Endothelium-dependent relaxation in isolated renal arteries of diabetic rabbits

1 In this study, we have investigated the vasodilator response to acetylcholine under diabetes conditions in isolated renal arteries of rabbits. We have also examined the contribution of endothelium-derived nitric oxide (EDNO) and endothelium-derived hyperpolarizing factor (EDHF) to the endothelium-dependent relaxation caused by acetylcholine in the renal arteries of alloxan-induced diabetic rabbits. 2 Acetylcholine (10(-10) - 10(-4) M) produced cumulative concentration-response curve in the renal arteries of both control and diabetic rabbits. The EC50 values and maximal responses to acetylcholine were not significantly different relative to diabetic conditions. In order to isolate the EDHF component of acetylcholine-induced vasodilator response, L-nitro-methyl arginine ester (L-NAME, 10(-4) M) and indomethacin (10(-6) M) were added to the Krebs' solution throughout the experiment. Under these conditions, acetylcholine induced vasodilatation in the isolated renal arteries from both control and diabetic rabbits. The vasodilator response to acetylcholine was not affected under diabetic conditions. 3 Sodium nitroprusside (SNP)-induced relaxation was increased in the diabetic rabbits compared with the control animals. 4 Tetrabutyl ammonium (TBA, 0.5 mM) produced a significant reduction in acetylcholine-induced vasodilatation in both preparations from control and diabetic animals, consistent with involvement of K+ channels in mediating this response. Glibenclamide (1 microM) attenuated acetylcholine-induced vasodilatation in preparations from control animals only, while iberiotoxin (0.05 microM) significantly reduced the vasodilator response to acetylcholine in preparations from both control and diabetic animals. 5 The role of EDNO in mediating acetylcholine-induced vasodilatation was examined. The vascular preparations were incubated with 20 mM K(+)-Krebs' solution to inhibit the EDHF contribution to acetylcholine-induced vasodilatation. Under this condition, acetylcholine induced a vasodilator response in both preparations from control and diabetic rats. Pretreatment with L-NAME (10(-4) M) attenuated acetylcholine-induced vasodilatation in both preparations, indicating an nitric oxide-mediated vasodilator response. 6 Our results indicated that acetylcholine-induced vasodilatation in the isolated renal arteries of alloxan-induced diabetic rabbits was not affected under diabetic conditions. Acetylcholine-induced vasodilatation is mediated by two vasodilator components; namely, EDHF and EDNO. The contribution of EDHF and EDNO to acetylcholine-induced vasodilatation was not affected under diabetic conditions and there was no indication of endothelial dysfunction associated with diabetes. EDHF component was found to act mainly through high conductance Ca(2+)-activated K+ channels under normal and diabetic conditions, while the adenosine triphosphate-dependent K+ channels were involved in mediating acetylcholine vasodilator response in the control preparations only.     

Endothelium-derived relaxing factor-mediated vasodilation in mouse mesenteric vascular beds

The endothelium in rat mesenteric vascular beds has been demonstrated to regulate vascular tone by releasing mainly endothelium-derived hyperpolarizing factor (EDHF), which is involved in the activation of K(+) channels and gap-junctions. However, it is unclear whether the endothelial system in mouse resistance arteries contributes to regulation of the vascular tone. The present study was designed to investigate the role of the endothelium using acetylcholine and A23187 (Ca(2+) ionophore) in mesenteric vascular beds isolated from male C57BL/6 mice and perfused with Krebs solution to measure perfusion pressure. In preparations with active tone produced by methoxamine in the presence of guanethidine, injections of acetylcholine, A23187, and sodium nitroprusside (SNP) caused a concentration-dependent decrease in perfusion pressure due to vasodilation. The vasodilator responses to acetylcholine and A23187, but not SNP, were abolished by endothelium dysfunction and significantly inhibited by N(ω)-nitro-L-arginine methyl ester (nitric oxide synthase inhibitor) and tetraethylammonium (K(+)-channel inhibitor) but not glibenclamide (ATP-sensitive K(+)-channel inhibitor). Indomethacin (cyclooxygenase inhibitor) significantly blunted only A23187-induced vasodilation, while 18α-glycyrrhetinic acid (gap-junction inhibitor) attenuated only acetylcholine-induced vasodilation. These results suggest that the endothelium in mouse mesenteric arteries regulates vascular tone by prostanoids, EDHF, and partially by nitric oxide, different from the endothelium of rat mesenteric arteries.     

