Effects of beta 1- and beta 2-adrenoceptor stimulation on hemodynamics in the anesthetized rat

β₂-Adrenoceptor Blockade Deteriorates Systemic Anaphylaxis by Enhancing Hyperpermeability in Anesthetized Mice

Purpose

Patients treated with propranolol, a nonselective β-adrenoceptor antagonist, develop severe anaphylaxis, but the mechanism remains unknown. We determined effects of β₁- and β₂-adrenoceptor antagonists on the anaphylaxis-induced increase in vascular permeability in mice.

Methods

In anesthetized ovalbumin-sensitized C57BL mice, mean arterial blood pressure (MBP) was measured, and Evans blue dye extravasation and hematocrit (Hct) were assessed at 20 minutes after antigen injection. The following pretreatment groups (n=7/group) were studied: (1) sensitized control (non-pretreatment), (2) propranolol, (3) the selective β₂-adrenoceptor antagonist ICI 118,551, (4) the selective β₁-adrenoceptor antagonist atenolol, (5) adrenalectomy, (6) the selective β₂-adrenoceptor agonist terbutaline, and (7) non-sensitized groups.

Results

The antigen injection decreased MBP, and increased Hct and vascular permeability in the kidney, lung, mesentery, and intestine, but not in the liver or spleen. Pretreatment with ICI 118,551, propranolol and adrenalectomy, but not atenolol, reduced the survival rate and augmented the increases in Hct and vascular permeability in the kidney, intestine, and lung as compared with the sensitized control group. Pretreatment with terbutaline abolished the antigen-induced alterations. Plasma epinephrine levels were increased significantly in the sensitize control mice.

Conclusions

Blockade of β₂-adrenoceptor can deteriorate systemic anaphylaxis by augmenting hyperpermeability-induced increase in plasma extravasation by inhibiting beneficial effects of epinephrine released from the adrenal glands in anesthetized mice.

Beta-adrenergic receptors are expressed across diverse cancers

Based largely on retrospective analyses and a handful of prospective case reports, pharmacological inhibition of the beta adrenergic receptors using beta blockers has shown clinical anti-cancer efficacy in reproductive cancers, as well as angiosarcoma and multiple myeloma. Because of the potential promise of beta blockers as an adjunct to standard anti-cancer therapy, it is imperative to identify other tumor types expressing beta adrenergic (β-AR) receptors so future preclinical and clinical studies can be directed at the most promising tumor targets. We performed immunohistochemical detection of β1-AR, β2-AR, and β3-AR across 29 of the most common human cancer types (389 tissues total) and 19 matching non-diseased controls (100 tissues total). Our analysis revealed all three β-AR receptors were expressed most strongly in melanoma relative to other cancer types. Other malignancies that revealed relatively higher levels of β-AR receptors were esophagus, pancreas, kidney, and lung cancers. Moreover, particular β-AR receptors exhibited significant overexpression in tumor tissue relative to their matching normal tissue in urogenital/reproductive malignancies including breast, endometrium, ovarian, and urothelial cancer, as well as colon, lung, and thyroid cancer. This study identifies several cancer types expressing the β-AR receptors which should be evaluated in future studies for susceptibility to beta blockade.

