Differences between vasorelaxant responses of the canine and human mesenteric arteries and veins to amrinone.

Amrinone induced similar degrees of relaxation in human mesenteric arteries and veins contracted with KCl, but exerted a more potent vasodilator effect in canine mesenteric veins than in arteries. Dibutyryl cAMP had a stronger effect on KCl-induced contraction in canine mesenteric veins than in arteries. These results suggest that amrinone promotes a different vasodilating action in human and canine mesenteric vessels, possibly via different sensitivity to cAMP in each vascular smooth muscle.     

Differential vasorelaxant effects of KR-30075, a new cyclic AMP-phosphodiesterase inhibitor,on guinea-pig pulmonary,bovine coronary and renal arteries

The vasorelaxant effects of KR-30075 in guinea-pig pulmonary, bovine coronary and renal arterial strips contracted by either K⁺-depolarization, phenylephrine, or prostaglandin F_2α (FGF_2α) were evaluated. KR-30075 was more potent than imazodan as a vasorelaxant against PGF_2α-induced contractions in bovine coronary and renal arteries, whereas against K⁺-induced contractions KR-30075 was less potent than imazodan in guineapig pulmonary arteries and more potent in bovine coronary arteries. KR-30075 was more potent against contractions induced by phenylephrine or PGF_2α than the contractions induced by K⁺. This profile of activity for KR-30075 was similar to that of imazodan and dissimilar from the calcium entry blocking agent nifedipine. There was no vascular selectivity of KR-30075 between bovine coronary and renal arterial strip preparations. In conclusion, this study shows that KR-30075 represents the vasorelaxant effects on guinea-pig pulmonary, bovine coronary and renal arteries without specific vascular selectivity. The vasorelaxant profile of KR-30075, with different sources of vascular smooth muscle, is unlike that of calcium entry blocking agent and more similar to the profile of the agent that inhibit cyclic nucleotide phosphodiesterase.     

Effects of dobutamine on isolated canine cerebral, coronary, mesenteric, and renal arteries

The effects of dobutamine on helical strips of isolated canine cerebral, coronary, mesenteric, and renal arteries was investigated. Dobutamine contracted only renal arterial strips under resting condition. When renal and mesenteric arterial strips were partially contracted with prostaglandin F2 alpha (PGF2 alpha), dobutamine caused further concentration-related contraction, while coronary arterial strips were relaxed. Cerebral arterial strips, on the other hand, did not significantly respond to dobutamine. After treatment with 10(-5) M dl-phenoxybenzamine hydrochloride (POB) for 1 h, dobutamine-induced contractions of partially precontracted mesenteric and renal arterial strips were converted to relaxations. Relaxations of coronary arteries were not potentiated by the alpha-antagonist, but were attenuated by treatment with 10(-6) M propranolol and 10(-6) M metoprolol to a similar extent. On the other hand, relaxations of mesenteric and renal arterial strips were not inhibited by metoprolol but by propranolol. Droperidol (3 X 10(-5) M) failed to significantly alter the concentration-response curve for dobutamine. These results suggest that dobutamine causes vasoconstriction mediated by apha-adrenergic receptor and vasodilatation mediated by beta 1- and beta 2-adrenoceptors. Dobutamine does not appear to act on dopamine receptors.     

Positive inotropic and vasodilating effects of amrinone and milrinone in isolated canine heart

The question has been raised whether the in vivo positive inotropic effect of amrinone and milrinone is a primary effect or secondary to vasodilation. The effects of each drug on isolated trabeculae and resistance vessels obtained from the same dog hearts were determined separately. The positive inotropic and vasodilatory effects coincided over the same concentration range for amrinone. The active isometric force of trabeculae and resistance vessels was increased, and respectively decreased by 20% at comparable concentrations of amrinone--20% ED (effective dose) ranging between 9.5 and 18 microM. By contrast, the vasodilatory properties of milrinone (20% ED, 15-34 microM) appeared only at concentrations at which milrinone had already evoked a maximal positive inotropic response in trabecular muscle (20% ED, 0.36-0.38 microM). Based on the present experiments and their limitations, it would thus appear that, in the intact heart and at therapeutic doses, positive inotropic and vasodilatory effects may be equally present for amrinone, whereas, for milrinone, the positive inotropic effect would largely predominate the vasodilatory effect.     