Endothelium-dependent relaxation in coronary arteries requires magnesium ions.

A great deal of interest has recently focused upon the mechanism(s) associated with the generation and action of endothelium-derived relaxant factors (EDRFs) in blood vessels. Since we have shown that extracellular magnesium ions ([Mg2+]o) are important in control of coronary vascular tone and reactivity, we wondered whether these divalent cations play any role in the generation or action of EDRF in coronary arterial smooth muscle. Using isolated canine coronary arterial rings, we have now found that removal of [Mg2+]o inhibits the ability of these vascular preparations to relax when challenged with acetylcholine; in the absence of [Mg2+]o, the relaxation concentration-response curves for acetylcholine are shifted markedly to higher concentrations with small maxima. It, thus, appears that [Mg2+]o is an important co-factor for acetylcholine-induced endothelium-dependent relaxation in canine coronary arteries. These findings support our previous hypothesis that dietary deficiency of Mg may be an important factor in aetiology of coronary vasospasm.     

Pharmacologic inhomogeneity between the reactivity of intramural coronary arteries and arterioles

We hypothesized that because of their size, anatomic location, and hemodynamic function, coronary arteries and arterioles would respond differently to vasoactive substances. Intramural arteries (281.7 +/- 23.1 microm) and arterioles (77.3 +/- 6.6 microm) of the left anterior descending coronary of rats were isolated and cannulated. Spontaneous tone was lower in arteries than in arterioles (81.1 +/- 5.7 vs. 53.0 +/- 3.9% of passive diameter, p < 0.05 at 60 mm Hg intraluminal pressure). Arterial tone was adjusted by the thromboxane receptor agonist U46619 (5 x 10(-8) M ) to reach an active tone close to that of arterioles. Bradykinin elicited dilations in both types of vessels. Acetylcholine (10(-6) - 10(-5) M ) dilated arteries (by 42.6 +/- 11.5 microm) but constricted arterioles (by 16.4 +/- 9.3 microm). Sodium nitroprusside and adenosine elicited significantly greater dilations in arterioles than in arteries (by 7.9 and 11.9%, respectively, p < 0.05), whereas dilations to norepinephrine were similar. Inhibition of nitric oxide synthesis caused a significantly smaller constriction in arteries (10.2 +/- 3.31%) than in arterioles (31.6 +/- 6.9%) and completely blocked bradykinin-and acetylcholine-induced dilations, whereas it did not affect dilations to sodium nitroprusside, adenosine, and norepinephrine. Compared with arteries, arterioles have a greater spontaneous tone and enhanced nitric oxide modulation of basal tone and exhibit greater responsiveness to nitric oxide and adenosine. In addition, nitric oxide synthase is activated differently by pharmacologic stimuli in these segments. The qualitative and quantitative differences among vasoactive responses of coronary arteries and arterioles demonstrated in this study suggest segment-specific roles for endothelial and metabolic factors in regulation of coronary vascular resistance.     

Stress susceptibility as a determinant of endothelium-dependent vascular reactivity in rat mesenteric arteries