Role of β3-adrenoceptors in regulation of retinal vascular tone in rats

The aim of this study was to determine the role of β(3)-adrenoceptors in the action of endogenous catecholamines (adrenaline and noradrenaline) on rat retinal arterioles in vivo. Using an original high-resolution digital fundus camera, the rat ocular fundus images were captured. The diameter of retinal arterioles contained in the images was measured. Both systemic blood pressure and heart rate were recorded continuously. Adrenaline (0.3-5.0 μg/kg/min, i.v.) increased the diameter of retinal arterioles, mean blood pressure and heart rate in a dose-dependent manner. Under blockade of β(1)/β(2)-adrenoceptors with propranolol (2 mg/kg, i.v. bolus followed by 100 μg/kg/min infusion), adrenaline decreased the diameter of retinal arterioles. Similar observation was made under treatment with the β(3)-adrenoceptor antagonist L-748337 (50 μg/kg, i.v.). The pressor response to adrenaline was enhanced by propranolol, but not by L-748337. The positive chronotropic action of adrenaline was markedly prevented by propranolol, whereas it was unaffected by L-748337. Noradrenaline (0.03-1.0 μg/kg/min, i.v.) decreased the diameter of retinal arterioles but increased the mean blood pressure and heart rate. The effects of noradrenaline on retinal arteriolar diameter and blood pressure were unaffected by propranolol or L-748337. The positive chronotropic action of noradrenaline was almost completely abolished by propranolol. These results suggest that β(3)-adrenoceptors play crucial roles in vasodilator responses to adrenaline of retinal arterioles but have minor or no effect on noradrenaline-induced responses. The results also indicate that the functional role of β(3)-adrenoceptors may be more important than that in peripheral resistance vessels.

Central and peripheral mechanisms underlying gastric distention inhibitory reflex responses in hypercapnic-acidotic rats

We have observed that in chloralose-anesthetized animals, gastric distension (GD) typically increases blood pressure (BP) under normoxic normocapnic conditions. However, we recently noted repeatable decreases in BP and heart rate (HR) in hypercapnic-acidotic rats in response to GD. The neural pathways, central processing, and autonomic effector mechanisms involved in this cardiovascular reflex response are unknown. We hypothesized that GD-induced decrease in BP and HR reflex responses are mediated during both withdrawal of sympathetic tone and increased parasympathetic activity, involving the rostral (rVLM) and caudal ventrolateral medulla (cVLM) and the nucleus ambiguus (NA). Rats anesthetized with ketamine and xylazine or α-chloralose were ventilated and monitored for HR and BP changes. The extent of cardiovascular inhibition was related to the extent of hypercapnia and acidosis. Repeated GD with both anesthetics induced consistent falls in BP and HR. The hemodynamic inhibitory response was reduced after blockade of the celiac ganglia or the intraabdominal vagal nerves with lidocaine, suggesting that the decreased BP and HR responses were mediated by both sympathetic and parasympathetic afferents. Blockade of the NA decreased the bradycardia response. Microinjection of kainic acid into the cVLM reduced the inhibitory BP response, whereas depolarization blockade of the rVLM decreased both BP and HR inhibitory responses. Blockade of GABA(A) receptors in the rVLM also reduced the BP and HR reflex responses. Atropine methyl bromide completely blocked the reflex bradycardia, and atenolol blocked the negative chronotropic response. Finally, α(1)-adrenergic blockade with prazosin reversed the depressor. Thus, in the setting of hypercapnic-acidosis, a sympathoinhibitory cardiovascular response is mediated, in part, by splanchnic nerves and is processed through the rVLM and cVLM. Additionally, a vagal excitatory reflex, which involves the NA, facilitates the GD-induced decreases in BP and HR responses. Efferent chronotropic responses involve both increased parasympathetic and reduced sympathetic activity, whereas the decrease in BP is mediated by reduced α-adrenergic tone.

Effects of β-adrenoceptor antagonists on anaphylactic hypotension in conscious rats

