Pharmacological evidence that NaHS inhibits the vasopressor responses induced by stimulation of the preganglionic sympathetic outflow in pithed rats.

It has been reported that i.v. administration of NaHS, a donor of H2S, elicited dose-dependent hypotension although the mechanisms are not completely understood. In this regard, several mechanisms could be involved including the inhibition of the vasopressor sympathetic outflow. Thus, this study was designed to determine the potential capability of NaHS to mediate inhibition of the vasopressor responses induced by preganglionic sympathetic stimulation. For this purpose, Wistar rats were anaesthetised, pithed and cannulated for drug administration. In animals pre-treated with gallamine, the effect of i.v. infusion of NaHS (310 and 560µg/kgmin) or its vehicle (phosphate buffer) was determined on the vasopressor responses induced by: (1) sympathetic stimulation (0.03-10Hz); (2) i.v. bolus injections of exogenous noradrenaline (0.03-3µg/kg); or (3) methoxamine (1-100µg/kg). The vasopressor responses induced by preganglionic sympathetic stimulation were dose-dependently inhibited by i.v. infusion of NaHS (310 and 560µg/kgmin), but not by vehicle, particularly at high frequencies. In marked contrast, the vasopressor responses to exogenous noradrenaline or methoxamine were not inhibited by the above doses of NaHS or its vehicle. The above results, taken together, demonstrate that NaHS inhibited the vasopressor responses induced by preganglionic sympathetic outflow by a prejunctional mechanism. This is the first evidence demonstrating this effect by NaHS that may contribute, at least in part, to the hypotension induced by NaHS.

Angiotensin-(1-7) reduces the perfusion pressure response to angiotensin II and methoxamine via an endothelial nitric oxide-mediated pathway in cirrhotic rat liver.

Recent studies have shown that, in cirrhosis, portal angiotensin-(1-7) [Ang-(1-7)] levels are increased and hepatic expression of angiotensin converting enzyme 2 (ACE2) and the Mas receptor are upregulated, but the effects of Ang-(1-7) on hepatic hemodynamics in cirrhosis have not been studied. This study investigated the effects of Ang-(1-7) on vasoconstrictor-induced perfusion pressure increases in cirrhotic rat livers. Ang II or the alpha 1 agonist methoxamine (MTX) were injected in the presence or absence of Ang-(1-7), and the perfusion pressure response was recorded. Denudation of vascular endothelial cells with sodium deoxycholate was used to investigate the contribution of endothelium to the effects of Ang-(1-7). Ang-(1-7) alone had no effect on perfusion pressure. However, it reduced the maximal vasoconstriction response and area under the pressure response curve to Ang II and MTX by >50% (P < 0.05). This effect of Ang-(1-7) was not blocked by Mas receptor inhibition with A779 or by Ang II type 1 and type 2 receptor and bradykinin B(2) receptor blockade and was not reproduced by the Mas receptor agonist AVE0991. D-Pro(7)-Ang-(1-7), a novel Ang-(1-7) receptor antagonist, completely abolished the vasodilatory effects of Ang-(1-7), as did inhibition of endothelial nitric oxide synthase (eNOS) with N(G)-nitro-L-arginine methyl-ester, guanylate cyclase blockade with ODQ and endothelium denudation. The functional inhibition by D-Pro(7)-Ang-(1-7) was accompanied by significant (P < 0.05) inhibition of eNOS phosphorylation. This study shows that Ang-(1-7) significantly inhibits intrahepatic vasoconstriction in response to key mediators of increased vascular and sinusoidal tone in cirrhosis via a receptor population present on the vascular endothelium that is sensitive to D-Pro(7)-Ang-(1-7) and causes activation of eNOS and guanylate cyclase-dependent NO signaling pathways.

The α1-, but not α2-, adrenoceptor in the nucleus accumbens plays an inhibitory role upon the accumbal noradrenaline and dopamine efflux of freely moving rats.

