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N S Cook

Publications and source records attributed to N S Cook.

31 records · Page 2Linked to original sources
PubMed1988

Pertussis toxin treatment does not inhibit the effects of the potassium channel opener BRL 34915 on rat isolated vascular and cardiac tissues.

The experiments were undertaken to determine whether the effects of the K+ channel opener BRL 34915 on rat isolated vascular smooth muscle and atria were sensitive to pertussis toxin (PTx). PTx treatment of rats (100 micrograms/kg, infused over 15 min) affected some baseline parameters of the isolated tissues: in the atria, heart rate was increased, contractile force was decreased and the basal efflux of 86Rb+ was increased; in portal veins, the spontaneous activity was decreased but the contractility of aortic rings was unaffected. In the isolated atria removed from saline-treated rats, carbamylcholine decreased heart rate and contractile force, shortened the action potential duration by increasing the maximum rate of repolarization and increased 86Rb+ efflux. These effects of carbamylcholine were completely abolished in the atria from PTx-treated rats, demonstrating the efficacy of the toxin. The ability of 300 microM BRL 34915 and of 55 mM KCl to increase atrial 86Rb+ permeability was, however, only slightly affected by PTx treatment. In portal veins from PTx-treated rats, the efficacy of BRL 34915 to inhibit spontaneous activity and to increase 86Rb+ efflux was the same as in control organs. Similarly, in aortic rings, the ability of BRL 34915 to inhibit contractions to low concentrations of KCl or to noradrenaline was unaffected by PTx treatment as was the 86Rb+ efflux response to BRL 34915 in this tissue. It is concluded that PTx treatment does not inhibit the effects of BRL 34915 in the tissues investigated. The results are compatible with the notion that BRL 34915 does not open K+ channels by acting through a PTx-sensitive G-protein.

Action Potentials↗
PubMed1988

Leiurus quinquestriatus venom inhibits BRL 34915-induced 86Rb+ efflux from the rat portal vein.

The effect of the crude venom of the Israeli scorpion Leiurus quinquestriatus hebraeus on the 86Rb+ efflux stimulated by the K+ channel opener BRL 34915 in the rat portal vein was examined. Applied alone, the venom greatly increased the spontaneous mechanical activity of and the concomitant 86Rb+ efflux from the vessel. When the excitability of the vein was suppressed by the dihydropyridine calcium antagonist, PN 200-110, the 86Rb+ efflux stimulated by BRL 34915 could be shown to be inhibited by the venom. From the concentration dependence of this inhibition an IC50 value of 0.17 +/- 0.01 mg/ml was estimated. This venom is thus the most potent blocker of BRL 34915-evoked 86Rb+ efflux reported so far.

Animals↗
PubMed1988

Similarities in the mechanism of action of two new vasodilator drugs: pinacidil and BRL 34915.

The present study compares the in vitro and in vivo activities of pinacidil with another new vasodilator drug, BRL 34915, claimed to act via the opening of K+ channels in vascular smooth muscle. In the rabbit aorta, BRL 34915 and pinacidil caused rightward shifts of the KCl concentration-response curve and noncompetitively antagonized angiotensin II contractions, yielding an IC50 of 5 and 10 microM, respectively. In 86Rb-loaded guinea pig portal veins, both BRL 34915 and pinacidil stimulated 86Rb+ efflux over the concentration range 0.3-30 microM. At saturating concentrations, the maximum efflux elicited by pinacidil was only one-third that of BRL 34915. Spontaneous activity measured simultaneously from the same vessels was inhibited by BRL 34915 and pinacidil with an IC50 of 12 and 32 nM, respectively. Effects on mechanical activity were thus observed at drug concentrations 100-fold lower than those required to stimulate 86Rb+ efflux. In anesthetized rats, both compounds rapidly lowered blood pressure, with BRL 34915 being threefold more potent than pinacidil in this respect. Tachycardia was more pronounced after BRL 34915 than after pinacidil. Angiotensin II pressor responses were poorly antagonized by these two vasodilators in rats, in marked contrast to the potent effects of Ca2+ antagonists on these responses at equieffective hypotensive doses. We conclude the pinacidil, like BRL 34915, is a potent antihypertensive that is able to enhance the K+ permeability of vascular smooth muscle. The similarities between these two drugs suggest they have a common mechanism of action. However, the discrepancy between the concentrations of these drugs necessary to stimulate 86Rb efflux, and those at which mechanical effects are seen in the portal vein, do not rule out the possibility that other actions may contribute to their vasodilator activities.