Anaphylactic shock is sometimes fatal or resistant to therapy in patients treated with propranolol, a nonselective β-adrenoceptor antagonist, against cardiovascular diseases. However, it remains unknown which subtype of β-adrenoceptors, β(1)- or β(2)-adrenoceptor, is primarily responsible for the detrimental effects of propranolol on anaphylactic hypotension. Effects of β(1)- and β(2)-adrenoceptor antagonists were therefore determined on the survival rate and systemic hypotension in conscious Sprague-Dawley rats that suffered from anaphylactic shock. Mean arterial pressure and portal venous pressure were simultaneously measured. The control rats showed a decrease in mean arterial pressure and an increase in portal venous pressure, but did not die within 48h after an injection of ovalbumin antigen. The survival rate of the rats pretreated with propranolol (1mg/kg; n=7), the selective β(2)-adrenoceptor antagonist ICI 118,551 (0.5mg/kg; n=7), or adrenalectomy (n=7) was significantly smaller than that with the selective β(1)-adrenoceptor antagonist atenolol (2mg/kg; n=7). However, the changes in mean arterial pressure and portal venous pressure were similar for 10min after antigen among any groups, although propranolol and atenolol attenuated the antigen-induced increase in heart rate. Furthermore, bolus injections of epinephrine (3μg/kg) at 3 and 5min after antigen prevented the death of the atenolol-pretreated rats, but only marginally prolonged the survival rates for the ICI 118,551- or propranolol-pretreated and adrenalectomized rats. In conclusion, in rat anaphylactic shock, inhibition of β(2)-adrenoceptor causes more detrimental effects than that of the β(1)-adrenoceptor. These β-adrenoceptor antagonists may exert detrimental effects on rat systemic anaphylaxis via inhibiting beneficial actions of catecholamines endogenously released from the adrenal gland.

The effect of dobutamine infusion on fractional diaphragm thickening and diaphragm blood flow during fatigue

BACKGROUND

Diaphragm fatigue (DF) has been implicated in respiratory failure in diseases that increase inspiratory resistance loading (IRL) and may complicate weaning of patients from mechanical ventilation.

OBJECTIVE

The purpose of this study was to examine the effects of dobutamine administration (10 micro g/kg/min) on DF and to identify the mechanisms by which dobutamine augments diaphragm shortening and diaphragm blood flow (DBF) during fatigue with a rat model.

METHODS

The study had an experimental design with 3 groups of Sprague-Dawley rats (n = 38) with 4 experimental periods: period 1, control; period 2, application of IRL; period 3, treatment; and period 4, recovery. DF was produced via IRL. During period 3 treatment, normal saline solution was infused in group I, dobutamine in group II, and dobutamine plus butoxamine hydrochloride in group III. The percent change in fractional diaphragm thickness (FDT) during inspiration reflected diaphragm shortening. DBF and aortic blood flow were determined with fluorescent microspheres. Diaphragm vascular resistance and systemic vascular resistance were calculated on the basis of Poiseuille's equation.

RESULTS

Results indicated infusion of dobutamine increased FDT (P =.01) and DBF (P =.009) with respect to fatigue levels. The effects of dobutamine on FDT and DBF were attenuated with infusion of butoxamine hydrochloride (a beta-2 adrenoceptor antagonist) with respect to fatigue.

CONCLUSION

Administration of dobutamine at a rate similar to that used clinically increased diaphragm muscle contractility (FDT) and DBF in diaphragms fatigued by IRL. The dobutamine effect on FDT may be the result of restoration of the balance between diaphragm energy consumption and energy delivery to the diaphragm by increasing DBF. Butoxamine hydrochloride attenuated the dobutamine-induced increase in DBF, indicating dobutamine produced vasodilatation via beta-2 adrenoceptors. Thus, the administration of intravenous dobutamine may be a useful adjunct in the treatment of DF.

Differential distribution of beta-adrenergic receptor subtypes in blood vessels of knockout mice lacking beta(1)- or beta(2)-adrenergic receptors

