of bradykinin. Glibenclamide (10 microM) alone increased N Po and partially inhibited the cromakalim-induced increase in N Po with respect to control.(ABSTRACT TRUNCATED AT 400 WORDS)

Pharmacological characterization of ATP receptors mediating vasodilation on isolated rabbit aorta

1. The effects of adenosine 5'-triphosphate (ATP), 2-methyl-thio-ATP and adenosine on rabbit aorta were examined in isolated preparations precontracted by noradrenaline, both in vessels where the endothelium was present and mechanically removed. 2. In the presence of endothelium, ATP (30 microM-3 mM) induced a relaxation that was reduced by removal of the endothelium. 3. The maximum endothelium-dependent relaxation of ATP was twice the maximum endothelial activity of the potent agonist at P2y-purinoceptors, 2-methyl-thio-ATP. 4. Adenosine which acts on P1 purinoceptors, induced a relaxant effect at 1 mM concentration, both in the vessels with and without endothelium. 5. It is concluded that relaxation by ATP is induced both via the endothelial P2y-purinoceptor and via a "nucleotide" receptor that is located on endothelium and on smooth muscle of rabbit aorta.

A nucleotide receptor in vascular endothelial cells is specifically activated by the fully ionized forms of ATP and UTP

Extracellular ATP causes an increase in the concentration of cytoplasmic free calcium ([Ca2+]i) in bovine pulmonary-artery endothelial (BPAE) cells that results in the synthesis and release of prostacyclin (PGI2), a potent vasodilator and inhibitor of platelet aggregation. We show here that PGI2 release in BPAE cells correlates with the concentration of the fully ionized form of extracellular ATP (ATP4-) and not with the concentration of other ionic forms of ATP. Concentrations as low as 10 nM-ATP4- elicited an increase in PGI2 release [EC50 (concn. giving half-maximal stimulation) 3 microM] in BPAE cells incubated in an iso-osmotic medium, pH 7.4, lacking Ca2+ and Mg2+. When the pH or the Mg2+ concentration of the medium was varied so as to maintain a constant level of ATP4-, while varying the concentration of proton-ATP (HATP3-) or MgATP2- respectively, PGI2 release remained constant. An inhibitory effect of extracellular Mg2+ on PGI2 release could be attributed solely to a decrease in the concentration of ATP4-. In contrast with Mg2+, extracellular Ca2+ stimulated PGI2 release induced by ATP. Several results suggest that extracellular Ca2+ modulates PGI2 release by increasing Ca2+ uptake through an ATP(4-)-activated plasma-membrane channel. In BPAE cells incubated in Ca(2+)-free medium, ATP elicited a transient increase in [Ca2+]i that declined to the basal level within 60 s. In cells incubated in Ca(2+)-containing medium, ATP caused an increase in [Ca2+]i that had two components: a transient peak in [Ca2+]i (0-60 s) and a sustained increase in [Ca2+]i that was maintained for several minutes after ATP addition. Increasing the concentration of extracellular calcium from 0.25 mM to 10 mM had no effect on the transient rise in [Ca2+]i induced by ATP, but significantly enhanced the magnitude of the sustained increase in [Ca2+]i. Alterations in the magnitude of the sustained increase in [Ca2+]i would likely modulate PGI2 release, which was not complete until 2 min after ATP addition. Extracellular Ca2+ also stimulated PGI2 release induced by bradykinin. Bradykinin caused a sustained increase in [Ca2+]i in BPAE cells in the presence of extracellular Ca2+. Finally, the magnitude of PGI2 release induced by UTP, a more potent agonist than ATP, correlated with the concentration of extracellular fully ionized UTP (UTP4-). These findings support the hypothesis that nucleotide receptors in BPAE cells recognize the fully ionized form of ATP and UTP and are coupled to signal-transduction pathways involving the mobilization of intracellular Ca2+, the influx of extracellular Ca2+ and the subsequent release of PGI2.

Stimulation of phosphatidic acid synthesis in bovine aortic endothelial cells in response to activation of P2-purinergic receptors

