Postsynaptic alpha 2-adrenergic receptor-mediated vasoconstriction in SHR tail arteries in vitro.

In isolated perfused tail arteries of spontaneously hypertensive rats (SHR), the selective alpha 2-adrenergic receptor antagonist idazoxan ( RX781094 ) at low concentrations antagonized vasoconstrictor responses induced by norepinephrine (NE) and low frequency periarterial field stimulation. The vasoconstrictor responses to the selective alpha 2-adrenergic receptor agonist TL99 or to phenylephrine were also antagonized by low concentrations of idazoxan . In contrast, idazoxan did not antagonize responses induced by the alpha 1-adrenergic receptor agonists amidephrine or methoxamine in perfused tail arteries of SHR. The alpha 1-adrenergic receptor antagonist prazosin was very potent against methoxamine or phenylephrine and responses to periarterial field stimulation in SHR and Wistar-Kyoto (WKY) rat tail arteries, but only showed a modest selectivity for TL99 -induced responses in SHR arteries. The results support the contention that postjunctional alpha 2-adrenergic receptors can be demonstrated in arterial smooth muscle in vitro and are particularly evident in arteries from hypertensive animals. In SHR tail arteries, postsynaptic alpha 2-adrenergic receptors contribute to the vasoconstrictor responses to exogenous NE and may be activated by endogenously released NE.

Differential blocking actions of idazoxan against the inhibitory effects of 6-fluoronoradrenaline and clonidine in the rat vas deferens.

The prejunctional inhibitory effects of clonidine and 6-fluoronoradrenaline (6-FNA) have been evaluated in the isolated prostatic segment of the rat vas deferens, against the twitch response evoked by low frequency (0.1 Hz) field stimulation. The inhibitory potency of 6FNA was significantly increased in the presence of cocaine (1 microM) or pargyline (10 microM), but was not modified in the vas deferens from rats pretreated with reserpine when the endogenous levels of noradrenaline (NA) were decreased by 97%. Clonidine was significantly more potent than 6-FNA as an inhibitory agonist, and the potency of clonidine was not modified after cocaine, pargyline or reserpine. The alpha 2-adrenoceptor blocking agent idazoxan, was a competitive antagonist against the inhibitory effects of clonidine under all experimental conditions. In contrast, the only antagonism shown by idazoxan against the inhibitory effects of 6-FNA was in the presence of cocaine (1 microM), and this antagonist effect of idazoxan was not concentration-related. Low concentrations of 6-FNA caused concentration-dependent facilitatory effects on the twitch response, which were significantly greater after treatment with idazoxan (1 microM) in reserpine-treated vas deferens. These facilitatory effects of 6-FNA were always observed in the presence of prazosin (300 nM) and also after treatment of the preparations with phenoxybenzamine (10 microM), a concentration which abolished the inhibitory actions of both clonidine and 6-FNA. The facilitatory effects on the twitch response induced by low concentrations of 6-FNA are therefore unlikely to be due to either alpha 1- or alpha 2-adrenoceptor stimulation. In conclusion, the failure of idazoxan to block the inhibitory effects of 6-FNA, while exerting a potent competitive antagonism of clonidine-induced inhibitory effects, supports the proposal that alpha 2-adrenoceptors may in fact be subdivided into two subclasses, involving imidazoline and phenylethylamine recognition sites.

Differential effects of endothelin-1 on the vasorelaxant properties of benzopyran and non-benzopyran potassium channel openers.

