Modification by yohimbine and prazosin of the mechanical response of isolated dog mesenteric, renal and coronary arteries to transmural stimulation and norepinephrine

Endothelium-dependent relaxation competes with alpha 1- and alpha 2-adrenergic constriction in the canine epicardial coronary microcirculation

BACKGROUND

The purpose of this study was to determine whether endothelium-dependent relaxation competes with alpha 1- and alpha 2-adrenergic coronary microvascular constriction in the beating heart in vivo.

METHODS AND RESULTS

Coronary microvascular diameters were measured using stroboscopic epi-illumination and intravital microscopy during fluorescein microangiography in open-chested dogs (n = 20). Both alpha 1- and alpha 2-adrenergic receptors were selectively activated by intracoronary infusions of norepinephrine (0.05 and 0.2 microgram.kg-1 x min-1) in the presence of the alpha 2-adrenergic antagonist rauwolscine (0.2 mg/kg) or the alpha 1-adrenergic antagonist prazosin (0.75 mg/kg) during beta-adrenergic blockade (1 mg/kg propranolol). Microvascular diameters during selective alpha-adrenergic receptor activation were measured under baseline conditions and after inhibition of endogenous nitric oxide synthesis by an analogue of L-arginine, either NG-nitro-L-arginine (L-NA, 30 mg/kg) or NG-nitro-L-arginine methyl ester (L-NAME, 30 mg/kg). Under baseline conditions, alpha 1-adrenergic activation constricted small arteries (vessels with diameters between 100 and 300 microns) (4 +/- 1% and 5 +/- 1% decrease in diameter for the low and high doses of norepinephrine, respectively, both p < 0.05) but did not change the diameter of arterioles (vessels with diameters < 100 microns). In contrast, alpha 2-adrenergic activation by the lower but not the higher dose of norepinephrine induced constriction of arterioles (6 +/- 2% and 3 +/- 4% decrease in diameter, p < 0.05 and NS, respectively) but not small arteries. Inhibition of nitric oxide synthase activity by either L-NA or L-NAME produced constriction of small coronary arteries (9 +/- 2% decrease in diameter, p < 0.01) and arterioles (6 +/- 1% decrease in diameter, p < 0.05). The dilatation of small arteries and arterioles by acetylcholine (0.05 microgram-1 x kg-1 x min-1 intracoronary infusion; 10 +/- 1% increase in diameter under baseline conditions, p < 0.05) was abolished by either analogue. Both alpha 1- and alpha 2-adrenergic coronary microvascular constriction were markedly potentiated after L-NA or L-NAME. alpha 1-Adrenergic constriction was unmasked in arterioles (7 +/- 3% and 10 +/- 4% decrease in diameter, p < 0.05), although it was not significantly increased in small arteries. Conversely, alpha 2-adrenergic constriction was unmasked in small arteries (8 +/- 1% and 6 +/- 2% decrease in diameter, both p < 0.05) and potentiated in arterioles (12 +/- 1% and 8 +/- 4% decrease in diameter, both p < 0.05). After L-NA or L-NAME, microvessels retained the ability to dilate to sodium nitroprusside (0.1 microgram.kg-1 x min-1 intracoronary infusion; 10 +/- 2% increase in diameter, p < 0.05). alpha-Adrenergic constriction was not accentuated by increased tone alone, since it was either attenuated or converted to dilatation during a similar degree of preconstriction by the endothelium-independent vasoconstrictor angiotensin II (p < 0.05 for both alpha 1- and alpha 2-adrenergic activation).

CONCLUSIONS

These data confirm that alpha-adrenergic receptors are widespread in the coronary microcirculation, with the baseline functional responses to alpha 1-adrenergic activation predominating in small arteries and those to alpha 2-adrenergic activation predominating in arterioles. Furthermore, coronary microvascular constriction caused by both alpha 1- and alpha 2-adrenergic receptor activation is significantly modulated by endothelium-dependent relaxation, being markedly potentiated by inhibition of nitric oxide synthase activity. The data imply that alpha-adrenergic activation will assume considerable importance as a determinant of coronary microvascular resistance in pathophysiological situations associated with coronary endothelial impairment.

