Positive inotropic action of angiotensin II in the pithed rat

Potential Mechanisms Involved in Palmitoylethanolamide-Induced Vasodepressor Effects in Rats

Palmitoylethanolamide is an endogenous lipid that exerts complex vascular effects, enhances the effects of endocannabinoids and induces a direct hypotension, but the mechanisms involved have been poorly explored. Hence, this study investigated in Wistar pithed rats the role of CB1, CB2, TRPV1 and GPR55 receptors in the inhibition by palmitoylethanolamide of the vasopressor responses produced by sympathetic stimulation or exogenous noradrenaline. Frequency- and dose-dependent vasopressor responses were analysed before and during intravenous (i.v.) continuous infusions of palmitoylethanolamide in animals receiving i.v. bolus of the antagonists NIDA41020 (CB1), AM630 (CB2), capsazepine (TRPV1), and/or cannabidiol (GPR55). Palmitoyletha-nolamide (0.1-3.1 μg/kg/min) dose-dependently inhibited the sympathetically induced and noradrenaline-induced vasopressor responses. Both inhibitions were: (i) partially blocked by 100 μg/kg NIDA41020, 100 μg/kg capsazepine, or 31 μg/kg cannabidiol; (ii) unaffected by 310 μg/kg AM630; and (iii) abolished by the combination NIDA41020 + capsazepine + cannabidiol (100, 100, and 31 μg/kg, respectively). The resting blood pressure was decreased by palmitoylethanolamide (effect prevented by NIDA41020, capsazepine or cannabidiol, but not by AM630). These results suggest that: (i) palmitoylethanolamide inhibits the vasopressor responses to sympathetic stimulation and exogenous noradrenaline and that it induces hypotension; and (ii) all these effects are mediated by prejunctional and vascular CB1, TRPV1 and probably GPR55, but not by CB2, receptors.

Angiotensin II type 1 receptor antagonists in animal models of vascular, cardiac, metabolic and renal disease

We have reviewed the effects of angiotensin II type 1 receptor antagonists (ARBs) in various animal models of hypertension, atherosclerosis, cardiac function, hypertrophy and fibrosis, glucose and lipid metabolism, and renal function and morphology. Those of azilsartan and telmisartan have been included comprehensively whereas those of other ARBs have been included systematically but without intention of completeness. ARBs as a class lower blood pressure in established hypertension and prevent hypertension development in all applicable animal models except those with a markedly suppressed renin-angiotensin system; blood pressure lowering even persists for a considerable time after discontinuation of treatment. This translates into a reduced mortality, particularly in models exhibiting marked hypertension. The retrieved data on vascular, cardiac and renal function and morphology as well as on glucose and lipid metabolism are discussed to address three main questions: 1. Can ARB effects on blood vessels, heart, kidney and metabolic function be explained by blood pressure lowering alone or are they additionally directly related to blockade of the renin-angiotensin system? 2. Are they shared by other inhibitors of the renin-angiotensin system, e.g. angiotensin converting enzyme inhibitors? 3. Are some effects specific for one or more compounds within the ARB class? Taken together these data profile ARBs as a drug class with unique properties that have beneficial effects far beyond those on blood pressure reduction and, in some cases distinct from those of angiotensin converting enzyme inhibitors. The clinical relevance of angiotensin receptor-independent effects of some ARBs remains to be determined.

Block of Kv1.7 potassium currents increases glucose-stimulated insulin secretion

Glucose-stimulated insulin secretion (GSIS) relies on repetitive, electrical spiking activity of the beta cell membrane. Cyclic activation of voltage-gated potassium channels (K_v) generates an outward, ‘delayed rectifier’ potassium current, which drives the repolarizing phase of each spike and modulates insulin release. Although several K_v channels are expressed in pancreatic islets, their individual contributions to GSIS remain incompletely understood. We take advantage of a naturally occurring cone-snail peptide toxin, Conkunitzin-S1 (Conk-S1), which selectively blocks K_v1.7 channels to provide an intrinsically limited, finely graded control of total beta cell delayed rectifier current and hence of GSIS. Conk-S1 increases GSIS in isolated rat islets, likely by reducing K_v1.7-mediated delayed rectifier currents in beta cells, which yields increases in action potential firing and cytoplasmic free calcium. In rats, Conk-S1 increases glucose-dependent insulin secretion without decreasing basal glucose. Thus, we conclude that K_v1.7 contributes to the membrane-repolarizing current of beta cells during GSIS and that block of this specific component of beta cell K_v current offers a potential strategy for enhancing GSIS with minimal risk of hypoglycaemia during metabolic disorders such as Type 2 diabetes.