1. The effects of endothelin-1 (ET-1) on the vasorelaxant properties of structurally different potassium channel openers (PCOs), BRL-38227, Ro 31-6930, SDZ PCO 400, EMD-52692, RP-49356 and pinacidil, were studied. 2. All PCOs evoked concentration-related relaxations of ET-1 (10 nM) or KCl (20 mM) contracted rat isolated aortic rings denuded of endothelium. BRL-38227, EMD 52692, SDZ PCO 400 and Ro 31-6930 were 11-42 times less potent in relaxing contractions to ET-1 than KCl. In contrast, this differential potency was not observed with RP-49356 or pinacidil. 3. BRL-38227 (0.06-3.0 microM), RP-49356 (0.3-3.0 microM) and pinacidil (0.3-3.0 microM) displaced KCl concentration-response curves to the right of controls, without modifying the maximum response. A subcontractile concentration of ET-1 (0.1 nM) prevented the inhibitory effects of low concentrations of BRL-38227 (0.06-0.1 microM) on KCl responses, but failed to modify those to RP-49356, pinacidil or high concentrations of BRL-38227 (0.3-3.0 microM). The inhibitory effects of BRL-38227 (0.1 microM) were also not changed by ET-3 (1.0 nM) or angiotensin II (0.1 nM). 4. In anaesthetized spontaneously hypertensive rats (SHR), cumulative bolus intravenous administrations of BRL-38227 (1-1000 micrograms kg-1, i.v.), Ro 31-6930 (1-1000 micrograms kg-1, i.v.), RP-49356 (10-1000 micrograms kg-1, i.v.) or nitrendipine (0.1-30 micrograms kg-1, i.v.) produced dose-dependent falls in diastolic blood pressure (DBP).ET-1 (i.v.) evoked a transient fall in DBP (1 pg kg- = 58 + 1 mmHg) which returnedto pre-administration levels within 4 min.5. Pretreatment of anaesthetized SHR with ET-l (1 pg kg-', i.v.) significantly increased the ED,5 (dose to evoke a 15% fall in DBP) values for BRL-38227 and Ro 31-6930. However, ET-l failed to modify the ED,5 values for RP-49356 or nitrendipine. The ED50 values for all of the vasodilators studied were not modified by ET-1.6. Infusion of BRL-38227 (2 pgkg-'min-', i.v.) or RP-49356 (4 pgkg-'min', i.v.) to anaesthetized SHR evoked dose-related falls in DBP, with a corresponding increase in descending aortic blood flow (DABF) and a decrease in total lower body vascular resistance (TLBVR). Pretreatment with ET-1 (1 ptg kg-', i.v.) significantly attenuated the decreases in DBP and TLBVR observed with low doses of BRL-38227, but not RP-49356 or high doses of BRL-38227. In contrast, ET-3 (3 pig kg-, i.v.) failed to modify the effects of BRL-38227 on DBP or TLBVR.7. In conscious SHR, the fall in DBP to BRL-38227 (30 pgkg-', p.o.) was significantly reduced following ET-1 (1 pig kg-', i.a.) treatment. ET-1 (1 pg kg-', i.a.) pretreatment, however, failed to modify the decrease in DBP induced by an equieffective oral dose of RP-49356 (1001pgkg-1).8. In conclusion, ET-1 selectively attenuated the vasorelaxant effects of the potassium channel opener,BRL-38227 and other substituted benzopyrans. The results are compatible with the hypothesis that benzopyran PCOs and ET-1 have affinity for a site that does not recognise RP-49356 or pinacidil. Thus,ET-l can differentiate between structurally unrelated potassium channel openers. The cardiovascular effects of some, but not all, PCOs might be radically modified in the clinical setting by elevated endogenous levels of ET-1 associated with certain diseased states.

Alpha-adrenoceptor subtypes in dog saphenous vein that mediate contraction and inositol phosphate production.

