Antagonist characterization of atypical beta adrenoceptors in guinea pig ileum: blockade by alprenolol and dihydroalprenolol

The present study was undertaken to further characterize the atypical beta adrenoceptor in guinea pig ileum. Tension was developed in isolated segments of ileum using transmural electrical stimulation of enteric cholingeric nerves. The ability of isoproterenol to relax the ileum, via beta-1 adrenoceptor and atypical beta adrenoceptor agonism, was measured. Propranolol (5 x 10(-6) M) and bromoacetylaprenololmetane blocked beta-1 adrenoceptors but, at the concentrations tested, were without affinity at atypical beta adrenoceptors. (-)-Alprenolol and (-)-dihydroalprenolol, however, acted as competitive antagonists at both sites (pA2 values of 8.2 and 8.81 at beta-1 adrenoceptors and 6.47 and 6.43 at atypical beta adrenoceptors, respectively). (-)-Alprenolol also exerted agonistic activity at the atypical beta adrenoceptor. [3H](-)-Dihydroproalprenolol failed to identify beta-1 adrenoceptors or atypical beta adrenoceptors but, instead, bound to a putative lipophilic site unrelated to ileal adrenoceptors. Before this study, nadolol (pA2 = 4.7) was the only documented antagonist at the atypical beta adrenoceptor in guinea pig ileum. Thus, the present results detail two additional pharmacological probes which exhibit about a 100-fold greater affinity than nadolol for the atypical site.     

Pharmacological analysis of the cardiac actions of xamoterol, a beta adrenoceptor antagonist with partial agonistic activity, in guinea pig heart: evidence for involvement of adenylate cyclase system in its cardiac stimulant actions

The pharmacological effects of xamoterol, a beta adrenoceptor antagonist with partial agonistic activity, were examined in guinea pig cardiac preparations and compared with those of isoproterenol to assess possible mechanisms of its cardiac stimulant actions. Xamoterol produced a positive inotropic effect in the papillary muscles and a positive chronotropic effect in the spontaneously beating right atria in a concentration-dependent manner. The maximum inotropic and chronotropic effects of xamoterol were about 33 and 35% of those of isoproterenol, respectively. Although xamoterol failed to produce a consistent increase in contractile force in the left atria, the positive inotropic effect of the agent was observed clearly in preparations obtained from reserpine-pretreated animals. The positive inotropic and chronotropic effects of xamoterol were antagonized by atenolol, but not by ICI 118,551. On the other hand, xamoterol antagonized competitively the positive inotropic and chronotropic responses to isoproterenol. In papillary muscles the increases in contractile force induced by xamoterol and isoproterenol were depressed markedly in the presence of carbachol or adenosine. In all of left atria, right atria and papillary muscles obtained from reserpine-pretreated animals, xamoterol caused a significant elevation in cyclic AMP levels, while inhibiting the isoproterenol-induced increase in cyclic AMP levels. Computer-assisted analysis of concentration-response curves for the inhibition by xamoterol of the binding of [125I]iodocyanopindolol in the membranes from guinea pig ventricles showed the existence of the 5'-guanylylimidodiphosphate sensitive, highly affinity site of beta adrenoceptors for xamoterol, suggesting that xamoterol may induce the formation of a ternary complex with the beta adrenoceptor and a stimulatory guanine nucleotide regulatory protein.(ABSTRACT TRUNCATED AT 250 WORDS)     

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.     

In vitro effects of beta adrenoceptor agonists and antagonists on the rat ovarian suspensory ligament