In order to investigate the consequences of stress susceptibility on vascular function, the authors assessed the respective contributions of nitric oxide (NO), prostanoids, and endothelium-derived hyperpolarizing factor to the vascular tone in rats with a constitutionally determined high and low susceptibility to behavioral stressors. In mesenteric resistance arteries mounted in a small vessel myograph and precontracted with l-phenylephrine hydrochloride (phenylephrine), the NO-synthase inhibitor N omega-nitro-l-arginine (l-NOARG, 100 microM) elicited a smaller increase of vascular tone in apomorphine-susceptible (APO-SUS) rats (P < 0.01). Addition of indomethacin (10 microM), in the presence of l-NOARG, resulted in a smaller decrease of vascular tone in APO-SUS rats (P < 0.01). Although acetylcholine-induced relaxation in phenylephrine-precontracted arteries was not different (P > 0.1), the individual components contributing to this relaxation were. In arteries precontracted with 125 mM K+, and incubated with indomethacin, acetylcholine-induced relaxation was not significantly different (pEC(50) and E(max): P > 0.1). Sensitivity (pEC(50): P < 0.05) and maximum relaxation (E(max): P < 0.001) to sodium nitroprusside, in the presence of 125 mM K+, was more pronounced in APO-SUS rats. In phenylephrine-precontracted arteries, in the presence of l-NOARG and indomethacin, maximum relaxation to ACh was reduced in APO-SUS rats (E(max): P < 0.05). This study showed that in rats with a high susceptibility to stressors, the contribution of NO to vascular tone was decreased as was the ratio of vasoconstrictor and vasodilator cyclooxygenase products in alpha-adrenergic precontracted arteries. End-organ sensitivity to NO was greater in APO-SUS rats, possibly due to up-regulation. Moreover, the contribution of endothelium-derived hyperpolarizing factor to acetylcholine-induced vasodilation was reduced in APO-SUS rat arteries.     

Flow- and acetylcholine-induced dilatation in small arteries from rats with renovascular hypertension--effect of tempol treatment

We investigated whether renovascular hypertension alters vasodilatation mediated by nitric oxide (NO) and endothelium-derived hyperpolarizing factor (EDHF) and the influence of the superoxide dismutase mimetic tempol on vasodilatation. One-kidney one-clip hypertensive Sprague-Dawley rats, treated with either vehicle or tempol (from weeks 5 to 10 after placement of the clip), and uninephrectomized control rats were investigated. In renal hypertensive rats systolic blood pressure increased to 171+/-6 mmHg (n=10), while in tempol-treated rats systolic blood pressure remained normal (139+/-7 mmHg, n=5). In isolated pressurized mesenteric small arteries NO-mediated dilatation was obtained by increasing flow rate and EDHF-mediated dilatation by acetylcholine. In arteries from hypertensive rats, flow-induced dilatation was blunted, as compared to normotensive and tempol-treated rats, while acetylcholine-induced dilatation remained normal. Measured by dihydroethidium staining there was an increased amount of superoxide in arteries from vehicle-treated rats, but not from tempol-treated rats. Expression by immunoblotting of endothelial NO synthase and the NAD(P)H oxidase subunit p47phox remained unaffected by high blood pressure and tempol treatment. Simultaneous measurements of NO-concentration and relaxation were performed in isolated coronary arteries from the same animals. As compared to vehicle-treated rats, both acetylcholine-induced relaxation and NO-concentration increased in arteries from tempol-treated animals, while only the relaxation was improved by the NO donor, S-nitroso-N-acetylpenicillamine (SNAP). In conclusion renovascular hypertension selectively inhibits flow-induced NO-mediated vasodilatation, while EDHF-type vasodilatation remains unaffected, suggesting that high blood pressure leads to increased generation of superoxide contributing to decreased NO bioavailability. Furthermore, the abnormal endothelium function can be corrected by tempol treatment, but this seems to involve mechanisms partly independent of NO.     

The actions of nifedipine and nisoldipine on the contractile activity of human coronary arteries and human cardiac tissue in vitro

We have studied the action of nifedipine and nisoldipine on the contractile activity of human isolated coronary arteries and human isolated auricular and ventricular muscles. Nisoldipine depressed dose dependently the spontaneous rhythmic contractions displayed by the coronary artery preparations and at 1 nM abolished these contractions. Nisoldipine was twenty times more potent than nifedipine as an inhibitor of increase in tone induced by depolarization (100 mM K+). The rhythmic activity induced by serotonin (10 microM) was about five times more sensitive to nisoldipine than to nifedipine. In both auricular and ventricular preparations, isoprenaline evoked an increase in the rate of force development and in the rate of relaxation. Nifedipine was five times (ventricular muscles) and ten times (auricular muscles) more potent than nisoldipine as a negative inotropic agent. The present observations in human isolated preparations indicate that nisoldipine shows a higher vascular selectivity than nifedipine.     