1. Studies have been made of the contractile responses to the alpha-adrenoceptor agonists phenylephrine (Phen), cirazoline (Cir) or BHT-920 (BHT) in dog isolated saphenous vein (DSV) rings, using the antagonists yohimbine (Yoh), idazoxan (Idaz), prazosin (Praz), WB-4101 (WB) and nitrendipine or zero Ca2+ medium. 2. Contractile concentration-response curves to Phen or BHT were displaced to the right of controls by Yoh (0.01-3 microM) with mean apparent antagonist dissociation constants (pKBs) of 7.9 and 8.6 respectively. Yoh did not show simple competitive antagonism against either agonist, since the Schild plot slopes were significantly less than unity. Neither the antagonist affinity of Yoh against Phen, nor the slope of the Schild plot was modified in the presence of catecholamine uptake inhibitors, nor in the presence of alpha,beta-methylene ATP, which desensitizes P2-purinoceptors, suggesting that Phen does not release ATP, or noradrenaline to cause contraction in DSV. In the presence of Praz (0.3 microM) the antagonist potency of Yoh (mean pKB 7.4) against Phen was slightly decreased. Yoh had low potency against responses induced by Cir (pKB 6.3). 3. WB (0.001-1.0 microM) was a very potent antagonist of Phen-induced contractions, however, the biphasic Schild plot against Phen could be separated into two affinity sites, a high pKB of 9.3 (equivalent to that obtained using Cir as the agonist; pKB 9.6) and a lower affinity (pKB 8.6). WB showed an even lower antagonist affinity (pKB 7.4) against BHT-induced contractions, suggesting that these effects might be mediated by alpha 2A-adrenoceptors. Praz also appeared to identify two sites using Phen-induced contractions, a high pKB of 8.4 was equivalent to that obtained with Cir (pKB 8.2) and a lower affinity site (pKB 7.7; pA2 7.6; slope 1.1) at which Praz showed competitive antagonism. Higher concentrations of Praz were required to antagonize contractions to BHT (pKB 5.9). 4. Idaz was a weak partial agonist in this tissue with threshold contractile effects at concentrations in excess of 3 microM. Idaz (0.1-1 microM) competitively antagonized the contractile effects of BHT, but showed low antagonist affinity against Phen at these concentrations. 5. Contractions to Phen were slightly antagonized by nitrendipine (1 microM), with a 36% decrease in Emax. Contractions to Phen and Cir were also markedly attenuated in zero calcium medium (with EGTA), but maximum responses of 4.2 +/- 0.1 and 3.6 +/- 0.1 g, could be obtained with these agonists respectively. Only part of the contractile effects to Phen or Cir are therefore due to calcium influx (but L-type channels are not totally implicated), while the contractile effects of BHT were abolished in zero Ca2 + medium. Yoh (0.1 microm) retained its antagonist effects on Phen-induced responses in zero Ca2 + medium. 6. The formation of inositol phosphates (InsPs) in the presence of lithium (10mM) was measured after incubation of intact DSV strips with myo-2-[3H]-inositol. Phen (1-1OO0 microM) and Cir (O.O1-1O microm) induced concentration-dependent increases in total labelled InsP1_3, but BHT showed minimal InsP stimulation. InsPs were recovered after Phen (100,M) stimulation (10min) as labelled InsP1 (71%), InsP2 (25%) and InsP3 (4%). Phen (100 microM)-stimulated InsP1-3 formation was significantly antagonized by Praz (10nM), but was not fully inhibited even after Praz 1 microM. Yoh and Praz (0.1 and 1.0 microM) were equipotent inhibitors of this response, while Idaz (0.3 microM) showed no effects. 7. The receptors in DSV which are stimulated by Phen to cause contraction show characteristics of the alpha lA-adrenoceptor (high pM antagonist affinity for WB-4101 and extracellular calcium sensitivity) and the alpha lB-adrenoceptor (contraction in calcium-free medium, increase in InsP and low nm antagonist affinity of WB). The paradoxical results obtained with Yoh (potent antagonist effects on Phen-stimulated PI and pKB 7.9 on contraction) and Praz (low affinity competitive antagonist of Phen-induced contraction, pKB 7.7 and failure to inhibit completely the PI response at 1 microM), cannot fully exclude an alpha 2B-subtype characterization of these responses. These pharmacological differences suggest that the adrenoceptor involved in the contractile and in particular the second messenger effects of Phen in DSV is not typically an alpha lB-adrenoceptor.

Alfuzosin, a selective alpha 1-adrenoceptor antagonist in the lower urinary tract.

1. Phenylephrine-induced contractions of rabbit isolated trigone and urethra were antagonized in a competitive manner by alfuzosin (pA2 7.44 and 7.30, respectively) and prazosin. 2. The characteristics of [3H]-prazosin binding to human prostatic adenoma tissue were evaluated. [3H]-prazosin was potently displaced by alpha 1-adrenoceptor specific agents including alfuzosin, its (+)- and (-)-enantiomers and prazosin (IC50 0.035, 0.023, 0.019 and 0.004 microM, respectively), but only weakly by alpha 2-adrenoceptor selective agents, for example, yohimbine (IC50 = 6.0 microM). 3. In the pithed rat, alfuzosin (0.03-0.3 mg kg-1, i.v.) markedly inhibited pressor responses produced by the alpha 1-selective agonist, cirazoline but inhibited only slightly responses to the alpha 2-selective agonist, UK 14,304. Alfuzosin (1 mg kg-1, i.v.) had minimal effects against responses mediated by stimulation of prejunctional alpha 2-receptors (UK 14,304-induced inhibition of sympathetic tachycardia). 4. In the anaesthetized cat, alfuzosin (0.001-1 mg kg-1, i.v.) and prazosin (0.001-0.3 mg kg-1, i.v.) produced dose-related inhibition of the increases in urethral pressure caused by stimulation of sympathetic hypogastric nerves. Prazosin was approximately 5 fold more potent than alfuzosin. When phenylephrine was employed to induce urethral and vascular alpha 1-mediated tone simultaneously, prazosin inhibited both stimuli with similar potency whereas alfusozin was 3-5 fold more potent against elevated urethral pressure. This functional uroselectivity of alfuzosin was more evident by the intraduodenal route, since doses of 0.03 and 0.1 mg kg-1 alfuzosin inhibited urethral pressure with minimal effects on arterial blood pressure. 5. Alfuzosin is a potent selective alpha1-adrenoceptor antagonist in tissues of the lower urinary tract including the human prostate. This provides a pharmacological basis for its use in the treatment of benign prostatic hypertrophy.

