Concentration-response Curve For Isoproterenol Research Articles

Phosphodiesterase inhibition by naloxone augments the inotropic actions of β‐adrenergic stimulation

In a shock state, naloxone generates the cardiovascular pressor effect by displacing the endogenous opiate-like peptide beta-endorphin, resulting in restoration of the normal response to catecholamines. In addition to this opioid antagonistic effect, the non-opiate receptor-mediated effect has also been proposed. The aim of this study was to define the mechanism of non-opiate receptor-mediated action of naloxone. In guinea-pig ventricular tissues, cumulative concentration-response curves for isoproterenol as well as for forskolin and 3-isobutylmethylxanthine (IBMX) were obtained by increasing the concentration stepwise. To assess the effect on the phosphodiesterase (PDE), the effects of naloxone on contractile forces induced by isoproterenol (0.05 microM) in the presence of IBMX, cilostamide (a PDE III inhibitor), or rolipram (a PDE IV inhibitor) were observed. Naloxone-induced changes in cAMP production by isoproterenol both in the absence and in the presence of IBMX were measured. Naloxone-induced changes in cAMP production by forskolin in the presence of IBMX were also measured. Naloxone (30 microM) produced a leftward shift of the isoproterenol concentration-response curve (0.01-2 microM) without changing the maximal response. Forskolin (0.5-10 microM) produced a concentration-dependent increase in contractile forces. Naloxone increased the maximal inotropic response of forskolin. Naloxone showed no effect on the IBMX concentration-response curve. In the presence of IBMX (200 microM), naloxone did not alter the contractions evoked by isoproterenol or forskolin. Whereas naloxone increased contractile forces significantly (approximately 25%) more than that of isoproterenol in the presence of rolipram, no alteration of contractile forces in the cilostamide-incubated muscles was observed. Naloxone caused a concentration-related increase of cAMP in the absence of IBMX, but caused no change in its presence. The enhancement of myocardial contractility by naloxone in the presence of stimulation of adenylyl cyclase activity appears to be mediated by inhibition of PDE, specifically PDE III.

β1 Adrenergic Receptor-Mediated Enhancement of Hippocampal CA3 Network Activity

Norepinephrine is an endogenous neurotransmitter distributed throughout the mammalian brain. In higher cortical structures such as the hippocampus, norepinephrine, via beta adrenergic receptor (AR) activation, has been shown to reinforce the cognitive processes of attention and memory. In this study, we investigated the effect of beta1AR activation on hippocampal cornu ammonis 3 (CA3) network activity. AR expression was first determined using immunocytochemistry with antibodies against beta1ARs, which were found to be exceptionally dense in hippocampal CA3 pyramidal neurons. CA3 network activity was then examined in vitro using field potential recordings in rat brain slices. The selective betaAR agonist isoproterenol caused an enhancement of hippocampal CA3 network activity, as measured by an increase in frequency of spontaneous burst discharges recorded in the CA3 region. In the presence of alphaAR blockade, concentration-response curves for isoproterenol, norepinephrine, and epinephrine suggested that a beta1AR was involved in this response, and the rank order of potency was isoproterenol > norepinephrine = epinephrine. Finally, equilibrium dissociation constants (pK(b)) of subtype-selective betaAR antagonists were functionally determined to characterize the AR subtype modulating hippocampal CA3 activity. The selective beta1AR antagonists atenolol and metoprolol blocked isoproterenol-induced enhancement, with apparent K(b) values of 85 +/- 36 and 3.9 +/- 1.7 nM, respectively. In contrast, the selective beta2AR antagonists ICI-118,551 and butoxamine inhibited isoproterenol-mediated enhancement with apparent low affinities (K(b) of 222 +/- 61 and 9268 +/- 512 nM, respectively). Together, this pharmacological profile of subtype-selective betaAR antagonists indicates that in this model, beta1AR activation is responsible for the enhanced hippocampal CA3 network activity initiated by isoproterenol.