Functional distribution of alpha 1- and alpha 2-adrenergic receptors in the coronary microcirculation

BACKGROUND

The goal of this study was to determine the functional distribution of alpha 1- and alpha 2-adrenergic receptors in the epicardial coronary microcirculation. This goal was accomplished by intracoronary administration of the selective alpha 1-adrenergic agonist phenylephrine and the selective alpha 2-adrenergic agonist BHT-933 during measurements of coronary microvascular diameters in the beating heart.

METHODS AND RESULTS

Experimental measurements were made under conditions with intact vasomotor tone and during coronary hypoperfusion (i.e., under conditions with autoregulatory mechanisms intact and blunted, respectively). Administration of selective alpha 1- and alpha 2-adrenergic antagonists, prazosin and SKF 104078, respectively, confirmed that the agonists were preferentially activating the desired adrenergic receptor subtype because the vasoconstrictor effects of the agonists were completely blocked by the appropriate antagonist. With baseline coronary vasomotor tone intact, phenylephrine caused constriction (8 +/- 3% decrease in diameter, p less than 0.05) of small coronary arteries (vessels greater than 100 microns in diameter) but did not produce constriction of coronary arterioles (vessels less than 100 microns in diameter). During coronary hypoperfusion, phenylephrine caused constriction (p less than 0.05) of both small coronary arteries and arterioles, 6 +/- 2% and 11 +/- 3% decreases in diameter, respectively. BHT-933 did not cause significant changes in microvascular diameters under control conditions but substantially and selectively decreased arteriolar diameters during hypoperfusion (24 +/- 6% decrease in diameter, p less than 0.05).

CONCLUSIONS

In the intact, autoregulating coronary circulation, coronary arterioles escape from the effects of adrenergic activation but coronary arteries do not; rather, they can exhibit alpha 1-adrenergic coronary vasoconstriction. During coronary hypoperfusion, when autoregulatory adjustments are blunted, coronary arterioles are sensitive to both alpha 1- and alpha 2-adrenergic agonists, demonstrating significant constrictor responses. Also, the magnitude of coronary alpha 2-adrenergic arteriolar constriction (24% decrease in diameter) is significantly greater than that of alpha 2-adrenergic constriction (11% decrease in diameter) (p less than 0.05). Thus, alpha 1- and alpha 2-adrenergic activation produce different constrictor effects in the coronary microcirculation under baseline conditions when autoregulatory adjustments are intact and during coronary hypoperfusion when autoregulation is blunted. The data suggest that alpha 2-adrenergic receptors are preferentially distributed in arterioles, whereas alpha 1-adrenergic receptors are located throughout the coronary microcirculation. Importantly, the data also suggest that intrinsic autoregulatory adjustments in tone (i.e., autoregulatory escape) can override either alpha 1- or alpha 2-adrenergic constriction in coronary arterioles.

Alpha- and beta-adrenergic control of large coronary arteries in conscious calves

Large and small coronary arteries are subject to control by alpha- and beta-adrenergic mechanisms. However, controversy exists as to the distribution and physiological effects of alpha- and beta-adrenergic receptor subtypes in large coronary arteries. Studies in our laboratory have addressed these questions in conscious calves, chronically instrumented to measure large coronary artery diameter and coronary blood flow. Additionally, adrenergic receptor subtype distribution was determined using ligand binding assays in membrane preparations isolated from large coronary arteries of calves. Physiological results demonstrate, in contrast to the results of most previous studies, that both alpha 1- and alpha 2-adrenergic receptors elicit constriction of the large coronary artery. Studies with ganglionic blockade indicate that the constriction was unaltered by autonomic reflexes or presynaptic release of neurotransmitters. Selective beta-adrenergic receptor activation demonstrated that both beta 1- and beta 2-adrenergic receptors elicit dilation of large coronary arteries, and that the vasodilation was direct, i.e., it was not mediated by increases in coronary blood flow. Biochemical characterization of adrenergic subtype density indicated the presence of both alpha 1- and alpha 2-, as well as beta 1- and beta 2-adrenergic receptor subtypes. Thus, both biochemical and physiological data support the concept that large coronary arteries are regulated by both alpha 1- and alpha 2-, as well as beta 1- and beta 2-adrenergic receptor subtypes.