Vascular β-Adrenoceptor-mediated Relaxation and the Tone of the Tissue in Canine Arteries

The aim of the present investigation was to study the influence of the tone in the response to β-adrenoceptor activation of four different canine arteries: coronary, pulmonary, mesenteric and splenic. Five different levels of tone were produced (of about 35, 50, 65, 80 and 95% of the maximum) by adding phenylephrine (0·6, 1·0, 2·0, 4·0, and 10 μm , respectively) to the bath. In the coronary artery at spontaneous tone, low concentrations of noradrenaline or adrenaline (1–3 nm ) caused either relaxation or contraction, while after induced tone, both noradrenaline and adrenaline caused concentration-dependent relaxations, noradrenaline being more potent (EC50 of 0·16 (0·13–0·20) and 0·38 (0·28–0·67) μm , respectively; n = 6; P < 0·05). Only in the coronary artery did isoprenaline relax the tissue irrespective of the previous level of tone. In all the other arteries, isoprenaline was able to cause concentration-dependent relaxations only if the previous tone was submaximal. At 80% of the maximum, isoprenaline caused relaxation in the mesenteric and pulmonary arteries, but in the splenic artery it caused relaxation only when the tone was of about 65% of the maximum or less. While in the coronary artery atenolol and ICI-118,551 (erythro-dl 1(7-methylindan-4-yloxy)-3-isopropylaminobutan-2-ol) were equipotent in antagonizing isoprenaline, noradrenaline and adrenaline, in the other vessels ICI-118,551 was from 58 (splenic artery) to 525 (mesenteric artery) times more potent than atenolol against the isoprenaline relaxant effect. We conclude that: the tone of the vessel represents a critical factor deciding the sense of the response (coronary artery; low concentrations of noradrenaline or adrenaline), the level at which the relaxant effect is triggered (mesenteric = 80% vs splenic = 65%), and the magnitude of the relaxant effect (always). β-Adrenoceptors predominate in the mesenteric, pulmonary and splenic arteries, while β₁-adrenoceptors predominate in the coronary artery. It is unlikely that adrenaline is able to cause vasodilation in any of the vascular beds studied.     

Vascular effects of potassium cardioplegic solutions

Experiments have been performed on isolated human umbilical and sheep coronary arteries, studying the effects of increasing potassium concentration in steps from 10 to 160 mM. Comparative studies were also done on rat-heart papillary-muscle preparations. The results revealed a dose-dependent increase in smooth muscle tone of the blood vessels. Maximal contraction was reached at 80 mM K+. Papillary-muscle preparations showed a dose-dependent decrease in contractile activity. Complete stoppage of contractile activity was observed at 30 mM K+. The corresponding increase in the vascular smooth muscle tone at 30 mM K+ was 10-25%, showing that vascular smooth muscle cells are less easily affected by increasing K+ than the heart. Further confirmation of the results were obtained by exposing the preparations (papillary muscle and arteries) to a locally-developed depolarizing K+-cardioplegic solution.     

Different contribution of endothelial nitric oxide in the relaxation of human coronary arteries of ischemic and dilated cardiomyopathic hearts

Coronary artery disease and congestive heart failure (CHF) have been associated with a reduction in nitric oxide (NO) release or bioavailability from the vascular endothelium. The objectives of this study were to compare the role of NO in human coronary vessels isolated from nonischemic dilated (DCM) (n = 10) and ischemic (ICM) (n = 12) cardiomyopathic hearts. Segments were mounted on a wire myograph to record changes in isometric tension. All experiments were performed in the presence of indomethacin (10 microM). Contractions induced by angiotensin II (0.1 microM) or a depolarizing physiologic solution containing 40 mM KCl, were of similar amplitude in DCM and ICM. In vessels precontracted with angiotensin II, acetylcholine (1 microM) caused an endothelium-dependent relaxation of rings from DCM but a paradoxical contraction of rings from ICM; NO synthase inhibition with Nomega-nitro-L-arginine (L-NNA, 100 microM) did not affect acetylcholine-induced relaxation or contraction of DCM or ICM vessels, respectively. By contrast, substance P (0.1 microM) induced an endothelium-dependent relaxation in both groups of vessels; this relaxation was prevented (p < 0.05) by L-NNA in vessels from ICM hearts but only reduced (p < 0.05) by L-NNA in vessels from DCM hearts. In depolarized conditions, acetylcholine contracted (p < 0.05) whereas substance P induced a complete relaxation (p < 0.05) of vessels from both groups: substance P-induced relaxation was abolished (p < 0.05) by L-NNA. Our data suggest that in the presence of indomethacin, NO does not contribute to acetylcholine-induced relaxation of human epicardial coronary arteries isolated from DCM hearts. Furthermore, whereas NO and a secondary endothelium-derived relaxing factor sensitive to high K+ contribute to substance P-induced relaxation of rings from DCM hearts, only NO is involved in ICM hearts.     

