Rat Heart Membranes Research Articles

Differential expression of α-CGRP and β-CGRP by primary sensory neurons and enteric autonomic neurons of the rat

Expression of the calcitonin gene-related peptide, α-calcitonin gene-related peptide (CGRP), and the homologous β-CGRP were compared in sensory and enteric nerves of the rat. Anatysis of CGRP-like immunoreactivity by cation exchange chromatography and radioimmunoassay showed that in the dorsal root ganglia, dorsal spinal cord and in those peripheral tissues where CGRP-like immunoreactivity is primarily localized to sensory libres, α-CGRP concentrations were three to six times greater than β-CGRP concentrations. In the intestine, however. β-CGRP concentrations were up to seven times greater than α-CGRP concentrations. Only β-CGRP was detected in the intestines of capsaicintreated rats. Northern blot and in situ hybridization to α-CGRP- and β-CGRP-speeific probes showed that while both α-CGRP and β-CGRP messenger ribonucleic acids occurred in the dorsal root ganglia, only β-CGRP messenger ribonucleic acid occurred in the intestine, where it was localized to enteric neurons. Receptor binding sites on membranes of rat heart and colon had approximately equal affinities for α-CGRP and β-CGRP. The two peptidcs were equipotent in increasing the rate and force of atrial contractions but α-CGRP was slightly (2.6 times) more potent than β-CGRP in relaxing colonie smooth muscle.Thus, both α-CGRP and β-CGRP occur in the rat nervous system and are both biologically active. Sensory neurons and enteric neurons have been identified as populations which preferentially express α-CGRP and β-CGRP. respectively.

Effects of amiloride and its analogues on [ 3H]batrachotoxinin-A 20-α benzoate binding, [ 3H]tetracaine binding and 22Na influx

The ability of amiloride and its analogues to inhibit [ 3H]batrachotoxinin-A 20-α benzoate ([ 3H]BTX-B) and [ 3H]tetracaine binding to rat synaptosomes and to a rat heart membrane preparation was tested. Their ability to inhibit 22Na influx was determined with rat synaptosomes. 5-N-substituted analogues were generally more potent in inhibiting [ 3H]BTX-B and [ 3H]tetracaine binding than compounds substituted on the guanidine group. However, the inhibition was not competitive. Amiloride and some of its analogues were as active or more active in inhibiting [ 3H]tetracaine binding than they were in inhibiting [ 3H]BTX-B binding. 22Na influx was inhibited with the same relative potencies as [ 3H]BTX-B binding and a good correlation was found between the two inhibitions. These results show an effect of amiloride and its analogues on the voltage-sensitive Na + channels, which could partly explain the inotropic effects of these drugs.

Muscarinic cholinergic receptors on intact human lymphocytes properties and subclass characterization

Saturable specific binding of tritiated N- methyl-scopolamine ( 3H-NMS) can be demonstrated on intact circulating human lymphocytes, with an average K D of 7 nmol/liter and an average density of about 15 fmol/10 6 cells. Specific 3H-NMS binding can be inhibited by cholinergic antagonists and agonists, is highly stereospecific for the enantiomers of the cholinergic antagonist quinuclidinyl benzilate (QNB), and is modulated by the stable guanosine triphosphate (GTP) analog GppNH p in a fashion similar to the specific binding of the same radioligand to rat heart membranes. The results indicate the presence of muscarinic cholinergic receptors on intact human lymphocytes, with a predominance of the M/ 2 subtype. As central muscarinic cholinergic receptors have been reported to play an important role in the pathogenesis of affective disorders, the lymphocyte might represent a suitable model for the study of muscarinic receptor functions in humans .

A bromoacetylated analogue of cyanopinoolol: An irreversible antagonist at rat beta-adrenoceptors

