Modulation of vasodilatation to levcromakalim by adenosine analogues in the rabbit ear: an explanation for hypoxic augmentation

Increase in the vasorelaxant potency of K(ATP) channel opening drugs by adenosine A(1) and A(2) receptors in the pig coronary artery

Myograph recording from ring segments of pig small coronary arteries was used to investigate the effects of adenosine receptor activation on the vasorelaxant potency of ATP-sensitive K(+) channel opening drugs. Receptor activation with 2-chloroadenosine (2-CA, 300 nM) increased the potency of both nicorandil and levcromakalim, shifting the pEC(50)s from 4.68+/-0.03 to 5.05+/-0.04 and from 6.34+/-0.06 to 6.72+/-0.06, respectively (P<0.05 in each case). Experiments with selective adenosine receptor agonists (2-chloro-N(6)-cyclopentyladenosine (CCPA), 2-p-(2-carboxyethyl)phenethylamino-5'-N-ethylcarboxamidoadenosine hydrochloride (CGS 21680)) and antagonists (8-cyclopentyl-1, 3-dipropylxanthine (DPCPX), 4-(2-[7-amino-2-(2-furyl)[1,2, 4]triazolo[2,3-a] [1,3,5]triazin-5-ylamino]ethyl)phenol (ZM 241385)) suggest that both A(1) and A(2a) receptors can increase the potency of nicorandil, while that of levcromakalim is increased only by A(2) receptors. Adenosine receptor activation did not affect the potency of pinacidil. Thus, adenosine receptor activation can increase the potency of some K(+) channel opening drugs to relax coronary arteries, but the details of the interaction with adenosine receptors depend on the particular drug.

Potassium channel activation and relaxation by nicorandil in rat small mesenteric arteries

1. We used whole-cell patch clamp to investigate the currents activated by nicorandil in smooth muscle cells isolated from rat small mesenteric arteries, and studied the relaxant effect of nicorandil using myography. 2. Nicorandil (300 microM) activated currents with near-linear current-voltage relationships and reversal potentials near to the equilibrium potential for K+. 3. The nicorandil-activated current was blocked by glibenclamide (10 microM), but unaffected by iberiotoxin (100 nM) and the guanylyl cyclase inhibitor LY 83583 (1 microM). During current activation by nicorandil, openings of channels with a unitary conductance of 31 pS were detected. 4. One hundred microM nicorandil had no effect on currents through Ca2+ channels recorded in response to depolarizing voltage steps using 10 mM Ba2+ as a charge carrier. A small reduction in current amplitude was seen in 300 microM nicorandil, though this was not statistically significant. 5. In arterial rings contracted with 20 mM K+ Krebs solution containing 200 nM BAYK 8644, nicorandil produced a concentration-dependent relaxation with mean pD2 = 4.77+/-0.06. Glibenclamide (10 microM) shifted the curve to the right (pD2 = 4.32+/-0.05), as did 60 mM K+. LY 83583 caused a dose-dependent inhibition of the relaxant effect of nicorandil, while LY 83583 and glibenclamide together produced greater inhibition than either alone. 6. Metabolic inhibition with carbonyl cyanide m-chlorophenyl hydrazone (30 nM), or by reduction of extracellular glucose to 0.5 mM, increased the potency of nicorandil. 7. We conclude that nicorandil activates KATP channels in these vessels and also acts through guanylyl cyclase to cause vasorelaxation, and that the potency of nicorandil is increased during metabolic inhibition.

Role for adenosine A1 and A2 receptors in femoral vasodilatation induced by intra-arterial adenosine in rabbits

The vasodilator effects of adenosine injected into the femoral artery (i.a.) of rabbits were analyzed. Single bolus i.a. doses of adenosine (0.3-10 microg) and 5'-(N-cyclopropyl)-carboxamidoadenosine (CPCA) (0.03-1 microg), an adenosine A2-receptor agonist, produced dose-dependent increases in femoral blood flow and decreases in resistance, almost without affecting blood pressure, heart rate, left ventricular (LV) pressure, and LVd P/dt max, even though CPCA elicited slight decreases in arterial blood pressure and LV pressure. On the other hand, bolus i.a. injections of N6-cyclopentyladenosine (CPA) (1-30 microg), an adenosine A1 receptor agonist, caused a relatively weak increase in blood flow, but markedly affected cardiac parameters, especially heart rate and LVd P/dt max. I.v. treatment with 3,7-dimethyl-1-propargylxanthine (DMPX)(2 mg kg(-1)), an antagonist of adenosine A2 receptors, or 8-phenyltheophylline (1 mg kg(-1)), an antagonist of adenosine A1 receptors, significantly attenuated the vasodilator response to adenosine, but not that to acetylcholine. Decreases in blood pressure, heart rate, LV pressure, LVdP/dt max and femoral vascular resistance, and increases in the blood flow elicited by CPA were not significantly modified by the DMPX treatment, but when this was combined with 8-phenyltheophylline, the responses to CPA were completely abolished. The present results indicate that the adenosine-induced femoral vasodilatation in rabbits may be mediated throughout activation of both adenosine A1 and A2 receptors.

