Effects of bunazosin on electrical responses of smooth muscle cells of the guinea-pig mesenteric artery and vein to perivascular nerve stimulation and to noradrenaline

Infarct size-reducing effect of ischemic preconditioning is related to alpha1b-adrenoceptors but not to alpha1a-adrenoceptors in rabbits

In rabbits and rats, both stimulation of alpha-adrenoceptors and ischemic preconditioning (PC) reduce infarct size. Activation of alpha1b-adrenoceptors play an important role in the PC effect on ventricular function in rats. However, the alpha1-adrenoceptors have not been reported to be related to the PC effect in rabbits, because the infarct size-reducing effect of PC is not blocked by the nonselective alpha-adrenoceptor antagonist, phenoxybenzamine (POB) or by the alpha1-adrenoceptor antagonist, BE2254. However, we speculated that alpha1b-adrenoceptors but not alpha1a-adrenoceptors may be related to the infarct size-reducing effect of PC in rabbit hearts. Thus we examined in rabbits whether the alpha1b-adrenoceptor blocker chloroethylclonidine (CEC), the alpha1a-adrenoceptor blocker 5-methylurapidil (5-MU), the selective alpha1-adrenoceptor antagonist bunazosin (BN), and the nonselective apha-adrenoceptor antagonist phenoxybenzamine (POB) can block the PC effect on infarct size. Eighty-eight anesthetized open-chest Japanese white male rabbits were subjected to 30-min coronary occlusion and 48-h reperfusion. In five PC groups, the rabbits were subjected to a single 5-min occlusion and 5-min reperfusion before 30-min sustained ischemia. In the PC groups, those with CEC (3 mg/kg, n = 10), 5-MU (3 mg/kg, n = 10), BN (0.3 mg/kg, n = 10), POB (4 mg/kg, n = 10), or placebo saline (n = 10) were pretreated before PC. In the non-PC groups, those with CEC (3 mg/kg, n = 7), 5-MU (3 mg/kg, n = 7), BN (0.3 mg/kg, n = 7), POB (4 mg/kg, n = 7), or placebo saline (n = 10) were pretreated before 30-min sustained ischemia. After a 48-h reperfusion, the infarct size was measured histologically and expressed as a percentage of the area at risk. PC caused a marked reduction of infarct size as compared with the non-PC control (10 +/- 3% vs. 42 +/- 2%; p < 0.05). The PC effect was completely blocked by CEC (36 +/- 2%) and by BN (42 +/- 4%) but not by 5-MU (14 +/- 1%) or POB (13 +/- 2%). None of the drugs by itself affected the infarct size. Stimulation of alpha1b-adrenoceptors but not of alpha1a-adrenoceptors during PC plays an important role in the PC effect on infarct size. This may explain the previous confusion concerning the PC blocking effect of various alpha1-blockers.

The mechanism of action of hepatic sympathetic nerves on ketone-body output from perfused rat liver. The effect of the interaction of noradrenaline with ATP on the release of beta-hydroxybutyrate

The regulatory mechanism of ketone-body output by the hepatic sympathetic nerves was studied in rat liver perfused in situ. Enrichment of the perfusion medium with 1 mM octanoate increased the basal ketone-body output from the liver up to 1.5 mumol.min-1.g liver-1. Under these conditions, electrical stimulation of the hepatic nerves (20 V, 20 Hz, 2 ms) decreased the output of both acetoacetate and beta-hydroxybutyrate, and was accompanied by an accumulation of beta-hydroxybutyrate in the liver. The effects of nerve stimulation were inhibited by the alpha 1-antagonist bunazosin (10 microM). However, noradrenaline, a typical sympathetic neurotransmitter, at a concentration of 1 microM decreased the output of acetoacetate but did not affect beta-hydroxybutyrate output. Prostaglandin F2 alpha at a concentration of 10 microM produced an effect similar to treatment with noradrenaline, without a decrease in beta-hydroxybutyrate output. ATP at 50 microM, however, decreased the output of both acetoacetate and beta-hydroxybutyrate and increased the tissue concentration of beta-hydroxybutyrate, mimicking the effect of nerve stimulation. Moreover, in the presence of 0.2 microM ATP, a concentration that produced neither metabolic nor hemodynamic changes, noradrenaline (1 microM) was shown to decrease the beta-hydroxybutyrate output. These results indicate the possible involvement of ATP in the action of hepatic sympathetic nerves on beta-hydroxybutyrate output from the liver, presumably through its interaction with noradrenaline.