A high affinity, chemically reactive cyanopindolol derivative, N 8-bromoacetyl-N 1-3′-(2-cyano-4-indolyloxy)-2′-hydroxypropyl-[Z]-1,8-diamino-p-menthane (Br-CYP) was synthesized and its interaction with β-adrenoceptors characterized. Studies with rat heart, lung, brain, and red blood cell membranes indicated that the compound displaced 3H-dihydroalprenolol ( 3H-DHA) from β-adrenoceptors with IC 50 values in the nanomolar range. The concentration of functional β-adrenoceptors in membranes was markedly reduced when membranes were preincubated with Br-CYP and then extensively washed prior to assay. (±)Alprenolol and (-)isoproterenol, not (+) isoproterenol, when included in the preincubation prevented this reduction in binding sites by Br-CYP. Br-CYP was active in vivo when injected intraperitoneally into rats. A dose of 10 μg/kg reduced the concentration of binding sites in membranes from heart by 30%, lung by 36%, and RBC by 70%, but did not affect sites on brain membranes 16 hours after injection. Higher doses blocked virtually all the 3H-DHA binding sites in the peripheral organs studied. Br-CYP reduced the concentration of β-adrenoceptors in membranes from these same tissues (but not brain tissue) as long as two weeks after injection with recovery of binding occurring more rapidly in heart tissue than lung and red blood cells. These results suggest that Br-CYP may be a useful compound for in vivo studies of the biochemistry and pharmacology of β-adrenergic systems.

Characterization of picomolar affinity binding sites for [125I]-human calcitonin gene-related peptide in rat brain and heart.

We have characterized picomolar affinity binding sites for human calcitonin gene-related peptide (CGRP) in rat brain and heart (atria and ventricle) membranes. By saturation analysis, apparent dissociation constant (KD) values of high affinity sites for [125I]-human CGRP are 9 approximately 15 pM (brain), 34 pM (ventricle) and 85 pM (atria). Low affinity sites with KD values of about 50 nM are found in rat brain and ventricle, but not in atria. Human and rat CGRP potently inhibited [125I]-human CGRP binding to these high affinity sites with apparent inhibition constant (Ki) values comparable to their KD values. Salmon calcitonin marginally inhibited these binding with Ki values between 0.1 microM and 1 microM. Extremely potent cardiovascular and gastrointestinal actions of CGRP might be mediated through CGRP binding sites with picomolar affinity which are similar to those we characterized in this study.

.beta.-Adrenoceptor antagonist activity of bivalent ligands. 1. Diamide analogs of practolol.

Two series of bivalent ligands (P-X-P) containing the (R,S)-3-[(4-aminoaryl)oxy]-1-(isopropylamino)propan-2-ol pharmacophore and a connecting alpha,omega-dicarbonylpoly(methylene) [X = -OC(CH2)nCO-] or alpha,omega-N,N'-bis(carbonylmethylene) polymethylenediamine [X = -OCCH2NH(CH2)nNHCH2CO-] spanner were synthesized and evaluated for beta-adrenoceptor antagonist activity in rat heart and lung membrane preparations. The target compounds were obtained as a mixture of stereoisomers in modest yields by using a three to four step sequence beginning with N-benzylpractolol. The results from the competitive binding studies indicated that binding affinity increased by a factor of up to 160 by increasing the length of the group spanning the pharmacophore moieties. Modest increases in cardioselectivity were also obtained. The data suggest that further increases in spanner length and lipophilicity and optical resolution may improve the potential of a labeled bivalent beta 1-adrenoceptor antagonist to function as a myocardial imaging agent.

Characterization of dermorphin binding to membranes of rat brain and heart

Binding of dermorphin to the two major opioid receptor types, μ and δ, in rat brain membranes was examined by displacement of [ 3H] [D-Ala 2,MePhe 4,Gly-(ol) 5]enkephalin (DAGO) and [ 3H]-[D-Ala 2,D-Leu 5]-enkephalin (DADLE) binding. Affinity of dermorphin binding to μ sites, K d = 1.24 nM, was almost 3 times greater than that of DAGO, K d = 3.35 nM. In contrast, the K d value of dermorphin binding to δ sites was 78 nM only, as compared to K d = 2.27 nM for DADLE. Dermorphin was ineffective in displacing [ 3H]ethylketocyclazocine (EKC) binding to κ receptors after prior blocking of [ 3H]EKC binding to μ and δ sites. Studies of dermorphin binding to μ sites revealed that the potency of dermorphin increased in the presence of Na + (+31%) but decreased in the presence of Mn 2+ (−81%) or Gpp(NH)p (−44%). Displacement of bound [ 3H]diprenorphine (DPN) by dermorphin from atrial membranes of the rat heart, left side, was detectable, suggesting the presence of μ sites in this section of the heart.

Effects of some antianginal and vasodilating drugs on sodium influx and on the binding of 3H-batrachotoxinin-A 20-alpha-benzoate and 3H-tetracaine.

