Effects of repeated footshock stress on the chronotropic responsiveness of the isolated pacemaker of the rat: role of beta-2 adrenoceptors

The effects of repeated footshock stress on the chronotropic responsiveness of the isolated right atria were studied. Repeated footshock stress was found to produce supersensitivity to isoproterenol and epinephrine (increase of 4.6- and 1.99-fold at pD2 level, respectively), but not to norepinephrine. Experiments using salbutamol, in the presence of an effective blocking concentration of metoprolol, showed that footshock stress increases the sensitivity (2.69-fold at pD2 level) to the selective beta-2 adrenoceptor agonist. Footshock stress had no effect on the atrial sensitivity to theophylline (1.09-fold at pD2 level). Addition of butoxamine (1 microM) suppressed footshock-induced pacemaker supersensitivity to isoproterenol and epinephrine. Footshock stress causes a small (3.23-fold) but pharmacologically unimportant increase in pacemaker pA2 value of metoprolol. However, footshock stress induces a large increase in the pacemaker beta-2 adrenoceptor affinity for butoxamine (11.48-fold, pA2 value). It is concluded that repeated footshock stress acts primarily to increase the chronotropic function of pacemaker beta-2 adrenoceptors, thus causing supersensitivity to isoproterenol, epinephrine and salbutamol.     

Beta adrenoceptors in the canine large coronary arteries: beta-1 adrenoceptors predominate in vasodilation

Beta adrenoceptors of the canine large coronary artery were characterized by observing the effects of the subtype selective antagonists, metoprolol (beta-1) and ICI 118,551 (beta-2), on the vasodilator responses of isolated and perfused preparations to beta adrenoceptor agonists and in the radioligand binding assay. The integrity of the endothelium was checked by acetylcholine-induced vasodilations. Without any precontraction, isoproterenol, norepinephrine, epinephrine and procaterol (selective beta-2 agonist) dilated the canine large coronary artery pretreated with phentolamine (10(-5) M). The rank order of agonist potency was isoproterenol greater than norepinephrine greater than epinephrine greater than procaterol. The pA2 values for metoprolol and ICI 118,551 were determined by the antagonisms of the vasodilator responses to isoproterenol and procaterol. The slopes of Schild plots for metoprolol and ICI 118,551 against isoproterenol and the value for ICI 118,551 against procaterol were not significantly different from unity, but the value for metoprolol against procaterol was significantly less than unity. The pA2 value for metoprolol against isoproterenol was 7.48 and those values for ICI 118,551 against isoproterenol and procaterol was 7.19 and 7.25, respectively. These pA2 values are typical for beta-1 adrenoceptors. The beta adrenoceptors of the canine large coronary artery were examined further using an antagonist [125I]iodocyanopindolol as a ligand for the binding of beta adrenoceptors. The [125I]iodocyanopindolol binding to the canine coronary artery smooth muscle membrane was saturable with a KD of 63.7 pM and a total number of radioligand binding sites of 44 fmol/mg of protein.(ABSTRACT TRUNCATED AT 250 WORDS)     

Altered vascular beta adrenoceptor-mediated relaxation in deoxycorticosterone-salt hypertensive rats

Beta adrenoceptor-mediated relaxation was studied on femoral and mesenteric arteries and thoracic aorta from deoxycorticosterone-salt hypertensive and age-matched normotensive rats. Maximum relaxations to isoproterenol (ISO), fenoterol and norepinephrine were less in hypertensive than in normotensive tissues. Mean negative log EC50 values of ISO and norepinephrine, but not fenoterol, were smaller in femoral and mesenteric strips from hypertensive rats than those from oil-water-treated rats. In thoracic aortas from hypertensive rats, the mean negative log EC50 values of ISO and fenoterol decreased, whereas those of norepinephrine increased as compared with those in oil-water-treated rats. In oil-salt- and deoxycorticosterone-water-treated rats, hypertension did not develop nor were the responses to ISO reduced. Relative potencies of the beta adrenoceptor agonists and Schild plot data for atenolol and butoxamine indicate that femoral arteries of normotensive and hypertensive rats possess beta-1 adrenoceptors mediating relaxation. It also is suggested that the mesenteric artery and thoracic aorta of both normotensive and hypertensive rats predominantly possess beta-2 adrenoceptors mediating relaxation. The present results suggest that an elevation in blood pressure is associated with the reduction of beta-1 adrenoceptor-mediated relaxation in femoral arteries and of beta-2 adrenoceptor-mediated relaxation in mesenteric arteries.     