The pharmacology of RS-15385-197, a potent and selective alpha 2-adrenoceptor antagonist.

1. RS-15385-197 ((8aR, 12aS, 13aS)-5,8,8a,9,10,11,12,12a,13,13a-decahydro- 3-methoxy-12-(methylsulphonyl)-6H-isoquino [2,1-g][1,6]-naphthyridine) was evaluated in a series of in vitro and in vivo tests as an antagonist at alpha 2-adrenoceptors. 2. RS-15385-197 had a pKi of 9.45 for alpha 2-adrenoceptors in the rat cortex (pA2 in the guinea-pig ileum of 9.72), whereas the 8aS, 12aR, 13aR enantiomer, RS-15385-198, had a pKi of only 6.32 (pA2 6.47) indicating a high degree of stereoselectivity. The racemate RS-15385-196 had a pKi of 9.18. 3. RS-15385-197 showed unprecedented alpha 2 vs. alpha 1 adrenoceptor selectivity in vitro. In the rat cortex, RS-15385-197 had a pKi of 9.45 in displacing [3H]-yohimbine and 5.29 in displacing [3H]-prazosin (alpha 2/alpha 1 selectivity ratio in binding experiments > 14000). The compound had a pA2 of 9.72 as a competitive antagonist of the inhibitory effects of UK-14,304 in transmurally-stimulated guinea-pig ileum and 10.0 against BHT-920-induced contractions in dog saphenous vein (DSV); this latter value was unaltered by phenoxybenzamine. An apparent pKB of 5.9 was obtained against cirazoline-induced contractions in DSV, whilst a pA2 of 6.05 was obtained against phenylephrine-induced contractions in the rabbit aorta (alpha 2/alpha 1 selectivity ratio in functional experiments > 4000). 4. RS-15385-197 was highly selective for alpha 2-adrenoceptors over other receptors: the compound showed low affinity for 5-HT1A (pKi 6.50) and 5-HT1D (pKi 7.00) receptor subtypes, and even lower affinity (pKi < or = 5) for other 5-HT receptor subtypes, dopamine receptors, muscarinic cholinoceptors, beta-adrenoceptors and dihydropyridine binding sites. RS-15385-197 was devoid of affinity for the non-adrenoceptor imidazoline binding site, labelled by [3H]-idazoxan, which provides further evidence that these sites are not related to alpha 2-adrenoceptors. In the DSV, contractile responses to 5-hydroxytryptamine (5-HT) were unaffected by a concentration of 1 microM RS-15385-197. 5. RS-15385-197 was non-selective for the alpha 2A- and alpha 2B-adrenoceptor subtypes in that the pKi for the alpha 2A-adrenoceptor in human platelets was 9.90 and the pKi for the alpha 2B-adrenoceptor in rat neonate lung was 9.70. However, RS-15385-197 showed lower affinity for the alpha 2-adrenoceptor subtype in hamster adipocytes (pKi 8.38). 6. In anaesthetized rats, RS-15385-197 was a potent antagonist of the mydriasis response induced by UK-14,304 or clonidine (AD50 5 and 7 microg kg-1, i.v., respectively; 96 microg kg-1, p.o.) and of UK-14,304-induced pressor responses in pithed rats (AD50 7 microg kg-1, i.v.); the compound therefore is both centrally and orally active. Even at a high dose (10 mg kg-1, i.v.), RS-15385-197 did not antagonize pressor responses to cirazoline in pithed rats, indicating that the selectivity for alpha2 vs. alpha1-adrenoceptors was maintained in vivo.8 RS-15385-197 is therefore a very potent, selective, competitive alpha2-adrenoceptor antagonist, both in vitro and in vivo, is orally active and readily penetrates the brain. It will thus be a powerful pharmacological tool for exploring the various physiological roles of alpha2-adrenoceptors.

