Effects Of Alpha-adrenergic Agonists Research Articles

Effects of alpha-adrenergic agonists on pain modulation in diffuse noxious inhibitory control.

Diffuse noxious inhibitory control (DNIC) is thought to be mediated by neural networks in supraspinal brain structures. The descending antinociceptive system (DAS) is an important component of the DNIC neural network, but the precise structure of the neural network and the related neurotransmitters have not been examined. The study was designed to examine whether systemic administration of the adrenergic agonists dexmedetomidine (DEX) and phenylephrine (PE) influences DNIC in the rat. Changes in the C-fiber reflex evoked by electromyographic activity were recorded following noxious tail immersion in hot water. Inhibition of the C-fiber reflex by the conditioning stimuli was reduced from 77.1 +/- 22.6% to 26.6 +/- 38.2% with continuous administration of DEX, and restored to 58.3 +/- 29.2% by intramuscular injection of atipamezole hydrochloride(APZ), a selective alpha 2-adrenoceptor antagonist. Inhibition of the C-fiber reflex was reduced from 75.6 +/- 25.8% to 22.7 +/- 38.9% with continuous administration of PE, and restored to 84.9 +/- 9.7% by intramuscular injection of phentolamine mesylate (PT), an alpha-adrenoceptor antagonist. The results show that clinical doses of DEX and PE inhibit DNIC, thereby affecting and modulating the intrinsic pain inhibition system. These findings suggest that adrenergic neurons are involved in DNIC.

Selectivity of oxymetazoline for urethral pressure vs blood pressure in the anaesthetized female rabbit.

The aim of this study was to test alpha-adrenergic reference agonists for tissue selectivity in the urethra and to pharmacologically characterize the functional alpha-adrenoceptor type of the female rabbit urethra in vivo. The effect of alpha-adrenergic agonists and antagonists on the urethral pressure was compared with that on blood pressure and heart rate measured simultaneously in the anaesthetized female rabbit. Oxymetazoline, NS-49, phenylephrine and phenylpropanolamine enhanced the urethral pressure in a dose-dependent manner. Phenylephrine and phenylpropanolamine also enhanced the blood pressure with significantly lower ED50 (dose that gives half of the maximal enhancing effect) values than for the urethral pressure. This was in contrast to oxymetazoline and NS-49. The ED50 values for oxymetazoline on urethral pressure, and systolic and diastolic blood pressure were 0.00067, 0.0030 and 0.0020 mg/kg, respectively. The ED50 values for NS-49 on urethral pressure, and systolic and diastolic blood pressure were 0.019, 0.21 and 0.18 mg/kg, respectively. Clonidine and UK 14,304 had no effect on urethral or blood pressure. The oxymetazoline-evoked increase in urethral pressure was inhibited by WB-4101 with an ID50 (dose that gives half of the inhibitory effect) significantly lower than that for rauwolscine. The results suggest that in the female rabbit in vivo activation of alpha1-adrenoceptors increased the urethral pressure. Phenylephrine and phenylpropanolamine, in contrast to oxymetazoline and NS-49, selectively enhanced blood pressure as compared with urethral pressure. Provided that the present results also have validity in humans, it would seem possible to develop urethra-selective drugs for treatment of stress incontinence with few or no cardiovascular side-effects.

Maturation of rat renal tubular response to alpha-adrenergic agonists and neuropeptide Y: a study on the regulation of Na+,K+-ATPase.