Differential effects of magnesium on basal and agonist-induced EDRF relaxation in canine coronary arteries.

In the present study the effects of alterations in extracellular magnesium concentration on spontaneous release of endothelium-derived relaxing factor (EDRF) and acetylcholine-, thrombin-, and calcium ionophore A23187-induced EDRF-dependent relaxation in isolated canine coronary arteries were determined. Increasing the extracellular magnesium concentration from the standard 1.2 to 2.4 mM did not alter the coronary contractile responses, while complete removal of extracellular magnesium resulted in an EDRF-dependent relaxation. The averaged percent relaxation of coronary arteries previously constricted with either prostaglandin F2 alpha, norepinephrine, or barium chloride was -75.9 +/- 4.4 (mean +/- SEM of 27 vessel segments), -43.1 +/- 4.9% (20 segments), and -25.8 +/- 6.9% (15 segments), respectively. Complete removal of extracellular magnesium also resulted in a slight but significant rightward shift of the concentration-response curves of acetylcholine- and thrombin-induced EDRF-dependent relaxation. The maximum relaxation was decreased to 86.5 +/- 1.7% and 69.5 +/- 4.6% of the control acetylcholine and thrombin relaxation, respectively. In contrast, the calcium ionophore A23187-induced EDRF-dependent relaxation was not altered following magnesium withdrawal, nor was the isoproterenol-induced endothelium-independent relaxation. These results suggest that extracellular magnesium exerts a direct inhibitory effect on the spontaneous release of EDRF and is apparently important for the expression of endothelium-dependent relaxation induced by acetylcholine and thrombin, but not calcium ionophore A23187, in canine coronary arteries.     

Mechanism of contractile responses to bradykinin in canine basilar arteries.

This study was undertaken to investigate the role of endothelium in the modulation of vascular responses to bradykinin and to elucidate the receptor types and mechanism of action of bradykinin in isolated basilar artery. The results showed a contractile response to bradykinin in basilar artery. This contractile response to bradykinin was partially modulated by endothelium in a dose-dependent manner. In addition, both des-Arg9-[Leu8]bradykinin and [d-Arg0,Hyp3,Thi5,8,D-Phe7]bradykinin significantly antagonized the responses to bradykinin. However, the blocking effect and the apparent affinity of [d-Arg0,Hyp3,Thi5,8,D-Phe7]bradykinin (pA2 = 9.6 +/- 0.4) were greater than those with des-Arg9-[Leu8]bradykinin (pA.2 = 7.8 +/- 0.3). These results suggest that two apparently distinct types of BK receptors may exist in basilar arteries. Furthermore, the contractile response to bradykinin in basilar artery was significantly inhibited by 2-nitro-4-carboxyphenyl-N,N-diphenylcarbamate (10(-5) M), H-7 (10(-5) M) and TMB-8 (10(-5) M), but not by indomethacin (10(-5) M) or nordihydroquariaretic acid (10(-5) M). On the other hand, nifedipine, Ca(2+)-free medium, EGTA and Ca(2+)-free medium/EGTA significantly reduced the bradykinin-induced contraction, indicating that part of the contractile response of basilar artery to bradykinin is dependent on extracellular Ca2+. In conclusion, the mechanism of the contractile responses to bradykinin in basilar artery may involve increased intracellular Ca2+ levels acting on the BK1 and BK2 receptor, followed by activation of the phosphoinositide pathway and receptor-mediated Ca2+ channel.     

Differential effects of prostaglandins on canine intrapulmonary arteries and veins

1 The sensitivity and contractility of isolated canine intrapulmonary arteries and veins to a variety of primary prostaglandin compounds was studied.

2 Intrapulmonary arteries produced no measurable contractile responses to prostaglandin A₁ (PGA₁), PGA₂, PGB₁, PGD₂, PGE₁, PGE₂ or to PGF_1α. However, high concentrations of both PGB₂ (> 10^-7 M) and PGF_2α (> 10^-6 M) elicited concentrated-related, but weak, contractile responses, measuring only 5-25% of KCl-induced maximum contractions.