In this study we used the bovine thoracic aorta endothelial cell line AG 4762 and primary bovine aortic endothelial cells to investigate the formation of phosphatidic acid (PA) in response to activation of P2-purinergic receptors. 2-Methylthio ATP (2MeSATP) stimulated the formation of [32P]-PA in bovine aortic endothelial cells labelled with 32P(i) for 2.5 hr. A comparison of the response to other ATP analogues suggests that this was mediated via a P2Y-purinergic receptor. Using various agonists at 30 microM there was a correlation between the formation of [32P]PA and of total inositol phosphates in the presence of lithium. The 2MeSATP-stimulated accumulation of [32P]PA showed an initial high rate, followed by a more sustained slower rate. The initial response was independent of extracellular calcium while the later response was dependent on calcium influx. The protein kinase C stimulator phorbol myristate acetate (PMA) produced only a very small enhancement of [32P]PA accumulation compared to 2MeSATP. The 2MeSATP stimulation of both inositol phosphates and [32P]PA was almost eliminated by the presence of PMA. Using cells prelabelled with [3H]methylcholine 2MeSATP produced only a small non-significant enhancement of [3H]choline formation; PMA by contrast formed a much larger amount of [3H]choline. There was no evidence of a change in [3H]phosphocholine. The dissociation between phospholipase D (PLD) activation and [32P]PA accumulation and the correlation between stimulation of [32P]PA accumulation and phospholipase C (PLC) activation all suggest that, using this protocol for labelling cells, the principle route of the stimulation of formation of [32P]PA is via the activation of PLC followed by metabolism of diacylglycerol (DAG) by DAG kinase. These results show that activation of P2Y-purinergic receptors on aortic endothelial cells leads to the formation of phosphatidic acid and that both PLD and PLC pathways are likely to contribute to this response.

Endothelium-dependent and -independent effects of exogenous ATP, adenosine, GTP and guanosine on vascular tone and cyclic nucleotide accumulation of rat mesenteric artery

1. The effects of exogenous guanosine 5'-triphosphate (GTP) and guanosine on vascular tone and cyclic nucleotide accumulation of noradrenaline-precontracted endothelium-intact and endothelium-denuded rat mesenteric artery rings were compared with the effects of the known purinoceptor agonists adenosine 5'-triphosphate (ATP) and adenosine. 2. GTP (10 microM-1 mM) dose-dependently relaxed endothelium-intact mesenteric artery rings by producing a rapid initial response followed by sustained relaxation resembling the relaxant response to acetylcholine. GTP also slightly relaxed endothelium-denuded artery rings. The acetylcholine- and GTP-induced relaxations of endothelium-intact rings were attenuated by NG-nitro L-arginine methyl ester (L-NAME, 330 microM) which attenuation was reversed with L-arginine (1 mM). 3. Guanosine (10 microM-1 mM) relaxed both endothelium-intact and -denuded artery rings in a dose-dependent manner. The relaxations were more pronounced in endothelium-intact preparations and were only slightly attenuated by L-NAME (330 microM). 4. ATP (1 microM-1 mM) and adenosine (10 microM-1 mM) dose-dependently relaxed endothelium-intact and -denuded artery rings. The responses were more pronounced in endothelium-intact vascular preparations. 5. GTP (100 microM) and guanosine (100 microM) increased guanosine 3':5'-cyclic monophosphate (cyclic GMP) accumulation in both endothelium-intact and -denuded artery rings corresponding to the relaxations observed. The concentrations of adenosine 3':5'-cyclic monophosphate (cyclic AMP) were not affected. 6. ATP (100 microM) increased cyclic GMP concentration of endothelium-intact artery rings. The concentrations of cyclic AMP were not affected by ATP (100 microM) and adenosine (100 microM) in endothelium-intact and -denuded vascular preparations.7. These results provide evidence that exogenous GTP and guanosine relax precontracted endothelium-intact and -denuded rat mesenteric artery rings by increasing cyclic GMP accumulation. The response to GTP of endothelium-intact rings can mainly be explained by the release of endothelium-derived relaxing factor (EDRF), but that of guanosine is only partly due to EDRF, and is a combination of endothelium-dependent and -independent effects. The endothelium-independent response of GTP and guanosine is a direct, unknown effect on smooth muscle and guanylate cyclase.

Characterization of acetylcholine-induced membrane hyperpolarization in endothelial cells

The characteristics of the hyperpolarization response to acetylcholine (ACh) in endothelial cells from the guinea pig coronary artery were studied by microelectrode recording technique. ACh (30 nM to 3 microM) induced membrane hyperpolarization in a dose-dependent manner. The sustenance of the response required the presence of external calcium. The hyperpolarization was not affected by nifedipine (1 microM) but was inhibited by the potassium channel blockers charybdotoxin (10 nM), tetraethylammonium (1 mM), and 4-aminopyridine (0.5 mM). Glibenclamide (10 microM) and apamin (1 microM) were not effective. The inhibitors of endothelium-derived relaxing factor/nitric oxide synthesis N omega-nitro L-arginine (50 microM) and NG-monomethyl L-arginine (30 microM) had no effect on the resting membrane potential or the ACh-induced responses. No hyperpolarization was observed with application of sodium nitroprusside (10 microM) or 8-bromo-cGMP (0.1 microM). Ouabain (10 microM) depolarized the membrane significantly by 5 mV, but the ACh hyperpolarization was not affected. Indomethacin (10 microM) was without effect on the resting membrane potential or the hyperpolarization to ACh. These results show that ACh-induced hyperpolarization is dependent on external calcium and can be inhibited by certain potassium channel blockers. The hyperpolarization response is not mediated by endothelium-derived relaxing factor/nitric oxide, cGMP, a cyclooxygenase product, or stimulation of the Na-K pump.