Isolated bovine cerebral arteries from rostral and caudal regions: distinct responses to adrenoceptor agonists

Responsiveness to norepinephrine and related agents was compared in isolated bovine anterior cerebral (ACA). middle cerebral (MCA), intracranial internal carotid (ICA), posterior communicating (PCOM), posterior cerebral (PCA) and basilar arteries (BA). Norepinephrine contracted the strips from ACA, MCA and ICA, but relaxed those from PCOM, PCA and BA. In the presence of propranolol, the amine-induced contractions tended to be potentiated in ACA, MCA and ICA, and the relaxations in PCOM, PCA and BA were reversed to contractions. The maximum contractions induced by norepinephrine in ICA and ACA treated with propranolol were significantly greater than those in PCA and BA, but the EC50 values did not differ among arteries. In ACA and MCA, the contractions induced by phenylephrine were greater than those induced by clonidine. The contractions induced by norepinephrine and phenylephrine were attenuated by prazosin but not influenced by yohimbine in ACA and MCA treated with propranolol. These findings indicate that the responses to norepinephrine evidently differ in bovine cerebral arteries of rostral (ACA, MCA and ICA) and caudal regions (PCOM, PCA and BA), possibly due to different functioning of alpha/beta receptors. The amine-induced contraction predominantly seen in the rostral arteries appears to be associated with activation of the alpha 1 adrenoceptor subtype.

A differential effect of yohimbine on adrenal and neuronal catecholamine release during bilateral carotid occlusion in the dog

This study reports on the effects of yohimbine and clonidine on the release of adrenal and renal catecholamines (epinephrine, E; norepinephrine, NE; and dopamine, DA) in response to bilateral carotid occlusion (BCO, 3 min) in vagotomized dogs anesthetized with sodium pentobarbital. The model used allowed us to simultaneously compare adrenal catecholamine secretion with neuronal NE release in the kidney. In control dogs, the net output (ng/min/g tissue) of adrenal E (70.5 +/- 19.7), NE (22.2 +/- 5.9) and DA (2.6 +/- 0.8) increased markedly (P less than 0.01) during BCO to a maximum level of 265.1 +/- 87.9, 97.4 +/- 30.6 and 10.5 +/- 3.2, respectively. Similarly, the net output (ng/min/g tissue) of renal NE (0.66 +/- 0.06) and DA (0.09 +/- 0.02) increased significantly (P less than 0.01) to 1.00 +/- 0.11 and 0.15 +/- 0.04, respectively. Aortic systolic pressure (mm Hg) (140.8 +/- 8.0) and heart rate (beats/min) (162.7 +/- 5.1) also increased (P less than 0.01) to 212.5 +/- 19.3 and 179.5 +/- 5.4, respectively. In dogs treated with yohimbine (0.3 mg/kg, i.v.), the net increase in adrenal catecholamine output was diminished by approximately 47% (P less than 0.05). In contrast, the net increase in renal NE output was potentiated by 41% (P less than 0.05). The net increase in heart rate was also enhanced significantly (P less than 0.01) in the presence of yohimbine. In dogs receiving clonidine (15 micrograms/kg, i.v.) the increases in net output of both adrenal and renal catecholamine were abolished. Similarly, pressor and heart rate responses were abolished in the presence of clonidine. The results indicate that yohimbine exerted a differential effect on renal sympathetic nerves (increase) and adrenal medullae (decrease) in modulating catecholamine release in response to BCO, while clonidine abolished both neural NE release and adrenal catecholamine secretion. This study suggests that a presynaptic alpha 2-adrenoceptor-mediated mechanism, the blockade of which enhances neural NE release at peripheral sympathetic nerve terminals in many tissues, may not be involved in the modulation of adrenal catecholamine secretion during BCO.