EFFECT OF INDOMETHACIN ON RESPONSES OF SHEEP ISOLATED CORONARY ARTERY TO ARACHIDONIC ACID

Indomethacin (2.8 μmol/1) did not consistently affect basal tone of sheep coronary artery strips, while a ten-fold higher concentration increased tension in 50% of the preparations tested. When acetylcholine was used as a spasmogen, oscillations in induced tone and relaxations produced by arachidonic acid (6.6 μmol/1) were abolished by indomethacin, 2.8 μmol/1 and 7 μmol/1, respectively. Prostacyclin (PGI₂) and prostaglandin E₁ decreased and PGE₂ increased arterial tension while PGF_2α was inactive. Responses to PGI₂ were reduced by indomethacin (28 μmol/1) but not by indomethacin (2.8 μmol/1). It is suggested that sheep isolated coronary arteries synthesize and release prostacyclin in the presence of acetylcholine and arachidonic acid and that such synthesis can be inhibited by indomethacin.     

Classification of beta-adrenoceptors in human isolated bronchus.

(+/-)-Isoprenaline (Iso), (-)-adrenaline (Ad), (-)-noradrenaline (NA), (+/-)-phenylephrine (Phe) and the beta 2-selective adrenoceptor agonist (+/-)-fenoterol (Fen) caused a concentration-dependent relaxation of human isolated bronchial preparations. Iso, Ad and NA caused complete relaxation of both spontaneous and carbachol-induced bronchial tone. Fen, which was only tested in preparations where tone was induced with carbachol, also caused complete relaxation. However, Phe was a partial agonist in all preparations tested. When relaxation responses to these amines were calculated as a % of their maximal effects, comparison of EC50 values showed that the order of potency was Iso greater than Ad = Fen greater than NA greater than Phe (92:27:25:1:0.2) in preparations with carbachol-induced tone and Iso greater than Ad greater than NA greater than Phe (112:38:1:0.3) in preparations with spontaneous tone. pA2 values determined for the beta-adrenoceptor antagonists propranolol (non-selective), atenolol (beta-selective) and ICI-118, 551 (beta 2-selective), using Iso as an agonist were, 9.3, 5.3 and 9.1 respectively. These results indicate that beta 2-adrenoceptors mediate relaxation of human isolated bronchus to sympathomimetic amines in preparations obtained 4-14 h post-mortem from non-diseased lung. alpha-Adrenoceptors were apparently sparse or absent in this tissue.     

Inhibitory role of endothelium-derived relaxing factor in rat and human pulmonary arteries.

1. The inhibitory role of endothelium-derived relaxing factor was studied in both rat and human pulmonary arteries in vitro by inhibiting its synthesis with the L-arginine analogue NG-monomethyl-L-arginine (L-NMMA). 2. In rat pulmonary arteries, L-NMMA pretreatment (10-300 microM) dose-dependently inhibited acetylcholine-induced relaxation (which is endothelium-dependent). NG-monomethyl-D-arginine (D-NMMA, 100 microM) was without effect. L-Arginine, but not D-arginine, dose-dependently reversed this inhibition. L-NMMA had no effect on relaxation induced by sodium nitroprusside. 3. In human small pulmonary arteries L-NMMA (100 microM) pretreatment similarly inhibited the acetylcholine-induced relaxation but had no effect on the sodium nitroprusside-induced relaxation. 4. In both rat and human pulmonary arteries, L-NMMA, but not D-NMMA, always caused contraction of preconstricted tissues whereas it had no effect on baseline tone. In the rat this contraction was completely prevented by prior treatment with L-arginine. 5. L-NMMA (100 microM) pretreatment mimicked the effect of endothelium removal on phenylephrine-induced vasoconstriction, both resulting in an increase in tension development at each concentration of phenylephrine. This enhancement was greatest at low concentrations of phenylephrine but was still present even at the highest concentrations. Pretreatment with L-NMMA (100 microM) also significantly increased the responses to single doses of phenylephrine. 6. These results suggest that endothelium-derived relaxing factor from endothelial cells both mediates the relaxation response to acetylcholine and also acts as a physiological brake against vasoconstriction in pulmonary vessels.     