Na+,K+-ATPase in tubular cells plays a pivotal role for the regulation of renal sodium excretion. In adult rats the activity of this enzyme is inhibited by natriuretic hormones and stimulated by antinatriuretic hormones. Here we have examined the tubular response to alpha-adrenergic agonists and neuropeptide Y (NPY) in both infant and adult rats. In the adult kidney, alpha-adrenergic agonists and NPY stimulate Na+,K+-ATPase activity via Ca2+-dependent pathways. Oxymetazoline, a selective alpha-adrenergic agonist, and NPY failed to stimulate proximal tubular (PT) Na+,K+-ATPase activity in 10-d-old rats in doses of 10(-8) to 10(-5) M and 10(-8) to 10(-6) M, respectively, but when tubules were incubated simultaneously with both oxymetazoline 10(-8) M and NPY 5 x 10(-9) M, stimulation was observed in both 10- and 40-d-old rat PT. This effect was abolished by FK 506, an inhibitor of Ca2+ and calmodulin-dependent protein phosphatase 2B in both age groups. A23187, a calcium ionophore, stimulated Na+,K+-ATPase in both infant and adult PT, but 10-fold higher doses were required for the infant tubules. The effect of alpha-adrenergic agonists and NPY on free intracellular Ca2+ was studied in PT cells in primary culture. The Ca2+ response to each agent was less pronounced in infant than in adult cells. Preincubation with NPY, which increases Ca2+ influx into the cells, enhanced the response to the alpha-adrenergic agonist in both infant and adult cells. The results support the concept that the systems regulating renal tubular Na+, K+-ATPase and sodium metabolism undergo postnatal maturation.

The effects of alpha-adrenergic agonists on hyperpnea-induced airway obstruction in dogs.

Two alpha-adrenergic agonists that inhibit hyperpnea-induced airway obstruction (HIAO) in asthmatic subjects were used to examine the role of bronchial blood flow in the development of HIAO in canine periphery airways. A bronchoscope was used to record peripheral airway resistance (Rp) in anesthetized dogs before and after hyperpnea with dry air. Hyperpnea increased Rp 64 +/- 8% (mean +/- SE) above baseline. Treatment with norepinephrine (NOR) either before or at various times after hyperpnea inhibited HIAO (p < 0.01). We also found that NOR inhibited acetylcholine-induced bronchoconstriction. However, beta-adrenergic blockade with propranolol completely eliminated these effects. Thus, NOR inhibited HIAO in canine peripheral airways via the stimulation of beta-adrenergic receptors and the attenuation of airway smooth muscle contractility. In contrast, pretreatment with methoxamine (MX) decreased HIAO by approximately 25% when compared with the vehicle control, and this effect was completely eliminated by alpha-adrenergic blockade with phentolamine. Relative to NOR, MX provides weak protection against HIAO via the direct stimulation of alpha-adrenergic receptors and their subsequent effect on either mucus secretion or bronchovascular tone. We conclude that bronchial blood flow plays at best a minor role in the development of HIAO.

Interactive effects of alpha-adrenergic agonists and increased cell volume on deoxyglucose uptake and phosphorylation in isolated ventricular myocytes.

Interactive effects of alpha-adrenergic agonists and increased cell volume on deoxyglucose uptake and phosphorylation in isolated ventricular myocytes.

Lack of role of endothelium-derived relaxing factor in effects of alpha-adrenergic agonists in cutaneous veins in humans.

In some blood vessels, the alpha 2-adrenergic agonist clonidine simultaneously activates vasoconstrictive alpha-adrenoceptors on smooth muscle cells and endothelial alpha 2-adrenoceptors mediating release of endothelium-derived relaxing factor (EDRF), with the net vascular response representing a balance between these two counteracting pathways. To investigate whether clonidine's modest constrictor effect in human veins is due to simultaneous release of EDRF, the dorsal hand vein compliance technique was used to measure vascular responses in healthy volunteers. Clonidine-induced venoconstriction was not potentiated by methylene blue, an inhibitor of EDRF-mediated relaxation. After preconstriction with angiotensin II, clonidine did not cause venodilation but rather promoted further constriction, which could be reversed by the alpha 1-antagonist labetalol. However, in veins preconstricted with the full alpha 1-agonist phenylephrine, clonidine induced venodilation, suggesting that clonidine is a partial agonist at venous alpha 1-adrenoceptors. In conclusion, we found no evidence for endothelial alpha 2-adrenoceptor-mediated release of EDRF in human hand veins. The results further suggest that postjunctional alpha 1-adrenoceptors participate in clonidine-induced venoconstriction in humans.