beta-Adrenergic receptors (beta-AR) are essential regulators of cardiovascular homeostasis. In addition to their prominent function in the heart, beta-AR are located on vascular smooth muscle cells, where they mediate vasodilating effects of endogenous catecholamines. In this study, we have investigated in an isometric myograph different types of blood vessels from mice lacking beta(1)- and/or beta(2)-adrenergic receptor subtypes (beta(1)-KO, beta(2)-KO, beta(1)beta(2)-KO). In wild-type mice, isoproterenol induced relaxation of segments from thoracic aorta, carotid, femoral and pulmonary arteries, and portal vein. The relaxant effect of beta-receptor stimulation was absent in femoral and pulmonary arteries from beta(1)-KO mice. In aortic and carotid arteries and in portal veins, the vasodilating effect of isoproterenol was reduced in mice lacking beta(1)- or beta(2)-receptors. However, in these vessels the vasodilating effect was only abolished in double KO mice lacking both beta(1)- and beta(2)-receptors. Vessel relaxation induced by forskolin did not differ between wild-type and KO mice. Similar contributions of beta(1)- and beta(2)-receptors to isoproterenol-induced vasorelaxation were found when vessels from KO mice were compared with wild-type arteries in the presence of subtype-selective beta-receptor antagonists. These studies demonstrate that beta(1)-adrenergic receptors play a dominant role in the murine vascular system to mediate vasodilation. Surprisingly, beta(2)-receptors contribute to adrenergic vasodilation only in a few major blood vessels, suggesting that differential distribution of beta-adrenergic receptor subtypes may play an important role in redirection of tissue perfusion.

Autonomic control of the venous system in health and disease: effects of drugs

The venous system contains approximately 70% of the blood volume. The sympathetic nervous system is by far the most important vasopressor system in the control of venous capacitance. The baroreflex system responds to acute hypotension by concurrently increasing sympathetic tone to resistance, as well as capacitance vessels, to increase blood pressure and venous return, respectively. Studies in experimental animals have shown that interference of sympathetic activity by an alpha1- or alpha2-adrenoceptor antagonist or a ganglionic blocker reduces mean circulatory filling pressure and venous resistance and increases unstressed volume. An alpha1- or alpha2-adrenoceptor agonist, on the other hand, increases mean circulatory filling pressure and venous resistance and reduces unstressed volume. In humans, drugs that interfere with sympathetic tone can cause the pooling of blood in limb as well as splanchnic veins; the reduction of cardiac output; and orthostatic intolerance. Other perturbations that can cause postural hypotension include autonomic failure, as in dysautonomia, diabetes mellitus, and vasovagal syncope; increased venous compliance, as in hemodialysis; and reduced blood volume, as with space flight and prolonged bed rest. Several alpha-adrenoceptor agonists are used to increase venous return in orthostatic intolerance; however, there is insufficient data to show that these drugs are more efficacious than placebo. Clearly, more basic science and clinical studies are needed to increase our knowledge and understanding of the venous system.

Endothelium-derived nitric oxide partially mediates salbutamol-induced vasodilatations

This study examined the ability of salbutamol (selective beta 2-adrenoceptor agonist) to cause endothelium-dependent relaxation in rat aortic rings and depressor response in conscious rats. Salbutamol (0.01-100 microM) concentration dependently relaxed preconstricted aortic rings. The relaxant response was partially attenuated by either mechanical removal of the endothelium or treatment with NG-nitro-L-arginine methyl ester (L-NAME, 100 microM). In conscious rats, either i.v. infused phenylephrine (5 micrograms/kg per min) or i.v. bolus injected L-NAME (12.8 mg/kg), but not the vehicle, caused similar sustained increases in mean arterial pressure (MAP). I.v. infused salbutamol (2-128 micrograms/kg per min, each dose for 5 min) dose dependently decreased MAP in vehicle-treated rats; the depressor responses were potentiated by hypertension induced by phenylephrine. In contrast, the magnitudes of the depressor response to salbutamol in L-NAME-treated rats were less than those in rats pretreated with phenylephrine or the vehicle. I.v. bolus injections of salbutamol (0.25-16 micrograms/kg) also caused dose-dependent and transient decreases in MAP in vehicle-treated rats. The magnitude but not the duration of the depressor response to salbutamol was less in rats treated with L-NAME, compared to those in rats given phenylephrine or the vehicle. These results suggest that endothelium-derived nitric oxide is partially involved in beta 2-adrenoceptor-mediated vasodilatation.