Effects of perivascular nerve stimulation on the flow rate in isolated epicardial coronary arteries of pigs

The effect of perivascular nerve stimulation on coronary contraction was examined by perfusing the isolated epicardial coronary artery of pigs. Coronary flow decreased in a frequency-dependent manner after electrical stimulation, with a maximum percent flow reduction of 23.8 +/- 1.1% (n = 10) at 20 Hz. The reduction in flow rate was not inhibited by phentolamine nor propranolol but was inhibited by atropine. Neostigmine enhanced the flow reduction induced by nerve stimulation. Acetylcholine reduced the flow rate dose dependently but norepinephrine showed no effect. We conclude that perivascular nerve stimulation of the epicardial coronary artery of pigs causes a modest flow reduction through activation of a cholinergic mechanism.

Role of alpha-adrenoceptors in constrictor responses of rat, guinea-pig and rabbit small arteries to neural activation

1. We have investigated the adrenoceptors mediating the force and electrical responses of rat mesenteric small arteries (i.d. 100-300 microns). Some mechanical experiments were also performed using guinea-pig and rabbit mesenteric small arteries. 2. Vessels were mounted on an isometric myograph and stimulated either with short (3 s) trains of electric field stimuli (ca. 0.2 ms pulse width) at 25 Hz (nerve stimulation) or with 10 microM-exogenous noradrenaline. 3. Nerve stimulation caused a force response equal to ca. 40% of the response to exogenous noradrenaline and, in the rat vessels, excitatory junction potentials (EJPs), which normally summated to give a depolarization of ca. 10 mV (although action potentials were sometimes seen). 4. Almost complete and reversible inhibition of the force responses of all vessels to both exogenous noradrenaline and to nerve stimulation was obtained using prazosin (0.1 microM) or phentolamine (1 microM). 5. Irreversible blockade of alpha 2-receptors enhanced the force response of all vessels to nerve stimulation by ca. 50%, but did not affect the force response of rat and guinea-pig vessels to exogenous noradrenaline. In the rabbit vessels this force response was abolished by alpha 2-blockade. 6. Following alpha 2-blockade, in the rat vessels the alpha-antagonists prazosin (0.1 microM), phentolamine (0.1 microM), phenoxybenzamine (0.01 microM) and benextramine (10 microM) all totally abolished the force response to exogenous noradrenaline, and inhibited the response to nerve stimulation by at least 80%. Similar effects of phentolamine were seen in the guinea-pig and, for the response to nerve stimulation, in the rabbit vessels. 7. In the rat vessels, alpha-adrenoceptor antagonists did not affect the EJPs, but did inhibit the small depolarization which resulted from several seconds of nerve stimulation. The ATP analogue alpha, beta-methylene-ATP (3 microM) abolished the EJPs, but only slightly reduced the force responses. 8. The results suggest that the force response to nerve stimulation of the rat mesenteric small arteries is mediated primarily through alpha-adrenoceptors, but also to a small degree through non-alpha-adrenoceptors, possibly ATP receptors.