Potentiating effect of nicorandil on the adenosine A2 receptor-mediated vasodepression in rats: potential role for KATP channels

The effects of nicorandil on systemic blood pressure (SBP) and heart rate (HR) responses to adenosine were compared with those to N6-cyclopentyladenosine (CPA), a selective adenosine A1 receptor agonist, and 5'-(N-cyclopropyl)-carboxamidoadenosine (CPCA), a selective adenosine A2 receptor agonist, in anesthetized rats. When injected intravenously (i.v.), single bolus doses of CPCA (0.01-1.0 micrograms/kg), like adenosine (30 micrograms/kg), elicited dose-dependent decreases in SBP scarcely affecting HR, while CPA (0.03-1.0 micrograms/kg) produced only reduction of HR without influencing SBP. The enhancement of the vasodepressor response to CPCA, like adenosine, was induced by the i.v. infusion of either nicorandil (10 micrograms/kg per min) or cromakalim (0.1 micrograms/kg per min), but the response to CPA in HR remained unmodified during the infusion of nicorandil as well as cromakalim. After i.v. treatment with glibenclamide (20 mg/kg), and adenosine triphosphate (ATP)-sensitive K+ channel blocker, or 3,7-dimethyl-1-propargylxanthine (DMPX) (1 mg/kg), a selective antagonist of adenosine A2 receptor, not only CPCA action but also the enhancement of CPCA action by nicorandil and cromakalim were significantly attenuated. Similar results were obtained in the case of single bolus i.v. adenosine. The present result indicates that the augmentation of the adenosine action by nicorandil appears to be mediated by activation of ATP-sensitive K+ channels, closely linked with stimulation on A2 receptors by adenosine.

Basal nitric oxide release differentially modulates vasodilations by pinacidil and levcromakalim in goat coronary artery

In the current investigation, the role of basal nitric oxide (NO) in modulating the vasorelaxant responses to pinacidil and levcromakalim was examined in goat isolated coronary artery. Pinacidil (10(-8) 10(-4) M) elicited concentration-dependent relaxations of the coronary artery ring segments (with intact endothelium) constricted with 30 mM K+ saline solution. The EC50 of the vasodilator was 2.57 x 10(-6) M (95% CL, 1.9-3.46 x 10(-6) M). The removal of endothelium by mechanical rubbing caused a rightward shift in the concentration-response curve of pinacidil with a corresponding increase in EC50 value (1.90 x 10(-5) M; 95% CL, 1.12-3.23 x 10(-5) M). Similar to endothelium removal, treatment of endothelium-intact rings either with the NO synthesis inhibitor L-NAME (NG-nitro-L-arginine methyl ester; 3 x 10(-5) M) or the guanylate cyclase inhibitor, methylene blue (3 x 10(-6) M) resulted in a marked inhibition in the relaxant responses to pinacidil. Hence, the EC50 values of the potassium channel opener were significantly higher in tissues treated either with L-NAME (7.41 x 10(-6) M; 95% CL, 6.02-9.12 x 10(-6) M) or methylene blue (2.29 x 10(-5) M; 95% CL, 1.58-3.31 x 109-5) M) as compared to untreated controls. The ATP-sensitive potassium (KATP) channel blocker glibenclamide, which caused a significant rightward shift in the concentration-relaxation curve of pinacidil in control tissues, was found to be less potent in antagonising the relaxant responses of the KATP channel opener in endothelium-denuded rings and in rings with intact endothelium but treated with either L-NAME or methylene blue. In contrast to the observations made with pinacidil, the vasodilator responses to another KATP channel opener, levcromakalim, were potentiated in the absence of basal NO. Thus, the EC50 of levcromakalim was 1.33 x 10(-8) M (95% CL, 0.8-2.21 x 10(-8) M) in control tissues with intact endothelium, which was significantly higher than those obtained in endothelium-deprived rings (4.81 x 10(-9) M; 95% CL, 4.04-5.73 x 10(-9) M) or endothelium intact rings treated either with L-NAME (2.63 x 10(-9) M; 95% CL, 1.58-4.36 x 10(-9) M) or methylene blue (2.82 x 10(-9) M; 95% CL, 1.7-4.68 x 10(-9) M). The selective modulation by basal NO of the arterial relaxations elicited with the KATP channel openers was evident from the findings that papaverine-induced relaxations were not affected in the absence of basal NO. Taken together, the results of the present study suggest that basal NO differentially modulates the interaction of pinacidil and levcromakalim with the KATP channels in goat coronary artery through a cGMP-dependent pathway.