Alpha-adrenoreceptor modulation of neurally evoked circular muscle responses of the guinea pig stomach

The effects of alpha-adrenergic agonists on transmural-evoked motor responses were investigated in guinea pig gastric corpus in vitro, using preparations stripped of mucosa and orientated to record changes in circular muscle tension. Three tetrodotoxin-sensitive components to a 10 s burst of transmural stimulation could be distinguished: an initial ‘on’ contraction, an ‘off’ contraction and a transient relaxation. The ‘on’ response was blocked by atropine (0.1 μM), while the ‘off’ response and relaxation were unaffected at this dose. A submaximal dose of acetylcholine was used to assess the sensitivity of the preparation. The alpha 1 agonist l-phenylephrine decreased the amplitude of the ‘off’ response while simultaneously increasing both the ‘on’ response and the relaxation, although the response to acetylcholine was unchanged. These effects were dose-dependent and reversed by pretreatment with prazosin. In marked contrast, the alpha 2 agonist clonidine inhibited the ‘on’ response in a dose-dependent manner without affecting the ‘off’ response, the relaxation or the response to acetylcholine. Yohimbine reversed the effect of clonidine. We conclude that the inhibitory action of alpha-agonists involves both cholinergic and non-cholinergic pathways, with alpha 2 and alpha 2 adrenoreceptors modulating different circuits within the enteric nervous system.

Characterization of renal alpha-adrenoceptor subtypes in sheep during development.

Intrarenal arterial infusion of alpha 1-adrenergic agonists decreases renal blood flow, glomerular filtration rate, and water and sodium excretion to a greater extent in fetus and newborn than in adult sheep. In vitro renal vascular effects of alpha 1- and alpha 2-adrenergic agonists are also greater in fetus than newborn or adult. The present studies were designed to examine the ontogeny of renal alpha-adrenoceptor subtypes in sheep for whom the renal effects of alpha-adrenergic agonists have been described at similar postconceptional ages. With the use of radioligand-binding techniques, specific binding of [3H]prazosin (alpha 1-adrenergic antagonist), [3H]idazoxan, and [3H]rauwolscine (alpha 2-adrenergic antagonists) was studied in fetus, lamb, and adult sheep kidneys. The specific binding of the three radioligands was greatest in fetal and least in adult kidneys. Analysis of Scatchard plots revealed a greater renal alpha 1-adrenoceptor density in fetuses than in lamb or adults. Renal alpha 2-adrenoceptor density was also greater in fetuses than in lambs. These studies suggest that the increased renal alpha 1- and alpha 2-adrenergic effects in fetal sheep are related to increased alpha-adrenoceptor density. Competition experiments and rank adrenergic antagonist potency suggested the presence of only the alpha 1b-adrenoceptor in fetal and adult sheep kidneys. The alpha 2-adrenoceptor that was found only in the fetal sheep had a low affinity to rauwolscine, which is unlike that described in most species for alpha 2-adrenoceptors.

Effects of alpha-adrenergic agonists on intracellular and intramitochondrial pH in rat proximal nephrons