The mesovarian suspensory ligament of the rat was used to compare the activities of beta adrenoceptor agonists and antagonists. The following beta adrenoceptor agonists, in descending order of potency, inhibited spontaneous activity in a dose-related manner: zinterol greater than isoproterenol much greater than dobutamine. Several noncardioselective, beta-2 adrenoceptor antagonists with intrinsic sympathomimetic activity (ISA) also inhibited the activity of the ligament: pindolol greater than alprenolol = bucindolol = oxprenolol greater than labetalol. Maximal relaxation induced by the antagonists was equivalent to that caused by the beta receptor agonists. Two cardioselective, beta adrenoceptor antagonists with ISA, acebutolol and practolol, did not inhibit the activity of the suspensory ligament but did increase the rate of the isolated right atrium of the rat. The maximal increases in atrial rate evoked by the antagonists were significantly less than those induced by the beta adrenoceptor agonists. Studies with ICI 118,551 or atenolol as beta-2 or beta-1 selective adrenoceptor blockers, respectively, suggest that the beta adrenoceptors of the suspensory ligament are predominantly of the beta-2 subtype. The possible relevance of these results to the induction of mesovarian leiomyomas in rats by noncardioselective beta adrenoceptor agonists and antagonists with ISA is discussed.     

In vitro characterization of the adrenoceptor activity of AY-28,925

The adrenoceptor activity of AY-28,925 (5-[1-hydroxy-2-[(1-methyl-3-phenylpropyl)amino]ethyl]-1 H-indole-7-carboxamide) was evaluated on various isolated vascular and nonvascular tissues. Based upon pA2 values derived from either the antagonism of the effect of isoproterenol on paced guinea pig left atria (pA2 = 7.3), spontaneously beating guinea pig right atria (pA2 = 7.4) or canine saphenous vein (pA2 = 7.3), AY-28,925 demonstrated nonselective beta adrenoceptor antagonist activity. Studies using the spontaneously beating guinea pig right atria and the prostaglandin F2 alpha-contracted canine saphenous vein indicated that AY-28,925 also possessed nonselective beta-adrenoceptor intrinsic efficacy. AY-28,925 was shown to be a selective alpha-1 adrenoceptor antagonist with a pA2 value which was greater against phenylephrine in the rabbit aorta (pA2 = 6.6) than against clonidine in the rat vas deferens (pA2 = 5.4) or B-HT 920 in the canine saphenous vein (pA2 = 5.3). Only in concentrations greater than 2000 times those required for adrenergic activity was AY-28,925 able to inhibit potassium chloride- or prostaglandin F2 alpha-induced contractions of the rabbit aorta. The compound had no significant negative intropic activity. These experiments indicate that AY-28,925 possesses, in decreasing orders of activity: nonselective beta-adrenoceptor properties; a selective alpha-1 adrenoceptor inhibitory effect; and a nonspecific direct relaxing action on vascular smooth muscle.     

A beta adrenoceptor with atypical characteristics is involved in the relaxation of the rat small intestine

In several studies in guinea pig ileum or rat colon a beta adrenoceptor with characteristics distinct from beta-1 or beta-2 receptors has been observed and has been denoted as "atypical" beta adrenoceptor. In this study the relaxation of the rat small intestine was investigated, using isolated segments of the rat jejunum. Several beta-1 or beta-2 agonists and antagonists were tested on the rat jejunum preparation, and it was found that nonselective and selective antagonists for beta-1 or beta-2 receptors showed a relatively low affinity, compared to their affinity for beta-1 or beta-2 receptors. BRL 37344, an agonist which has been reported to be selective for the atypical beta adrenoceptor, was more potent although a partial agonist compared to isoprenaline, whereas it is clearly less active than isoprenaline on beta-1 or beta-2 receptors. These findings indicate that beta adrenergic relaxation of the rat small intestine is mediated via atypical beta adrenoceptors. Efforts were made to confirm these findings with binding studies on small intestinal 45,000-g membranes. Competition radioligand binding experiments were performed with the radiolabeled ligand [125I]iodocyanopindolol and the various antagonists which were also tested in the intact rat jejunum preparations. Receptor binding experiments only revealed beta adrenoceptors of the beta-2-subtype, which does not correspond with the results obtained in the jejunum relaxation. Probably the beta-2 receptors found in the binding studies are located on circular smooth muscle cells or on epithelial cells, whereas longitudinal smooth relaxation is mediated by atypical beta adrenoceptors. Atypical beta adrenoceptors were not measured in binding studies probably because [125I]iodocyanopindolol is an unsuitable ligand to label atypical intestinal beta adrenoceptors.     