A study designed to explore the hypothesis that beta-1 adrenoceptors are "innervated" receptors and beta-2 adrenoceptors are "hormonal" receptors

It has been demonstrated that the chronotropic responses to sympathomimetic amines in rat atria were mediated only by beta-1 adrenoceptors. This was like guinea pig (beta-1) but unlike cat (beta-1 and beta-2). Extraneuronal uptake of isoproterenol into rat atrial myocardial cells was detected by fluorescence histochemistry but uptake was less than was seen in cat atria. Furthermore, it did not modulate responses of rat atrial preparations to isoproterenol. The lack of specific extraneuronal uptake in guinea-pig atrial myocardial cells was confirmed. Collation of data on the atria of these three species with data already available on guinea-pig trachea allowed the following conclusions. All four tissues contained a population of beta-1 adrenoceptors and were adrenergically innervated. Beta-2 adrenoceptors were present in cat atria and guinea-pig trachea and these tissues possessed a functionally effective extraneuronal metabolizing system for catecholamines. Beta-2 adrenoceptors were not detected in guinea-pig or rat atria and in these tissues extraneuronal uptake was either absent (guinea pig) or not functionally effective (rat). It is suggested that these data could support the hypotheses that 1) beta-1 adrenoceptors are "innervated" receptors mediating responses to neuronally released norepinephrine, whereas beta-2 adrenoceptors are "hormonal" receptors mediating responses to circulating epinephrine and 2) extraneuronal metabolizing systems are of particular importance in the dissipation of circulating catecholamines acting on beta-2 adrenoceptors.     

Beta-1 and beta-2 adrenoceptor-mediated responses in preparations of pulmonary artery and aorta from young and aged rats

Rat pulmonary artery contains both alpha adrenoceptors mediating contraction and beta-1 and beta-2 adrenoceptors mediating relaxation. Neither alpha nor beta adrenoceptor-mediated responses of this vessel to norepinephrine or epinephrine were potentiated by cocaine, despite evidence for the presence of some adrenergic nerves. Beta adrenoceptor-mediated relaxation of pulmonary artery, but not aorta, modulated alpha adrenoceptor-mediated contractions to epinephrine but not norepinephrine. Rat aorta also contains both beta-1 and beta-2 adrenoceptors mediating relaxation, in that Schild plots for atenolol (beta-1 selective antagonist) using a beta-1 selective agonist (norepinephrine) and a beta-2 selective agonist (fenoterol), respectively, were not superimposed. The Schild plot data for lCl 118,551 (beta-2 selective) indicated that the minor population (beta-1) was less important in aorta than in pulmonary artery. On preparations from 18-month-old rats, the maximum relaxation to isoproterenol (20.4%, aorta and 67.9%, pulmonary artery) was less than in preparations from young rats (79.8%, aorta and 96.9%, pulmonary artery), i.e., aging had reduced the beta adrenoceptor-mediated relaxation in both vessels, but particularly in aorta. Also, the negative log EC50 of fenoterol, but not of isoproterenol or norepinephrine, was less than in preparations from young rats. This could indicate that aging had affected beta-2 more than beta-1 adrenoceptor-mediated responses and may explain why aging depressed the maximum relaxation of aorta more than that of pulmonary artery.     