A comparison between haemodynamic effects of vasopressin analogues

Some analogues of arginine vasopressin (AVP) reportedly possess hypotensive properties, and two such peptides are Cys(1)-Tyr(2)-Phe(3)-Val(4)-Asn(5)-Cys(6)-Pro(7)- d-Arg(8)-Gly(9)-NH(2) (VD-AVP) and d(CH(2))(5)-Cys(1)- d-Tyr(Et)(2)-Arg(3)-Val(4)-Asn(5)-Cys(6)-Lys(7)-Lys(8)-ethylenediamine(9) (TA-LVP). In the present investigation we examined the effects of TA-LVP (0.3, 1.0 and 3.0 microg/kg/min), VD-AVP (0.3, 1.0 and 3.0 microg/kg/min) and AVP (1.0, 3.0, 10 ng/kg/min) on haemodynamics, blood volume (BV) and plasma troponin levels in anaesthetised rats. Infusion of TA-LVP significantly ( P<0.05) reduced blood pressure (-45+/-3%; n=8; mean +/- SEM), mean circulatory filling pressure ( P(mcf); -41+/-3%), and cardiac output (CO; -59+/-4%). The reduction in CO at a lower dose of TA-LVP was due to reduced venous tone, while at higher doses the reduction was predominantly the result of reduced BV (-35+/-4%). The large decrease in BV during the infusion of TA-LVP, substantially increased resistance to venous return (50+/-11%), which was the main contributor in reducing CO. Administration of AVP significantly increased blood pressure (41+/-4%) and arterial resistance (98+/-16%) without any impact on P(mcf) and BV, while significantly reducing CO (-26+/-5%). Infusion of VD-AVP did not produce hypotension, but produced a modest but significant reduction in CO (-18+/-5%) and insignificant but moderate increases in peripheral resistance (30+/-12%) and resistance to venous return (28+/-8%). Plasma troponin levels were not affected by any of the peptides. The hypotensive action of TA-LVP was due to a reduction in CO as a result of a reduced pre-load, while the pressor effect of AVP increased after-load sufficiently to impede flow, reducing CO. VD-AVP was devoid of any hypotensive effects, suggesting that V(2)-vasopressin receptors are most likely to play a limited role in the control of cardiac and vascular function in these animals.

Differential inotropic effects of endothelin-1, angiotensin II, and phenylephrine induced by crosstalk with cAMP-mediated signaling process in dog ventricular myocardium

Endothelin-1 (ET-1), angiotensin II (Ang II), and phenylephrine, an alpha1-adrenoceptor agonist, share the common signaling process, resulting in activation of Gq protein-coupled receptor (GqPCR) to activate the hydrolysis of phosphoinositide (PI). They do not elicit any inotropic effect in isolated dog ventricular muscle. In the presence of forskolin or IBMX (3-isobutyl-1-methylxanthine), ET-1 produced a dual effect, that is, a positive inotropic effect (PIE) and/or a negative inotropic effect (NIE) depending on concentrations of forskolin or IBMX present simultaneously with ET-1. Phenylephrine produced a definite PIE and Ang II induced a small and transient PIE in the presence of forskolin or IBMX, but they did not elicit a NIE. Facilitation of Ca2+ influx via L-type Ca2+ channel may play a crucial role in the crosstalk because GqPCR agonists produced, likewise a PIE in the presence of Bay k 8644. GqPCR agonists failed to induce a PIE in the presence of dihydroouabain or elevated [Ca2+]o. These findings indicate that the accumulation of cAMP or activation of L-type Ca2+ channels markedly modulates the inotropic response to GqPCR agonists in a manner that leads to a PIE in dog ventricular myocardium. In addition, ET-1, but not Ang II or phenylephrine, activates the signal transduction process that results in a NIE.

Effects of angiotensin II on myocardial contractility during short-term pressor responses to angiotensin II

BACKGROUND

Cardiac angiotensin AT1 receptors have been found in several animal species. In-vitro studies performed on cardiac preparations have shown that angiotensin II (ANG II) exerts a positive inotropic effect; however, in-vivo results have allowed no definitive conclusion to be drawn. The reasons behind these controversial results remain unknown, and could originate both from different experimental conditions and from the techniques used to assess myocardial contractility.