Heterogeneity of beta adrenoceptors in right atria isolated from cold-exposed rats

The chronotropic response of right atria isolated from 5-day-cold-exposed rats to isoproterenol and norepinephrine was studied. A large increase in the sensitivity of the pacemaker to isoproterenol and a decrease in the sensitivity to norepinephrine occurred. Determination of pA2 values of propranolol and metoprolol using isoproterenol and norepinephrine as agonists and analysis of the slopes of Schild plots suggested that in atria isolated from control rats the chronotropic effect of isoproterenol and norepinephrine resulted from the preferential interaction of the catecholamines with a homogeneous beta-1 adrenoceptor population. After cold exposure the affinity of atrial adrenoceptors for propranolol increased when the agonist was isoproterenol and decreased when norepinephrine was used. The slopes of the Schild plots of metoprolol when the agonists were isoproterenol or norepinephrine were not unitary unless the experiments were performed in the presence of butoxamine. However, butoxamine prevented the demonstration of cold-induced super-sensitivity to isoproterenol, leaving the subsensitivity to norepinephrine unaffected. It is concluded that cold-induced heterogeneity of the atrial beta adrenoceptors is responsible for the increased sensitivity to isoproterenol. Probably, subsensitivity to norepinephrine resulted from conformational alterations of the atrial beta-1 adrenoceptors.     

Adrenoceptor blocking properties of the stereoisomers of amosulalol (YM-09538) and the corresponding desoxy derivative (YM-11133)

The antagonist potencies of amosulalol (YM-09538), its stereoisomers and the corresponding desoxy derivative (YM-11133) have been compared at alpha-1, alpha-2, beta-1 and beta-2 adrenoceptors in isolated organs in vitro and in radioligand binding experiments. In isolated peripheral tissues, (+/-)-, (-)- and (+)-amosulalol and YM-11133 are selective alpha-1 adrenoceptor antagonists over alpha-2 adrenoceptors by two orders of magnitude and are nonselective beta adrenoceptor antagonists. (+)-Amosulalol and YM-11133 were 14 and 9.3 times more potent than (-)-amosulalol as alpha-1 adrenoceptor antagonists but approximately 50 and 40 times less potent antagonists at beta adrenoceptors than (-)-amosulalol, respectively. The adrenoceptor blocking profile of the racemate is approximately 2-fold less potent than that of the (+)-isomer at alpha and that of the (-)-isomer at beta adrenoceptors. The affinities of (+/)-, (-)- and (+)-amosulalol and YM-11133 obtained from radioligand binding experiments (pKi) using rat brain membrane correlated well with those obtained from in vitro experiments (pA2) at alpha-1 (r = 0.884), alpha-2 (r = 0.977), beta-1 (r = 0.993) and beta-2 (r = 0.971) adrenoceptors. These results indicate that the stereochemical requirements of alpha and beta adrenoceptors differ for the stereoisomers of amosulalol with the alpha adrenoceptor subtypes favoring the (+)-isomer and the desoxy form and the beta subtypes favoring the (-)-isomer.     

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)     

Intrinsic sympathomimetic activity of the partial agonist prenalterol in relation to beta adrenoceptor interaction in various tissues, in vitro