3 Intrapulmonary arteries, partially contracted by 5-hydroxytryptamine (5-HT), exhibited concentration-related relaxations in response to PGE₁; PGE₂, PGA₁ or PGA₂ produced only weak superimposed contractions.

4 In contrast to intrapulmonary arteries, intrapulmonary veins contracted in a concentration-related fashion to all prostaglandins tested, where the contractile sensitivity was (based on EC₅₀ s and threshold concentrations): PGB₂ > PGB₁ > PGD₂ > PGF_2α > PGA₂ >> PGA₁ > PGF_1α > PGE₂ > PGE₁.

5 In terms of the ability to generate maximum contractile responses on intrapulmonary veins, the prostaglandins were also variable, with PGA₂ and PGB₂ being the most potent and PGD₂ the least potent.

6 Intrapulmonary veins, partially contracted by 5-HT, exhibited concentration-related relaxations to PGE₁ at low concentrations, followed by secondary contractile responses at higher concentrations.

7 Neither PGA₁ nor PGA₂ (3.4 × 10^-8 to 3.4 × 10^-5 M) inhibited or potentiated 5-HT responses of intrapulmonary arteries.

8 These data suggest that there are species, regional and major qualitative and quantitative, differences in the responsiveness of intrapulmonary arteries and veins to prostaglandin.

     

Differential antagonism by glibenclamide of the relaxant effects of cromakalim,pinacidil and nicorandil on canine large coronary arteries

Summary The relaxant mechanisms of action of cromakalim, pinacidil and nicorandil, potassium channel openers, on large epicardial coronary arteries were investigated in isolated canine left circumflex arteries contracted by 10^–7 mol/l U46619, a thromboxane A₂ analogue, or addition of 25 mmol/l KCl in comparison with nitroglycerin.Cromakalim (3 × 10^–8–3 × 10^–5 mol/l), pinacidil (10^–6–10^–4 mol/l), nicorandil (3 × 10^–6–10^–3 mol/l) and nitroglycerin (3 × 10^–8–10^–5 mol/l) all produced a concentration-dependent relaxation in both U46619- or KCl-contracted arteries. At their maximum effects pinacidil, nicorandil and nitroglycerin produced full relaxation in arteries contracted by either means. In contrast, cromakalim produced about a 73% relaxation in KCl-contracted arteries, although it caused full relaxation in U46619-contracted ones. In the presence of glibenclamide the concentration-relaxation curves for cromakalim in U46619- or KCl-contracted arteries underwent rightward parallel shifts. Schild regression had a slope of 1.00 and yielded a pA₂ of 7.47 for glibenclamide in U46619-contracted arteries, and corresponding values obtained in KCl-contracted arteries were 0.86 (not significantly different from unity) and 7.28. The concentration-relaxation curves for pinacidil in U46619-contracted arteries also underwent rightward parallel shifts in the presence of glibenclamide, however, Schild regression had a slope of 0.60. The concentration-relaxation curves for pinacidil in KCl-contracted arteries underwent rightward parallel shifts only to a limited extent in the presence of glibenclamide. The concentration-relaxation curves for nicorandil and nitroglycerin in U46619- or KCl-contracted arteries were not affected by glibenclamide in concentrations which antagonized cromakalim. The concentration-relaxation curves for nicorandil or nitroglycerin in U46619-contracted arteries were shifted by methylene blue (10^–5 mol/l) to the right without suppression of the their maximum effects. Similar curves for cromakalim were not affected at all by this concentration of methylene blue. The concentration-relaxation curves for nicorandil in U46619-contracted arteries determined in the presence of methylene blue (10^–5 mol/l) and glibenclamide (3 × 10^–7, 10^–6 and 3 × 10^–6 mol/l) were not significantly different from those in the presence of methylene blue alone.These results indicate the following: In canine large epicardial coronary arteries (1) cromakalim produced relaxation by the mechanism antagonized by glibenclamide, probably opening ATP-sensitive potassium channels, (2) pinacidil did so by the mechanism shared with cromakalim and by one not antagonized by glibenclamide as well, and (3) nicorandil did so exclusively by the mechanism of action as a nitrate. Send offprint requests to N. Taira at the above address     

Effects of a cardiotonic agent, TA-064, on isolated canine cerebral, coronary, femoral, mesenteric, and renal arteries