Acetylcholine induces constriction of epicardial coronary arteries in anesthetized dogs after removal of endothelium

The acetylcholine-induced relaxation of isolated coronary arteries is reversed to contraction in the absence of endothelium. The importance of endothelium for the regulation of coronary blood flow remains unclear. We thus tested the effects of acetylcholine on epicardial arteries and on coronary resistance vessels in situ in 8 anesthetized dogs. The left circumflex coronary artery was perfused at constant pressure. Epicardial vasomotion was evaluated by sonomicrometry, the vasomotion of coronary resistance vessels by calculated end-diastolic resistance. Acetylcholine (1 microgram/kg/min i.c.) decreased epicardial resistance by 8.6 +/- 1.6% and end-diastolic resistance by 65.8 +/- 6.3%. The epicardial coronary segment was perfused with distilled water for 65 +/- 5 s to denude it of endothelium. After removal of epicardial endothelium, the decrease in end-diastolic resistance caused by acetylcholine was unchanged (59.6 +/- 1.2%); however, epicardial resistance was increased by 7.7 +/- 1.7%. Application of glyceryl trinitrate (5 micrograms/kg/min i.c.) induced a similar decrease of epicardial resistance before and after removal of endothelium:7.9 +/- 1.4 and 6.2 +/- 1.9%, respectively. We conclude that acetylcholine-induced dilation of epicardial coronary arteries is endothelium-dependent in vivo. However, the constriction of epicardial coronary arteries in the absence of endothelium is insufficient to reduce blood flow and to induce myocardial ischemia.     

Diabetes differentially modulated receptor- and non-receptor-mediated relaxation in rat renal artery.

In this study, we have investigated the vasodilator response to acetylcholine under diabetic conditions in isolated renal arteries of Wistar rats. The effect of nitric oxide synthase (NOS) inhibition on acetylcholine-induced vasodilator response was investigated. We have also examined the effects of two endothelium-dependent agonists which induce receptor-dependent and receptor-independent vasodilator responses.Acetylcholine (10(-10) to 10(-4)M) produced a cumulative concentration-response curve in the renal arteries of both control and diabetic rats. The EC(50) values and maximal responses to acetylcholine were reduced relative to diabetic conditions. The vasodilator response to sodium nitroprusside (SNP) (10(-10) to 10(-5)M) was also investigated. SNP produced a cumulative concentration-dependent vasodilator response, which was not affected under diabetic conditions.To confirm the nitric oxide component of acetylcholine-induced vasodilator response, L-nitro-methyl arginine ester (L-NAME) (10(-4)M) was added to the Krebs' solution. The maximal vasodilator response to acetylcholine was reduced in the presence of L-NAME (10(-4)M) in both control and diabetic renal preparations, with greater attenuation in the diabetic conditions.In order to examine the possible contribution of receptor dysfunction in diabetes, the vasodilator response to ADP (receptor-dependent agonist) and the calcium ionophore A23187 (receptor-independent agonist) were investigated. ADP (10(-10) to 10(-5)M) produced a concentration-dependent vasodilator response in preparations from both control and diabetic rats. The maximal vasodilator response to ADP was significantly reduced in the renal arteries from diabetic animals. However, A23187 (10(-10) to 10(-5)M); the receptor-independent agonist, produced a concentration-dependent vasodilator response in both control and diabetic rat preparations. There was no significant change in the EC(50) values or maximal vasodilator responses to A23187 under diabetic conditions.In conclusion, our results indicated that acetylcholine-induced vasodilatation in the isolated renal arteries of streptozotocin (STZ)-induced diabetic rats was attenuated under diabetic conditions. The reduction in acetylcholine-induced vasodilatation may be attributed to acetylcholine receptor dysfunction. This is based on the results from this study in which the vasodilator response to the receptor-independent agonist A23187 were maintained, while that of the receptor-dependent agonist ADP was attenuated under diabetic conditions.     