The intrinsic sympathomimetic activity (ISA) of prenalterol was studied in isolated tissues from different species, including tissues containing predominantly beta-1 adrenoceptors (cardiac preparations from cat, rabbit, rat and guinea pig) and tissues characterized by beta-2 adrenoceptor predominance (cat skeletal muscle and rat uterus). The ISA of prenalterol, varying between 0 and 94% in the various tissues, was found to be positively correlated to the stimulatory potency (-log EC50) of isoproterenol and prenalterol. In the cardiac preparations from the rabbit there was an interindividual variation in the ISA of prenalterol, which was also positively correlated to the stimulatory potency of the beta agonists. The density of beta adrenoceptors in the tissues studied correlated neither to the variable ISA of prenalterol nor to the -log EC50 values of isoproterenol or prenalterol. The affinities of isoproterenol and prenalterol for the beta adrenoceptors were subject to less variation than were the stimulatory potencies of the agonists. The degree of separation between the concentration-effect curves for beta adrenoceptor occupancy and mechanical performance, expressed as the ratios Kd/EC50 for both agonists, were positively correlated to the corresponding ISA of prenalterol in various tissues. However, a considerably steeper relationship between occupancy/potency ratio and ISA was seen with prenalterol than with isoproterenol. The present data suggest that the level of ISA of the partial agonist, prenalterol, depends upon the efficiency of signal transmission from the activated receptor to the final end-organ response. The separation between the concentrations of the full agonist, isoproterenol, required for receptor occupancy and response serves as an index of the efficiency of coupling between the stimulus, elicited by activation of the receptor, and the response.     

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.     

Vascular alpha-1 antagonistic and agonistic effects of beta adrenoceptor agonists in rabbit common carotid arteries.

The stainless-steel cannula-inserting method was used to observe vascular effects of mixed and selective beta adrenoceptor agonists, isoproterenol, procaterol and denopamine, on isolated, perfused rabbit common carotid arteries. In phenylephrine-preconstricted preparations, the three beta agonists induced a dose-dependent vasodilation which was not suppressed by treatment with beta antagonists, atenolol, a selective beta-1 antagonist and ICI 118551, a selective beta-2 antagonist. On the other hand, in prostaglandin F2 alpha-preconstricted preparations, these agonists produced no vasodilation and revealed weak vasoconstrictions which were readily suppressed by bunazosin, a selective alpha-1 antagonist. Moreover, these agonists caused a shift of the dose-response curve for phenylephrine to the right in a parallel fashion in non-preconstricted preparations. The relative pA2 values for isoproterenol, procaterol and denopamine calculated from the displacement curve were 7.47, 7.59 and 8.17, respectively. Thus, it is concluded that 1) there are little functional beta adrenoceptors in the rabbit common carotid arteries, 2) beta adrenoceptor agonists have both antagonistic and agonistic properties for alpha-1 adrenoceptor activation, 3) denopamine possesses a higher potency as an alpha-1 antagonist and 4) beta agonists generally act as vasodilators in rabbit cerebral circulation.     

CH-38083, a selective, potent antagonist of alpha-2 adrenoceptors

The selectivity and specificity of CH-38083 [7,8-(methylenedioxi)-14-alpha-hydroxyalloberbane HCl], a berbane derivative for alpha adrenoceptors has been studied and compared with yohimbine and idazoxan in peripheral tissues and in the central nervous system. In isolated tissue experiments CH-38083 was a competitive antagonist at presynaptic alpha-2 adrenoceptors on the axon terminals of the rat vas deferens (pA2 against xylazine = 8.17 +/- 0.06) and of the longitudinal muscle strip of guinea pig ileum (pA2 against xylazine = 8.07 +/- 0.20). As far as its postsynaptic alpha-2 adrenoceptor antagonistic activity is concerned its affinity in rat vas deferens (pA2 = 4.95 +/- 0.11) against l-phenylephrine and in rabbit pulmonary artery (pA2 = 5.38 +/- 0.33 against l-norepinephrine) was markedly less than that displayed for presynaptic sites. From pA2 values obtained in rat vas deferens the calculated alpha-1/alpha-2 adrenoceptor selectivity ratios for yohimbine, idazoxan and CH-38083 were 4.7, 117.5 and 1659, respectively. CH-38083 failed to show any affinity for histamine and muscarinic receptors and it even potentiated the effect of serotonin on atropinized longitudinal muscle strip of guinea pig ileum. It enhanced the release of [3H]norepinephrine from electrically stimulated mouse vas deferens loaded previously with labeled [3H]norepinephrine. In binding studies carried out in rat brain membrane preparations using [3H]prazosin and [3H]idazoxan, the selectivity ratios (Ki alpha-1/Ki alpha-2) proved to be 32.5, 289.5 and 1368 for yohimbine, idazoxan and CH-38083, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)     