Comparative analysis of beta-1 adrenoceptor agonist and antagonist potency and selectivity of cicloprolol, xamoterol and pindolol

The partial beta adrenoceptor agonist properties of cicloprolol, xamoterol and pindolol have been compared in vivo (anesthetized catecholamine-depleted or pithed rats) and in vitro (guinea pig or rat right atria and guinea pig tracheal muscle preparations) conditions. All three compounds increased heart rate in the former preparations, and their intrinsic activities relative to isoproterenol were 0.7, 0.65 and 0.45, respectively. The positive chronotropic effects of cicloprolol or xamoterol were competitively antagonized by betaxolol or propranolol; however, part of those induced by pindolol were resistant to these beta adrenoceptor antagonists. None of these compounds increased the spontaneous beating rate of isolated guinea pig atria; however, xamoterol only increased heart rate in isolated rat atria, and its intrinsic activity with respect to isoproterenol was 0.4. Pindolol, xamoterol and cicloprolol behaved as competitive beta-1 adrenoceptor antagonists against isoproterenol-induced tachycardia in a pithed rat model. In order to mimic the intrinsic effects of the partial agonist drugs, control dose-response curves for isoproterenol were determined in pithed rats in which the base-line heart rate was elevated by thoracic spinal cord stimulation. In this in vivo preparation, xamoterol and pindolol were more potent beta-1 adrenoceptor antagonists than cicloprolol; however, cicloprolol and xamoterol, in contrast to pindolol, were selective for beta-1 adrenoceptors. In isolated spontaneously beating guinea pig right atria, cicloprolol and xamoterol were equipotent beta-1 adrenoceptor antagonists but were about 50 times less potent than pindolol. In isolated rat atria, the beta-1 adrenoceptor antagonist potency of xamoterol was greater (pA2 = 8.7) than in guinea pig atria (pA2 = 7.8). The potencies of cicloprolol and pindolol did not vary between these species. In catecholamine-depleted rats, high i.v. doses of cicloprolol had vasodilator activity that was partly mediated by beta-2 adrenoceptors. In carbachol-contracted guinea pig trachea, cicloprolol and xamoterol, in contrast to pindolol, were relatively inactive against isoproterenol-induced relaxation. In conclusion, cicloprolol and xamoterol, similarly to pindolol, behave as agonists and antagonists of beta-1 adrenoceptors. However, only cicloprolol and xamoterol show an elevated degree of selectivity toward the beta-1 adrenoceptor subtype.     

Supersensitivity and changes in the active population of beta adrenoceptors in rat right atria in early sepsis.

We have investigated the effect of sepsis induced by cecal ligation and puncture on the chronotropic actions of beta adrenoceptor agonists on isolated right atria. The present findings show that right atria obtained from rats in an early stage of sepsis were supersensitive to the chronotropic actions of the beta-agonists, isoproterenol (ISO), fenoterol (FEN) and prenalterol (PREN). The supersensitivity to the chronotropic actions of ISO and FEN was much greater than that which developed to PREN. The positive chronotropic actions of isobutylmethylxanthine and forskolin were not affected by sepsis. The receptor subtypes mediating the responses to ISO, FEN and PREN by control and septic right atria were characterized by functional assays using selective beta-1 and beta-2 antagonists. The results showed that the chronotropic response produced by all three agonists on right atria obtained from control rats were mediated by beta-1 receptors. In contrast, the chronotropic actions of ISO and FEN on atria from septic rats were mediated by what appears to be beta-2 receptors and those of PREN by beta-1 receptors.     

ICI 147,798: a slowly dissociable beta adrenoceptor antagonist that causes insurmountable beta-1 and surmountable beta-2 adrenoceptor antagonism in isolated tissues