Animals↗
PubMed1988

Cardiovascular effects of apamin and BRL 34915 in rats and rabbits.

1. The cardiovascular effects of apamin, a selective blocker of certain calcium-activated potassium channels, and BRL 34915, a vasodilator thought to act by opening of potassium channels, have been investigated in vivo in rats and rabbits. 2. In anaesthetized normotensive rats, apamin (0.05 and 0.15 mg kg-1, i.v.) potentiated angiotensin II pressor responses but did not modify baseline blood pressure or heart rate. 3. Apamin (0.15 mg kg-1, i.v.) was without cardiovascular effects in rabbits. 4. BRL 34915 (0.1 and 0.3 mg kg-1, i.v.) lowered blood pressure in rats dose-dependently and caused reflex tachycardia. The heart rate increase was abolished by prior administration of the beta-adrenoceptor blocker bopindolol (0.1 mg kg-1, i.v.). 5. In anaesthetized rabbits, regional blood flow measurements (with radioactive tracer microspheres) showed that BRL 34915 (3 to 30 micrograms kg-1, i.v.) caused marked vasodilatation in the stomach, with increases in flow also to the heart and small intestine. Brain blood flow also tended to increase. Blood flow to the kidneys was reduced by BRL 34915, whereas flow to skeletal muscle was unchanged. 6. Apamin pretreatment did not modify the blood pressure lowering activity of BRL 34915 in rats. The site at which BRL 34915 acts to cause vasodilatation in vivo thus appears to be apamin-insensitive.

Animals↗
PubMed1988

Anti-vasoconstrictor effects of the K+ channel opener cromakalim on the rabbit aorta--comparison with the calcium antagonist isradipine.

1. Contractile responses of rabbit aortic rings elicited by KCl-depolarization, angiotensin II (AII), 5-hydroxytryptamine (5-HT) and noradrenaline (NA) have been investigated in the presence of cromakalim (BRL 34915) and isradipine (PN 200-110). 2. Above 10(-6)M, cromakalim inhibited contractile responses to low (less than or equal to 32 mM) but not to higher KCl concentrations. The 5-HT and AII concentration-response curves were antagonized noncompetitively by cromakalim (10(-7)-10(-5)M) and the maximal responses were inhibited by 40 and 55%, respectively. 3. Isradipine caused less inhibition of AII and 5-HT contractile responses than cromakalim, and in the presence of isradipine (10(-7)M), cromakalim was still able to antagonize further the contractions to AII in this vessel. 4. NA-induced contractions were relatively insensitive to inhibition by cromakalim and isradipine, both drugs causing a small rightward shift of the NA concentration-response curve. This result suggests that NA utilizes different Ca2+ pools from those involved in AII- and 5-HT-induced contractions of this vessel. 5. The sustained (tonic) part of the NA response was inhibited in a concentration-dependent manner by cromakalim (10(-7)-10(-5)M), but not by isradipine. 6. In aortic rings partially depolarized with 3.5 x 10(-2)M KCl, the ability of cromakalim, but not of sodium nitroprusside, atriopeptin III or hydralazine, to inhibit AII- and tonic NA-induced contractions was abolished. 7. Antivasoconstrictor activity of cromakalim on the rabbit aorta appears to involve factors in addition to an indirect inhibition of Ca2+ entry through dihydropyridine-sensitive Ca2+ channels. 8. The ability of cromakalim to open K+ channels and thereby modify the membrane potential would appear to underlie these antivasoconstrictor effects. This mechanism of action of cromakalim clearly differs from that of other vasodilators such as sodium nitroprusside and hydralazine.

Angiotensin II↗
PubMed1988

Mechanism of action and systemic and regional hemodynamics of the potassium channel activator BRL34915 and its enantiomers.

BRL34915 (BRL) is a vasodilator with a novel structure. Its mechanism of action, its effects on depolarization-induced and receptor-mediated blood vessel contraction, and its hemodynamic effects were investigated. In the rat portal vein, BRL inhibited spontaneous mechanical activity [IC50 0.013 +/- 0.001 microM (mean +/- SEM) for (-)-BRL], the initial effect being a reduced frequency of contraction. At higher concentrations, the spontaneous contractions were abolished and 86Rb+ efflux was increased. These results suggest that BRL preferentially acts on the pacemaker cells, the K+ channels in other cells being activated only at higher BRL concentrations in this vessel. In experiments on the rabbit aorta, (-)-BRL shifted the KCl concentration-response curve to the right and noncompetitively inhibited responses to angiotensin II. A concentration of 3 microM (-)-BRL reduced maximal angiotensin II contractions by around 50%, higher concentrations having little further effect. This inhibition of angiotensin II contractions is notably greater than that seen with Ca2+ antagonists in this vessel. In anesthetized rabbits, (-)-BRL was a peripheral vasodilator at doses of 3-30 micrograms/kg, but it had no relevant effects on heart rate and myocardial contractile force. This suggests tissue selectivity of this compound or this mechanism of action. BRL preferentially dilated the coronary, gastrointestinal, and cerebral vessels but not those of the kidneys or skeletal muscle as measured with tracer microspheres. This profile of activity is different from that of calcium antagonists or nonspecific vasodilators like dihydralazine. All effects were stereoselective, the (-)-enantiomer being 100 to 200 times more active than the (+)-enantiomer.