Functional Characterization of the β-Adrenergic Receptor Subtypes Expressed by CA1 Pyramidal Cells in the Rat Hippocampus

Recent studies have demonstrated that activation of the beta-adrenergic receptor (AR) using the selective beta-AR agonist isoproterenol (ISO) facilitates pyramidal cell long-term potentiation in the cornu ammonis 1 (CA1) region of the rat hippocampus. We have previously analyzed beta-AR genomic expression patterns of 17 CA1 pyramidal cells using single cell reverse transcription-polymerase chain reaction, demonstrating that all samples expressed the beta2-AR transcript, with four of the 17 cells additionally expressing mRNA for the beta1-AR subtype. However, it has not been determined which beta-AR subtypes are functionally expressed in CA1 for these same pyramidal neurons. Using cell-attached recordings, we tested the ability of ISO to increase pyramidal cell action potential (AP) frequency in the presence of subtype-selective beta-AR antagonists. ICI-118,551 [(+/-)-1-[2,3-(dihydro-7-methyl-1H-inden-4-yl)oxy]-3-[(1-methylethyl)amino]-2-butanol] and butoxamine [alpha-[1-(t-butylamino)ethyl]-2,5-dimethoxybenzyl alcohol) hydrochloride], agents that selectively block the beta2-AR, produced significant parallel rightward shifts in the concentration-response curves for ISO. From these curves, apparent equilibrium dissociation constant (K(b)) values of 0.3 nM for ICI-118,551 and 355 nM for butoxamine were calculated using Schild regression analysis. Conversely, effective concentrations of the selective beta1-AR antagonists CGP 20712A [(+/-)-2-hydroxy-5-[2-([2-hydroxy-3-(4-[1-methyl-4-(trifluoromethyl)-1H-imidazol-2-yl]phenoxy)propyl]amino)ethoxy]-benzamide methanesulfonate] and atenolol [4-[2'-hydroxy-3'-(isopropyl-amino)propoxy]phenylacetamide] did not significantly affect the pyramidal cell response to ISO. However, at higher concentrations, atenolol significantly decreased the potency for ISO-mediated AP frequencies. From these curves, an apparent atenolol K(b) value of 3162 nM was calculated. This pharmacological profile for subtype-selective beta-AR antagonists indicates that beta2-AR activation is mediating the increased AP frequency. Knowledge of functional AR expression in CA1 pyramidal neurons will aid future long-term potentiation studies by allowing selective manipulation of specific beta-AR subtypes.

Pharmacological characterization of beta-adrenoceptors mediating relaxation of the rat urinary bladder in vitro.

1. Isoproterenol relaxed KCl-precontracted rat bladder strips with a pD2 of 7.21 leaving a residual contractile response of 3.2% after 30 microM. The selective beta1-agonist, T-0509 (pD2 : 6.24, 10.1% residual contraction after 100 microM), beta2-agonist, terbutaline (pD2 : 5.43, 13.7% residual contraction after 100 microM), and beta3-agonists, BRL 37344A (pD2 : 6.60, 17.3% residual contraction after 100 microM), and SR 58611A (pD2 : 5.15, 34.0% residual contraction after 100 microM), also relaxed bladder strips. 2. The relaxant response to isoproterenol was weakly but significantly antagonized by 1 microM propranolol which produced a 3 fold shift of the concentration-response curve to the right, and significantly antagonized by the beta1-selective antagonist, metoprolol (10 microM, 3 fold shift), and the beta2-selective antagonist, butoxamine (100 microM, 6 fold shift). A combination of 10 microM metoprolol and 100 microM butoxamine caused a 15 fold shift of the concentration-response curve for isoproterenol to the right. Incubation with the beta3-antagonist, SR 59230A (1 microM), caused a 6 fold shift of the concentration response curve for isoproterenol to the right. 3. The non-conventional partial agonist, CGP 12177A, weakly relaxed KCl-precontracted bladder strips (pD2 : 3.31, 51.3% residual contraction after 300 microM); the relaxation was resistant to blockade by 1 or 10 microM propranolol. 4. In the presence of 200 microM propranolol, CGP 12177A (20 microM) or SR 59230A (10 microM) antagonized surmountably the relaxant effects of BRL 37344A. 5. The data suggest that rat urinary bladder body contains beta1, beta2, and beta3-adrenoceptors, all of which mediate relaxation.