Antagonism by tetrahydro-beta-carboline of the vasoconstrictor responses to tryptamine in rat tail arteries.

Tryptamine and 5-HT mediate vasoconstriction in the isolated perfused rat tail artery. These agonists were differentiated by the classical serotonin receptor antagonists methysergide and ketanserin which were significantly more potent against 5-HT-induced responses. Neither prazosin nor RX 781094 alone or in combination reduced the vasoconstriction produced by tryptamine. Tetrahydro-beta-carboline (THBC) was devoid of agonist properties and behaved as a competitive antagonist of tryptamine but not of 5-HT. THBC also had alpha-adrenoceptor blocking properties in this preparation.

No evidence for central histamine-receptor involvement with the hypotensive effect of clonidine in cats.

In conscious hypertensive cats, intraventricular (i.c.v.) administration of clonidine (25 mug), induced hypotension and bradycardia. Pretreatment with metiamide (2 mg i.c.v.) did not significantly antagonise either the hypotension or bradycardia induced by clonidine (25 mug), but induced marked behavioural changes. Central pretreatment with mepyramine (200 mug, i.c.v.) or procaine (600 mug i.c.v.), reduced the hypotension evoked by clonidine (25 mug), but no antagonism of the clonidine-induced bradycardia was apparent. Central phentolamine (200 mug, i.c.v.) or tolazoline (200 mug, i.c.v.) antagonised the hypotension and bradycardia evoked by i.c.v. clonidine.

Central hypertensive action of histamine in conscious normotensive cats.

In conscious normotensive cats intraventricular (i.c.v.) administration of histamine (2.0-50.0 mug) induced dose-related rises in blood pressure, with no increase in heart rate. The hypertensive response elicited by a sub-maximal dose of histamine (10.0 mu i.c.v.) was significantly antagonised by central pretreatment with the H1-receptor antagonist mepyramine maleate (200 mug i.c.v.) but not by the H2-receptor antagonist metiamide hydrochloride (1.0 mg i.c.v.). Behavioural responses were obtained in response to to histamine (10.0 and 50.0 mug i.c.v.), which were not antagonised by these antihistamine pretreatments.

Reserpine-resistant responses to nerve stimulation in the cat nictitating membrane involve the release of newly synthesized noradrenaline.

Preganglionic sympathetic nerve stimulation in cats pretreated with reserpine resulted in significant frequency-dependent contractions of the nictitating membrane, despite a severe depletion of tissue noradrenaline content. These residual responses to sympathetic nerve stimulation were potentiated by cocaine or pargyline, and were antagonised by the tyrosine hydroxylase inhibitor alpha-methyl-p-tyrosine. In contrast to the residual responses to sympathetic stimulation in the nictitating membrane, the tachycardia evoked by postganglionic cardiac nerve stimulation was totally abolished by pretreatment with reserpine, even after the administration of cocaine. The alpha-adrenoceptor antagonists phentolamine or prazosin reduced, but did not abolish the reserpine-resistant responses of the nictitating membrane, suggesting the nerve-mediated release of a co-transmitter. In addition to this co-transmitter, a neuronal pool of neurotransmitter which is reserpine-resistant and involves newly synthesised noradrenaline, contributes to the residual responses of the nictitating membrane, following depletion of the neurotransmitter stores by the administration of reserpine. administration of reserpine.

5HT-receptor antagonist properties of SCH 23390 in vascular smooth muscle and brain.

The dopamine D1-receptor antagonist SCH 23390 was a potent competitive antagonist of 5HT-induced vasoconstriction in the isolated perfused rat tail artery preparation (pA2 8.17) but a very weak antagonist of phenylephrine-induced responses (pA2 5.94). In rat brain cerebral cortex, SCH 23390 inhibited 5-HT2-sensitive [3H]spiperone binding with an IC50 of 112 nM. Binding of [3H]5HT to 5HT1 receptors in the cortex was inhibited by SCH 23390 with an IC50 of 2.49 microM. SCH 23390 has significant affinity for 5HT receptors in addition to the reported selective dopamine D1-receptor antagonist properties.

Inhibitory effects of alpha, beta-methylene ATP on nerve-mediated contractions of the nictitating membrane in reserpinised cats.