In vivo microdialysis was used to analyse the role of the α(1)- and α(2)-adrenoceptor subtypes in the regulation of noradrenaline and dopamine efflux in the nucleus accumbens of freely moving rats. Intra-accumbal infusion of α(1)-adrenoceptor agonist methoxamine (24pmol) failed to alter the noradrenaline efflux, but decreased the dopamine efflux. The intra-accumbal infusion of α(1)-adrenoceptor antagonist prazosin (6, 600 and 6000pmol) produced a dose-related increase and decrease of the noradrenaline and dopamine efflux, respectively. An ineffective dose of prazosin (6pmol) counteracted the methoxamine (24pmol)-induced decrease of dopamine efflux. The prazosin (6000pmol)-induced increase of noradrenaline efflux, but not the decrease of dopamine efflux, was suppressed by the co-administration of an ineffective dose of methoxamine (0.024pmol). Neither the α(2)-adrenoceptor agonist clonidine (300pmol) and UK 14,304 (300pmol) nor the α(2)-adrenoceptor antagonist RX 821002 (0.6, 3, 600 and 6000pmol) significantly affected the accumbal noradrenaline and dopamine efflux. The doses mentioned are the total amount of drug over the 60-min infusion period. The present results show that (1) accumbal α(1)-adrenoceptors which are presynaptically located on noradrenergic nerve terminals inhibit the accumbal noradrenaline efflux, increasing thereby the accumbal dopamine efflux, (2) accumbal α(1)-adrenoceptors which are postsynaptically located on dopaminergic nerve terminals inhibit the accumbal dopamine efflux, and (3) accumbal α(2)-adrenoceptors play no major role in the regulation of accumbal efflux of noradrenaline and dopamine.

The effect of losartan and carvedilol on vasopressor responses to adrenergic agonists and angiotensin II in the systemic circulation of Sprague Dawley rats.

1 Interaction between renin-angiotensin (RAS) and sympathetic nervous systems (SNS) was investigated by examining the effect of cumulative blockade of angiotensin II (Ang II) and adrenergic receptors in normal Sprague Dawley rats. 2 Rats were treated with losartan (10 mg/kg), carvedilol (5 mg/kg), or losartan plus carvedilol (10+5 mg/kg) orally for 7 days. On day 8, the animals were anaesthetized with pentobarbitone and prepared for systemic haemodynamic study. Dose-response relationships for the elevation of mean arterial pressure or change in heart rate (HR) in response to intravenous injections of noradrenaline (NA), phenylephrine (PE), methoxamine (ME) and Ang II were determined. 3 Losartan or the combination of losartan with carvedilol blunted vasopressor responses to ME and Ang II. Dose-response relationships for agonist action on HR were significantly inhibited by all treatments except for the combination of losartan and carvedilol on the decrease in HR induced by PE. Carvedilol decreased vasopressor responses to NA, PE and Ang II, and HR responses to NA, ME and Ang II. Combination treatment produced similar effects to losartan on the vasopressor and HR responses but had a greater effect on vasopressor responses to ME and Ang II, and on HR responses to NA and Ang II than carvedilol alone. 4 It is concluded that peripheral vasoconstriction induced by Ang II is partly mediated by adrenergic action and that the vasopressor responses to adrenergic agonists depend on an intact RAS. These observations suggest an interactive relationship between RAS and SNS in determining systemic haemodynamic responses in 'normal' rats.

Age-related disappearance of the inhibitory effect of vascular endothelium on agonist-induced vasoconstriction in rat mesenteric vascular beds.

We previously reported that endothelium-derived hyperpolarizing factor (EDHF)-mediated response time-dependently suppressed methoxamine-induced vasoconstriction in mesenteric vascular beds isolated from 8-week-old rats. We investigated age-related changes in endothelial regulation of methoxamine-induced vasoconstriction. Mesenteric vascular beds isolated from young (8-week-old) to adult (16-week-old) rats were perfused, and changes in perfusion pressure induced by continuous perfusion of methoxamine or high KCl (60 mM) were measured over 180 min. In young preparations with intact endothelium, methoxamine-induced vasoconstriction time-dependently decreased to 20% of the initial levels, while time-dependent reduction was not observed in adult preparations. High KCl-induced vasoconstriction in young and adult preparations did not show time-dependent reduction. Endothelium removal abolished time-dependent reduction of methoxamine-induced vasoconstriction in young preparations and significantly attenuated vasoconstriction in adult preparations. Indomethacin, seratrodast, or tempol but not catalase significantly reduced methoxamine-induced vasoconstriction in adult preparations with endothelium. A23187 (Ca(2+)-ionophore)-, but not acetylcholine-, induced endothelium-dependent vasodilation in the presence of N(G)-L-nitro arginine methyl ether in adult preparations was significantly smaller than that in young preparations. These findings suggest that the inhibitory effect of mesenteric vascular endothelium on methoxamine-induced vasoconstriction disappears with aging by reducing EDHF and increasing endothelium-derived contracting factors and reactive oxygen species.