Effects of hydrogen peroxide on isolated trachealis muscle of horses

SUMMARY During acute bouts of recurrent airway obstruction (heaves) in horses, neutrophils that are capable of increased production of reactive oxygen species accumulate in the airways. In the study reported here, the effect of hydrogen peroxide (H2O2; 1 µM to 0.1M), one of these reactive oxygen species products, on the responses of isolated trachealis muscle of horses was determined. Before and after incubation with H2O2, contractile responses to acetylcholine, electrical field stimulation (efs), 127 mM KCl, and relaxation responses to isoproterenol and activation of the nonadrenergic noncholinergic inhibitory response (iNANC) were evaluated. Beginning at 1 mM, H2O2 contracted trachealis muscle in a concentration-dependent manner. This contraction was unaffected by atropine (1 µM), tetrodotoxin (1 µM), or 1 µM meclofenamate. Contraction of trachealis muscle in response to H2O2 is, therefore, not attributable to release of prostaglandins, acetylcholine, or other neurotransmitters. Above a concentration of 0.1 mM, H2O2 depressed the responses to efs, acetylcholine, and KCl in a concentration-dependent manner. At 0.1M, H2O2 decreased the maximal responses to efs, acetylcholine, and KCl by 62.7 ± 7.2, 60.58 ± 6.12, and 37.8 ± 9.54%, respectively. In the presence of meclofenamate (1 µM), partial but significant protection against 1 to 100 mM H2O2 was observed. In tracheal strips contracted with 0.3 µM methacholine, H2O2 had no effect on the isoproterenol concentration-response curve. Up to a concentration of 100 mM, H2O2 had no effect on iNANC response. However, in the presence of 100 mM H2O2, this response was abolished in 2 of 4 horses. We conclude that high concentrations of H2O2 affected the responses of airway smooth muscle by actions on neurotransmission, muscarinic receptors, and downstream from receptors; some of the H2O2 effects were in part mediated by cyclooxygenase products; and H2O2 had no effect on β-adrenergic or iNANC-induced relaxation.

Role of prostaglandin E 2 in alterations of the β-adrenergic system from rat eclamptic uterus

The inotropic effect of isoproterenol, as well as the β-adrenoceptor population, was measured in pregnant uterine tissue from female spontaneous hypertensive rats (SHR) (control group: C) and female SHR that were grafted with skin from Holtzman male rats (eclamptic group: E). The K d value of the concentration response curve of isoproterenol was higher for uteri from E rats than C rats. This phenomenon was not accompanied by a modification in the expression of β-adrenoceptors. Inhibition of the synthesis of prostaglandins prevented the hyporeactivity to isoproterenol during eclampsia. Moreover, uteri from E rats generated and released greater amounts of prostaglandin E 2 (PGE 2) than uteri from C rats, even in the presence or absence of isoproterenol. In addition, whereas isoproterenol administered alone increased basal cyclic AMP (cAMP) production from C uteri, PGE 2 administered alone enhanced cAMP production in E uterine tissue. These results suggest that the decrease in β-adrenergic response to the agonist in E rats is ascribed to PGE 2 production. The abnormal reactivity to the β-agonist could be associated with a heterologous desensitization of uterine β-adrenoceptors exerted by PGE 2 overload in uteri from E rats. These results bear directly on the regulation of uterine motility during pregnancy, since an impaired response to β-adrenergic innervation could lead to increased uterine motility, impairing the maintenance of pregnancy.

Multiple effects of long-term morphine treatment on postsynaptic β-adrenergic receptor function in hippocampus: an intracellular analysis

We previously reported that β-adrenergic receptors are increased in cerebral cortex and hippocampus in rats treated chronically with morphine and subsequently down-regulated after morphine withdrawal [22,23]. The changes in receptor density in hippocampus were accompanied by a corresponding super- and subsensitivity, respectively, in β-adrenergic responsiveness, as assessed electrophysiologically by measuring the ability of isoproterenol to augment population spike responses in the slice. In this study, we compared the ability of isoproterenol to reduce the Ca 2+-activated K + slow afterhyperpolarization (slow AHP) in pyramidal neurons in hippocampal slices from opiate-naive and chronic morphine-treated rats to determine whether such changes in β-adrenergic receptor function are localized postsynaptically. Chronic treatment of rats with morphine produced a 3.5-fold parallel shift to the left in the concentration-response curve for isoproterenol and reduced the EC 50 from 4.8 ± 1.3 to 1.4 ± 0.5 nM. In contrast, sensitivity and maximal responsiveness to isoproterenol was markedly decreased in pyramidal neurons recorded in slices from morphine withdrawn animals. The concentration-response curves for inhibition of the slow AHP by carbachol or forskolin were not affected by chronic morphine treatment. However, blockade of the slow AHP by forskolin was significantly reduced in pyramidal neurons studied after morphine withdrawal. These data suggest that the increase in electrophysiological responsiveness to β-adrenergic receptor stimulation found in hippocampus after chronic morphine treatment most likely resulted from an up-regulation in postsynaptic membrane receptors, whereas alterations occurring beyond the receptor level may be involved in the desentization that is associated with morphine withdrawal.