We investigated the effects of a newly synthesized cardiotonic agent, TA-064, on helical strips of isolated canine cerebral, coronary, femoral, mesenteric, and renal arteries. TA-064 had no effect on isolated arterial strips under resting tension. When the arterial strips were partially contracted with prostaglandin F2 alpha, TA-064 caused markedly significant concentration-related relaxations in coronary arterial strips. However, the maximum relaxation induced by TA-064 in renal, mesenteric, and femoral arterial strips was only one-third or less of the coronary artery. On the other hand, cerebral arterial strips generated negligible responses to TA-064. Relaxation of renal, mesenteric, and femoral arteries was not potentiated by pretreatment with 10(-5) M phenoxybenzamine. The concentration-response curve for TA-064 in coronary artery was shifted to the right to a similar extent by exposure to 2 X 10(-7) M propranolol and 2 X 10(-7) M metoprolol. On the other hand, relaxation of renal arterial strips was only slightly attenuated by metoprolol but was inhibited by propranolol. Droperidol (3 X 10(-5) M) failed to significantly alter the concentration-response curve for TA-064 in coronary artery. These results indicate that TA-064 causes coronary arterial vasodilatation mediated by beta 1-adrenoceptors. It would further appear that the same mechanism may be responsible for the positive inotropic action of TA-064.     

Inhibitory effects of nitroglycerin on contractile responses to methoxamine and clonidine in isolated rabbit renal and femoral arteries

The inhibitory effect of nitroglycerin (NG) on contractile responses to methoxamine (MO) and clonidine (CL) was investigated in isolated rabbit renal and femoral arteries. NG (10(-7)-10(-5) mol/l) inhibited the responses to MO and CL in a noncompetitive manner and its inhibitory effect on CL-responses was much greater than that on the MO-responses. In the presence of phenoxybenzamine, however, NG (10(-5) mol/l) markedly inhibited the residual response to MO. Contractile responses to KCl or added Ca2+ in a Ca2+-free medium containing KCl were slightly inhibited by NG. In the presence of nifedipine, NG (10(-5) mol/l) markedly inhibited residual responses to CL but it only slightly inhibited the response to MO. In a Ca2+-free medium with EGTA, MO (10(-5) mol/l) or CL (10(-5) mol/l) induced a phasic contraction. NG had a greater inhibitory effect on the response to CL than MO. In a Ca2+-free medium with EGTA, nifedipine and MO (10(-5) mol/l) or CL (10(-5) mol/l), Ca2+ induced a tonic contraction. NG inhibited the Ca2+-response in the presence of CL, but it had little or no effect on the Ca2+-response in the presence of MO. These results suggest that in rabbit renal and femoral arteries, the potent inhibitory effect of NG on the responses to CL as compared to MO may be due to differences in the amount of receptor reserves that exist for both the agonists. In addition, the inhibition of voltage-dependent Ca2+ channels may not play a major role in the vasoinhibitory action of NG. Further, NG inhibits contractile responses due to mobilization of intracellular Ca2+ much more than the responses due to receptor-activated, nifedipine-insensitive Ca2+-movement.     

Differential vasoactive effects of sildenafil and tadalafil on cerebral arteries

Phosphodiesterase 5 (PDE5) is associated with migraine pathophysiology, stroke recovery and vasospasm treatment. The potential vascular interplay of PDE5 inhibitors sildenafil, tadalafil and UK-114,542 was studied by intra- versus extra-luminal administration in rat middle cerebral arteries in vitro and on middle meningeal arteries in vivo. By Western blot PDE5 was detected in both cerebral and meningeal arteries, though with minor variations in band intensity between vascular beds. Rat middle cerebral artery diameter was investigated using pressurised arteriography, applying UK-114,542, sildenafil, and tadalafil intra- or extra-luminally. Effects on the dural middle meningeal artery were studied in the in vivo closed cranial window model. At high concentrations, abluminal sildenafil and UK-114,542, but not tadalafil, induced dilatation of the middle cerebral artery. Luminal application elicited a contraction of 4% (sildenafil, P=0.03) and 10% (tadalafil, P=0.02). In vivo, sildenafil, but not tadalafil, dose-dependently dilated middle meningeal artery concomitant to blood pressure reduction (1-3mg/kg);1mg/kg sildenafil inducing 60 ± 14% (P=0.04) and vehicle (DMSO) 13 ± 6% dilatation. In conclusion, PDE5 inhibitors applied luminally had minor contractile effect, whereas abluminal sildenafil induced middle cerebral artery dilatation above therapeutic levels. In vivo, sildenafil dilated middle meningeal artery concomitant with a reduction in blood pressure. Tadalafil had no dilatory effects. PDE5 inhibitors show differential vascular activity in cerebral arteries from healthy animals; arterial dilatation is seen primarily above therapeutic levels. Such findings support clinical studies showing no vasodilator effects of sildenafil on cerebral arteries in healthy subjects.     