Rat proximal tubular segments were used to examine alpha-adrenoceptor alterations in Na+-H+ exchange by monitoring intracellular pH (pHi) and mitochondrial matrix pH (pHm). To obtain pHi, tubules were incubated with the cell-permeant fluorescent probe, 2',7'-bis(2-carboxyethyl)-5(6) carboxyfluorescein acetoxymethyl ester in a HCO3--free Na+ buffer. The intracellular distribution of the weak acid [2-14C] 5,5-dimethyloxazolidine-2,4-dione was used to calculate pHm, using values of medium pH, pHi, cell volume, and matrix content. Several selective alpha 1- and alpha 2-adrenoceptor agonists and the endogenous mixed agonist, norepinephrine, all produced dose-related increases in pHi. With each of the agonists tested, a maximum increase in pHi was observed at 1 microM final concentrations, with peak effects occurring in less than 1 min. Pretreatment with ethylisopropyl amiloride (EIPA, 10 microM), a specific inhibitor of proximal Na+-H+ exchange, blocked receptor-stimulated increases in pHi, as well as stimulation of Na+-H+ exchange by phorbol ester (PMA, 0.1 microM). Similarly, selective alpha 1- (prazosin, 0.1 microM) and alpha 2-(idazoxan, 0.1 microM) adrenoceptor antagonists inhibited alterations in agonist-induced pHi changes, whereas PMA-stimulated increases in pHi remained unaffected. Neither alpha 1- nor alpha 2-adrenoceptor agonists produced differences in pHm. Adrenoceptor agonist-induced pHi changes were also assessed at various concentrations of external Na+ (0-135 mM). It was observed that 0 and 10 mM external Na+ concentrations significantly reduced alpha 1- and alpha 2-adrenoceptor-stimulated pHi changes; Km values for the alpha 1-agonist phenylephrine and the alpha 2-agonist B-HT 933 were 18.0 +/- 2.1 and 22.7 +/- 2.6, respectively. In summary, stimulation by alpha-adrenergic agonists may be blocked at the receptor level with specific alpha-antagonists or at the exchanger with EIPA. The increase in cellular pH induced by these agonists is sensitive to external Na+ and reflects alpha-adrenoceptor activation of the Na+-H+ exchanger.

An in Vivo Evaluation of Alpha Adrenergic Receptors in Canine Prostate

The role of alpha2 adrenergic receptors in the prostate in vivo is unknown. A model was developed to measure canine prostatic urethral pressure in vivo, and to assess the ability of various alpha adrenergic blocking agents to affect prostatic pressure. In this model, an esophogeal pressure catheter is inserted into the prostatic urethra to record prostatic urethral pressure. We investigated the effects of alpha adrenergic agonists and antagonists on prostatic pressure using this model. Dose-response curves were generated for epinephrine, and were then repeated in the presence of either prazosin (alpha1 antagonist), yohimbine (alpha2 antagonist) or SK&F-86466 (alpha2 antagonist). Prazosin was the most potent of the three drugs in competitively blocking epinephrine-induced contraction of the prostate, with an inhibition constant of 0.24µg./kg. calculated from the double reciprocal plot. Clonidine, an alpha2 adrenergic agonist, caused contraction of the prostate, which was also blocked by prazosin. Furthermore, the specific alpha2 agonist BHT-920 was totally inactive in our system. These results demonstrate that the increase in urethral pressure caused by alpha-adrenergic agonists can be blocked by alpha adrenergic antagonists. However, the specific alpha1 antagonist, prazosin, is more potent than alpha2 antagonists, and is also effective against an alpha2 agonist, clonidine. This suggests that blockade of alpha1 receptors may be a more useful strategy for causing relaxation of the prostate than blockade of alpha2 receptors.

Renal proximal tubular alpha-adrenergic receptors oppose urinary 3,5'-cyclic adenosine monophosphate response to parathyroid hormone in vivo.