Effect of shear stress on cytosolic Ca2+ of calf pulmonary artery endothelial cells

The purpose of the present study was to determine if hemodynamic shear stress increases free cytosolic Ca2+ concentration ([Ca2+]i) of cultured pulmonary artery endothelial cells exposed to steady laminar fluid flow in a parallel plate chamber. Average [Ca2+]i was estimated by measuring cell-associated fura-2 fluorescence using microfluorimetric analysis. To determine [Ca2+]i close to the membrane surface, 86Rb+ efflux via Ca(2+)-dependent K+ channels was measured. Upon initiation of flow or upon step increases in flow, no change in [Ca2+]i was observed using fura-2. However, increases in shear stress produced a large, transient increase in 86Rb+ efflux. The shear stress-dependent increase in 86Rb+ efflux was not blocked by either tetrabutylammonium ions (20 mM) or by charybdotoxin (10 nM), two specific inhibitors of the Ca(2+)-dependent K+ channel of vascular endothelial cells. These results demonstrate that shear stress per se has little effect on either the average cytosolic [Ca2+]i as measured by fura-2 or on [Ca2+]i close to the cytoplasmic surface of the plasmalemma as measured by the activity of Ca(2+)-dependent K+ channels.

Effect of hemorrhagic hypotension on cerebrovascular reactivity and ultrastructure in the cat

BACKGROUND AND PURPOSE

The goal of this study was to determine the alterations in contractile and dilatory responses and ultrastructure of the feline middle cerebral artery after hemorrhagic hypotension.

METHODS

In the sodium pentobarbital anesthetized cats, a steady 50 mm Hg level of hypotension was reached by bleeding into a reservoir and maintained at this level by further bleeding or autotransfusion for 2 hours. Rings of the arteries, from control animals and from animals after hypotension, were suspended for isometric tension recording in organ chambers filled with modified Krebs-Henseleit solution, aerated with 95% O2-5% CO2 at 37 degrees C, and their reactions to contractile and relaxant agents were tested. Vascular ultrastructure was studied by electron microscope.

RESULTS

Endothelium-dependent relaxations induced by 10(-8) M acetylcholine were enhanced, whereas there was a marked inhibition of the relaxation at 10(-6) M. Relaxations induced by adenosine triphosphate and adenosine showed an impairment. Contractions induced by norepinephrine and prostaglandin F2 alpha remained unchanged, whereas 5-hydroxytryptamine caused a more pronounced contraction after hypotension. No alterations in the morphology of endothelium or smooth muscle were found after hemorrhage. There was, however, a marked decrease in the number of transmitter vesicles in the perivascular nerve terminals.

CONCLUSIONS

The present results show marked alterations in cerebrovascular reactivity and ultrastructure of the adventitia after hypotension. These alterations might play an important role in the development of cerebral vasoconstriction during and after this hemorrhagic state.

M3 cholinoceptors and P2y purinoceptors mediating relaxation of arteries in spontaneously hypertensive rats at prehypertensive stages

Muscarinic cholinoceptor and purinoceptor subtypes were determined in femoral and mesenteric artery strips from spontaneously hypertensive rats (SHR) at prehypertensive stages. Acetylcholine (ACh), carbachol and oxotremorine induced endothelium-dependent relaxations during contractions evoked by 5-hydroxytryptamine. The order of relative potency of the agonists was ACh greater than carbachol = oxotremorine in the prehypertensive SHR and Wistar Kyoto rats (WKY). Schild plot data for muscarinic cholinoceptor antagonists obtained with carbachol or oxotremorine as agonist indicated that the arteries of prehypertensive SHR and WKY possessed M3 cholinoceptors mediating relaxation. The relaxation responses of the prehypertensive SHR and WKY arteries to ATP were inhibited by reactive blue 2 but not altered after P2x purinoceptors had been desensitized by alpha,beta-methylene ATP, which suggests that the arteries possessed P2y purinoceptors mediating relaxation. The responses of the femoral and mesenteric arteries from prehypertensive SHR to ACh, carbachol and ATP were similar to those of WKY arteries, although the response of the prehypertensive SHR artery to oxotremorine was significantly increased compared to that of the WKY artery. The results suggest that M3 cholinoceptor- and P2y purinoceptor-mediated relaxations are not changed in prehypertensive SHR arteries as compared to those in WKY arteries.