Indications for vascular alpha- and beta-2 adrenoceptors in synapses of the muscarinic pathway in the pithed normotensive rat

In the pithed normotensive rat the adrenoceptors involved in the hypertensive and tachycardic effects of the indirectly acting sympathomimetic agent tyramine and of electrical stimulation of the spinal cord (TH5-L4 or C7-Th1) were analyzed. The tools used for the identification of the adrenoceptors were the selective alpha-1 adrenoceptor blocking drug prazosin, the selective alpha-2 adrenoceptor antagonist rauwolscine, the beta-1 blocker atenolol and the selective beta-2 adrenoceptor blocking agent ICI 118,551. The participation of vascular alpha-2 adrenoceptors in the pressor response of tyramine was shown. The increase in blood pressure induced by electrical stimulation of the spinal cord (TH5-L4) was used to demonstrate that alpha-2 adrenoceptors were activated via ganglionic muscarinic receptors. The tachycardia evoked by electrical stimulation of the spinal cord (C7-Th1) was not influenced by beta-2 adrenoceptor blockade. It was enhanced, however, by the alpha-2 adrenoceptor antagonist rauwolscine. It is hypothesized that activation of ganglionic nicotinic receptors leads to stimulation of the nearest varicosities interfering with alpha-1 adrenoceptors. However, activation of ganglionic muscarinic receptors may lead to an additional release of neurotransmitter in the more distant varicosities endowed with alpha-2 or beta-2 adrenoceptors.     

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.     

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)     

Specialization in beta-1 and beta-2 adrenoceptor distribution in veins of the rabbit face: relationship to myogenic tone and sympathetic nerve innervation

Studies were performed on several superficial veins from the rabbit face to examine the relationship between beta adrenoceptor subtype distribution, intrinsic myogenic tone and sympathetic nerve innervation. Experiments using selective beta adrenoceptor agonists and antagonists indicate that the dorsal nasal and angularis oculi veins possess a homogeneous population of beta-2 adrenoceptors. Sympathetic nerve stimulation in these segments results only in contraction mediated through postjunctional alpha adrenoceptors. These segments are devoid of intrinsic myogenic tone. In the facial vein, of which both veins are tributaries, both beta-1 and beta-2 adrenoceptors are found. Studies with the beta-1 adrenoceptor antagonist, betaxolol, and beta-2 adrenoceptor antagonist, ICI 118,551, indicate that the prominent relaxation observed in this tissue to sympathetic nerve stimulation is mediated through postjunctional beta-1 adrenoceptors. At physiological temperatures, the facial vein possesses a marked intrinsic myogenic tone that is inhibited by this beta-1 adrenoceptor-mediated sympathetic activity. Considering the anatomical relationship between these vessels and the unique association between beta adrenoceptor subtype, intrinsic myogenic tone and sympathetic innervation, it is possible that facial blood redistribution in the rabbit can be markedly affected by sympathetic nerve stimulation. Such a process could have an important role in cranial thermoregulation.     

Localization of beta adrenoceptor subtypes in rat kidney by light microscopic autoradiography

The characteristics and localization of beta adrenoceptor subtypes in rat kidney sections have been examined using [125I]cyanopindolol and in vitro labeling combined with autoradiography. Binding was stereoselective since the (-)-isomers of propranolol and pindolol were some two orders of magnitude more effective as competitors than the (+)-isomers. Competition curves obtained using the subtype selective antagonists ICI 118,551 (beta-2) and betaxolol (beta-1) had low pseudo Hill coefficients and were resolved into two distinct components representing beta-1 (63%) and beta-2 adrenoceptors (37%). Combined autoradiographic and histochemical studies using nuclear emulsion-coated coverslips and alkaline phosphatase staining showed that the majority of receptors were in the renal cortex and in the outer band of the medulla with fewer receptors associated with the inner medulla, papilla and renal blood vessels. Delineation of beta-1 and beta-2 adrenoceptor subtypes with the selective antagonists betaxolol and ICI 118,551 indicated that the highly localized receptors were predominately of the beta-1 subtype, associated with glomeruli and with tubules that from their staining characteristics and topography represent distal and cortical collecting tubules with few if any receptors associated with proximal tubules. Beta-2 adrenoceptors were more diffusely distributed in the cortex and there were minor areas of localization in the inner medulla. Although some glomerular beta adrenoceptors probably play a role in control of renin release, their distribution throughout this structure indicates that they also control other functions. The distribution of beta adrenoceptors in tubules corresponds well with the known distribution of beta adrenoceptor-stimulated adenylate cyclase in rat kidney and indicates that these receptors subserve a physiological function.     