In both man and rat, urinary cAMP (U cAMP) level increases in response to PTH. The increased cAMP arises largely by secretion from the proximal tubule where cAMP synthesis is stimulated by PTH through adenylate cyclase-coupled receptors. We have previously demonstrated alpha 2-adrenergic receptors which inhibit PTH-stimulated adenylate cyclase in rat renal cortex membranes in vitro. In the present study, the effects of alpha-adrenergic agonists and antagonists on the U cAMP response to PTH were investigated in anesthetized rats in vivo. Injection of PTH (15 U/kg iv) produced an increase in U cAMP from 1.7 +/- 0.3 to 7.4 +/- 0.7 nmol cAMP/mumol creatinine (n = 6), (P less than 0.001). This rise was largely due to an increase in nephrogenous cAMP which increased 10-fold. Infusion of the alpha 2-adrenergic agonist clonidine at 1 microgram/kg X min caused a decrease in the cAMP response to PTH to 3.6 +/- 0.5 nmol cAMP/mumol creatinine (n = 12) (P less than 0.001). Infusion of the alpha 2-selective catecholamine alpha-methylnorepinephrine (1 microgram/kg X min) caused a similar reduction in U cAMP response to that observed with clonidine. The alpha-adrenergic antagonist phentolamine (100 micrograms/kg X min) reversed the effects of clonidine and, when administered in the absence of alpha-agonists, caused an increased cAMP response to PTH. These results demonstrate the presence of alpha-receptors in the rat proximal convoluted tubule which oppose the actions of PTH in vivo.

Demonstration of proximal tubular alpha-adrenoceptors in vivo.

Receptors for parathyroid hormone (PTH) in the renal tubule are coupled to adenylate cyclase stimulation. PTH causes a rise in urinary cAMP by leakage from adenylate cyclase stimulation. PTH causes a rise in urinary cAMP by leakage from cells of the proximal convoluted tubule. The effects of alpha-adrenergic agonists and antagonists on the urinary cAMP response to PTH were investigated in anaesthetized rats in vivo. Injection of PTH (15 u/kg i.v.) produced an increase in urinary cAMP from 1.7 (s.e.m. = 0.3, n = 6) to 7.47 (s.e.m. = 0.7, n = 6) nmol cAMP/mumol creatinine in 30 min urine samples. Infusion of the alpha 2-adrenoceptor agonist clonidine at 1 microgram/kg per min caused a decrease in the cAMP response to PTH to 3.6 (s.e.m. = 0.5, n = 12) nmol cAMP/mumol creatinine. Infusion of the alpha 2-selective catecholamine alpha-methylnoradrenaline (1 microgram/kg per min) caused a similar reduction in urinary cAMP response to that observed with clonidine. The alpha-adrenoceptor antagonist phentolamine (100 micrograms/kg per min) enhanced the cAMP response to PTH and reversed the decreased response caused by clonidine. These results demonstrate the presence of alpha-receptors in the rat proximal convoluted tubule which oppose the actions of PTH in vivo.

The action of alpha-adrenergic agonists on plasma-membrane calcium fluxes in perfused rat liver.

The effect of alpha-adrenergic agonists on Ca2+ fluxes was examined in the perfused rat liver by using a combination of Ca2+-electrode and 45Ca2+-uptake techniques. We showed that net Ca2+ fluxes can be described by the activities of separate Ca2+-uptake and Ca2+-efflux components, and that alpha-adrenergic agonists modulate the activity of both components in a time-dependent manner. Under resting conditions, Ca2+-uptake and -efflux activities are balanced, resulting in Ca2+ cycling across the plasma membrane. The alpha-adrenergic agonists vasopressin and angiotensin, but not glucagon, stimulate the rate of both Ca2+ efflux and Ca2+ uptake. During the first 2-3 min of alpha-agonist administration the effect on the efflux component is the greater, the net effect being efflux of Ca2+ from the cell. After 3-4 min of phenylephrine treatment, net Ca2+ movements are essentially complete, however, the rate of Ca2+ cycling is significantly increased. After removal of the alpha-agonist a large stimulation of the rate of Ca2+ uptake leads to the net accumulation of Ca2+ by the cell. The potential role of these Ca2+ flux changes in the expression of alpha-adrenergic-agonist-mediated effects is discussed.

Release of a common source of intracellular Ca2+ by alpha-adrenergic agonists and dinitrophenol in rat liver slices.