Characterization of the inhibitory effect of vascular endothelium on agonist-induced vasoconstriction in rat mesenteric resistance arteries.

Vascular endothelium regulates vascular tone by releasing endothelium-derived vasoactive substances. We performed this study to characterize the inhibitory effect of the endothelium on vasoconstrictor stimuli in rat mesenteric vascular beds. Changes in perfusion pressure induced by continuous perfusion of Krebs solution containing methoxamine (alpha(1)-adrenoceptor agonist) or high KCl were measured over 180 min. In preparations with intact endothelium, methoxamine-induced vasoconstriction was time-dependently decreased to cause 60% - 80% reduction of the initial vasoconstriction level, while no reduction was observed in high-KCl-induced vasoconstriction. Endothelium removal significantly blunted the time-dependent reduction of methoxamine-induced vasoconstriction without affecting high-KCl-induced vasoconstriction. Neither a nitric oxide synthase inhibitor (L-NAME) nor indomethacin (cyclooxygenase inhibitor) altered the time-dependent reduction of vasoconstriction. High KCl, K(+)-channel inhibitors tetraethylammonium and apamin plus charybdotoxin, and 18alpha-glycyrrhetinic acid (18alpha-GA, a gap-junction inhibitor) significantly inhibited the time-dependent reduction of methoxamine-induced vasoconstriction. In preconstricted preparations, bolus injection of acetylcholine and Ca(2+)-ionophore A23187 (A23187) evoked a sharp vasodilation, which was inhibited by endothelium removal, high KCl and tetraethylammonium, but not indomethacin, L-NAME, or 18alpha-GA. However, 18alpha-GA plus L-NAME inhibited vasodilation induced by A23187, but not acetylcholine. These findings suggest that endothelium-derived hyperpolarizing factor (EDHF) via gap junctions mainly counteracts vasoconstriction induced by methoxamine in mesenteric resistance arteries.

The contribution of adrenoceptor subtype(s) in the renal vasculature of diabetic spontaneously hypertensive rats.

1. Diabetes and hypertension are both associated with an increased risk of renal disease and are associated with neuropathies, which can cause defective autonomic control of major organs including the kidney. This study aimed to examine the alpha(1)-adrenoceptor subtype(s) involved in mediating adrenergically induced renal vasoconstriction in a rat model of diabetes and hypertension. 2. Male spontaneously hypertensive rats (SHR), 220-280 g, were anaesthetized with sodium pentobarbitone 7-day poststreptozotocin (55 mg x kg(-1) i.p.) treatment. The reductions in renal blood flow (RBF) induced by increasing frequencies of electrical renal nerve stimulation (RNS), close intrarenal bolus doses of noradrenaline (NA), phenylephrine (PE) or methoxamine were determined before and after administration of nitrendipine (Nit), 5-methylurapidil (5-MeU), chloroethylclonidine (CEC) and BMY 7378. 3. In the nondiabetic SHR group, mean arterial pressure (MAP) was 146+/-6 mmHg, RBF was 28.0+/-1.4 ml x min(-1) x kg(-1) and blood glucose was 112.3+/-4.7 mg x dl(-1), and in the diabetic SHR Group, MAP was 144+/-3 mmHg, RBF 26.9+/-1.3 ml(-1) min x kg(-1) and blood glucose 316.2+/-10.5 mg x dl(-1). Nit, 5-MeU and BMY 7378 blunted all the adrenergically induced renal vasoconstrictor responses in SHR and diabetic SHR by 25-35% (all P<0.05), but in diabetic rats the responses induced by RNS and NA treated with 5-MeU were not changed. By contrast, during the administration of CEC, vasoconstrictor responses to all agonists were enhanced by 20-25% (all P<0.05) in both the SHR and diabetic SHR. 4. These findings suggest that alpha(1A) and alpha(1D)-adrenoceptor subtypes contribute in mediating the adrenergically induced constriction of the renal vasculature in both the SHR and diabetic SHR. There was also an indication of a greater contribution of presynaptic adrenoceptors, that is, alpha(1B)-, and/or alpha(2)-subtypes.