Role of the baroreflex in beta 2-sympathomimetic induced tachycardia in male rats

The aim of the study was to determine whether tachycardia which is associated with the use of beta 2-sympathomimetic tocolytic agents is caused by baroreflex activation or by direct stimulation of cardiac beta-adrenoceptors. In conscious male rats, tachycardiac responses following intravenous injection of hexoprenaline, ritodrine and fenoterol were compared following (i) bilateral sinoaortic denervation (SAD) or (ii) sham-operation, and (iii) ganglionic-blockade using hexamethonium. Dose-ranges were chosen to result in similar reductions in diastolic blood pressure (DAP). Furthermore, following ganglionic blockade, the relative contribution of beta 1-adrenoceptor stimulation was assessed using the selective beta 2-receptor antagonist ICI 118,551. In intact rats, increases in HR induced by all beta-adrenoceptor agonist were comparable. In SAD and ganglion-blocked rats, DAP fell more pronounced at even lower doses. The corresponding increases in HR were approximately 3 times smaller than in intact rats but not different between agents. During ganglionic blockade ICI 118,551 significantly inhibited HR responses to fenoterol and hexoprenaline but not ritodrine. The conclusion is that in intact male rats, baroreflex activation is the major determinant of tachycardia following injection of ritodrine, fenoterol or hexoprenaline. Increasing the beta 2-selectivity of these drugs will not limit the tachycardic effects.

Soleus muscle alterations in spontaneously hypertensive rats are not dependent on activation of beta 2-adrenergic receptors

The effects of selective beta 2-adrenergic blockade with ICI 118,551 on the histochemical, biochemical, and contractile properties of slow-twitch soleus muscle from spontaneously hypertensive (SHR) and normotensive (WKY) rats were examined from birth to 8-10 weeks of age. Chronic treatment of normotensive rats with ICI 118,551 caused an impairment in the differentiation of slow type fibers during development but failed to alter the fiber type distribution of hypertensive rats. beta 2-Adrenoreceptor blockade was ineffective in reducing the enhanced glycolytic and oxidative capacities of soleus in hypertensive rats. The suggestion can be made that beta 2-adrenoreceptor activation is not responsible, at least directly, for the histochemical and biochemical alterations of slow muscle from hypertensive rats.

Sepsis-induced insulin resistance in rats is mediated by a beta-adrenergic mechanism

Bacterial infection decreases insulin-mediated glucose uptake (IMGU) by skeletal muscle and produces whole body insulin resistance. Because circulating catecholamines are elevated by the septic insult, the present study was performed to determine the potential role of the beta-adrenergic system in eliciting these changes. Before induction of sepsis, an infusion containing saline, propranolol, or atenolol was started and continued throughout the experimental protocol. Sepsis increased the basal rate of glucose production and utilization and impaired IMGU by peripheral tissues. The peripheral insulin resistance in septic rats was manifested by an increase in the dose producing 50% of maximal response and a decrease in the maximal responsiveness. Infusion of propranolol, a nonselective beta-adrenergic antagonist, attenuated the sepsis-induced increase in basal glucose turnover by 70% and completely prevented the decrease in IMGU by the whole body. In contrast, atenolol, a selective beta 1-antagonist, did not alter the glucose metabolic response to infection. Under basal conditions, propranolol prevented or attenuated the increase in glucose uptake by the gastrocnemius, diaphragm, skin, liver, lung, spleen, and ileum normally observed in septic rats. In addition, propranolol prevented the decrease in IMGU by various muscles and skin in septic animals. These results suggest that adrenergic stimulation, probably mediated by a beta 2-adrenergic mechanism, is partially responsible for the sepsis-induced increases in basal whole body glucose turnover and plays a prominent role in the development of peripheral insulin resistance in this condition.