The effects of alpha-adrenergic agonists on 45Ca efflux from slices of rat liver were studied and the results compared to earlier studies on the rabbit aorta. Norepinephrine and phenylephrine (PE) cause a stimulation of 45Ca efflux which was due to alpha 1-receptor activation. The mitochondrial uncoupler dinitrophenol (DNP) also stimulated 45Ca efflux while caffeine had little or no effect. PE and DNP effects were due to the release of intracellular 45Ca stores, and both agents released a similar quantity of Ca2+. 45Ca/40Ca exchange in both PE- and DNP-released sources was similar (t1/2 approximately or equal to 16 min). DNP and PE effects were not additive, and previous exposure to one agent reduces the response to the other. These results suggest that alpha 1-agonists and DNP release a common source of intracellular Ca2+ in rat liver. Since in rabbit aorta alpha-agonists primarily release Ca2+ from a nonmitochondrial source, our results suggest alpha 1-receptors act via the generation of a Ca2+-releasing substance which subsequently mobilizes Ca2+ from different organelles in different tissues.

Inhibition of dibutyryl cyclic amp-induced insulin release by alpha-2 adrenergic stimulation

Effects of alpha adrenergic agonists and antagonists on dibutyryl cAMP (Bt 2 cAMP)-induced insulin release were investigated with isolated rat pancreatic islets. Bt 2 cAMP (4 mM) produced 2-fold stimulation of insulin release in all the concentrations of glucose examined (3.3–16.7 mM). Clonidine but not phenylephrine inhibited the Bt 2cAMP-stimulated insulin release in a concentration-dependent manner with approximate EC 50 of nanomolar range. Yohimbine but not prazosin antagonized the inhibitory effect of clonidine on the Bt 2-cAMP-induced insulin release. These results suggest that alpha-2 adrenergic mechanisms are involved in a step distal to cAMP generation.

The effects of alpha-adrenergic agonists on the regulation of the branched chain alpha-ketoacid oxidation in the perfused rat liver.

The effects of alpha-adrenergic agonists on the regulation of the branched chain alpha-ketoacid oxidation in the perfused rat liver.

Alpha-Adrenoceptor-mediated inhibition of noradrenaline release in rabbit brain cortex slices. Receptor properties and role of the biophase concentration of noradrenaline.

Brain cortex slices from rabbits were preincubated with [3H]noradrenaline and then superfused and stimulated electrically at 3Hz. In the presence of cocaine 30 microM, unlabelled noradrenaline, alpha-methylnoradrenaline, clonidine, oxymetazoline, xylazine and guanabenz decreased, whereas yohimbine, corynanthine, phentolamine, tolazoline and azapetine increased the stimulation-evoked overflow of tritium. Phenylephrine and prazosin had no effect on the evoked overflow except at concentrations that greatly accelerated the basal outflow of tritium. The results indicate that the noradrenergic axons of rabbit brain cortex are endowed with presynaptic alpha-adrenoceptors which are exclusively of the alpha 2-type. Addition of various concentrations of cocaine, addition of pargyline, or stimulation at different current strengths was used to obtain either a high or low stimulation-evoked overflow of tritium. Independently of the method used, a low evoked overflow coincided with a large percentage inhibition produced by 0.1 micro M clonidine, whereas a high evoked overflow coincided with a smaller percentage inhibition produced by clonidine. The results indicate that drugs which block the re-uptake of noradrenaline diminish the presynaptic inhibitory effect of alpha-adrenergic agonists by increasing the biophase concentration of released noradrenaline.

V. Liquid chromatography with electrochemical detection for plasma norepinephrine and epinephrine

One of the most significant advances in catecholamine methodology in recent years has been the development of the electrochemical detector in combination with liquid chromatography. In this report the practicability of this system for the routine quantitative determination of norepinephrine and epinephrine in plasma is described. The method has been found to be of particular value in evaluating the effect of alpha adrenergic agonists on norepinephrine and epinephrine levels in the plasma of dogs.