Beta 2-adrenergic receptor and angiotensin II receptor modulation of sympathetic neurotransmission in human atria.

The aim of the present study was to investigate beta-adrenergic receptor and angiotensin II (Ang II) receptor modulation of norepinephrine release in human atria. Slices of human atrial appendages were incubated with [3H]norepinephrine, superfused with Krebs-Henseleit solution, and electrically stimulated in superfusion chambers. Pretreatment of the tissue with 6-hydroxydopamine (1.2 mmol/L) before the [3H]norepinephrine incubation to destroy sympathetic nerves reduced the uptake of radioactivity and abolished the stimulation-induced (S-I) outflow of radioactivity. Furthermore, S-I outflow of radioactivity was prevented by the addition of tetrodotoxin (1 mumol/L) to and omission of extracellular Ca2+ from the superfusion solution. Separation of [3H]norepinephrine from its metabolites revealed that the S-I outflow of radioactivity was mainly composed of intact [3H]norepinephrine. Thus, the S-I outflow of radioactivity was taken as an index of norepinephrine release. Isoproterenol (0.001 to 0.1 mumol/L) dose-dependently enhanced the S-I outflow of radioactivity. The concentration-response curve of isoproterenol was shifted to the right by the selective beta 2-adrenergic receptor antagonist ICI 118551 (0.01 and 0.1 mumol/L) but not by the beta 1-adrenergic receptor-selective antagonist atenolol (0.3 and 30 mumol/L). Ang II (0.001 to 1.0 mumol/L) also dose-dependently enhanced S-I outflow of radioactivity. The facilitatory effect of Ang II was blocked by either the peptide Ang II receptor antagonist saralasin (1.0 mumol/L) or EXP 3174 (0.1 mumol/L), the in vitro active form of the nonpeptide Ang II receptor antagonist losartan. The cell-permeable cAMP analogue 8-bromo-cAMP (30 to 300 mumol/L) dose-dependently enhanced S-I outflow of radioactivity.(ABSTRACT TRUNCATED AT 250 WORDS)

Selectivity and potency of opioid peptides in regulating human chorionic gonadotropin release from term trophoblast tissue

The in vitro effect of three opioid peptides on hCG release from term trophoblast tissue was investigated. These peptides were prototypic for opioid receptors of the following types: kappa [dynorphin(1–13)], mu (DAMGO) H-Tyr- d-Ala 2-Gly-N-Me-Phe-Gly 5-ol, and delta (DPDPE) H-Tyr-O-Pen-Gly-Phe- d-Pen-OH[ d-Pen 2, d-Pen 5]enkephalin. All peptides stimulated hCG release and their concentration-response curves were bell shaped. Their order of potency was kappa ⋙ mu > delta . Stimulation of hCG release by any of the peptides was totally reversed by opioid antagonists, indicating that the action of peptides is mediated by placental opioid receptors. In order to confirm the specificity of opioid regulation of hCG release, three nonopioid drugs (cocaine, nicotine, and isoproterenol), with binding proteins and receptors known to be present in trophoblast tissue membranes, were also investigated. Stimulation of hCG release caused by certain concentrations of nonopioid drugs was not reversed by opioid antagonists, demonstrating that their effect is not mediated by opioid receptors. Furthermore, the concentration-response curve of isoproterenol was biphasic, suggesting the presence of a mechanism regulating hCG release that is not mediated by placental β-adrenergic receptors. Data presented in this manuscript indicate that placental opioid receptors mediate one of the mechanisms regulating hCG release from trophoblast tissue and confirm our earlier results using opioid drugs.