Residual responses of the cat nictitating membrane to nerve stimulation were obtained after reserpine pretreatment (40% of controls), in spite of a pronounced reduction in noradrenaline content. The putative ATP-receptor desensitising agent alpha, beta-methylene ATP (alpha, beta-MATP), administered intraarterially through the lingual artery produced a contraction of the nictitating membrane and subsequently inhibited the residual responses evoked by sympathetic nerve stimulation in reserpinised cats. These doses of alpha, beta-MATP did not modify the contractions evoked by exogenous noradrenaline (i.a.) but antagonized the contractions of the nictitating membrane elicited by beta, gamma-methylene ATP, which is an agonist at P2 receptors. These results are compatible with a co-transmitter role for ATP in the neurally mediated contractile responses of the nictitating membrane following depletion of endogenous noradrenaline stores by pretreatment with reserpine.

Dopamine preferentially stimulates postsynaptic alpha 2-adrenoceptors in the femoral vascular bed, but alpha 1-adrenoceptors in the renal vascular bed of the anaesthetised dog.

The decrease in blood flow in response to dopamine (DA) injected intraarterially (i.a.) into the femoral or renal vascular beds was examined in the anaesthetised dog. DA or noradrenaline (NA) were 10 times more potent as vasoconstrictor agents in the femoral than in the renal vasculature. In the femoral bed, the DA induced vasoconstriction was completely resistant to antagonism by prazosin (30-300 micrograms/kg i.v.), but was dose-dependently blocked by the alpha 2-receptor antagonist idazoxan (30-300 micrograms/kg i.v.). In the renal bed the vasoconstrictor effects of DA were resistant to blockade by idazoxan, but were prazosin sensitive indicating that alpha 1-adrenoceptors were involved in this response. The alpha-receptor agonist profile for DA was not modified in the femoral bed after blockade of dilatory D1-receptors with SCH 23390 (0.5 mg/kg i.v. and 0.1 mg/kg per h i.v.). However, this antagonist significantly increased the vasoconstrictor potency for DA in the renal bed. The decrease in femoral blood flow induced by an injection of DA, appears to be mediated by alpha 2-adrenoceptors. In the renal vascular bed where the predominant alpha-adrenoceptor corresponds to the alpha 1-subtype and there are few postsynaptic alpha 2-receptors subserving vasoconstriction, DA can stimulate alpha 1-receptors but this action requires higher doses of agonist than those needed for alpha 2-adrenoceptor stimulation.

Effect of acidosis on alpha 1- and alpha 2-adrenoceptor-mediated vasoconstrictor responses in isolated arteries.

We have investigated the effect of reducing the pH (from 7.5 to 7.0 by addition of HCl) on vasoconstrictor responses to noradrenaline in cat middle cerebral artery (in which responses are mediated almost entirely by alpha 2-adrenoceptors) and in rabbit pulmonary artery (in which responses are mediated by alpha 1-adrenoceptors). In the cerebral artery, a reduction in pH caused a pronounced inhibition of the responses to noradrenaline, and the antagonistic effect of idazoxan (100 nM) was increased 10-fold. In contrast, in the pulmonary artery, a reduction in pH had no effect on the responses to noradrenaline and the antagonistic effect of prazosin (100 nM) was not altered. We conclude that acidosis selectively reduces the vasoconstriction mediated by alpha 2-adrenoceptors in vitro.

Effects of ATP-dependent K+ channel modulators on an ischemia-reperfusion rabbit isolated heart model with programmed electrical stimulation.

The effects of glibenclamide and BRL-38227 were studied in isolated rabbit hearts subjected to ischemia and programmed electrical stimulation. Coronary artery occlusion over 24 min decreased the ventricular effective refractory period in the ischemic zone. BRL-38227 (0.1 microM) showed significant coronary vasodilator effects, but failed to modify the ventricular effective refractory period under these conditions. A higher concentration (5 microM) of BRL-38227 potentiated the ischemia induced ventricular effective refractory period shortening effects. Glibenclamide (0.1 and 1 microM) delayed the onset of the ischemia-induced ventricular effective refractory period shortening. Glibenclamide (1 microM) inhibited the potentiated ventricular effective refractory period shortening effects of BRL-38227 (5 microM) during ischemia, but failed to antagonise the coronary vasodilator effects of BRL-38227 (5 microM). A higher incidence of ventricular fibrillation was inducible when an extra beat was applied in the ischemic zone through programmed electrical stimulation. The incidence of programmed electrical stimulation induced ventricular fibrillation was increased by BRL-38227 (5 microM) and antagonised by glibenclamide (1 microM). The results suggest that high concentrations of KATP-activators can accentuate ischemia-induced decreases in refractory period and increase the susceptibility of hearts to ventricular fibrillation when an extra beat is applied to the ischemic myocardium. These effects did not occur at lower coronary vasodilating concentrations of BRL-38227.