Vascular responsiveness of rabbit common carotid, renal and femoral arteries to alpha-adrenoceptor agonists

The stainless steel cannula inserting method was used to observe vascular effects of alpha-adrenoceptor agonists, phenylephrine (PE), methoxamine (ME), clonidine (CL) and xylazine (XY), on the isolated, perfused rabbit common carotid, renal and femoral arteries. PE and ME induced a dose-dependent vasoconstriction that was readily suppressed by treatment with bunazosin, an alpha 1-adrenoceptor blocker. CL induced a constriction only in common carotid arteries, and this was readily suppressed by bunazosin. In preparations preconstricted by phenylephrine, CL and XY dose-dependently induced vasodilations in the 3 types of arteries, and these vasodilations were not modified by a potent alpha 2-adrenoceptor antagonist, midaglizole. In preparations preconstricted by prostaglandin F2 alpha, CL and and XY did not produce any significant vasodilation, but CL induced a vasoconstriction in common carotid arteries that was completely blocked by bunazosin. Thus, it is concluded that: 1) alpha 1-adrenoceptors are functionally predominant in rabbit peripheral arteries, 2) the alpha 2-adrenoceptor agonist-induced vasodilation may be due to an antagonistic action towards alpha 1-mediated constrictions, and 3) clonidine has alpha 1-adrenoceptor stimulating properties in rabbit common carotid artery but not in renal and femoral arteries.

A comparison of guanfacine, bunazosin, atenolol and nadolol on blood pressure and plasma noradrenaline responses to cold pressor testing

1. The role of the presynaptic adrenoceptor subtypes in man was investigated based on observation of the changes in blood pressure (delta BP) and plasma noradrenaline concentration (delta NA) with the cold pressor test (CPT). 2. The CPT was well reproducible for BP and NA when performed at a 2 week interval in patients with mild hypertension. 3. After administration for 4 weeks, guanfacine (Gf; alpha 2-adrenoceptor agonist) decreased the delta NA response to CPT. 4. After administration for 2 or 4 weeks, bunazosin (Bu; alpha 1-adrenoceptor antagonist) atenolol (At; beta 1-adrenoceptor antagonist) and nadolol (Nd; non-selective beta-adrenoceptor antagonist) did not affect the delta NA response to CPT. 5. Both Gf and Bu decreased the systolic blood pressure response (delta SBP) to CPT after 4 weeks of the administration. Neither At nor Nd significantly changed the delta SBP response to CPT. 6. It is likely that Gf stimulated the presynaptic alpha 2 adrenoceptors at the sympathetic nerve endings as well as the central alpha 2 adrenoceptors, inhibiting the release of noradrenaline. It is unlikely that Bu, At and Nd exerted any clearly defined action on the presynaptic adrenoceptors in human hypertensive subjects.

Effects of α-adrenoceptor antagonists and clonidine on the haemodynamic response to acute hypovolaemia in conscious rabbits

In conscious rabbits an inferior vena caval cuff was progressively inflated so cardiac output fell at a constant approximately 8% of its baseline value. There was a biphasic haemodynamic response, consisting of an initial compensatory phase during which there was progressive systemic vasoconstriction and tachycardia, followed by a decompensatory phase in which systemic vasoconstriction failed abruptly, blood pressure plummeted and heart rate declined. We tested the effects on the haemodynamic response of prior 4th ventricular, and in some cases intravenous, infusions of saline, yohimbine, clonidine, yohimbine plus clonidine, and bunazosin. From the results we conclude that a yohimbine-sensitive mechanism in the brainstem, possibly alpha 2-adrenoceptor-mediated, may be an essential element of the cardiac receptor-mediated decompensatory phase of acute central hypovolaemia, but does not contribute to the arterial baroreflex-mediated compensatory phase.

Comparison of neurotransmission with nerve trunk and transmural field stimulation in guinea-pig mesenteric artery

1. Intracellular electrical and contractile responses to sympathetic nerve trunk stimulation (NTS) and transmural electrical field stimulation (TMS) were compared in the guinea-pig mesenteric artery in vitro. 2. Step increases in voltage with NTS gave rise to excitatory junctional potentials (EJPs) which reached a plateau amplitude of 5-10 mV, whereas with TMS larger amplitude EJPs and sometimes action potentials were obtained. 3. EJPs of equal amplitude (1-7 mV) elicited with TMS and NTS had the same rise time, duration and decay half-time. 4. Slow depolarization obtained with repetitive stimulation was significantly greater in amplitude with TMS than with NTS. 5. Equal amplitude EJPs were obtained throughout the preparation with NTS. With TMS, the amplitude of responses declined substantially with distance from the stimulating electrodes. 6. Tetrodotoxin (TTX) completely blocked EJPs, slow depolarization and contraction with NTS; however, with TMS a TTX-resistant component was observed. The TTX-resistant response to TMS was abolished in the presence of a low-Ca2+ superfusion solution but was not affected by endothelium removal. 7. Phentolamine or prazosin abolished slow depolarization but not EJPs with NTS or TMS. Prazosin abolished contraction with NTS and reduced but did not abolish contraction with TMS. 8. alpha, beta-Methylene ATP abolished EJPs with NTS, whereas with TMS only EJPs obtained with low stimulus intensities were abolished. alpha, beta-Methylene ATP did not block contraction with either NTS or TMS. 9. Combined TTX, prazosin and alpha, beta-methylene ATP abolished EJPs initiated with TMS at all but the highest stimulus intensities (12-20 V, 0.3 ms duration). 10. It is concluded that responses obtained with NTS can be reliably attributed to the release of transmitter by the conduction of action potentials in paravascular nerves, whereas activation by TMS is a more complex phenomenon dependent upon stimulus strength and probably involving multiple forms of activation.