Effects of Tertatolol on the Responsiveness of Isolated Femoral, Mesenteric, and Renal Resistance Arteries to Adrenergic Stimuli

We evaluated effects of the beta-adrenoceptor antagonist tertatolol on responses of resistance arteries to isoproterenol and phenylephrine and compared them with those of propranolol. The experiments were performed in femoral, mesenteric, and renal resistance arteries that had been isolated from adult Wistar-Kyoto rats, chemically sympathectomized and mounted for recording of contractile reactivity in vitro. In renal resistance arteries that had been pretreated with phenoxybenzamine, isoproterenol did not affect contractile responses to potassium. Under these conditions, isoproterenol induced relaxation in mesenteric resistance arteries. Low concentrations of both SR-tertatolol and SR-propranolol shifted concentration-response curves for isoproterenol to the right. Affinity for competitive beta-adrenergic antagonism did not differ between both agents. Unlike SR-propranolol, SR-tertatolol reduced maximal relaxing responses to isoproterenol. As compared with R-tertatolol, S-tertatolol was 100 times less potent as a competitive beta-adrenergic antagonist and failed to affect maximal relaxing responses to isoproterenol. In femoral, mesenteric, and renal resistance arteries preconstricted by phenylephrine, concentrations of SR-tertatolol greater than 1 microM induced relaxation. This relaxing effect did not differ between SR-tertatolol and SR-propranolol or between S-tertatolol and R-tertatolol. These data indicate that in resistance arteries tertatolol is a stereoselective beta-adrenoceptor antagonist that exhibits both a competitive and a noncompetitive mechanism of action. In addition, high concentrations of tertatolol relax resistance arterial smooth muscle. This effect was neither stereoselective nor selective for the renal vascular bed and was also observed with propranolol.

Isoproterenol sensitivity of isolated cardiac myocytes from rats with monocrotaline-induced right-sided hypertrophy and heart failure

Rats treated with the alkaloid monocrotaline developed right ventricular hypertrophy with a left: right ventricle weight ratio of 1.35 ± 0.10 (mean ± s.e.m., n = 25) compared with 3.83 ± 0.40 ( n = 14) in diet-matched controls ( P < 0.001). Urine volume and sodium content were reduced and body water increased consistent with heart failure. In 10 out of 26 treated rats pleural, pericardial or peritoneal effusions were present. Urine norepinephrine content was significantly raised ( P < 0.02) but epinephrine was unchanged. Plasma norepinephrine levels were raised though not significantly. Myocytes isolated from the right ventricle had a reduced myosin Ca 2+-activated ATPase ( P < 0.05) activity and a shift towards slower V2 and V3 myosin isofroms. There was no decrease in maximum contraction amplitude with calcium or isoproterenol in either left or right ventricular cells of treated rats. Right ventricular cells from treated rats showed a reduced rate of contraction in maximum isoproterenol ( P < 0.05) and a significant rightward shift in PD 2 ( P < 0.05) representing a two-fold increase in EC50 for isoproterenol compared with right ventricular cells from control animals. There was no shift in EC50 for isoproterenol in left ventricle cells. In parallel experiments, myocytes isolated from both ventricles of rats treated with isoproterenol for one week showed a rightward shift of more than 50-fold in the isoproterenol concentration-response curve and a depressed response to maximum isoproterenol. In the rat monocrotaline model of right-sided cardiac hypertrophy and failure, changes in sensitivity to beta-adrenoceptor agonists are slight, and present only in the right ventricle. The lack of change in the left ventricle seems to suggest that this functional desensitisation is not a consequence of raised circulating catecholamines.

An Explanation of Bell-Shaped, Dose-Positive Inotropic Effect Curves for Denopamine in Canine Right Ventricular Muscle

We investigated mechanisms by which denopamine produces bell-shaped, cumulative concentration-response curves for positive inotropic effect in canine right ventricular muscle. As in our previous study, the curves ascended at 10(-8) to 10(-6) M, reached a maximum at 3 X 10(-6) to 10(-5) M, and descended at higher concentrations. The ascending limb gave 6.58 +/- 0.21 for the pD2 value. Denopamine, when administered cumulatively up to 3 X 10(-5) to 3 X 10(-4) M, caused rightward shifts of concentration-response curves for the positive inotropic effect of isoproterenol. Application of a two-component fitting model to the concentration-response curves for isoproterenol in the presence of denopamine yielded two pA2 values of denopamine; 6.59 +/- 0.19 for the high- and 5.05 +/- 0.21 for the low-affinity state of beta 1 receptors. The bell-shaped curves for denopamine were successfully computer-simulated with the following assumptions. (a) Denopamine produces a positive inotropic effect by binding to the high-affinity state of beta 1 receptors with an apparent dissociation constant of 10(-6.59) M. (b) The fraction of the high-affinity state decreases as denopamine binds to the low-affinity state with an apparent dissociation constant of 10(-5.05) M. Cyclic AMP levels were changed by denopamine, isoproterenol, and a combination of both, roughly in a parallel way to changes in positive inotropic effect.