ICI 147,798 has been shown to exhibit both diuretic and beta-antagonist properties in vivo. The present study investigated the nature and selectivity of the beta-antagonism in a variety of isolated tissues. ICI 147,798 produced a concentration-dependent suppression of the maximum chronotropic response of norepinephrine in guinea pig right atria (beta-1 adrenoceptor). ICI 147,798 caused a concentration-dependent shift to the right of the salbutamol concentration-response curve in the guinea pig trachea (beta-2 adrenoceptor), and Schild analysis suggested competitive inhibition. Propranolol produced parallel shifts to the right of the norepinephrine concentration-response curve in guinea pig right atria, except at relatively high concentrations. The inhibitory effects of propranolol in guinea pig right atria were reversed by greater than 95%, whereas the effects of ICI 147,798 were only slightly reversed after a 6-hr washout period. Preincubation of propranolol with ICI 147,798 in guinea pig right atria prevented completely the suppression of the norepinephrine maximum chronotropic response. Postincubation of propranolol with ICI 147,798 partially reversed the suppression of the maximum chronotropic response. ICI 147,798 had no effect on the maximum chronotropic responses of either histamine (H2-receptor) or forskolin (adenylate cyclase activation) in guinea pig right atria and had no effect on agonist responses in a variety of other receptor systems. The insurmountable beta-1 adrenoceptor antagonism was evaluated based on the assumptions of irreversible competitive antagonism, mixed competitive and noncompetitive antagonism and slowly dissociating competitive antagonism ("hemi-equilibrium" conditions). Concentration-dependent changes in norepinephrine KA values suggested the first three possibilities were unlikely.(ABSTRACT TRUNCATED AT 250 WORDS)     

Beta-2 adrenoceptor-mediated relaxation of the isolated human saphenous vein

On isolated strips of human saphenous vein, pretreated with 5 microM phenoxybenzamine and contracted with 10 mM KCl, the beta adrenoceptor mediating the relaxant effects of isoproterenol, procaterol and norepinephrine was characterized using the selective beta-1 adrenoceptor antagonist, bisoprolol, and the selective beta-2 adrenoceptor antagonist, ICI 118,551. All three agonists produced concentration-dependent relaxations of the isolated saphenous vein with an order of potency: procaterol (pD2 value, 7.69) greater than isoproterenol (pD2 value, 7.41) much greater than norepinephrine (pD2 value, 5.30). ICI 118,551 (3 X 10(-10) to 3 X 10(-9) M) was nearly 100 times more potent than bisoprolol (10(-7) to 10(-6) M) in antagonizing the relaxant effects of isoproterenol and procaterol. The slopes of the Schild plots for the antagonistic effects of ICI 118,551 and bisoprolol against isoproterenol- and procaterol-induced relaxations were not significantly different from unity indicating interaction with a homogeneous population of beta adrenoceptors. The pA2 value for ICI 118,551 amounted to 9.11 to 9.20 and for bisoprolol to 6.50 to 6.63. In addition, the concentration-response curve for the relaxant effect of norepinephrine was significantly shifted to the right by 10(-9) M ICI 118,551, but not affected by 10(-7) M bisoprolol. These results indicate that on the isolated strips of the human saphenous vein the beta adrenoceptor mediating relaxation is of the beta-2 subtype.     

Beta adrenoceptor subtype in isolated human, monkey and dog epicardial coronary arteries

Isolated human, Japanese monkey and dog epicardial coronary arteries and dog renal and mesenteric arteries treated with phentolamine responded to isoproterenol with a concentration-related relaxation. The KB values of metoprolol, a beta-1 antagonist, in the coronary arteries from different mammals did not differ, but were appreciably smaller than those in the dog renal and mesenteric arteries. Treatment with butoxamine, a beta-2 antagonist, inhibited the relaxation of dog mesenteric arteries to a greater extent than that of monkey and dog coronary arteries. Terbutaline, a beta-2 agonist, produced a greater relaxation in monkey mesenteric and dog renal and mesenteric arteries than in human, monkey and dog coronary arteries. Norepinephrine relaxed the monkey and dog coronary arteries dose-dependently via mainly beta-1 adrenoceptors, but elicited a contraction or a minute relaxation in dog mesenteric arteries even when treated with high concentrations of phentolamine. Contractile responses to electrical stimulation of adrenergic nerves in monkey coronary arteries were potentiated by treatment with metoprolol and propranolol, whereas the contractions in dog mesenteric arteries were unaffected. It is concluded that the amine-induced relaxation of human and monkey epicardial coronary arteries is mediated mainly by beta-1 adrenoceptor subtype, as is the response of dog coronary arteries. Involvement of beta-1 subtype in coronary artery relaxations would be a mechanism underlying potentiation by beta antagonists of the contraction caused by norepinephrine released from adrenergic nerves in primates.     