On the mechanism of the involvement of endothelium in reactive hyperemia

Experiments on anesthetized dogs and on vascular test-preparations demonstrated that reactive hyperemia (RH) was accompanied by the appearance of vasodilator in the blood, and that the level increased with the duration of occlusion of the artery. Removal of the endothelium of the part of the vascular bed studied using saponin, decreased the RH and relaxation of a test-preparation. A rise of pressure in the vascular bed, and a decrease in the deformability of the endothelium resulting from pretreatment with dimerized glutaraldehyde, affected both the hyperemia and the reaction of the vascular preparation in a similar way. It was concluded that the RH resulted from the secretion of vasoactive substances by the endothelium in response to a fall in intravascular pressure.

Endothelium-dependent and endothelium-independent vasodilatation of the hepatic artery of the rabbit

1. The isolated hepatic artery of the rabbit contracted to exogenously applied noradrenaline (NA). There was no significant difference in the maximal contraction or the EC50 value in vessels where the endothelium was present and in endothelium-denuded preparations. 2. Acetylcholine (ACh) induced a vasodilatation of vessels preconstricted with NA which was entirely dependent on the endothelium. 3. Adenosine 5'-triphosphate (ATP), 2-methylthio ATP, adenosine and sodium nitroprusside induced concentration-dependent, sustained relaxations of vessels in which tone had been induced with NA. The relaxation responses were not reduced after removal of the endothelium. 8-Phenyltheophylline antagonized the relaxation response produced by adenosine, but not that due to ATP at lower concentrations. The maximum response to ATP was reduced in the presence of 8-phenyltheophylline. 4. alpha,beta-Methylene ATP produced further contraction of vessels preconstricted with NA in both endothelium-denuded preparations and in vessels where the endothelium remained intact. 5. Immunohistochemical analysis was used to show the presence of nerve fibres containing substance P (SP), calcitonin gene-related peptide (CGRP) and vasoactive intestinal polypeptide (VIP) in the hepatic artery. Application of SP induced a concentration-dependent relaxation which was entirely dependent on the presence of an intact endothelium. CGRP and VIP, however, elicited concentration-dependent relaxations which were independent of the endothelium. 7. It is concluded that in the rabbit hepatic artery, responses to ACh are dependent on the presence of intact endothelium. P1-, P2x- and P2y-purinoceptors, mediating relaxation to adenosine, vasoconstriction to ATP and vasodilatation to ATP respectively, are located on vascular smooth muscle. Furthermore, CGRP and VIP mediate a direct vasodilatation of smooth muscle both in the absence and the presence of the endothelium, whereas SP produces a relaxation via receptors located on the endothelium.

Endothelium-derived relaxing factor: basic review and clinical implications

EDRF is a potent, endogenous vasodilator that is produced and released from endothelial cells and subsequently causes the relaxation of VSM through the activation of soluble guanylate cyclase and an increase in VSM cyclic GMP. Structurally, EDRF is likely to be NO or a related nitrogen oxide-containing compound. It is synthesized in endothelial and other cell types from L-arginine by a calcium-calmodulin and NADPH-dependent enzyme. Its action is very similar to the nitrovasodilators that act directly on VSM. EDRF is present in all vascular beds, large and small vessels, and in a wide range of species. Its role in human vascular physiology and pathophysiology is just beginning to be understood. EDRF is a potent endogenous vasodilator and inhibitor of platelet aggregation and adhesion. Its activity is impaired in hypertension and atherosclerosis, and its absence due to endothelial damage may play a role in cerebral and coronary vasospasm. It is a mediator of flow-dependent vasodilation, and its inhibition by hypoxia may contribute to the hypoxic pulmonary vasoconstrictor response. Endothelial cell damage and impairment of EDRF production may also contribute to acute and chronic pulmonary hypertension. A further understanding of the chemical nature and synthetic pathways of EDRF should lead to the production of analogs and antagonists, which may play an important role in future treatments for atherosclerosis, myocardial infarction, angina, hypertension, and other vascular diseases. The recent realization that EDRF serves as the second messenger for guanylate cyclase activation and cyclic GMP production in a variety of cell types outside of the cardiovascular system, including renal and respiratory epithelium, cerebellar neurons, macrophages, and adrenocytes, suggests even broader implications. The importance of EDRF to the anesthesiologist may go beyond an understanding of its role in cardiovascular physiological and pathophysiological states. Initial studies have shown that the endothelium may play a role in mediating the vascular actions of anesthetics, and that anesthetics can inhibit the production, release, or action of EDRF. How are these interactions mediated? Are there significant differences between anesthetics with regard to their effects on EDRF? Is there a clinically significant effect of anesthetics on basal activity of EDRF, or only in response to exogenous stimulation? Conversely, it is important to determine if alterations in endothelial cell function by various disease states such as hypertension, atherosclerosis, adult respiratory distress syndrome, cerebral vasospasm, and others cause changes in the vascular actions of anesthetics. The potential interactions of anesthetics with EDRF production and action in cell types other than the endothelium have not yet been explored.(ABSTRACT TRUNCATED AT 400 WORDS)