Intramural nerve-mediated inotropic responses of left atria of rats and guinea pigs: demonstration of alpha adrenergic and nonadrenergic noncholinergic responses in guinea pigs

The effects of transmural nerve stimulation (TNS) on contractile responses of rat and guinea pig atria were analyzed pharmacologically. Isolated left atria were electrically driven through AgAgCl field electrodes and TNS was performed by brief introduction of defined stimulation patterns through the same electrodes. Step elevations in stimulating voltage induced biphasic inotropic responses in the left atria of both species: an initial negative component which was usually overwhelmed by a subsequent positive one. The transient negative inotropic response was induced by parasympathetic cholinergic nerve excitation, inasmuch as it was abolished by atropine. In the left atrium of the rat, the TNS-induced positive inotropic response was due exclusively to adrenergic nerve excitation through activation of beta-1 adrenoceptors. In contrast, analysis of the time course of responses in guinea pig left atria after nerve stimulation at 10 Hz revealed a positive inotropic response consisting of two phases; rapid and delayed phases were superimposed upon each other. The rapid phase was reduced by atenolol, a beta-1 antagonist, and attenuated further by prazosin, an alpha-1 antagonist. In the presence of both atenolol and prazosin, TNS of guinea pig left atria still induced a positive inotropic response but it had a slow onset and decay. This is termed the delayed phase response. TNS induced a similar delayed inotropic response in atria from surgically sympathectomized or reserpine-pretreated guinea pigs, from which catecholamine-fluorescence nerves and responses to tyramine were absent. These results demonstrate that TNS excitated adrenergic, cholinergic and nonadrenergic noncholinergic nerves in guinea pig left atria.(ABSTRACT TRUNCATED AT 250 WORDS)     

Further evidence for a homogeneous population of beta-1-adrenoceptors in bovine coronary artery

Isolated rings from the proximal and distal ends of the bovine epicardial anterior descending coronary artery and guinea pig tracheal rings were mounted in tissue baths for the measurement of isometric contraction and relaxation, and spontaneously beating rabbit atria were prepared for measurement of chronotropic response. All tissues were exposed to phenoxybenzamine to block tissue uptake of catecholamines and alpha adrenoceptors. The arterial and tracheal rings were contracted with 25 mM K+ in order to observe agonist-induced relaxation. Beta adrenergic agonist dose-response curves were obtained in all tissues in the presence and absence of the beta-2 selective antagonist, ICI 118,551. The orders of agonist potencies in the proximal and distal coronary arteries and the rabbit atria were the same: namely, isoproterenol (ISO) greater than norepinephrine (NE) much greater than procaterol. In contrast, that in the guinea pig trachea was procaterol greater than ISO greater than NE. Procaterol was a weak, partial agonist in the bovine coronary artery and rabbit atria and a potent, full agonist in the guinea pig trachea. The ICI 118,551 pA2 values in the proximal segment of the bovine coronary artery were the same against both NE and fenoterol, 7.38. In the distal segment, the values were 7.33 and 7.14, respectively. ICI 118,551 -log KB values in the rabbit atria, 6.81 and 6.83 for NE and ISO, respectively, were slightly below those in the coronary artery segments, whereas -log KB values in the guinea pig trachea were substantially higher, 8.19 and 8.81 for fenoterol and procaterol, respectively.(ABSTRACT TRUNCATED AT 250 WORDS)