Animals↗
PubMed1986

Effects of the enantiomers of the dihydropyridine derivative 202-791 on contractility, coronary flow and ischemia-related arrhythmias in rat heart.

The (+)-enantiomer of the dihydropyridine 202-791 produced coronary constrictor and positive inotropic effects in Langendorff-perfused rat hearts whereas (--)-202-791 showed coronary dilator and negative inotropic effects. Both enantiomers showed the vascular effects at lower concentrations than the myocardial effects. Furthermore, the (+)-enantiomer significantly increased whilst the (--)-enantiomer reduced the number of ventricular ectopic beats occurring during coronary ligation. Arrhythmia-enhancing effects must therefore be considered when the potential therapeutic uses of calcium channel activators are discussed.

Animals↗
PubMed1985

Cardioprotection by the calcium antagonist PN 200-110 in the absence and presence of cardiodepression.

The globally-ischaemic Langendorff rabbit heart model has been used to study the cardioprotective effects of the dihydropyridine PN 200-110 (PN) at two doses, one having no negative inotropic effect and a higher dose causing a 62 +/- 5% reduction in contractility. Following 45 min no-flow global ischaemia, recovery was monitored for a period of 90 min reperfusion. Hearts were paced at a constant rate throughout experiments. Contractile force and coronary flow were recorded continuously. Tracer microspheres were injected at regular intervals to assess regional flow distributions, drill biopsies were taken to determine tissue high energy phosphate content, and enzyme leakage in the coronary effluent measured during the first 15 min of reperfusion. Untreated hearts recovered 21 +/- 2% of their initial contractile force and flow to all heart regions was reduced. In particular, endocardial flow fell to 20% of its pre-ischaemic level, with the ratio of flow to the endocardium (endo)/epicardium (epi) decreasing from ca. 1.0 to 0.4. Hearts treated with 2 X 10(-8)M PN (included in the perfusate from 30 min before ischaemia until 30 min after ischaemia) recovered 49 +/- 2% of their initial, pretreatment contractile force, and following the ischaemia the endo/epi ratio was not significantly changed from the pre-ischaemic value. The lower PN dose (3 X 10(-10)M) afforded a lesser degree of protection, contractility recovering to 29 +/- 4% of the initial level, with an endo/epi ratio of 0.7 after 90 min reperfusion. 6 The two PN doses afforded a similar degree ofprotection against enzyme leakage which was in both cases significantly less than in untreated hearts. 7 Myocardial ATP and creatine phosphate content was markedly reduced by the ischaemic episode. Neither PN dose modified this depletion. 8 These results suggest that whilst cardiodepression may well offer protection against ischaemic damage, this is not the sole mechanism wherby PN (and possibly other calcium antagonists) can protect the heart. Preservation of blood flow to the inner layers of the left ventricular wall is likely to be one of the major factors underlying the enhanced recovery shown by PN.

Animals↗
PubMed1985

Effects of apamin, quinine and neuromuscular blockers on calcium-activated potassium channels in guinea-pig hepatocytes.