The cardiovascular effects of intraventricularly administered histamine in the anaesthetised rat.

In urethane-anaestetised rats intraventricular (i.c.v.) injections of histamine (0.1-10.0mug) elicited dose-related rises in both the resting blood pressure and heart rate. These cardiovascular effects of histamine were antagonised in a dose-dependent manner by i.c.v. pretreatments with the histamine H1-receptor antagonists mepyramine (10, 50 and 100 mug) and diphenylpyraline (100 and 200mug). Pretreatment with the histamine H2-receptor antagonist metiamide (100 and 200 mug i.c.v.) failed to modify either of the responses. A dose-related antagonism of the hypertensive response to histamine i.c.v. was elicited by phentolamine (100 and 200 mug i.c.v.) but the positive chronotropic effect was not modified by this pretreatment. The cardiovascular responses to histamine i.c.v. were abolished by mecamylamine (5.0 mg/kg i.v.) and greatly reduced by 6-hydroxydopamine (3 X 250 mug i.c.v.), but only the tachycardia was significantly modified by atropine (100 mug i.c.v.) and propranolol (1 mg/kg i.v.). Propranolol (100 mug i.c.v.), bilateral vagotomy, or acute bilateral adrenal demedullation failed to modify the cardiovascular responses to histamine i.c.v. The results suggest that histamine is able to modify the resting blood pressure and heart rate by independent central modes of action, which involve central adrenergic and cholinergic mechanisms.

Differential effects of alpha-beta-methylene ATP on responses to nerve stimulation in SHR and WKY tail arteries.

The effects of alpha,beta-,methylene-adenosine triphosphate, (alpha,beta-methylene ATP, a P2-receptor desensitising agent) have been evaluated on vasoconstrictor responses elicited by exogenous agonists or electrical field stimulation in isolated perfused SHR or WKY tail arteries and on tritium release elicited by electrical field stimulation in SHR-tail arteries pre-labeled with 3H-noradrenaline. Exposure to alpha,beta-methylene ATP (0.1 mumol/l) significantly inhibited vasoconstrictor responses to electrical field stimulation in SHR tail arteries. These inhibitory effects were not further increased at a higher concentration of alpha,beta-methylene ATP (1 mumol/l). In WKY tail arteries, alpha,beta-methylene ATP (1 mumol/l) failed to significantly inhibit vasoconstrictor responses to electrical stimulation. In SHR tail arteries prelabelled with 3H-noradrenaline, alpha,beta-methylene ATP (1 mumol/l) did not inhibit the stimulation evoked release of tritium. However, at this concentration, alpha,beta-methylene ATP significantly antagonized the vasoconstrictor responses of SHR tail arteries induced by exogenous ATP (1 mumol/l), beta,gamma-methylene ATP (30 mumol/l), a stable agonist at P2-receptors, or 60 mmol/l KCl. These effects of alpha,beta-methylene ATP on contractile responses to KCl were not observed in WKY-tail arteries. In tail arteries obtained from reserpine pretreated SHR, despite a 85-95% decrease in endogenous noradrenaline tissue content, the vasoconstrictor responses induced by periarterial field stimulation were greatly diminished, but not abolished. These residual responses to periarterial field stimulation were not antagonized by prazosin (0.1 mumol/l), but were practically abolished by the addition of alpha,beta-methylene ATP (1 mumol/l). In tail arteries from WKY rats pretreated with reserpine, exposure to prazosin (0.1 mumol/l) further reduced the residual responses elicited by electrical field stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Preferential antagonism by diltiazem of alpha 2-adrenoceptor mediated vasoconstrictor responses in perfused tail arteries of spontaneous hypertensive rats.