The effects of antianginal drugs, especially arylalkylamines and structurally related derivatives, on 3H-batrachotoxinin-A 20-alpha-benzoate (3H-BTX-B) binding and on 3H-tetracaine binding were studied on rat synaptosomal and heart membrane preparations. The effect of the same drugs on the Na+ influx induced by protoveratrine B was studied on the rat synaptosomal preparation. Antianginal drugs tested inhibited 3H-BTX-B binding in rat synaptosomes, arylalkylamine derivatives being the most potent: IC50 values were 27 nM for flunarizine, 32 nM for prenylamine, 79 nM for cinnarizine. Similarly, these drugs were the most potent when tested in cardiac membrane preparations. All the drugs tested were very weak inhibitors of 3H-tetracaine binding (IC50 ranging from 0.01 mM to more than 1 mM) except for guanabenz, which was more potent (IC50:0.3 microM on the synaptosomal preparation). The various drugs tested inhibited the 22Na+ influx induced by protoveratrine B, with IC50 values ranging from 15 microM (prenylamine) to 110 microM (verapamil), with the exception of nifedipine which had an IC50 of more than 0.1 mM. The inhibition of 22Na+ influx correlated well with the inhibition of 3H-BTX-B binding. These findings suggest that some antianginal drugs, especially the arylalkylamines may have, in addition to their calcium antagonist activity, direct effects on sodium channels.

Identification of two calcium channel receptor sites for [3H]nitrendipine in mammalian cardiac and smooth muscle membrane.

Various Ca-channel blockers differ in cardiovascular action despite common effects at the Ca channel. Many investigators have reported only a single high-affinity receptor for binding of [3H]nitrendipine, a dihydropyridine Ca-channel blocker. Its equilibrium dissociation constant (Kd) does not match the concentration of nitrendipine needed for a physiological effect on the mammalian cardiac Ca channel. The purpose of these studies was to clarify the existing discrepancy between pharmacological properties of nitrendipine receptors and the physiological effects of the dihydropyridine blockers. Of particular importance in this regard was to provide a pharmacological correlate for electrophysiological studies demonstrating multiple voltage-dependent conformational states of the Ca channel, which show differing affinities for the dihydropyridine Ca-channel blockers. By use of an improved ligand binding assay, our studies demonstrate both "high-affinity" and "low-affinity" [3H]nitrendipine receptors with Kd values corresponding well with observed physiologically effective nitrendipine concentrations. We detected two distinct populations of nitrendipine receptors in rat heart and bovine aortic membrane. A high-affinity Kd value of 0.2-0.3 nM was found, which seems to correspond to the physiologically functional state of the Ca channel in smooth muscle, since the Kd value is similar to the concentration at which nitrendipine inhibits contraction. However, in contrast to numerous other studies, we observed that the predominant component of [3H]nitrendipine binding (95-99%) had a low-affinity Kd value (235 nM). This putative low-affinity [3H]nitrendipine receptor may correspond to the physiologically functional state of the Ca channel in cardiac muscle.

Allosteric Modulation of [3H]Nitrendipine Binding to Cardiac and Cerebral Cortex Membranes by Amiodarone

The possible interaction between the antianginal and antiarrhythmic drug amiodarone and the slow calcium channel was investigated by competition binding experiments in guinea-pig cerebral cortex and rat heart membranes using [3H]nitrendipine as radioligand. Amiodarone displaced specifically bound [3H]nitrendipine from cerebral cortex and cardiac membranes in an apparently competitive manner. In saturation binding experiments, apparent affinity for [3H]nitrendipine progressively decreased with increasing concentrations of amiodarone, whereas maximal binding capacity (Bmax remained unchanged. Both diltiazem and verapamil reversed the inhibitory effect of amiodarone on [3H]nitrendipine binding to cerebral cortex membranes. Together these results suggest that amiodarone exerts a pseudocompetitive inhibition on [3H]nitrendipine binding by acting at a site in allosteric interaction with the 1,4 dihydropyridine binding site associated with the calcium channel. The data are compatible with the existence of a common binding site for diltiazem, verapamil, and amiodarone. These observations are discussed in connection with the pharmacological properties of the drug.

Propylbenzilylcholine mustard is selective for rat heart muscarinic receptors having a low affinity for agonists.