Characterization of beta-1 and beta-2 adrenoceptors in guinea pig atrium: functional and receptor binding studies

The selective beta-2 adrenoceptor agonist procaterol produced positive inotropic and chronotropic responses over a concentration range of 1 nM to 0.1 mM in spontaneously beating right atria and in three of seven electrically driven left atria. The pD2 values (right atria, 7.30; left atria, 7.18) were midway between its known affinities at beta-1 and beta-2 adrenoceptors and are evidence that positive inotropic and chronotropic responses involve a minor beta-2 adrenoceptor component. The pKB values for procaterol against (-)-isoproterenol in the right atria (5.59) and left atria (5.29) were consistent with its affinity for beta-1 adrenoceptors and suggest that these are responsible primarily for positive inotropic and chronotropic responses. Receptor binding studies in right atrial homogenates showed that [125I]cyanopindolol binding was saturable (KD = 36.2 pM, maximal density of binding sites = 49.2 fmol mg-1 protein) and stereoselective with respect to the isomers of propranolol. Competition binding curves for the beta-1 adrenoceptor antagonist CGP 20712A and beta-2 selective antagonist ICI 118,551 against [125I]cyanopindolol binding were resolved into two components using iterative curve fitting techniques. Binding sites with the characteristics of beta-1 and beta-2 adrenoceptors were present in the proportions of approximately 75 to 25%. These studies indicate either that the beta-1 adrenoceptor is coupled more efficiently to the positive inotropic and chronotropic response than the beta-2 adrenoceptor or that a proportion of the beta-2 adrenoceptors subserve other functions.     

Selective desensitization of cardiac beta adrenoceptors by prolonged in vivo infusion of catecholamines in rats

The effects of prolonged in vivo infusion of isoproterenol (400 micrograms/kg/hr) or norepinephrine (200 micrograms/kg/hr) from a minipump on the physiological reactivity and binding properties of cardiac beta and alpha-1 adrenoceptors were tested in rats. Infusion of either catecholamine significantly reduced the in vitro inotropic and chronotropic potency of isoproterenol in isolated left and right atria, respectively; desensitization was near maximal as early as after 2 hr of infusion. No significant change in the density of [3H]dihydroalprenolol-labeled beta receptors was evident at this time point in either atrial or ventricular tissue, although isoproterenol did decrease binding site density after 7 days of infusion. There was no change in the binding affinity or physiological blocking potency of dihydroalprenolol after isoproterenol infusion. The inotropic potency of phenylephrine in the presence of dihydroalprenolol was unaffected by infusion of either isoproterenol or norepinephrine and methoxamine failed to increase right atrial rate either in control or in isoproterenol-infused rats. There was also no change in the density and affinity of [3H]prazosin binding sites after isoproterenol infusion. These results indicate selective desensitization of cardiac beta receptors without changes in alpha-1 receptors by prolonged in vivo stimulation with catecholamines. This reaction pattern is different from the well documented effects of hypothyroidism, which include decreased sensitivity of cardiac beta and increased sensitivity of cardiac alpha-1 receptor-mediated responses in rats. Thus, the mechanisms responsible for altered receptor function in the two conditions appear to be different.     