Vasoconstrictor responses mediated by the alpha 2-adrenoceptor agonist TL99, were particularly sensitive to blockade by the calcium antagonist drug diltiazem in isolated perfused tail arteries of spontaneously hypertensive rats (SHR). In contrast, the vasoconstrictor responses induced by the alpha 1-adrenoceptor agonist methoxamine were significantly more resistant to antagonism by diltiazem. At higher concentrations (greater than 300 nmol/l) diltiazem became an effective antagonist of all alpha-adrenoceptor mediated responses. In normotensive Wistar Kyoto (WKY) or Sprague-Dawley (SD) rats diltiazem was significantly less potent against vasoconstrictor responses to TL99 than in SHR. The blockade of alpha 1-adrenoceptor mediated vasoconstriction by diltiazem was not significantly different when normotensive rats and SHR were compared. The vasoconstrictor responses evoked by 5HT in the perfused tail arteries were particularly resistant to blockade by diltiazem in SHR arteries. The responses to endogenously released noradrenaline, evoked by electrical field stimulation, were significantly antagonised by diltiazem (30 nmol/1-3 mumol/l) in SHR-tail arteries, while they were not modified in WKY-tail arteries. At the concentrations of diltiazem which blocked end organ responses to field stimulation, there was no modification of total tritium overflow from SHR-tail arteries after labelling the tissue with 3H-noradrenaline, indicating that diltiazem does not inhibit transmitter release at these concentrations. The tail artery preparation of SHR contains a population of postsynaptic alpha 2-adrenoceptors which mediate contraction in this blood vessel and the calcium entry blocker diltiazem is a potent antagonist of vasoconstrictor responses mediated by vascular alpha 2-adrenoceptors in hypertensive rats. These findings may be relevant to the antihypertensive action of diltiazem.

Possible involvement of presynaptic alpha 1-adrenoceptors in the effects of idazoxan and prazosin on 3H-noradrenaline release from tail arteries of SHR.

The effects of several alpha-adrenoceptor antagonists have been examined on tritium release elicited by electrical stimulation from isolated perfused SHR tail artery preparations prelabelled with 3H-noradrenaline (3H-NA). Phentolamine and yohimbine potently facilitated the stimulation evoked release of tritium at low frequencies of stimulation, but the alpha 2-adrenoceptor antagonist idazoxan was only weakly active at 1 mumol/l, despite antagonising the clonidine-evoked inhibition of 3H-release at a lower concentration of 0.1 mumol/l. The alpha 1-adrenoceptor antagonists prazosin and corynanthine also increased stimulation evoked tritium release in this preparation, suggesting the presence of prejunctional alpha 1-adrenoceptors. Furthermore, the alpha 1-adrenoceptor agonist methoxamine (3 mumol/l) caused a significant inhibition of tritium-evoked release, an effect which was blocked by prazosin (10 nmol/l). When alpha 1-adrenoceptors were blocked in the presence of prazosin, idazoxan (0.1 mumol/l) produced a significant facilitatory effect on the electrically-evoked release of 3H-transmitter. On the other hand, when alpha 2-adrenoceptors were blocked in the presence of yohimbine, exposure to idazoxan (0.1 mumol/l) reduced significantly the stimulation-evoked release of tritium elicited by electrical stimulation. The results indicate that in the SHR tail arteries, idazoxan has a partial agonist inhibitory activity on transmitter release, which can mask the facilitatory effects due to blockade of presynaptic alpha 2-adrenoceptors. The inhibitory effects of idazoxan appear to involve presynaptic alpha 1-adrenoceptors, which when stimulated, reduce 3H-NA release in SHR tail arteries.

An in vitro method for the evaluation of antiarrhythmic and antiischemic agents by using programmed electrical stimulation of rabbit heart.

A new model using isolated rabbit hearts perfused at constant flow in the Langendorff mode with the sinus node destroyed and under constant (2 Hz) pacing is described. Ventricular ischemia (24 min) was induced by ligation of the left ventricular branch of the coronary artery (LVB), followed by reperfusion (15 min). The programmed electrical stimulation (PES) technique was used to induce arrhythmias in the ischemic zone (IZ). Three agents with different mechanisms of action were tested to validate this model: dl-sotalol (10(-6) and 10(-5) M), oxfenicine (10(-6) M), and lidocaine (10(-5) M). These compounds were administered 15 min before the ligature and maintained until the end of the experiment. Ventricular effective refractory period (VERP), PES-induced ventricular fibrillation (VF), and coronary perfusion pressure (CPP) were monitored. PES-induced VF was only observed in ischemic tissue. Sotalol slightly reduced VF incidence only during reperfusion. Oxfenicine prevented PES-induced VF during the ischemia, but not during reperfusion, while lidocaine prevented VF during ischemia and throughout the reperfusion period. In conclusion, the rabbit heart model where PES is applied to normal and ischemic myocardium, appears useful to discern different mechanisms involved in ventricular arrhythmias. In addition, this model is considerably cheaper than equivalent dog models.