Enhancement of the vasorelaxant potency of nicorandil by metabolic inhibition and adenosine in the pig coronary artery

OBJECTIVE

Nicorandil is used clinically to treat angina and acts in part by opening ATP-sensitive K+ channels whose opening is also enhanced by metabolic compromise. We have therefore investigated whether treatments that mimic conditions in ischaemia can increase the potency of nicorandil to dilate coronary arteries.

METHODS

Ring segments from pig small coronary arteries were mounted on a myograph, contracted with 20 mM K+ Krebs solution containing 200 nM BAYK 6844, and relaxations to cumulative doses of nicorandil were measured.

RESULTS AND CONCLUSIONS

Nicorandil produced a dose-dependent relaxation with a mean pEC50 (-log EC50, M) of 4.76 +/- 0.02. Inhibition of metabolism with carbonyl cyanide m-chlorophenyl hydrazone (CCCP, 100 nM) or by removal of extracellular glucose significantly increased the potency of nicorandil (pEC50s of 5.11 +/- 0.08 and 5.08 +/- 0.06, p < 0.05 in each case). The adenosine analogue 2-chloroadenosine (2-CA, 300 nM) had a similar effect (pEC50 = 5.17 +/- 0.06, p < 0.05). Reducing extracellular pH to 6.8 also significantly increased the potency of nicorandil, but to a smaller extent. Glibenclamide reduced the potency of nicorandil (pEC50 = 3.81 +/- 0.01, n = 7), and abolished its enhancement by CCCP, zero glucose, 2-CA or pH 6.8 solution. 2-CA did not affect the potency of nicorandil in relaxing contractions to 80 mM K+ or the potency of glyceryl trinitrate. We conclude that the potency of nicorandil to cause coronary vasorelaxation is increased under conditions of metabolic inhibition. This effect appears to result from the K+ channel opening action of the drug, and may have significant consequences for its therapeutic effectiveness.

The effects of levcromakalim and pinacidil on the human internal mammary artery

The present study was undertaken to examine the effects of pinacidil and levcromakalim, two potassium, channel openers, on human internal mammary artery (HIMA) obtained from patients undergoing coronary artery bypass surgery, and to clarify the contribution of different K+ channel subtypes in pinacidil and levcromakalim action in this blood vessel. Pinacidil and levcromakalim induced a concentration-dependent relaxation of the precontracted arterial segments (pEC50 = 5.77 +/- 0.05 and 6.89 +/- 0.03, respectively), 4-Aminopyridine (3 mM), a non-selective blocker of K+ channels, induced significant shifts to the right of the concentration-response curves for pinacidil and levcromakalim. Tetraethylammonium (6 mM), charybdotoxin (0.4 microM) and apamin (0.1 microM), blockers of Ca(2+)-sensitive K+ channels, had no effect on the pinacidil- and levcromakalim-evoked relaxation. Glibenclamide (0.1-10 microM), a selective blocker of adenosine triphosphate (ATP)-sensitive K+ channels, competitively antagonized the response to levcromakalim (pKB = 7.92 +/- 0.07). In contrast, glibenclamide, in significantly higher concentrations (3-30 microM), non-competitively antagonized the response to pinacidil. High concentrations of pinacidil (> 10 microM) relaxed arterial rings bathed by a medium containing 100 mM K+ with maximum response 83 +/- 6%. Under the same conditions, the maximum levcromakalim-induced relaxation on HIMA was almost abolished (15 +/- 2%). It is concluded that pinacidil and levcromakalim do not relax the HIMA through the same subtype of K+ channel. ATP-sensitive K+ channels are probably involved in levcromakalim- but not in a pinacidil-induced relaxation in the HIMA. In addition, in pinacidil-induced relaxation of the HIMA, K+ channel-independent mechanisms seem to be involved.