Characterization of the prejunctional beta adrenoceptors in canine bronchial smooth muscle

Prejunctional beta adrenoceptors in canine bronchi (3rd to 6th order) were characterized by observing the effects of beta receptor agonists and antagonists on field stimulation-induced contractions and excitatory junction potentials (EJPs). Contractions were antagonized by norepinephrine (IC50 = 9.4 X 10(-7) M), isoproterenol (IC50 = 1.9 X 10(-8) M) or salbutamol (IC50 = 4.0 X 10(-8) M). EJPs were also decreased by all three agonists, with little or no effect on resting membrane potential or on carbachol-induced depolarization when used at concentrations sufficient to eliminate EJPs. These inhibitory effects were blocked by propranolol or timolol, as well as by the selective antagonists ICI 89,406 (beta-1-selective) and ICI 118,551 (beta-2-selective); pA2 values for the selective antagonists were 8.4 and 7.2 (norepinephrine as agonist) or 6.5 and 9.0 (salbutamol as agonist), respectively. Control responses were also sometimes potentiated by the nonselective antagonists. Schild plot analysis of the data indicated clearly that both beta-1 and beta-2 receptors are involved in the inhibitory effect. Electron microscopic studies showed this tissue to be densely innervated by adrenergic and cholinergic nerves with close apposition of adrenergic and cholinergic nerve varicosities, providing a structural basis for prejunctional interactions between them. From the data presented, we conclude that catecholamines act on prejunctional beta-1 and beta-2 receptors leading to inhibition of cholinergic neurotransmission in canine bronchi.     

Beta-1 and beta-2 adrenoceptors mediate smooth muscle relaxation in bovine isolated mesenteric lymphatics.

The relaxant effects of beta adrenoceptor agonists were investigated in isolated bovine mesenteric lymphatics which had been contracted by 5-hydroxytryptamine. Addition of isoproterenol (a nonselective beta agonist), denopamine (a selective beta-1 agonist) and procaterol (a selective beta-2 agonist) caused concentration-dependent relaxations in the lymphatic preparations. There was no significant difference in the relaxant responses to the beta adrenoceptor agonists between the preparations with and without endothelium. Treatment with 10(-7) to 3 x 10(-6) M metoprolol (a selective beta-1 antagonist) shifted the concentration-response curve for denopamine to the right, whereas 10(-9) to 3 x 10(-8) M ICI 118,551 (a selective beta-2 antagonist) did not affect the relaxant response to denopamine. The relaxations of bovine mesenteric lymphatics induced by isoproterenol were suppressed by both metoprolol and ICI 118,551. The procaterol-induced relaxations were inhibited by 10(-9) to 3 x 10(-8) M ICI 118,551 but not by 10(-7) to 3 x 10(-6) M metoprolol. Schild plot analyses showed that the slope and pA2 values for metoprolol against denopamine were 1.10 and 7.59, respectively, and that those for ICI 118,551 against procaterol were 0.91 and 9.96. These results suggest that both beta-1 and beta-2 adrenoceptors are located on the smooth muscle cells in bovine mesenteric lymphatics and that stimulation of either receptor produces a marked relaxation.     

Developmental changes in the accumulation of norepinephrine and the chronotropic effects of catecholamines in isolated rat atria as influenced by dietary lipid

Pregnant rats were fed semisynthetic diets enriched in either saturated fat (coconut oil) or polyunsaturated fat (sunflower oil) and sympathetic function was observed in the resulting neonatal pups. The neuronal accumulation of [3H]norepinephrine was significantly decreased in atria from neonates (11, 24 and 37 days of age) of dams receiving dietary sunflower oil compared with coconut oil diet. The nonspecific accumulation of [3H]norepinephrine occurring in the presence of desipramine (5 X 10(-5)M) was not changed by dietary lipid treatment. No differences in the accumulation of [3H]norepinephrine were observed in atria from day 7 neonates or in atria from adults which had received the diets since birth. When the beating rate of isolated atria was studied upon exposure to norepinephrine and isoproterenol, it was observed that the EC50 for these agonists was not changed upon exposure to the diets but that the maximal effect of both agonists was increased in atria from adult animals fed coconut oil relative to the sunflower oil diet. No significant differences were observed in the chronotropic effects of either norepinephrine or isoproterenol in atria from 11-day-old rats. A developmental analysis of the chronotropic response of isolated atria to a maximally effective dose of norepinephrine (10 microM) revealed a gradual onset of the altered maximum effects found in adult rat atria. The results of this study indicate that sympathetic nerve regulation of myocardial function can be modified during development by changing the dietary fat composition. The results are discussed in reference to the perturbation of specific membrane-associated processes that may be brought about by dietary lipid treatment and the possible importance of each of these effects in the changes observed in the accumulation of norepinephrine and the chronotropic effects of catecholamines in isolated rat atria.     