OBJECTIVE

To investigate, by means of echocardiographic measurements, whether ANG II, administered to intact and to sinoaortic denervated isoflurane-anesthetized rabbits, was able to directly increase myocardial contractility.

METHODS

The effect of ANG II on cardiac contractility was assessed with the use of simultaneous pressure measurements and Doppler-echocardiographic recordings. Specifically, we used both the relationship between the left ventricular end-systolic wall stress and the velocity of heart-rate-corrected circumferential fiber shortening (VCFC) and the maximum rate of rise of the ventricular pressure as indices of changes in myocardial contractility. Cardiac contractility was evaluated both in intact and in chronically sinoaortic denervated isoflurane-anesthetized rabbits under basal conditions and after ANG II infusion (50 ng/kg per min).

RESULTS

After ANG II infusion, increases in mean arterial blood pressure, left ventricular end-diastolic diameter and pressure were observed both in intact and in chronically sinoaortic denervated rabbits. The left ventricular end-systolic wall stress (a function of the mean arterial pressure and chamber size) and the maximum rate of rise of the ventricular pressure rose markedly in rabbits of both groups, whereas the VCFC decreased significantly. However, when compared the VCFC under ANG II infusion with that calculated at the same level of left ventricular afterload under basal conditions, we observed that ANG II infusion induced no significant change in VCFC either in intact of in chronically sinoaortic denervated rabbits.

CONCLUSION

Our results indicate that administration of ANG II to isoflurane-anesthetized rabbits induces a marked rise in ventricular pre- and after-loads and exerts no significant effect on the cardiac contractility. In light of this, it is reasonable to assume that the short-term increase in arterial blood pressure can be ascribed mainly to the increase in peripheral arterial resistance.

Activation of protein kinase C by angiotensin II decreases beta 1-adrenergic receptor responsiveness in the rat heart

Cardiac beta-adrenergic receptors are the primary driving force for the enhancement of contractility in response to sympathetic stimulation. Angiotensin II influences cardiac function by modulating sympathetic activity and by activating cardiac angiotensin II receptors. The aim of this study was to determine whether activation of cardiac angiotensin II receptors modulates the responsiveness of the heart to beta-adrenergic receptor activation. Male Sprague-Dawley rats were anesthetized and the hearts isolated and perfused with oxygenated Krebs-Henseleit buffer (KHB). Coronary artery perfusion pressure, left ventricular pressure (LVP), left ventricular dP/dtmax, and heart rate (HR) were measured. Bolus administration of the beta-adrenergic receptor agonists, isoproterenol, dobutamine, and salbutamol, produced dose-related increases in LVP, LV dP/dt(max), and HR. Addition of angiotensin-II (10-100 nM) to the KHB slightly increased coronary perfusion pressure but did not alter baseline LVP, LV dP/dt(max), or HR. Angiotensin II reduced the increase in LVP, LV dP/dt(max), and HR elicited by isoproterenol and dobutamine but did not affect responses to salbutamol. The inhibitory effect of angiotensin II was blocked by the AT1-receptor antagonist, losartan, and the protein kinase C inhibitor, calphostin C (50 nM). Activation of protein kinase C with phorbol-12, 13-dibutyrate (PDBu; 10 nM) reduced cardiac responses to all three agonists, although the effects were less on responses elicited by salbutamol. These data suggest that activation of protein kinase C by angiotensin II decreases the responsiveness of the rat heart to beta 1-adrenergic stimulation and that angiotensin II-mediated protein kinase C activation may differ from that activated by phorbol esters.

Effects of endothelin-1 on [Ca2+]i-shortening trajectory and Ca2+ sensitivity in rabbit single ventricular cardiomyocytes loaded with indo-1/AM: comparison with the effects of phenylephrine and angiotensin II