The Y1 antagonist BIBP 3226 inhibits potentiation of methoxamine-induced vasoconstriction by neuropeptide Y.

We investigated the interaction of neuropeptide Y (NPY) with the alpha 1-adrenoceptor agonist, methoxamine, in control of mean arterial pressure, renovascular resistance and mesenteric vascular resistance in anaesthetized rats. Infusion of 3.0 but not 0.3 microgram/kg/min NPY enhanced the elevations of all three haemodynamic parameters caused by bolus injections of methoxamine (10-100 micrograms/kg). These enhancements largely involved a prolongation of the methoxamine effects. While infusion of the Y1 NPY receptor-selective antagonist, BIBP 3226 (10 micrograms/kg/min), alone did not alter methoxamine-induced vasoconstriction, it inhibited the potentiation by NPY. We conclude that NPY can potentiate methoxamine-induced vasoconstriction in vivo. This is mediated predominantly, if not exclusively, via the Y1 receptor. Endogenously released NPY does not appear to reach sufficient concentrations to cause tonic systemic vasoconstriction or potentiation thereof in the anaesthetized rat.

Propranolol stereoselectively inhibits alpha-adrenoceptor-mediated vasoconstriction in mesenteric arterial beds of rats.

1. The optical isomers of propranolol were compared for their effects on pressor responses to adrenergic and non-adrenergic vasoconstrictors in mesenteric arterial beds of rats. 2. R(+)-propranolol (10(-7)-10(-5) M) had no effect on vessel preparations stimulated with noradrenaline, methoxamine, or arginine-vasopressin. 3. S(-)-propranolol 10(-7) M did not alter pressor responses to noradrenaline. However, S(-)-propranolol 10(-6) and 10(-5) M inhibited vasoconstriction induced by noradrenaline. This effect was similar in the presence of indomethacin 3 x 10(-6) M. 4. S(-)-propranolol 10(-5) M also inhibited vasoconstriction induced by methoxamine, shifting the dose-response curves to the right, but did not affect pressor responses to arginine-vasopressin. 5. Schild analysis for equilibrium vasoconstrictor responses to methoxamine indicated stereoselective competitive alpha-adrenoceptor antagonism by propranolol. 6. These data suggest selective inhibition of alpha-adrenoceptor-mediated vasoconstriction by S(-)-propranolol at higher concentrations by competitive alpha-adrenoceptor antagonism.

Effect of methoxamine on noradrenaline release in the caudal artery of hypertensive rats.

1. The effect of methoxamine, an alpha1-adrenoceptor agonist, on the overflow of endogenous noradrenaline (NA) was examined in the electrically field stimulated (EFS) caudal artery obtained from Wistar rats, Wistar-Kyoto rats (WKY) and age-matched spontaneously hypertensive rats (SHR). 2. Methoxamine inhibited the EFS-evoked release of endogenous NA in the arteries from Wistar rats and WKY, but not in the arteries of SHR. 2-chloroadenosine, a purinoceptor agonist, also inhibited the NA release in the arteries from normotensive rats but not in the arteries of SHR. 3. The inhibitory effect of methoxamine was blocked by adenosine deaminase and potentiated by adenosine uptake inhibitor, dipyridamole. 4. Methoxamine caused the release of adenine nucleotides and adenosine from the caudal arteries of WKY and SHR. 5. These suggest that the inhibitory effect of methoxamine on NA release is mediated by endogenous adenyl purines and that the failure of methoxamine to inhibit NA release in the caudal artery of SHR is due to a dysfunction of the prejunctional purinoceptors on sympathetic nerve terminals.

Prevention by specific chemical classes of alpha 1-adrenoceptor antagonists of veratrine-contractures in rat left atria independently of alpha 1-adrenoceptor blockade.