Cardiac responsiveness to alpha and beta adrenergic amines: effects of carbachol and hypothyroidism

Previous reports of cardiac beta to alpha adrenoceptor interconversion secondary to hypothyroidism left open the alternative possibility of a functional influence by hypothyroidism on the inotropic and chronotropic effects of adrenergic amines through a different mechanism. To test this possibility, the effects of hypothyroidism (thyroidectomy) were compared with those of acute carbachol pretreatment on the responses of isolated rat atria to the selective beta and alpha adrenoceptor agonists isoproterenol and methoxamine. Both hypothyroidism and acute carbachol pretreatment (3 X 10(-7) -10(-6) M): 1) reduced basal right atrial rates and left atrial tensions; 2) caused an apparent decrease in the inotropic and chronotropic potencies of isoproterenol; 3) reduced the degree of antagonism by propranolol of the responses to isoproterenol; 4) increased the maximum inotropic response of left atria to methoxamine; and 5) converted a lack of response to a positive chronotropic response of right atria to methoxamine. Equivalent reductions of basal rates by hypothermia, or of basal tensions by lowered calcium ion concentrations, did not affect the responses to isoproterenol or methoxamine. The results suggest that both carbachol pretreatment and hypothyroidism functionally antagonize the responses to isoproterenol and enhance the responses to methoxamine by means other than adrenoceptor interconversion.     

In vivo cardiovascular responses to isoproterenol, dopamine and tyramine after prolonged infusion of isoproterenol

Effects of prolonged in vivo infusion of isoproterenol on acute cardiovascular responses to isoproterenol, dopamine and tyramine were studied in pithed rats. Isoproterenol infusion resulted in a significant decrease in control values for maximum left ventricular dP/dt; heart rate and left ventricular systolic blood pressure were not altered. This treatment also depleted both atrial and ventricular stores of norepinephrine and caused cardiac hypertrophy. Isoproterenol infusion resulted in a desensitization of drug-induced cardiovascular responses. The acute in vivo effects of isoproterenol on maximum left ventricular dP/dt, heart rate and left ventricular systolic blood pressure responses to isoproterenol were severely attenuated. The ED50 for maximum left ventricular dP/dt was increased 36-fold and maximal responses were reduced by half; changes in heart rate occurred in a parallel fashion. By contrast, ED50 values for inotropic responses to tyramine and dopamine were increased 14- and 4-fold, respectively, whereas increases in heart rate were blunted. Tyramine and dopamine-mediated increases in heart rate were completely attenuated by desensitization; chronotropic effects were again evident after pretreatment with the selective alpha-1 blocker prazosin. In addition, prazosin blocked the inotropic responses to tyramine and dopamine after desensitization and this antagonism was only slightly enhanced by addition of propranolol (prazosin + propranolol); propranolol alone was ineffective. These results are consistent with the down-regulation of beta adrenoceptors after prolonged exposure to catecholamines and indicate that under such conditions the alpha-mediated cardiovascular responses may be unmasked. Compared to pure beta agonists, agents with a degree of alpha-1 activity might be superior inotropes in heart failure patients who characteristically present with depleted stores of myocardial norepinephrine and minimal beta adrenoceptor reserve.     