New alpha 1-adrenergic receptor antagonists for the treatment of hypertension: role of vascular alpha receptors in the control of peripheral resistance

The pharmacology, clinical efficacy and safety of new alpha-adrenergic receptor antagonists for the treatment of hypertension was reviewed (Table XIV). Although all these agents block alpha 1 receptors, some of them have additional effects on histamine, serotonin, dopamine, and alpha 2 receptors. These other actions account for the differences in the side effect profiles observed, i.e., increased incidence of central nervous system side effects found with indoramin, ketanserin, and urapidil, as well as for some additional beneficial effects of ketanserin (i.e., antiplatelet aggregation activity). The magnitude of BP reduction observed with antagonists of alpha 1-adrenergic receptors is modest. In most studies, the degree of BP reduction is comparable to that of prazosin, but less than that achieved with thiazide diuretics, beta-receptor antagonists, or methyldopa. Studies on the comparative efficacy and safety of new alpha 1 antagonists with converting enzyme inhibitors or calcium-channel blockers are not available. In general, alpha 1 antagonists produce greater reductions in standing than in supine BP, an effect due to the venodilatory action of these drugs. New alpha 1 antagonists appear to have equal efficacy in black and white hypertensive individuals. Their comparative efficacy and safety in young vs elderly hypertensive individuals requires further investigation. No information about the possible development of tolerance during treatment with new alpha 1 blockers was encountered. The effects of alpha 1 antagonists on HR are variable and depend on how long after the oral dose the measurements were obtained. In most studies, no significant HR changes are noticed for readings obtained 24 hours post dose; whereas tachycardia has been observed at the time of peak hypotension. Since alpha 1 antagonist-induced tachycardia is most likely of reflex nature, i.e., mediated to an increase in sympathetic activity, the increased HR may be associated with increases in myocardial contractility and in myocardial oxygen consumption. Consequently, a 24-hour HR monitoring during treatment with alpha 1 antagonists should be required for evaluation of new agents. The hemodynamic, humoral, and hormonal effects of the newer alpha 1-receptor antagonists are comparable to those of prazosin. The most consistent finding is a reduction in total peripheral resistance associated with either no change or with only small increases in cardiac index. These agents have been shown either not to change or to increase renal blood flow.(ABSTRACT TRUNCATED AT 400 WORDS)

Studies on the anti-vasoconstrictor activity of BRL 34915 in spontaneously hypertensive rats; a comparison with nifedipine

1. The blood pressure lowering and anti-vasoconstrictor effects of BRL 34915 and nifedipine were compared in female spontaneously hypertensive rats (SHR). 2. In conscious SHR, intravenous injection of BRL 34915 (0.1, 0.3 mg kg-1) produced rapid, dose-related falls in mean arterial pressure of greater than 3 h duration. Nifedipine, at the same intravenous dose levels, also evoked rapid anti-hypertensive effects, though these responses were of lesser magnitude and duration than those observed for BRL 34915. 3. In anaesthetized, ganglion-blocked SHR, BRL 34915 (0.1, 0.3 mg kg-1 i.v.) dose-dependently antagonized the pressor responses to incremental intravenous infusions of noradrenaline (3.8-28.5 ng min-1) or phenylephrine (120-907 ng min-1) but did not inhibit pressor responses to incremental infusions of methoxamine (0.47-3.63 micrograms min-1), angiotensin II (7.0-52.9 ng min-1) or vasopressin (0.27-2.0 mu min-1). 4. In anaesthetized, ganglion-blocked SHR, nifedipine (0.1, 0.3 mgkg-1 i.v.) antagonized the pressor responses to each of the infused vasoconstrictor agents, being most effective against responses to noradrenaline or angiotensin II. 5. In pithed SHR, both BRL 34915 and nifedipine (each at 0.3 mg kg-1 i.v.) reduced the basal blood pressure level and produced marked inhibition of frequency-dependent pressor responses evoked by electrical stimulation of the spinal cord sympathetic outflow (0.25-4.0 Hz). Restoration of the basal diastolic blood pressure to within the control range, using a continuous intravenous infusion of vasopressin (0.98 mu min-1), prevented the inhibitory effect of BRL 34915. In the case of nifedipine, however, even raising the basal blood pressure to a level exceeding that recorded in control rats (with vasopressin, 2.0 mu min-1), did not reverse the inhibitory effect of the drug on frequency-dependent pressor responses. 6. It is concluded that the anti-hypertensive properties of BRL 34915 in SHR are probably unrelated to an anti-vasoconstrictor action. In contrast, it is suggested that the broadly-based anti-vasoconstrictor properties of nifedipine may contribute substantially to the anti-hypertensive properties of this drug.