[3H]bunazosin, a novel selective radioligand of alpha 1 adrenoceptors in human prostates

The binding properties of a new radioligand, [3H]bunazosin, were studied in membranes of human prostates with benign prostatic hypertrophy (BPH). Specific binding of [3H]bunazosin was saturable, reversible, and of high affinity (Kd = 0.55 +/- 0.04 nM). The density of [3H]bunazosin binding sites (Bmax) was 676 +/- 33 fmol/mg. protein. [3H]Bunazosin rapidly associated with its binding sites in membranes of human prostates and reached steady state by 20 min. at 25C. The rate constants for association and dissociation of [3H]bunazosin binding were calculated to be 0.11 +/- 0.01/nM/min. and 0.05 +/- 0.02/min. (n = 4), respectively. Seven alpha 1 adrenoceptor antagonists competed with [3H]bunazosin for the binding sites in the rank order: R-(-)-YM-12617 greater than prazosin greater than SGB-1534 greater than bunazosin greater than terazosin greater than naftopidil greater than urapidil. In parallel studies with [3H]bunazosin, the Kd and Bmax values for [3H]prazosin binding in human prostates were slightly lower. There was a similarity in the potency and rank order of seven alpha 1, adrenoceptor antagonists for the inhibition of [3H] bunazosin and [3H]prazosin binding in human prostates. The new [3H]bunazosin binding assay in human prostates is remarkable for its low degree of nonspecific binding as compared to [3H]prazosin, especially at high ligand concentrations. Thus, [3H]bunazosin may become a useful radioligand for the further analysis of the alph 1 adrenoceptor binding sites in human prostates.

Intrapulmonary bronchial circulation during hemorrhage

Experiments were conducted on 119 anesthetized and artificially ventilated rats to evaluate effects of a physiological stimulus (hemorrhage) to the sympothoadrenal system on the bronchial circulation. In the presence of a sufficient dose of a vasopressin V1-receptor antagonist, moderate (81 mmHg on average, 33 rats) or severe hypotension (69 mmHg, 28 rats) was produced by controlled hemorrhage (11 or 9 rats, respectively), or by treatment with phenoxybenzamine (0.1 mg/kg, i.v., 12 rats, or 1.0 mg/kg, 10 rats), or the highly selective alpha 1-adrenoceptor antagonist, bunazosin (0.01 mg/kg i.v., 10 rats, or 0.1 mg/kg, 9 rats). During hypotension, the intrapulmonary bronchial blood flow (microsphere method) was decreased in a dose-dependent manner in the two antagonist-treated groups. However, these decreases were only of a moderate degree compared to the severe decrease in the hemorrhage group. Although the bronchovascular resistance was not significantly changed after treatment with either antagonist, this variable was greatly elevated during severe hemorrhagic hypotension, reaching 240 +/- 51% (P less than 0.001 with either antagonist study) of its baseline level. Changes in the pulmonary arterial and left atrial pressures, plasma vasopressin concentration, and renin activity were found to be less influential on these responses in 58 rats. Overall, we concluded that the sympathoadrenal mechanism powerfully increased the resistance and decreased the blood flow of the intrapulmonary bronchial circulation.

Alpha-1 blockers. A new generation of antihypertensive agents

Alpha-1 blockers have certain disadvantages over conventional and antihypertensive therapies in their haemodynamic profile and metabolic effects. This paper reviews the development of alpha-blockade, the therapeutic efficacy of prazosin, the prototype alpha-1 blocker, and the rationale for the once-daily antihypertensive compounds, terazosin and doxazosin. These drugs offer a useful alternative to first- or second-line therapy in suitable hypertensive patients, particularly with their potentially beneficial effects on serum lipids.

Bunazosin, an alpha 1-adrenoceptor blocker, differentially releases co-transmitters in dog mesenteric vessels

The effects of bunazosin on the electrical and mechanical responses of smooth muscle cells elicited by exogenously applied noradrenaline (NA) and by perivascular nerve stimulation were studied in the isolated mesenteric artery and vein of the dog. NA (above 10(-7) M in the artery and above 3 X 10(-8) M in the vein) depolarized the membrane. Perivascular nerve stimulation evoked an excitatory junction potential (e.j.p.) and slow depolarization in both vessels. Bunazosin and prazosin inhibited the NA-induced depolarization and slow depolarization in the artery but not in the vein. The NA actions in the vein were inhibited by yohimbine. Bunazosin (above 10(-6) M) increased the amplitude of the e.j.p. but decreased the outflow of NA during nerve stimulation. The amplitude and conduction velocity of the compound action potential of perivascular nerves were inhibited by higher concentrations of bunazosin (above 10(-5) M). The results provide evidence that bunazosin has selective inhibitory actions at alpha 1-adrenoceptors. This drug exerted differential effects on the release of co-transmitters which generate the e.j.p. and the slow depolarization, as bunazosin increased the former and decreased the latter. This suggests that e.j.p. is generated by a substance other than NA.