Active sensitisation modifies β-adrenoceptor reactivity in guinea-pig trachea

The occurrence of beta-adrenoceptor desensitisation in clinical experience, after long term therapy with specific beta 2-agonists, is still an open question, even if this phenomenon is easily observed in different experimental models. Since the majority of these experiments have been performed in normal animals, we investigated the possible occurrence of beta-adrenoceptor desensitisation in the ovalbumin actively sensitised guinea-pig model of experimental asthma. The isoproterenol concentration-response curves performed in pilocarpine contracted guinea-pig trachea in vitro were shifted to the right by the beta-adrenoceptor desensitisation procedure, which was achieved by the in vitro administration of isoproterenol (10(-5) M x 2 times x 20 min each) both in normal and ovalbumin sensitised tissues. The same desensitisation procedure markedly affected epinephrine-relaxing capacity in both normal and ovalbumin actively sensitised guinea-pig tracheae. However, the ovalbumin sensitised tissue seemed to be more sensitive than normal to the specific beta 2-agonist procaterol; in parallel the beta 2-mediated relaxation was more impaired by isoproterenol-induced beta-adrenoceptor down regulation in ovalbumin sensitised trachea when compared to normal. Similar results have been obtained using salbutamol as the beta 2-adrenoceptor desensitising agent. The changes in beta-adrenoceptor reactivity between normal and ovalbumin sensitised guinea-pig tracheae seemed to depend on the active sensitisation process. No difference in the degree of beta-adrenoceptor down regulation was observed in passively ovalbumin sensitised guinea-pig trachea as compared to normal. These data suggest that, in this model of experimental asthma, beta-adrenoceptor reactivity is in some way modified and that this phenomenon might contribute to the genesis of asthma.

Comparison of the effect of beta adrenergic antagonists with different ancillary properties on isolated canine and human coronary arteries

Experiments were performed on canine and human isolated coronary arteries to characterise human coronary beta adrenoceptors and to determine whether or not beta blocking agents with different ancillary properties unmask the alpha adrenergic effect of noradrenaline in a similar way. The inhibitory effects of atenolol (a beta1 selective antagonist), epanolol (a beta1 selective antagonist with modest intrinsic sympathetic activity), and propranolol (a non-selective antagonist) were assessed on isoproterenol concentration-response curves. Regression analysis provided slopes not significantly different from unity and similar pA2 values for each agent in both preparations. In a second group of experiments, the effects of noradrenaline (10 mumol.litre-1) were assessed in the absence and presence of beta blockade. At equipotent doses (1 log or 2 log units from the pA2 values) each beta blocking agent unmasked the alpha effect of noradrenaline in the same way. This alpha effect of noradrenaline (10 mumol.litre-1) was completely abolished by prazosin 1 mumol.litre-1 in canine coronary arteries but only partially antagonised in human coronary arteries. Thus the property of a beta blocking agent to unmask the alpha adrenergic effect of adrenaline is mainly related to its affinity for the coronary smooth muscle beta adrenoceptors. These beta adrenoceptors were very similar in both preparations and appear to be mainly beta1. The alpha adrenoceptors seem, nevertheless, to be different and resistant to prazosin in human preparations.

Facilitation of isoproterenol induced airway smooth muscle relaxation by nifedipine

The facilitating effect of nifedipine on isoproterenol induced airway smooth muscle relaxation was studied in guinea pig tracheas. For isometric force measurement, 4 mm tracheal cylinders were suspended in incubation chambers in oxygenated physiologic medium. After 90 minutes of equilibration under 2 grams resting tension, at a temperature of 37°C and pH of 7.4, concentration response curves for isoproterenol were performed with and without the addition of a 1 × 10 −5M nifedipine dose. The experiments were then repeated using tissues precontracted with histamine. Our data show that in the nifedipine pretreated tissues, the EC 50 of isoproterenol is shifted to the left (p < 0.05) probably due to further reduction in cytosolic calcium by nifedipine. Our findings suggest that nifedipine might have a role in the treatment of asthma and obstructive airway disease.