In most mammalian species, activation of myocardial endothelin as well as alpha1-adrenergic and angiotensin receptors leads to an increase in contractile function and myocardial cell hypertrophy, in association with acceleration of PI hydrolysis and with resultant production of IP3 and diacylglycerol. Therefore, these receptors may share a common intracellular signal transduction process in cardiac regulation. Although the pathophysiological relevance of endothelin- and angiotensin-mediated signal transduction has been postulated to play a key role in the progress of congestive heart failure, the details of the regulation are still controversial. We carried out experiments to further study the regulation induced by activation of these receptors. In spite of a wide range of species-dependent variation among mammals in the induction of the cardiotonic effect via these receptors, there is an excellent correlation between the extent of acceleration of PI hydrolysis and the positive inotropic effect (associated with a negative lusitropic effect) of the respective receptor agonists under most experimental conditions in rabbit ventricular myocardium. In isolated rabbit ventricular cardiomyocytes loaded with indo-1/AM, activation of these receptors elicited a very similar changes in the relationship between [Ca2+]i and cell shortening: the [Ca2+]i-shortening trajectory was shifted mainly upwards and the relationship of peak shortening vs peak [Ca2+]i was shifted to the left, an indication that the PIE of these agonists is consistently associated with an increase in [Ca2+]i and in the sensitivity of myofilaments to Ca2+ ions under the same experimental condition. Pieces of evidence in biochemical and pharmacological analyses imply that the products of PI hydrolysis, namely diacylglycerol and subsequent activation of protein kinase C, might play a crucial role in the regulation of cardiac function that is induced upon activation of endothelin, angiotensin and alpha-adrenergic receptors in the rabbit ventricular myocardium.

Direct positive chronotropic effects of angiotensin II and angiotensin III in pithed rats and in rat isolated atria

1. The direct positive chronotropic effects of angiotensin II (AII) and its degradation products angiotensin III (AIII) and angiotensin IV (AIV) were established in pithed rats and in rat spontaneously beating right atria. 2. In pithed rats, AII, AIII and AIV caused dose-dependent tachycardia with similar maximal responses (110 beats min-1). The beta-adrenoceptor antagonist propranolol (3.37 x 10(-6) mol kg-1) but not the alpha 1-adrenoceptor antagonist prazosin (2.38 x 10(-7) mol kg-1) significantly reduced these effects (P < 0.05; n = 7-8), but 20-25% of the responses could not be blocked by propranolol. 3. In isolated atria, AII, AIII and AIV caused concentration-dependent increases in beating rate with similar maximal responses to AII and AIII (34.3 +/- 0.4 and 34.7 +/- 0.4 beats min-1; n = 9-10), and a lower maximal response to AIV (26.8 +/- 0.6 beats min-1; P < 0.05; n = 8). AIII was about 9 times less potent than AII, whereas AIV proved approximately 3800 times less potent than AII. Neither propranolol (1 microM) nor prazosin (1 microM) could influence the effects of the angiotensin peptides. 4. In isolated atria, the selective AT1-receptor antagonist, losartan (10, 100 and 300 nM) caused parallel rightward shifts of the concentration-response curves for AII and AIII, whereas the selective AT2- receptor antagonist PD123177 (1 microM) did not influence the effects of AII and AIII. The aminopeptidase-A and -M inhibitor amastatin (10 microM), significantly steepened the slope of the AIII curves and increased the potency of AIII about 6 fold. Amastatin did not influence the responses to AII. 5. Our results indicate that both in vivo and in vitro, exogenous AII and AIII induced a direct dose-dependent chronotropic effect, which is independent of the adrenergic system. This chronotropic effect is mediated by AT1-subtype receptors.

Cardiac alpha(1)-adrenoceptors that regulate contractile function: subtypes and subcellular signal transduction mechanisms