The vasorelaxant effects of milrinone and other vasodilators are attenuated by ouabain

The purpose of this study was to investigate the effect of ouabain pretreatment (1 microM) on relaxation induced by milrinone and other reference vasorelaxants in guinea pig aorta. Pretreatment with ouabain for 1 h significantly reduced the threshold concentration and increased the vasoconstrictor potency of phenylephrine. Relaxation by milrinone of aortic rings contracted by phenylephrine or by an equieffective concentration of phenylephrine in the presence of ouabain was significantly attenuated in the presence of ouabain. The effect of all other vasorelaxants tested, which included isosorbide dinitrate, hydralazine, sodium nitroprusside, forskolin, HA 1004, isoproterenol and verapamil, were also significantly reduced in the presence of ouabain. The vasorelaxant effects of milrinone and verapamil were also evaluated in K+-contracted guinea pig aorta. In contrast to results obtained with phenylephrine-contracted vessels, milrinone and verapamil were equipotent as vasorelaxants in K+-contracted vessels in the presence or absence of ouabain. The results show that ouabain not only potentiates the effect of the vasoconstrictor phenylephrine, but also reduces the potency of drugs that cause vasorelaxation in phenylephrine-contracted tissues, regardless of the mechanism of action of the vasorelaxant. These data may have clinical relevance to the concomitant use of vasodilators and digitalis in the treatment of congestive heart failure.     

Actions of amrinone and milrinone on the guinea-pig ileum

The pharmacological activity of the cardiotonic agents amrinone and milrinone has been examined on the isolated guinea-pig ileum. Both compounds antagonised the presynaptic inhibitory action of adenosine on electrically evoked contractions, milrinone being the more potent. Higher concentrations of amrinone or milrinone produced contractions of the ileum which were prevented by atropine. Contractile responses to adenosine 5'-monophosphate were not inhibited by atropine. It is concluded that amrinone and milrinone can antagonise some effects of purines but this does not represent their only pharmacological action.     

Differential inhibitory effects of nicorandil, a new antiangina agent, on the contractile responses to alpha-1- and alpha-2-adrenoceptor agonists in isolated rabbit renal and femoral arteries

In the renal and femoral artery, nicorandil (10(-6)-10(-4) mol/l) had a much greater inhibitory effect on the responses to clonidine (CL) and BHT-920 than on the response to methoxamine (MO) but the inhibitory action of nifedipine on the responses was no different. In the presence of nifedipine, nicorandil further inhibited the responses to all agonists. In the tissue pretreated with phenoxybenzamine, nicorandil inhibited the residual maximum response to MO in the femoral artery but not in the renal artery. Nicorandil had no effect on the residual response to high concentrations of MO in the renal artery pretreated with phenoxybenzamine and nifedipine. Relationship between maximum contraction and percent-receptor occupancy was found to be nonlinear for MO but was close to linear for CL. The inhibitory effect (pA2) of prazosin on MO and CL was much greater than that of yohimbine. In both preparations, nicorandil had only a slight inhibitory effect on the responses to potassium and Ca2+. It is concluded that the responses induced by MO, CL and BHT are due to activation of alpha 1-adrenoceptors and that the differential effect of nicorandil on the responses to alpha 1- and alpha 2-agonists may be the result of differences in the amount of receptor reserves and/or efficacy of receptor-contraction coupling that exists for MO, CL and BHT.     

Differential effects of the antianginal drug nicorandil on canine arteries and veins.