Comparison of the beta-adrenoceptors in the myocardium and coronary vasculature of the kitten heart.

The relative potencies of (minus) noradrenaline, (minus)-adrenaline, (plus or minus)-isoprenaline and (plus or minus)-salbutamol have been assessed for their positive inotropic and chronotropic actions on kitten isolated atria. These relative potencies have been compared with those obtained for the relative coronary dilator potencies in two preparations. These were intact hearts perfused by Langendorff's method and isolated perfused coronary arteries from the kitten. The relative molar potencies for inotropic effects were noradrenaline 1: adrenaline1: iso-prenaline 7: salbutamol 0.6. The observed ratios for chronotropic effects were not significantly different from those for inotropic effects. The relative potencies of noradrenaline, adrenaline and isoprenaline as myocardial stimulants were similar to their relative potencies as coronary dilators in the intact heart. Similar relative potencies for noradrenaline and isoprenaline were also found in the isolated coronary artery but adrenaline was only one third as active as noradrenaline on this preparation. In both the intact heart and in the isolated coronary artery salbutamol was about one tenth as active as noradrenaline. It was therefore less active relative to noradrenaline as a coronary dilator than as a myocardial stimulant. In spite of these differences in relative potency ratios for myocardial and coronary dilator effects, similar dissociation constants (Kb values) for practolol against isoprenaline were found in driven atria and in isolated coronary arteries. Myocardial and coronary vascular beta-adrenoceptors thus can both be placed in the general classification of beta1-adrenoceptors.     

Purine-mediated relaxation and constriction of isolated rabbit mesenteric artery are not endothelium-dependent

The distribution of P1- and P2-purinoceptors in isolated transverse ring preparations of rabbit mesenteric artery was studied by comparing responses to ATP, alpha, beta-methylene ATP and adenosine in the presence and absence of endothelium. At resting tone in the presence of endothelium, ATP, alpha, beta-methylene ATP and acetylcholine, but not adenosine, produced transient concentration-dependent contractions. Alpha, beta-Methylene ATP was 5000 times more potent than ATP in this respect. Contractions to ATP or alpha, beta-methylene ATP in intact preparations were not significantly different from preparations without endothelium, but contractions to acetylcholine were significantly enhanced in the absence of endothelium. In the presence of endothelium, after desensitisation of the preparations to alpha, beta-methylene ATP, contractions to ATP, but not those to noradrenaline, were either greatly reduced or abolished. In preparations with tone raised by noradrenaline, acetylcholine, ATP and adenosine each produced relaxation in the presence of endothelium; however, in the absence of the endothelium, relaxations to acetylcholine were abolished, while those to ATP and adenosine were not significantly altered. 8-Phenyltheophylline significantly antagonised relaxation to adenosine in the presence of endothelium, whereas relaxation to ATP was unaffected. In raised tone preparations, with and without endothelium, alpha, beta-methylene ATP caused further contraction. However, in raised tone preparations with or without endothelium, that had been desensitised to alpha, beta-methylene ATP before elevation of muscle tone, ATP still caused relaxation, whereas alpha, beta-methylene ATP had no effect.(ABSTRACT TRUNCATED AT 250 WORDS)     

Neuropeptide Y potentiates contraction and inhibits relaxation of rabbit coronary arteries