Interaction between ATP-sensitive K+ channels and nitric oxide on pial arterioles in piglets

The interaction between ATP-sensitive K+ channels (KATP) and nitric oxide (NO) was studied in pial arterioles of piglets. We examined the effects of N omega-nitro-L-arginine methyl ester (L-NAME), a general inhibitor of nitric oxide synthase (NOS), and 7-nitroindazole (7-NI), a selective inhibitor of neuronal NOS, on aprikalim-induced cerebral vasodilation. Topically applied, aprikalim, a selective activator of KATP, dilated arterioles by 11 +/- 7% at 10(-8) M and 17 +/- 6% at 10(-6) M. After L-NAME treatment (15 mg/kg, i.v.), the response was reduced (4 +/- 4% and 12 +/- 7%, respectively; n = 8, p < 0.05). Administration of 7-NI (50 mg/kg, i.p.) did not change pial arteriolar responsiveness to aprikalim. However, both L-NAME and 7-NI reduced the vasodilator responses to 10(-4) M N-methyl-D-aspartate (NMDA) (by 73% and by 36%, respectively). Furthermore, 7-NI treatment abolished the glutamate-induced dilatation of pial arterioles. Administration of L-NAME reduced the NOS activity in the cerebral cortex by 88%, whereas the reduction after the 7-NI treatment was 44%. Pre-treatment and coadministration of 10(-5) M glibenclaminde, a specific inhibitor of KATP or L-NAME administration, did not change the dilatory response to sodium nitroprusside. We conclude that NO may be involved in aprikalim-induced dilation of pial arterioles.

Modulation of vasorelaxant responses to potassium channel openers by basal nitric oxide in the rat isolated superior mesenteric arterial bed

1. We have used the isolated buffer-perfused mesenteric arterial bed of the rat to assess the modulation of vasorelaxation to potassium channel openers (KCOs) by basal nitric oxide. 2. The dose-response curves to the KCOs, levcromakalim and pinacidil, in preconstricted preparations were significantly shifted to the left in the presence of the nitric oxide synthase inhibitor (100 microM) NG-nitro-L-arginine methyl ester (levcromakalim, ED50 = 4.47 +/- 0.70 nmol vs. 1.73 +/- 0.26 nmol, P < 0.001; pinacidil, ED50 = 16.1 +/- 4.8 nmol vs. 5.43 +/- 1.10 nmol, P < 0.001). The vasorelaxant responses to papaverine, a vasodilator which acts independently of potassium channels was unaffected by NG-nitro-L-arginine methyl ester (L-NAME). 3. Removal of the endothelium, by perfusion with the detergent CHAPS (0.3%), significantly (P < 0.001) increased the potency of levcromakalim as a vasodilator (ED50 4.47 +/- 0.70 nmol vs. 2.59 +/- 0.31 nmol). The subsequent administration of L-NAME following perfusion with CHAPS did not lead to any additional enhancement of responses to levcromakalim. 4. The presence of the non-selective adenosine antagonist, 8-phenyltheophylline (8-PT, 10 microM) significantly (P < 0.001) shifted the dose-response curve to levcromakalim to the left (ED50 4.47 +/- 0.70 nmol vs. 1.11 +/- 0.32 nmol). In the presence of both L-NAME and 8-PT, the dose-response curve to levcromakalim was also significantly (P < 0.01) shifted to the left compared with control (ED50 in the presence of both L-NAME and 8-PT was 0.42 +/- 0.08 nmol). 5. The presence of 8-bromo cyclic GMP (10 microM) reversed the increase potency of levcromakalim, observed following inhibition of nitric oxide synthase (ED50 in the presence of L-NAME was 0.59 +/- 0.01 nmol and in the presence of 8-bromo cyclic GMP plus L-NAME the ED50 was 3.17 +/- 0.80 nmol). However in the absence of L-NAME, the cell permeable analogue of cyclic GMP, 8-bromo cyclic GMP, did not affect the dose-response curve to levcromakalim compared with control (control ED50 value was 4.16 +/- 0.52 nmol vs. 3.85 +/- 1.13 nmol in the presence of 8-bromo cyclic GMP). 6. The present investigation demonstrates that both basal nitric oxide and adenosine modulate vasorelaxation to the KCOs levcromakalim and pinacidil. The modulatory effect of nitric oxide may be mediated via cyclic GMP.