Activation of alpha(1)-adrenoceptors as well as endothelin (ET) and angiotensin II (Ang II) receptors in cardiac muscle is coupled to acceleration of the hydrolysis of phosphoinositide (PI), with resultant production of inositol 1,4,5-trisphosphate (IP(3)) and diacylglycerol. There is an excellent correlation between the extent of acceleration of the PI hydrolysis and the positive inotropic effect (PIE) under most experimental conditions after the administration of a-adrenoceptor agonists, ET and Ang II in the rabbit ventricular muscle. The PIE of the alpha-adrenoceptor agonists, ET and Ang II is associated with a negative lusitropic effect and an increase in the sensitivity of myofilaments to Ca(2)+ ions. The PIE can be selectively inhibited by inhibitors of protein kinase C (PKC) such as staurosporine, NA 0345 and H-7, with little effect on the PI hydrolysis and the PIE of isoproterenol and Bay k 8644. Surprisingly, an activator of PKC, phorbol 12,13-dibutyrate (PDBu), selectively and more completely inhibited the PIE and acceleration of PI hydrolysis induced by the alpha-adrenoceptor agonists as well as by ET and Ang II in the rabbit. These receptor agonists consistently cause intracellular alkalinization by activation of Na+-H+ exchange, while the effects on membrane ion channel activities are divergent. For example, alpha-adrenoceptor agonists cause monophasic prolongation of the action potential, the time course of which coincides well with that of the PIE, while ET and Ang II produce a biphasic change in action potential duration, i.e., the long-lasting prolongation preceded by a transient abbreviation. Alpha-adrenoceptor agonists scarcely affect I(ca), whereas ET elicits a biphasic alteration of the current. In addition, the potassium current, I(K1), is markedly suppressed by alpha-adrenoceptor agonists, but this effect is not revealed with Ang II under the same experimental condition. These results indicate that the effects of alpha(1)-adrenoceptor stimulation are partially shared by those of FT and Ang II receptor activation in the heart. Approximately 60% of the total population of alpha(1)-adrenoceptors in the rabbit ventricle are composed of alpha(1A) subtype, which is susceptible to chlorethylclonidine (CEC) and is predominantly responsible for the alpha(1)-mediated PIE and PI hydrolysis. The remaining fraction that belongs to alpha(1A) subtype is further subclassified into the WB 4101-sensitive (partly coupled to PI hydrolysis) and the niguldipine-sensitive (PI hydrolysis-unrelated) subtypes.

In vivo pharmacological characterization of UP 269-6, a novel nonpeptide angiotensin II receptor antagonist

UP 269-6, 5-methyl-7-propyl-8(-)[2'-(1H-tetrazol-5-yl)biphenyl-4- yl)methyl]-1,2,4-triazolo]1,5-c]pyrimidin-2(3H)-one is a novel nonpeptide angiotensin II receptor antagonist. In vivo studies were performed to evaluate UP 269-6 for its angiotensin II antagonistic action. In pithed rats, i.v. administration of UP 269-6 (0.03-1 mg/kg) shifted dose dependently to the right the dose-pressor response curve for angiotensin II and decreased the maximum response. The angiotensin II antagonistic effect of UP 269-6 was as potent as that of L-158,809 (5,7-dimethyl-2-ethyl-3(-)[[2'- (1H-tetrazol-5-yl)biphenyl-4-yl]methyl]-imidazo[4,5-b]pyridine) and 10 times more potent than that of losartan. UP 269-6 antagonized the angiotensin II sympathetic-mediated tachycardiac response. UP 269-6 inhibited dose dependently the pressor response to angiotensin II with an ID50 of 4.5 micrograms/kg, i.v. in conscious normotensive dogs. Oral administration of UP 269-6 (0.1 to 30 mg/kg) resulted in a dose-dependent and long-lasting inhibition of the angiotensin II-induced pressor response in conscious normotensive rats and dogs. Compared to losartan, UP 269-6 presented a more rapid onset of action. UP 269-6 caused similar angiotensin II antagonistic effects in rats and dogs but the duration of the effect was greater in dogs than in rats. UP 269-6 did not alter the tachycardiac response to isoproterenol and the pressor response to vasopressin. UP 269-6 was demonstrated to be devoid of agonistic properties in rats and dogs. Furthermore, UP 269-6 did not induce hypotension and did not cause alteration in heart rate and ECG waveforms in dogs even at a dose 1000 times higher than the angiotensin II antagonistic effective dose. These results demonstrate that UP 269-6 is a potent and specific angiotensin II receptor antagonist and dose not possess agonistic properties.