Differential relaxant effects of isoproterenol on methacholine- versus leukotriene D 4-induced contraction in the guinea-pig trachea

The ability of isoproterenol to relax guinea-pig trachea was examined in tissues precontracted with three concentrations of methacholine or three corresponding equieffective concentrations of leukotriene D 4. Although increasing the concentration of either contractile agent progressively shifted isoproterenol concentration-response curves to the right, the shifts produced by methacholine were greater than those produced by leukotriene D 4. Thus, the sensitivity of tracheal smooth muscle to β-agonists depends upon the initial contractile state of the tissue, and the type of agent used to induce tone.

The effect of bromoacetylalprenololmentane on rat atrial tension development and β-adrenoreceptors

In isolated rat atria, 1 μM bromoacetylalprenololmentane shifted the tension development curve for isoproterenol to the right of the control curve. After a 2 h washout period, no significant recovery of the isoproterenol concentration-response curve had occurred. Pretreatment of rat atrial membranes with 1 μM bromoacetylalprenololmentane resulted in an 83% decrease in the concentration of β-adrenoceptors with no change in the KD value for [125I]iodocyanopindolol binding to the receptors left. The results indicated that bromoacetylalprenololmentane was an irreversible β-blocker.

Hydrocortisone and isoproterenol effects on trachealis cAMP and relaxation

Isoproterenol concentration-response curves for cAMP formation and relaxation were determined in control and hydrocortisone-treated strips of canine tracheal smooth muscle. Adenosine 3'5'-cyclic monophosphate(cAMP) formation and muscle relaxation were well correlated, and both responses were enhanced proportionally by hydrocortisone treatment. Guanosine 3'5'-cyclic monophosphate was unchanged by isoproterenol but was increased to a small but significant extent by hydrocortisone. Prostaglandin E2 (not a beta-adrenergic agonist) relaxed the muscle strips, but this effect was not enhanced by hydrocortisone pretreatment. Our data are compatible with the concept that cAMP is an obligatory intermediate in the chain of events by which beta-adrenergic agonists relax airway smooth muscle. The action of hydrocortisone on this process is localized at or before cAMP formation, since it enhanced both cAMP formation and relaxation to the same extent.

Responsiveness of isolated tracheal smooth muscle in a canine model of asthma.

Mongrel dogs were exposed to aerosols ofAscaris suum antigen and classified as aerosol antigen reactive or nonreactive on the basis of pulmonary resistance changes. Two weeks later the same dogs were anesthetized and strips of canine trachealis airway smooth muscle were prepared for in vitro studies. Low concentrations of methacholine caused significantly stronger isometric contractions of strips from aerosol reactive animals than of control strips. Contractile responses to maximal concentrations of methacholine were identical in both groups. Strips were precontracted with a half-maximal effective concentration of methacholine. Cumulative isoproterenol concentration-response curves were then run and strips from control dogs relaxed to a significantly greater degree than those from aerosol reactive dogs. The results indicate that the in vitro pharmacological responsiveness of the trachealis muscle can be correlated with the in vivo responsiveness of these animals to Ascaris suum aerosol bronchial provocation.

Micropuncture evaluation of beta-receptors in pial arteries of cats.

The existence of β-receptors and their possible contribution to the vascular tone of pial arteries was investigated using the microapplication technique combined with the measurement of vascular diameter. Concentration response curves for (+) and (−)propranolol revealed an identical course with no vascular reaction occurring between 2.5×10−10 and 2.5×10−5 M propranolol. Since both (+) and (−)propranolol were without effect on vascular diameter over a wide concentration range it is concluded that the vascular tone is not influenced by β-receptors under these experimental conditions. The increase in vascular diameter (60%) observed at 2.5×10−4 M (+) and (−)propranolol is supposed to be due to an unspecific effect. Concentration response curves for isoproterenol revealed no vascular reaction between 2.5×10−11 and 2.5×10−8 M, dilations of 4–5% at 2.5×10−7 and 2.5×10−6 M, constrictions of about 5% at 2.5×10−4 and of 3% at 1.25×10−3 M isoproterenol. These weak vascular reactions cannot be explained by reduced biological activity of isoproterenol due to autoxidation during storage. It is concluded that β-receptors are of little or no physiological significance for regulation of the vascular tone of pial arteries.