1. The putative direct protective effects of a series of chemically diverse alpha 1-adrenoceptor antagonists against veratrine alkaloid-induced tetanic contractures in rat isolated left atria have been investigated. 2. Atria were mounted in organ baths containing normal, oxygenated physiological salt solution (20 ml, pH 7.4), for isometric tension recording. Atria were electrically driven at 4 Hz and were maintained at 34 degrees C. Veratrine (100 micrograms ml-1) was applied to the atria to elicit tetanic (diastolic) contracture. 3. Concentration-dependent protective effects against veratrine-contractures, in the absence of negative inotropic responses, were observed with the quinazoline congeners, prazosin and doxazosin and with the benzodioxane-related compounds, WB 4101 and its thio analogue, benoxathian. IC50 concentrations and apparent Hill coefficients of all four drugs ranged from 0.27 to 0.93 microM, and from 0.86 to 1.09, respectively, and are consistent with interaction at a single site. 4. In contrast, no protective activity versus veratrine-contractures was observed with corynanthine, 5-methyl-urapidil, phenoxybenzamine, phentolamine or chloroethylclonidine (10 microM). 5. Contractures were prevented by prazosin at concentrations 2-3 log units higher than those which antagonized methoxamine-evoked inotropic responses. In addition, concomitant alpha 1-adrenoceptor occupancy by high concentrations of methoxamine (100 microM), phentolamine (10 microM, inactive per se in preventing contracture), or both drugs together, failed, in each case, to modify significantly the protective effects of prazosin or WB 4101 against veratrine-contractures. 6. Our findings demonstrate that alpha 1-adrenoceptor antagonists which prevent veratrine-contractures belong to specific chemical classes of the quinazoline- and benzodioxane-type. The mechanism by which these drugs afford protection is apparently independent of an interaction with defined alpha 1-adrenoceptors.

Comparison of the hemodynamic effects of urapidil and flesinoxan in healthy volunteers.

The possible role of peripheral 5HT1A-receptors in the vasodilation caused by urapidil was studied by means of venous occlusion plethysmography in the forearm vascular bed of healthy volunteers. Urapidil is known to be an alpha 1-adrenoceptor antagonist and an agonist of 5HT1A-receptors. The hemodynamic effects of urapidil were compared with those of flesinoxan, a selective 5HT1A-receptor agonist virtually devoid of alpha 1-adrenoceptor antagonistic activity, and with the selective alpha 1-adrenoceptor antagonist doxazosin, which has no affinity for 5HT1A-receptors. Urapidil, as well as doxazosin, caused a dose-dependent decrease in forearm vascular resistance (FVR), thus reflecting vasodilation. Both urapidil and doxazosin were competitive antagonists of the vasoconstrictor effect of the selective alpha 1-adrenoceptor agonist methoxamine. On a molar base doxazosin proved more potent than urapidil (more than 10-fold). Flesinoxan slightly decreased FVR only at high doses. The nitric oxide (NO)-synthase inhibitor NG-monomethyl-L-arginine acetate (L-NMMA) depressed the vasodilatation caused by serotonin and also that by high-dose flesinoxan. The serotonin-induced vasodilatation is known to be NO-dependent. From the experiments it is concluded that peripheral 5HT1A-receptors cannot play an important role in the vasodilator response caused by urapidil, which is predominantly the result of postsynaptic alpha 1-adrenoceptor blockade. 5HT1A-receptors are clearly not involved in the NO-dependent dilatation caused by serotonin. During chronic treatment of hypertension with urapidil central but not peripheral 5HT1A-receptors may be assumed to play a role.

Aminophylline preferentially inhibits chloroethylclonidine-insensitive alpha-adrenoceptor-mediated contractions in rat aorta.

1. In rat thoracic aortae, contractions induced by methoxamine were inhibited by chloroethylclonidine, whereas oxymetazoline-induced contractions, which were more dependent on Ca(2+)-entry, were insensitive to chloroethylclonidine. 2. Aminophylline inhibited the contractions and 45Ca(2+)-uptake induced by both methoxamine and oxymetazoline. However, oxymetazoline-induced contractions were more sensitive to inhibition by aminophylline and D600. 3. Thus, the partial selectivity of aminophylline for the chloroethylclonidine-resistant, highly dependent on extracellular Ca2+, oxymetazoline-mediated responses may be explained by a preferential inhibition of agonist-induced Ca2+ entry as compared to inhibition of other transduction pathways.

Differential attenuation of the responses to adenosine and methoxamine in isolated rabbit aorta.