The effects of various drugs on the myocardial inotropic response

1. The signal transduction process mediated by cyclic AMP that leads to the characteristic positive inotropic effect (PIE) in association with a positive lusitropic effect (acceleration of rate of twitch relaxation) has been well established. Relationships between accumulation of cyclic AMP, changes in intracellular Ca2+ transients and the PIE differ, however, depending on the mechanism of particular drugs that affect different steps in the metabolism of cyclic AMP. Selective partial agonists of beta 1-adrenoceptors and inhibitors of phosphodiesterase (PDE) III cause the accumulation of less cyclic AMP for a given PIE than does isoproterenol. In addition, in aequorin-microinjected canine ventricular muscle, selective inhibitors of PDE III, OPC 18790 and Org 9731, produced smaller decreases in the responsiveness of myofilaments to Ca2+ ions than isoproterenol, while a partial agonist of beta 1-adrenoceptors, denopamine, elicits a decrease in Ca2+ responsiveness of the same extent as does isoproterenol. 2. Activation of myocardial alpha 1-adrenoceptors, as well as stimulation of receptors for endothelin and angiotensin II, which accelerates hydrolysis of phosphoinositide (PI) to result in production of inositol 1,4,5-trisphosphate (IP3) and diacylglycerol (DAG) are associated with very similar inotropic regulation: (1) the dependence on the species of animals of induction of the PIE; (2) an excellent correlation between the extent of acceleration of hydrolysis of PI and the PIE; (3) isometric contraction curves associated with a negative lusitropic effect; (4) the PIE associated with increases in myofibrillar responsiveness to Ca2+ ions; and (5) the selective inhibition of the PIE by an activator of protein kinase C (PKC), phorbol 12,13-dibutyrate (PDBu), with little effect on the PIE of isoproterenol and Bay k 8644. 3. A novel class of cardiotonic agents, namely, Ca2+ sensitizers such as EMD 53998 and Org 30029, act on the Ca(2+)-binding site of troponin C, increasing the affinity of these sites for Ca2+ ions, or at the actin-myosin interface to facilitate the cycling of cross-bridges. These agents produce a PIE with little change or decrease in Ca2+ transients and may bring about a significant breakthrough in the development of drugs for reversal of myocardial failure in the treatment of congestive heart failure.

Postjunctional regulation by angiotensin II of alpha 1-adrenoceptor-mediated pressor responses in the rat

The effects of angiotensin II on the vasopressor responses to the selective alpha 1-adrenoceptor agonist, phenylephrine, in intact and sympathectomized rats were investigated. Infusion of angiotensin II at subpressor doses significantly enhanced the pressor effects of phenylephrine in intact rats. We also found that in the chemically sympathectomized rat, where prejunctional sympathetic function is impaired, the effects of angiotensin II infusion on the pressor effects of phenylephrine were similar to those obtained in intact rats. Furthermore, pretreatment with valsartan ((S)-N-valeryl-N-([2'-(1H-tetrazol-5-yl)biphenyl-4-yl]-methyl)-val ine), a new selective angiotensin AT1 receptor antagonist, antagonized the effects of angiotensin II on phenylephrine-mediated pressor responses, whereas the administration of the selective angiotensin AT2 receptor antagonist, PD 123319 (1-[[4-(dimethylamino)-3-methylphenyl]-methyl]-5-(diphenylacetyl)- 4,5,6,7-tetrahydro-1H-imidazo[4,5-c]-pyridine-6-carboxylic acid, ditriflouroacetate, monohydrate), injected in bolus doses of 100 micrograms/kg, did not antagonize the enhancing effect of angiotensin II. Collectively, these data suggest that angiotensin II modulates the response to phenylephrine primarily at a postjunctional level through the activation of angiotensin AT1 receptors and that the suggested prejunctional facilitation mediated by angiotensin receptors is quantitatively much less important in the intact animal.

Dual action of angiotensin II on coronary resistance in the isolated perfused rabbit heart

We studied the functional role of angiotensin II (AII) receptor subtypes and vasodilatory endothelial autacoid release in response to AII in isolated perfused rabbit hearts. AII infusion induced biphasic changes in coronary perfusion pressure (CPP): an initial increase was followed by a decrease until a plateau was reached. At higher concentrations of AII (> or = 10 nmol/l) this plateau phase was lower than the initial CPP level. AII infusion elicited inverse changes in peak left ventricular pressure (LVP): coronary constriction was associated with a transient decline, and during the plateau phase LVP was clearly increased. AII also moderately augmented prostacyclin (PGI2) release from the coronary vascular bed. The AII-induced changes in CPP, LVP, and PGI2 release were effectively inhibited by the AT1 receptor subtype antagonist ICI D8731 (30 nmol/l), but not by the AT2 receptor antagonist CGP 42112 (30 nmol/l). The adenosine A1 receptor antagonist 8-phenyltheophylline (0.1 mumol/l) attenuated the decline in CPP following the constriction phase without affecting the changes in LVP during AII infusion. The cyclooxygenase inhibitor diclofenac (1 mmol/l) had no effect on the AII-induced changes in CPP, whereas the nitric oxide-synthase inhibitor NG-nitro-L-arginine (30 mumol/l) markedly potentiated the vasoconstriction but was without effect on the plateau phase of the response. In contrast to AII, the thromboxane analogue U46619 elicited sustained increases in CPP which were associated with slight decreases in LVP.(ABSTRACT TRUNCATED AT 250 WORDS)