Nicorandil, a potent coronary vasorelaxant used in the treatment of angina, has differential effects on arteries and veins in vivo. To explain this phenomenon, experiments were designed to characterize the relaxant and inhibitory actions of this compound on canine isolated arteries and veins. Paired rings of canine coronary, femoral, and saphenous arteries and saphenous veins were suspended at optimal length for isometric tension recording in organ chambers containing physiologic salt solution at 37 degrees C and gassed with 95% O2-5% CO2. In certain experiments, one ring of each pair was denuded of the endothelium. Removal of the endothelium did not affect nicorandil-induced relaxations of contracted blood vessels. Nicorandil exerted a differential relaxant effect on arteries and veins contracted with KCl (order of potency: saphenous vein greater than coronary artery greater than femoral artery). No difference in sensitivity to nicorandil was observed in arteries and veins contracted with prostaglandin F2 alpha. Contractions of saphenous arteries and veins to norepinephrine (NE) were equally sensitive to the inhibitory action of nicorandil. However, contractions of saphenous veins induced by sympathetic nerve stimulation were more sensitive to nicorandil than were contractions of saphenous arteries. Furthermore, nicorandil did not affect contractions to phenylephrine in saphenous veins, although contractions to B-HT 920 were virtually abolished by the compound. Saphenous veins contracted with St 587 were more sensitive to the relaxant action of nicorandil than when contracted with phenylephrine. These results suggest that nicorandil inhibits preferentially contractions of canine arteries and veins mediated by alpha 2- rather than alpha 1-adrenoceptors.(ABSTRACT TRUNCATED AT 250 WORDS)     

Possible role of endothelial thromboxane A2 in the resting tone and contractile responses to acetylcholine and arachidonic acid in canine cerebral arteries

The amount of immunoreactive thromboxane B2 (iTXB2) released from isolated canine arteries was determined by radioimmunoassay. The amount of iTXB2 released from the cerebral, coronary, mesenteric, and saphenous arteries was 47.0 +/- 7.2, 4.0 +/- 0.6, 4.9 +/- 0.5, and 2.7 +/- 0.4 pg/mg wet weight tissue/30 min, respectively. The release of iTXB2 from the cerebral artery was decreased to less than 50% by the administration of indomethacin (10(-5) M) or OKY-046 (10(-4) M), and by intimal rubbing. The release of iTXB2 was enhanced nearly twofold by the addition of arachidonic acid (AA) (10(-5) M) to the medium, but not by the addition of acetylcholine (ACh) (10(-6) M). The cerebral arterial strips maintained the resting tone, which was reduced maximally by papaverine (10(-4) M). The resting tone was also reduced dose dependently by a cyclooxygenase inhibitor (indomethacin), a thromboxane A2 (TXA2) synthetase inhibitor (OKY-046), and a TXA2 antagonist (ONO-3708). The resting tone of rubbed strips was about half that of intact strips. ACh and AA induced similar transient contractions in the cerebral artery. Contractions produced by these agents were attenuated by indomethacin (10(-7) M), aspirin (5 X 10(-5) M), OKY-046 (10(-6) M), and ONO-3708 (10(-8) M), and abolished by intimal rubbing. From these results, it is concluded that TXA2 is produced in the endothelial cells and may be involved in maintaining the resting tone and contractile response to AA in the canine cerebral artery.     

Endothelins: vasoconstrictor effects and localization in canine cerebral arteries.