This article describes the functional antagonism between the responses to adenosine (through adenosine A2 receptors) and methoxamine (through alpha-1 adrenoceptors) in the adventitia- and endothelium-denuded isolated rabbit thoracic aorta. Rings were contracted with different concentrations of methoxamine and cumulative relaxation concentration-response curves (CRC) to adenosine were constructed. This protocol allowed the authors to rearrange the same data, which yielded contractile CRCs to methoxamine in the presence of adenosine. A 32-fold increase in the [methoxamine] markedly attenuated the maximal response to adenosine (80% decrease) and shifted the CRC to adenosine 10-fold to the right. By contrast, a 3000-fold increase in the [adenosine] shifted the CRC to methoxamine 3.25-fold to the right and attenuated the maximal response by a modest 18%. Analysis of these data by the operational model of agonism indicated that the efficacy parameter, tau, for adenosine or methoxamine was reduced by 99% or 71%, respectively, under these conditions. The agonist dissociation constant, KA, for adenosine (80 microM) or methoxamine (33 microM) by functional antagonism was also estimated. Use of an irreversible alpha-1 adrenoceptor antagonist allowed for the estimation of the KA for methoxamine by the receptor inactivation method using the operational model (40 microM), the Furchgott equation (48 microM) and the nested equations (42 microM) described by James et al. These results suggest that this tissue preparation is a good model to study functional antagonism quantitatively and that the functional antagonism between the responses mediated by these two receptors allows for the reliable estimation of the agonist dissociation constant for alpha-1 adrenoceptor agonists.

Methamphetamine antagonistic property of (+)- and (-)-4-phenyltetrahydroisoquinoline in rat anococcygeus muscle.

The antagonistic effects of (+)- and (-)-4-phenyl-1,2,3,4-tetrahydroisoquinoline (4PTIQ) on methamphetamine in the rat anococcygeus muscle were compared with those of cocaine and nomifensine. Methamphetamine contracted the anococcygeus muscle through the release of norepinephrine from noradrenergic nerve terminals. (+)-4PTIQ inhibited the methamphetamine-induced contractions more strongly than cocaine and nomifensine. (+)-4PTIQ had no potentiating effects on exogenous norepinephrine-induced contraction, which was considered to be an index of amine neuronal uptake blockade. On the other hand, (-)-4PTIQ, cocaine and nomifensine produced a significant leftward shift of the norepinephrine concentration-response curve, i.e. they showed a strong blocking effect on amine neuronal uptake. These results suggest that the inhibitory effects of (+)-4PTIQ on the action of methamphetamine are mediated by a mechanism other than inhibition of amine neuronal uptake.

Effect of TMB-8 on alpha-adrenoceptor agonist and KC1 induced-contractions in isolated rabbit aorta.

1. TMB-8 (10(-6) M-10(-4) M) depresses the contractile effect of the selective alpha 1-adrenoceptor agonists methoxamine and phenylephrine in the isolated rabbit aorta. 2. TMB-8 also depresses contractions evoked by 80 mM KCl in this tissue when used at similar concentrations. 3. The calcium antagonist nifedipine potentiates the inhibitory effect of TMB-8 on the alpha 1-contractions. 4. In preparations mounted in Ca-free solution containing 0.5 mM EGTA, 10(-4) M TMB-8 markedly depressed the contractions caused by both alpha 1-adrenoceptor agonists. 5. The Ca2+ agonist BAY K 8644 (10(-6) M) partially prevented the inhibitory effect of TMB-8 on 80 mM KCl contractions.

[Hypotensive effect of dihydrolycorine].

Dihydrolycorine-HCL(DL) is a derivative of lycorine, an alkaloid isolated from Lycoris radiata Herb. DL iv 20, 40, and 80 mg.kg-1 lowered the mean arterial pressure in pentobar-bitone-anesthetized normotensive rats, cats, and conscious rats either normotensive or renovascular hypertensive. In pithed rats DL depressed the hypertensive effect of reaction to electric stimulation to the T7-9 segments of spinal cord. DL also depressed the hypertensive effect of phenylephrine. In rabbit aorta ring and rat anococcygeus muscle, DL antagonized the contractile effect of methoxamine, with pA2 values of 5.93 and 6.35, respectively. These results suggest that DL can block alpha 1-adrenoceptors.

Effect of pinacidil on spontaneous and evoked contractile activity.