The involvement of ATP-sensitive potassium channels and adenosine in the regulation of coronary flow in the isolated perfused rat heart

1. The roles of adenosine 5'-triphosphate (ATP)-sensitive potassium channels (KATP) and endogenous adenosine in the regulation of coronary flow have been assessed in the isolated, buffer-perfused heart of the rat. 2. In the presence of glibenclamide 10 microM there was a significant (P < 0.001) reduction in coronary flow from a baseline value of 8.78 +/- 0.76 ml min-1 g-1 to 3.89 +/- 0.59 ml min-1 g-1. This change was accompanied by a significant (P < 0.01) reduction in cardiac mechanical performance as shown by the decrease in the pressure-rate product from 21,487 +/- 2,577 mmHg min-1 to 6,950 +/- 1,104 mmHg min-1. 3. The non-selective adenosine antagonist 8-phenyltheophylline (10 microM) also caused a significant (P < 0.001) reduction in coronary flow from a basal value of 10.4 +/- 0.6 ml min-1 g-1 to 6.32 +/- 0.60 ml min-1 g-1. The subsequent addition of glibenclamide, in the presence of 8-phenyltheophylline, brought about a further significant (P < 0.001) reduction in coronary flow to 3.05 +/- 0.55 ml min-1 g-1 and this value was similar to that in the presence of glibenclamide alone. 4. In hearts perfused under constant flow conditions, exogenous adenosine caused dose-related reductions in coronary perfusion pressure described by a maximum reduction in pressure of 30.7 +/- 3.9 mmHg and an ED50 of 977 +/- 813 pmol. Addition of glibenclamide caused a significant (P < 0.01) increase in coronary perfusion pressure of 44.7 +/- 7.2 mmHg and a significant (P < 0.05) rightward shift of the dose-response curve for the depressor effects of adenosine (ED50 = 13.5 +/- 3.8 nmol), with a depression (P < 0.05) of the maximum (16.3 +/- 2.4 mmHg). 5. In conclusion, both KATP and endogenous adenosine make major contributions towards coronary vascular tone and the regulation of coronary flow in the rat isolated heart. Furthermore, in the coronary vasculature a significant proportion of the vasodilator action of adenosine is mediated through the activation of KATP.

Effects of potassium channel openers in isolated human cerebral arteries

The objective of this study was to compare the relaxant effects of the K+ channel openers pinacidil and lemakalim in isolated human pial arteries with the effects of the dihydropyridines nifedipine and nimodipine and the prostacyclin analog iloprost. Relaxation was measured in vessels contracted by 40 mmol/L K+. In contrast to the potent and consistent relaxant effects of nifedipine, nimodipine, and iloprost, the potency of pinacidil and lemakalim proved to be highly variable and inversely correlated with the onset velocity of the preceding contractions of K+ as well as with the endothelium-dependent relaxation of carbachol. Thus, in contrast to dihydropyridines and iloprost, pinacidil and lemakalim selectively elicited potent relaxations in those arteries that exhibited signs of altered vascular wall functions.

Endogenous modulation of excitatory amino acid responsiveness by tachykinin NK1 and NK2 receptors in the rat spinal cord

1. The effects of selective tachykinin (neurokinin, NK) NK1 and NK2 receptor antagonists have been examined on spinal neurones in alpha-chloralose anaesthetized, spinalized rats. They were tested for effects on responses both to excitatory amino acids (EAA) and to noxious heat stimuli. They were also tested for their ability to reverse the actions of selective NK agonists. 2. The NK1-selective antagonists GR82334 (peptide) and CP-99,994 (non-peptide), when applied by microiontophoresis, both reduced responses to kainate > AMPA > NMDA. Intravenous CP-99,994 (3 mg kg-1) also reduced responses to kainate but had inconsistent effects on nociceptive responses. 3. GR82334, applied microiontophoretically, reduced the enhancement by the selective NK1 agonist, GR73632 of both responses to EAAs and background activity. Systemic CP-99,994 (< or = 10 mg kg-1) failed to reverse the effects of GR73632. 4. The selective peptide NK2 antagonist, GR103537, had no consistent effects on responses to EAAs when applied by iontophoresis. In contrast, the non-peptide NK2 antagonist, GR159897, administered systemically (0.5-2 mg kg-1, i.v.) enhanced responses to kainate (but not NMDA); responses to noxious heat were enhanced only weakly. 5. Iontophoretically-administered GR103537 attenuated the effects of the NK2 agonist GR64349, which selectively reduced responses to kainate compared to those to NMDA. Systemically administered GR159897 (< or = 2 mg kg-1, i.v.) caused little antagonism of the effects of GR64349. 6. The data indicate that under these conditions the non-peptide antagonists are not reliable at reversing the actions of selective NK agonists. 7. These results suggest that there is a tonic release of endogenous tachykinins that can modulate glutamatergic neurotransmission in the spinal cord. They provide further support for the hypothesis that release of endogenous NKs acting on NK1 and NK2 receptors can promote NMDA receptor mediated glutamatergic transmission.