The bee venom peptide, apamin, has been radiolabelled with 125I, the monoiodinated derivative purified, and its binding to intact guinea-pig liver cells studied. At 37 degrees C 125I-monoiodoapamin associated with, and dissociated from, guinea-pig hepatocytes remarkably rapidly. The association and dissociation rate constants were 1.4 X 10(8) M-1 s-1 and 0.035 s-1 respectively. Equilibrium binding studies demonstrated a saturable binding component compatible with 1:1 binding to a single class of site and having an equilibrium dissociation constant (KL) of 390 pM. The maximal binding capacity was 1.1 fmol mg-1 dry wt. of tissue. Unlabelled apamin displaced bound 125I-monoiodoapamin with a KI of 380 pM, which is consistent with the concentration of apamin required to inhibit Ca2+-activated K+ permeability (PK(Ca) ) in these cells. Inhibitable binding of 125I-monoiodoapamin to rat hepatocytes was much less than to guinea-pig hepatocytes and could not be reliably quantified. Neither was there any discernible inhibitable binding to human erythrocytes. This is in keeping with the reported lack of apamin-sensitive Ca2+-activated K+ channels in these cell types. Various agents were tested for their ability to inhibit monoiodoapamin binding to, and Ca2+-mediated K+ efflux from, guinea-pig hepatocytes. All compounds tested which inhibited binding also blocked K+ efflux at similar concentrations. TEA and quinine affected hepatocytes only at high concentration (KI = 5.8 and 0.51 mM respectively). 9-aminoacridine, quinacrine and chloroquine were slightly more effective (KI = 70-180 microM). By far the most active compounds (apart from apamin) were the neuromuscular blocking agents; tubocurarine, pancuronium and atracurium (KI = 7.5, 6.8 and 4.5 microM respectively). Gallamine was slightly less effective (KI = 14 microM) and decamethonium and hexamethonium much less so (KI = 620 and 760 microM respectively). 3,4-diaminopyridine, alpha-bungarotoxin and tetrodotoxin were among several compounds which showed little or no affinity for apamin binding sites or inhibition of K+ efflux in guinea-pig hepatocytes. The saturable binding of 125I-monoiodoapamin to guinea-pig hepatocytes corresponds to about 1700 sites per cell. Assuming, tentatively, that binding sites correspond to channels the rate of K+ loss observed following agonist action can readily be explained if these channels have unitary conductances in the range reported for PK(Ca) in other tissues.

Animals↗
PubMed1983

High affinity binding of [125I]monoiodoapamin to isolated guinea-pig hepatocytes.

The bee venom neurotoxin apamin has been labelled with 125I and its binding to isolated guinea-pig hepatocytes measured under physiological conditions. A single saturable component of [125I]monoiodoapamin binding with a Kd of 350 pM and Bmax of 0.99 fmol/mg dry wt was identified. Native apamin displaced labelled apamin with a Kd of 376 pM which is broadly in keeping with the concentrations found to inhibit K loss from guinea-pig hepatocytes. These observations, together with the binding found in other tissues, suggest that specific binding of labelled apamin is particularly associated with apamin-sensitive, Ca-activated K-channels.

Animals↗
PubMed1983

Calcium-activated potassium channels in liver cells.

Activation of certain membrane receptors increases the concentration of Ca2+ in the cytosol of hepatocytes. Since in most species these cells possess a PK(Ca) mechanism, the outcome is a rise in PK. This can be blocked by quinine, apamin and certain neuromuscular blocking agents. The binding of labelled apamin to hepatocytes has been studied under physiological conditions, and the relationship between the binding sites and K+ channels is discussed. The physiological role of the PK(Ca) mechanism in hepatocytes is unclear, though it is largely responsible for 'adrenaline hyperkalaemia'.

Animals↗
PubMedDate not supplied

Stereoselectivity at the calcium channel: different profiles of hemodynamic activity of the enantiomers of the dihydropyridine derivative PN 200-110.

The enantiomers of PN 200-110 (PN), a highly potent calcium antagonist, were synthesized by a stereoselective synthesis. Cross-contamination was less than 0.2% for the (S)-enantiomer and 0.5% for the (R)-enantiomer. Both isomers inhibited depolarization-induced contraction of rabbit aorta, the (S)-enantiomer being much more active: The pD'2 values were 9.1 and 6.9 for the (S)- and the (R)-enantiomer, respectively. This activity of (R)-PN on potential-operated channels can be attributed to the contamination with 0.5% of (S)-PN. In anesthetized cats, the (S)-enantiomer lowered blood pressure and heart rate dose-dependently (0.3-10 micrograms/kg i.v.). The (R)-enantiomer had almost no effect on heart rate (HR) and blood pressure (BP) at doses up to 300 micrograms/kg. However, both enantiomers increased cardiac output and blood flow to the heart and the brain. The effects on regional blood flow were tissue-dependent, the (R)-enantiomer being surprisingly potent in the subendocardium of the left ventricle. The qualitative and quantitative differences between the activities of the two enantiomers suggest that the results obtained cannot merely be explained by traces of the more active enantiomer contaminating the less active enantiomer. These results, together with those found with the enantiomers of a nitro-substituted dihydropyridine, suggest that calcium channels appear to be able not only to discriminate between enantiomers but also to respond differently to them.

Animals↗