Uninhibited bladder contractions have been associated with a variety of bladder dysfunctions including outlet obstruction, neurogenic bladder, incontinence, and other neurologic and nonneurogenic bladder disorders. One class of compounds that is gaining popularity and support for the treatment of hyperreflexia is potassium channel openers, such as pinacidil and cromakalim. In general, these agents act by hyperpolarizing the smooth muscle membrane, resulting in an increase in membrane stability which in turn would be expected to inhibit spontaneous and evoked contraction. It is the purpose of this study to compare the potency and selectivity of pinacidil at inhibiting both hyperreflexia in vivo, and several forms of in vitro contractile stimulation in the rabbit. The following is a summary of the results. (1) Pinacidil is an effective inhibitor of hyperreflexia in the in vivo rabbit model. (2) Pinacidil is a substantially more potent inhibitor of the amplitude of the hyperreflexia than the frequency. (3) Pinacidil was substantially more potent at inhibiting the contractile response to 2-Hz stimulation than to 32-Hz stimulation, but was equally effective at inhibiting field stimulation of the bladder base and body. (4) Pinacidil was significantly more potent at inhibiting the peak response to field stimulation than the rate of tension generation. (5) Pinacidil was equally potent and effective at inhibiting the phasic and tonic components of the response to field stimulation. (6) Pinacidil was a more potent inhibitor of methoxamine stimulation of the bladder base than bethanechol stimulation of the bladder body. (7) Pinacidil was a noncompetitive or mixed inhibitor of both methoxamine and bethanechol stimulation, whereas it was a competitive inhibitor of KCl stimulation.(ABSTRACT TRUNCATED AT 250 WORDS)

Comparative study of postsynaptic alpha-adrenoceptors in aorta obtained from human and other mammalian species.

The alpha-adrenoceptor populations in aortic strips from humans, rats, guinea-pigs and rabbits were investigated in vitro, using specific agonists and antagonists. In rabbit and human preparations the activities of the two agonists tested, noradrenaline and methoxamine, were competitively antagonized by prazosin, whereas in the other animal species prazosin showed a competitive antagonism for methoxamine induced contraction, but an uncompetitive behaviour against noradrenaline. The alpha 2-selective agonists B-HT 920 and detomidine did not elicit any effect on aortic strips up to 10(-3) M. On the basis of these results, alpha 1-receptors seem to represent an homogeneous population in human and rabbit aortae, but not in rat and guinea-pig tissues. On the other hand, the contractile response of noradrenaline and methoxamine on aortic strips from the four animal species examined cannot be ascribed to the activation of postsynaptic alpha 2-receptors. Moreover we present preliminary evidence that rat and guinea-pig aortae do not contain a clear subdivision in alpha 1a- and alpha 1b-receptors.

Calcium-dependent inhibition of renin secretion: TMB-8 is a non-specific antagonist.

Intracellular Ca (Cai) is an inhibitory second messenger in renin secretion, and it has been hypothesized that some first messengers--especially angiotensin II [A-II] and antidiuretic hormone [ADH], and possibly A1-adenosine receptor antagonists as well--increase Cai and thereby inhibit renin secretion by causing the release or mobilization of Ca from intracellular sites of sequestration. The present experiments were designed to test this hypothesis, by using 3,4,5-trimethoxybenzoic acid 8-(diethylamino)-octyl ester (TMB-8), a putative antagonist of Ca release from intracellular sequestration sites. The rat renal cortical slices preparation was used. Basal renin secretory rate was unaffected by 1 and 10 microM TMB-8, but more than doubled in response to 100 microM TMB-8. Basal renin secretory rate was inhibited by A-II (1 microM), by ADH (200 units/1), by an A1-adenosine receptor agonist (N6-cyclohexyladenosine, or CHA; 0.5 microM), and by an alpha-adrenergic agonist (methoxamine; 10 microM). Only the inhibitory effect of methoxamine was blocked by 1 and 10 microM TMB-8, but these concentrations had no effect on basal secretory rate. At 100 microM, TMB-8 blocked the inhibitory effects of ADH as well as of methoxamine, but failed to block the inhibitory effects of CHA and A-II. However, these observations cannot be taken as evidence that methoxamine and ADH, but not CHA and A-II, inhibit renin secretion by a mechanism involving release of Ca from intracellular sequestration sites, because 100 microM TMB-8 clearly had non-specific effects. Among them, it completely blocked the inhibitory effect of K-depolarization on renin secretion. Collectively, at least three separate actions of TMB-8 must be invoked to explain the present results. Likely candidates are an Na-channel blocking effect and a Ca channel blocking effect in addition to antagonism of the release of Cai.