Propylbenzilylcholine mustard (PrBCM), an irreversible muscarinic antagonist, inactivated receptors with a low affinity for agonists faster than those with a high affinity in rat heart membranes. This result was obtained using either: (a) a low ionic strength buffer (allowing heterogeneity among antagonist binding sites, (b) the same buffer enriched with GTP, or (c) a high ionic strength buffer (where antagonists showed similar binding characteristics to all receptors). These data suggest either that PrBCM is a 'selective' antagonist which detects conformational differences between low and high affinity receptors, or that the agonist affinity of cardiac muscarinic receptors is determined, in part, by the relative concentrations of receptor and GTP binding protein.

Solubilization and hydrodynamic characterization of the dihydropyridine receptor from rat ventricular muscle.

The dihydropyridine receptor-calcium channel complex, prelabeled with (+)-[3H]PN200-110, was solubilized from rat heart membranes with a detergent mixture of digitonin and Triton X-100. The dissociation of (+)-[3H]PN200-110 was slow enough to permit the hydrodynamic characterization of the complex by means of sucrose gradient sedimentation and gel filtration. The hydrodynamic properties of the complex were determined in several detergents, including Tween 80, 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonic acid (CHAPS), and digitonin. S20,w values of 12.5, 15.4, and 21.0 S were obtained in sucrose gradients prepared in Tween 80, CHAPS, and digitonin, respectively. A Stokes radius of 86-87 A was obtained in each of the three detergents. Determination of the partial specific volume of the protein-detergent complex in each case revealed that the differences in S20,w values could be explained by the differences in the properties of the bound detergent species. Partial specific volumes of 0.796, 0.730, and 0.730 ml/g, corresponding to molecular weights of 595,000, 540,000, and 740,000 were obtained for the complex in Tween 80, CHAPS, and digitonin, respectively. This indicated that Tween 80 readily exchanged for the solubilizing mixture of digitonin and Triton X-100, whereas CHAPS did not. Detergent exchange with Tween 80 made it possible to determine the fractional contribution of the receptor protein to the molecular weight of the protein-detergent complex. The molecular weight of the dihydropyridine receptor-calcium channel complex was estimated to be 370,000. The protein-detergent complex was found to have a frictional coefficient of 1.39, consistent with a large transmembrane protein.

Hormone regulation of cardiac energy metabolism: I. Creatine transport across cell membranes of euthyroid and hyperthyroid rat heart

Hyperthyroid rat heart was studied with the purpose of identifying the mechanism for the significant decrease in total creatine (free creatine plus phosphocreatine) observed in this pathology and its consequences on heart function. Administration of l-thyroxine in doses of 50–100 μ g 100 g of body weight during a week resulted in a reversible decrease of the total creatine by 40–50%. Simultaneously, remarkable changes in the creatine transport system across the cardiac cell membranes were observed: both the maximal rate of its active uptake and its passive movement along its concentration gradient were enhanced. In euthyroid hearts, the parameters of creatine uptake ( K m ⋍ 0.05 m m, V max = 20 nmole/min/g dry weight) were similar to those for skeletal muscle and the passive movement of creatine was negligible. In hyperthyroid hearts the latter rate was enhanced to 0.4 μmole min/g dry weight, this showing reversible damages in the cell membrane structure induced by l-thyroxine. This conclusion is consistent with observed penetration of colloidal lanthanum into the cells of hyperthyroid hearts. Perfusion of hyperthyroid rat hearts with 50 m m creatine significantly restored creatine content in the cells. Hyperthyroid hearts with decreased creatine content were found to develop ischemic contracture more rapidly and in higher extent than the euthyroid hearts. Increased sensitivity to ischemic damage may be related to decreased efficiency of energy channeling via phosphocreatine pathway.

Endogenous noradrenaline masks beta-adrenergic receptors in rat heart membranes via tight agonist binding

The tight binding of noradrenaline (NA) to β-adrenergic receptors was studied in membranes from the left ventricular myocardium of the rat. Addition of GTP (0.1 mM) to membrane preparations from control rats (NA concentration 8.5 ± 2.1 nM) caused a 4–35% (N = 8) increase (P < 0.01) in the number of specific binding sites of [ 125I](−)pindolol. In contrast, addition of GTP did not cause any changes in the number of β-adrenergic receptors in heart membranes from reserpinized animals (NA concentration < 0.1 nM). In heart membranes from reserpinized animals, preincubation with NA (followed by washing) revealed a time- and concentration-dependent decrease with a maximum of 35–40% in the [ 125I](−)pindolol-binding sites. This agonist-mediated decrease in the number of receptors was prevented if GTP was also present in the NA-preincubation medium. It is concluded that NA can undergo tight binding to β-adrenergic receptors in rat heart membranes. The heart-membrane preparations contain endogenous NA which, via tight agonist binding, is responsible for masking part of the β-adrenergic receptor population.