1. The vascular effects of endothelin and localization of endothelin-like immunoreactivity were characterized in isolated cerebral arteries of dogs. 2. Endothelin-like immunoreactivity was detected in a few populations of endothelial cells of dog basilar artery. 3. Endothelin-1, endothelin-2 and endothelin-3 contracted isolated ring preparations of cerebral arteries in a dose-dependent manner independently of the presence of endothelium. The ED50 values (and 95% confidence intervals) for the contraction were 411 pM (242-697 pM) and 478 pM (295-776 pM) for endothelin-1 and endothelin-2, respectively. Endothelin-3 induced vascular contraction at a higher concentration (ED50 = 26.5 nM, 95% confidence interval = 15.7-45.7 nM). 4. The increases in tone induced by endothelin-1 and endothelin-2 did not return to the resting level after repeated washings, while a rinse with Krebs solution reversed the vasoconstrictor response to endothelin-3. The endothelins did not cause any vasodilator response in arteries precontracted with uridine 5'-triphosphate even in the presence of intact endothelial cells. 5. NiCl2 (1 mM) attenuated the contractions induced by endothelin-3 (10-300 nM) and those to relatively low doses (1 nM) but not higher doses (10-100 nM) of endothelin-1 and endothelin-2. The contractions in response to endothelin-1, endothelin-2 and endothelin-3 were greatly attenuated in Ca(2+)-free solutions although high concentrations of endothelin-1 and endothelin-2 still evoked contractions. 6. These results suggest that the vasoconstriction induced by endothelin-3 and lower doses of endothelin-1 and endothelin-2, largely depends on the influx of Ca2+ ions. The apparent insensitivity to Ni2+ shows that additional distinct mechanisms also operate in the vasconstrictor responses to high concentrations of endothelin-1 and endothelin-2. 7. The presence of endothelin-like immunoreactivity in endothelial cells suggests that endothelin is a potential endogenous spasmogen.     

In vivo and in vitro effects of amrinone and milrinone on hepatic xenobiotic metabolism in rats

Studies were performed on the response of hepatic xenobiotic metabolizing enzymes to in vitro and in vivo exposure to amrinone and milrinone, two new inotropic compounds used in congestive heart failure. Both drugs exerted selective effects on various cytochrome P-450-dependent metabolic activities as well as conjugating pathways. Aminopyrine N-demethylation was selectively inhibited by in vitro addition of milrinone but not amrinone, and laurate hydroxylation was inhibited by both drugs. Cytosolic glutathione-S-transferase activity was profoundly inhibited by in vitro addition of both drugs. In vivo administration of either drug did not lead to significant inhibition of the pathways studied other than laurate hydroxylation which was depressed 20-30%. Irreversible binding of [14C]-amrinone-derived radioactivity to microsomal protein was partially NADPH-dependent. Inhibition by SKF 525-A, alpha-naphthoflavone and various antioxidants was observed. No binding of [14C]-milrinone-derived radioactivity was seen. It is suggested that amrinone may selectively inhibit certain hepatic drug-metabolizing enzymes through metabolic electrophilic intermediates.     

Comparison of contractile responses to donitriptan and sumatriptan in the human middle meningeal and coronary arteries

Donitriptan is a potent, high efficacy agonist at 5-HT(1B/1D) receptors. We investigated the contractile effects of donitriptan and sumatriptan on human isolated blood vessels of relevance to therapeutic efficacy in migraine (middle meningeal artery) and coronary adverse events (coronary artery). Furthermore, using the concentration-response curves in the middle meningeal artery, we predicted the plasma concentration needed for the therapeutic effect of donitriptan. Both donitriptan and sumatriptan contracted the middle meningeal artery with similar apparent efficacy (E(max): 103+/-8% and 110+/-12%, respectively), but the potency of donitriptan (pEC(50): 9.07+/-0.14) was significantly higher than that of sumatriptan (pEC(50): 7.41+/-0.08). In the coronary artery, the contraction to donitriptan was biphasic with a significantly higher maximal response (E(max): 29+/-6%) than sumatriptan (E(max): 14+/-2%; pEC(50): 5.71+/-0.16), yielding two distinct pEC(50) values (8.25+/-0.16 and 5.60+/-0.24). Incubation with the 5-HT(2) receptor antagonist ketanserin (10 microM) eliminated the low affinity component of the concentration-response curve of donitriptan and the resultant E(max) and pEC(50) were 9+/-2% and 7.33+/-0.21, respectively. Ketanserin was without effect on the sumatriptan-induced contraction. Based on the middle meningeal artery contraction, concentrations (C(max)) of donitriptan that may be expected to have a therapeutic efficacy equivalent to that of 50 and 100 mg sumatriptan are predicted to be around 2.5 and 4.3 nM, respectively. Such concentrations are likely to induce only a small coronary artery contraction of 2.9+/-1.5% and 3.8+/-2.0%, respectively; these are not different from those by C(max) concentrations of sumatriptan (1.7+/-0.4% or 2.2+/-0.4%). The present results suggest that, like sumatriptan, donitriptan exhibits cranioselectivity and would be effective in aborting migraine attacks with a similar coronary side-effect profile as sumatriptan.