Beta-2 adrenergic responses to tulobuterol in airway smooth muscle, vascular smooth muscle and adrenergic nerves

Experiments were designed to determine the mechanism of action of the bronchodilator drug tulobuterol. Tissues were suspended in organ chambers for isometric tension recording. Tulobuterol caused concentration-dependent relaxations of guinea pig tracheae, canine saphenous veins and canine bronchi; the compound relaxed canine coronary arteries only at high concentrations and did not affect spontaneously beating guinea pig atria. A metabolite of tulobuterol, 4-hydroxytulobuterol, was more potent in relaxing guinea pig tracheae than tulobuterol, salbutamol and isoproterenol. Other metabolites (3-hydroxy-, 5-hydroxy- and 4,5-dihydroxytulobuterol) were less efficacious than 4-hydroxytulobuterol. Both tulobuterol and 4-hydroxytulobuterol acted as partial agonists. The effects of tulobuterol in the saphenous vein (but not in the coronary artery) were antagonized by the selective beta-2 adrenergic blocker ICI 118,551 but were not affected by the selective beta-1 adrenergic inhibitor metoprolol. In bronchi, removal of the epithelium reduced the relaxations caused by tulobuterol. The drug did not inhibit responses of canine bronchi to electrical stimulation of the cholinergic nerves more than those to exogenous acetylcholine. Tulobuterol caused a moderate augmentation of the evoked release of [3H]norepinephrine in canine saphenous veins previously incubated with the labeled transmitter. Thus, tulobuterol is a selective beta-2 adrenergic agonist with minimal nonselective inhibitory effect on airway and vascular smooth muscle. It also facilitates adrenergic neurotransmission, which may help to explain its bronchodilator effect in the intact organism. Tulobuterol does not activate beta-1 adrenoceptors and has no direct positive chronotropic effect. A metabolite of tulobuterol, 4-hydroxytulobuterol, is more active than the parent compound.     

Responses mediated via beta-1 adrenoceptors but not beta-2 adrenoceptors exhibit supersensitivity after chronic reserpine pretreatment

The purpose of this study was to examine whether reserpine pretreatment induces supersensitivity of both beta-1 and beta-2 adrenoceptor-mediated responses. Guinea pigs received reserpine (0.5 mg kg-1 s.c. or i.p.) daily for 7 days. Isolated tissues were set up in the presence of phentolamine (5 microM) and metanephrine (10 microM) and the sensitivity to isoproterenol and, where possible, a partial agonist (ritodrine, salbutamol or prenalterol) was determined. The beta adrenoceptor-mediated responses were recorded as the increase in rate and tension of right and left atria, inhibition of carbachol-induced contractions of ileum, relaxation of aortic spirals contracted with histamine, inhibition of transmurally stimulated vas deferens and relaxation of tracheal spirals and lung strips with intrinsic tone. The atria exhibited supersensitivity after reserpine pretreatment (s.c. and i.p.) as a leftwards shift of the isoproterenol concentration-response curve and elevation of the prenalterol maximum response. The ileum was also supersensitive, but only when tissues from animals receiving i.p. reserpine were compared with shams, which themselves were subsensitive or when reserpine was administered s.c. The trachea was also supersensitive, but not the aorta, lung and vas deferens, the responses of which are mediated via beta-2 adrenoceptors. In contrast, beta-1 adrenoceptors are involved in the atrial, ileal and tracheal responses. Therefore, only responses mediated via beta-1 adrenoceptors exhibited reserpine-induced supersensitivity which supports the hypothesis that beta-1 but not beta-2 adrenoceptors receive a sympathetic innervation.     

Characteristics of adaptive supersensitivity in the left atrium of the guinea pig

Chronic pretreatment of guinea pigs with reserpine (0.1 mg/kg/day for 7 days) induces inotropic supersensitivity in left atria. The sensitivity is increased to isoproterenol, norepinephrine, 5'-guanylylimidodiphosphate and forskolin, and the maximum response is increased to the partial agonist albuterol. These results, coupled with data in the literature, suggest that adaptive supersensitivity in the guinea pig heart is due to a change in one or more of the components of the adenylate cyclase system that is specifically coupled to beta adrenoceptors. The results indicate that the supersensitivity is demonstrable when the concentration-response curves for agonists are determined in isolated whole left atria but not when they are determined in strips cut from left atria. This explains a discrepancy in the literature. It is suggested that cellular changes, possibly in electrolyte distribution, resulting from the additional manipulation of cutting the atria into slices obscure the sensitivity difference between the control and experimental tissues.