Different sensitivity of blocking effects of alpha-adrenoceptor blocking agents on vascular responses to intraluminal norepinephrine and periarterial stimulation in isolated dog arteries

The stainless steel cannula inserting method was used to examine the effects of periarterial electrical nerve stimulation and intraluminal norepinephrine in the isolated and perfused mesenteric artery of the dog. The optimum conditions for inducing an increase in perfusion pressure over 50 mmHg to periarterial electrical stimulation were 3-5 msec duration, 40-50 volts and 10-20 Hz. After the treatment with phentolamine or prazosin, the vasoconstrictor response to norepinephrine was readily inhibited completely at a relative small dose of 0.3-3 micrograms. Periarterial stimulation-induced vasoconstriction was also significantly suppressed by a relatively large dose of 100 micrograms of phentolamine or prazosin in concentrations 30 to 100 times larger than that required for blocking the norepinephrine-induced constriction. Yohimbine in relatively small doses potentiated the stimulation-induced vasoconstriction, but rather suppressed it in a large dose. The constrictor response to periarterial stimulation was significantly suppressed by 1 and 10 micrograms of intraluminal tetrodotoxin. It is concluded that periarterial electrical stimulation in the cannula inserting method is useful for studying autonomic pharmacology and physiology in vasculature with due regard to the characteristics.

Heterogeneity in the response to vasoconstrictors of isolated dog proximal and distal middle cerebral arteries

Responses to vasoconstrictor agents were compared in helical strips of proximal and distal middle cerebral arteries isolated from the same dogs. Contractions induced by 30 mM K+ per cross-sectional areas in the large and small artery strips did not differ significantly so that contractions induced by the agents could be compared relative to those induced by K+. The contractions induced by serotonin were greater in the proximal than in the distal arteries. Histamine produced a greater contraction in the proximal arteries under resting conditions. Distal arteries partially precontracted with prostaglandin F2 alpha responded to histamine with relaxations. The histamine-induced contraction was attenuated by chlorpheniramine, whereas the relaxation was suppressed by cimetidine. Prostaglandin F2 alpha or carbocyclic thromboxane A2 produced similar magnitudes of contractions in the proximal and distal arteries. Thrombin contracted the distal arteries to a greater extent. Contractions induced by norepinephrine or bradykinin did not differ in the proximal and distal arteries. It may be concluded that the ratio of histaminergic H1 and H2 receptor functions decreases peripherally along middle cerebral arteries, whereas the contractile response to bradykinin was not influenced in these arteries of a relatively short distance. The functions of receptor sites for serotonin and thrombin appear to be quite different in large and small cerebral arteries of such a short distance, but alpha-adrenoceptor functions seem to be identical.

Neuroeffector actions of prostaglandin D2 on isolated dog mesenteric arteries

Treatment with prostaglandin (PG) D2 in concentrations (10(-8) to 10(-7) M) insufficient to alter the basal tone potentiated the contractile response of helical strips of dog mesenteric arteries to transmural electrical stimulation but did not influence the response to norepinephrine. The potentiating effect of PGD2 was not prevented by treatment with diphloretin phosphate, a PG antagonist, whereas contractions of dog cerebral arteries induced by PGD2 were suppressed. The 3H-overflow evoked by transmural stimulation in superfused mesenteric arterial strips previously soaked in 3H-norepinephrine containing media was significantly increased in PGD2. It is concluded that PGD2 increases the stimulation-evoked release of norepinephrine from adrenergic nerves innervating the arterial wall. PGD2 appears to act differently on receptive sites responsible for increasing the release of norepinephrine and for producing arterial contraction.