The effects of neuropeptide Y (NPY) on contraction and relaxation of isolated rabbit coronary arteries were studied. NPY alone caused a weak contraction of coronary arteries with a mean EC50 value of 29 +/- 2.0 nM. Following exposure of coronary arteries to 30 nM NPY, the potencies of norepinephrine (in the presence of 3 microM timolol) and histamine in causing contraction were increased twofold, with no change in maximal contraction. After half-maximal contraction of coronary arteries with histamine and addition of 30 nM NPY, relaxation produced by norepinephrine (in the presence of 3 microM phentolamine), adenosine, and acetylcholine was inhibited. Concentration-response curves for all vasodilators were shifted to the right 10-22-fold by 30 nM NPY. Maximal relaxation caused by adenosine and norepinephrine was not changed by NPY, whereas the maximal response to acetylcholine was 37% less in the presence of NPY. Correlation of the tension produced by NPY with the shift in agonist contraction or relaxation concentration-response curves indicated that NPY-induced increases in baseline tone had no effect on the degree of shift in agonist concentration-response curves. These results show that NPY causes a modest potentiation of agonist-induced contraction and a dramatic blockade of vasodilator-induced relaxation of rabbit coronary arteries.     

The effects of adenosine triphosphate (ATP) and related purines on human isolated subcutaneous and omental resistance arteries.

1. Human resistance arteries were obtained from specimens of omentum and subcutaneous fat removed at surgery. They were studied in vitro by use of a myograph technique to determine the effects of purines on the arteries. 2. In preparations where tone had been raised with noradrenaline, low concentrations (1 nM-1 microM) of adenosine triphosphate (ATP) and 2-methylthioATP, but not alpha,beta-methyleneATP, produced concentration-dependent relaxation. There was a lack of relationship between the relaxation response to acetylcholine and that to ATP. 3. In preparations under basal tone, high concentrations (1 microM-1 mM) of ATP, 2-methylthioATP and alpha,beta-methyleneATP produced concentration-dependent contractions. 4. The rank order of potency of the purine nucleotide analogues for the relaxation response was 2-methylthioATP > ATP > alpha,beta-methyleneATP and for the contractile response it was alpha,beta-methyleneATP > ATP = 2-methylthioATP. 5. Adenosine produced concentration-dependent relaxation in preparations under raised tone and was less potent than ATP but did not produce contraction in preparations at basal tone. Relaxation responses to adenosine, but not to ATP, were antagonized by 8-phenyltheophylline. 6. These results indicate the presence of vasodilator P2y- and P1-purinoceptors and vasoconstrictor P2x-purinoceptors on human resistance arteries isolated from omental and subcutaneous sites.     

Thiorphan enhances bradykinin-induced vascular relaxation in hypoxic/hyperkalaemic porcine coronary artery

Relaxation induced by bradykinin is diminished by hypoxia in epicardial coronary arteries. The bradykinin-degrading enzyme, neutral endopeptidase (NEP, EC.3.4.24.11), is a potential target for coronary artery vasodilators. In this study, we examined the effect of thiorphan, an inhibitor of NEP, on the tone of porcine isolated coronary artery under hypoxic conditions. Endothelium-intact porcine isolated coronary artery rings were isometrically contracted with a prostaglandin F(2alpha) analogue (U46619, 0.75 microM) and potassium chloride (KCl, 30 mM), and relaxed with bradykinin (1-1000 nM) under normoxic (partial pressure of oxygen, pO(2) approximately 90-100 mmHg) and moderately hypoxic (pO(2) approximately 50-60 mmHg) conditions. Experiments were performed to study the effects of 30 min pre-treatment with the NEP-inhibitor, thiorphan (10 microM), both at physiological and at low pO(2)s. Hypoxia inhibited the bradykinin-induced relaxation in porcine epicardial coronary arteries. In normoxia, thiorphan significantly enhanced the decrease of coronary tone produced by bradykinin (1-10 nM) when U46619 was used as contractile agent. Under hypoxic conditions, in U46619 contracture, thiorphan did not influence, but in KCl contracture it enhanced the magnitude of relaxations induced by bradykinin. In the absence of bradykinin, thiorphan had no significant effect on the basal, KCl- and U46619-elevated tones and on the hypoxia-induced decrease of coronary artery tone. Inhibition of NEP-enzyme activity may effectively improve the relaxing capacity of epicardial coronary arteries under hypoxic/hyperkalemic conditions. This effect could be potentially utilized when the endothelial function and relaxation of the coronary arteries are impaired under clinical conditions.