5'-Nucleotidase from rat heart membranes. Inhibition by adenine nucleotides and related compounds.

ADP and ATP and their analogues were evaluated as inhibitors of 5'-nucleotidase purified from heart plasma membrane. ADP analogues are more powerful inhibitors than the corresponding ATP analogues. The most powerful inhibitor found is adenosine 5'-[alpha beta-methylene]diphosphate (AOPCP) for which the enzyme shows a Ki of 5 nM at pH 7.2. Measurements of pKi values for ADP and AOPCP as a function of pH indicate that the major inhibitory species of both nucleotides is the dianion. In the physiological range of pH values, AOPCP is a more powerful inhibitor than ADP principally because a higher percentage of AOPCP exists in the dianion form. The methylenephosphonate analogue of AMP (ACP), though not a substrate, is a moderately effective inhibitor. The corresponding analogues of ADP (ACPOP) and ATP (ACPOPOP) are as good inhibitors as ADP and ATP respectively. The thiophosphate analogues of ADP all inhibit 5'-nucleotidase, although not as powerfully as ADP, the most effective of these analogues being adenosine 5'-O-(1-thiodiphosphate) diastereoisomer B (ADP[alpha S](B)]. Other nucleotides inhibit the enzyme, but none is as effective as AOPCP. Inorganic tripolyphosphate and methylenediphosphonate are better inhibitors of the enzyme than is inorganic pyrophosphate. Inorganic thiophosphate is a better inhibitor than is orthophosphate. Hill plots of the ADP and AOPCP inhibition yield slopes close to 1; Hill plots of the ATP inhibition yield slopes of about 0.6. MgADP- is not an inhibitor, and MgATP2- is at best a very weak inhibitor of the enzyme.

Increase in Mg2+ affinity as a possible mechanism of chymotryptic activation of adenylate cyclase in rat heart membranes.

Chymotryptic activation of adenylate cyclase from rat heart was examined with respect to guanosine triphosphate (GTP), adenosine triphosphate (ATP) and Mg2+ concentrations. The activation was inhibited by aprotinin. In the absence or presence of GTP, chymotrypsin stimulated the cyclase at the same ratio. GTP exerted stimulatory (up to 0.1 mM) and inhibitory (at 1 mM) effects on the cyclase. Both effects were not affected by chymotrypsin. ATP at low concentrations increased the cyclase activity dose dependently and at high concentrations decreased the cyclase activity. Chymotrypsin retarded the appearance of the decreasing phase. This effect of chymotrypsin was similar to that of increasing the concentration of Mg2+ in the reaction mixture. According to double-reciprocal plots of Mg2+ concentrations and the cyclase activity, chymotrypsin diminished Km for Mg2+ with little effect on the Vmax. Hill plots of the same data showed that the Hill coefficient was about 1 and was not altered by chymotrypsin. These results suggest that chymotryptic activation of adenylate cyclase is mainly due to increasing the affinity for Mg2+ of the system.

Studies on the Affinity and Selectivity of Denopamine (TA-064), a New Cardiotonic Agent, for β-Adrenergic Receptorst

-Denopamine is a new orally active cardiotonic agent. The present experiment was carried out to characterize the binding affinity and selectivity of this drug for β-adrenergic receptor subtypes. Binding studies were performed using 3H-dihydroalprenolol as the radioligand. Binding affinities of denopamine and some β-agonists for rat heart membranes (KiH), which contain predominantly the β1-subtype, were in the order of isoproterenol (Iso, 14.1 nM) >prenalterol (158) >norepinephrine (Nor, 227) ≥epinephrine (Epi, 248) >denopamine (545) ≥ dobutamine (645)>procaterol (1440)>terbutaline (6420). In rat lung membranes (predominantly β2-subtype), the order of potency (KiL) was Iso (20.6 nM) >procaterol (70.2) >Epi (136) >prenalterol (412) >dobutamine (735) ≥Nor (744)>denopamine (2205)>terbutaline (2500). The β1/β2-selectivity as judged from the KiL/KiH values was in the order of denopamine (4.1)>Nor (3.3)>pre-nalterol (2.6)>Iso (1.5) >dobutamine (1.1)>Epi (0.55) >terbutaline (0.39) > procaterol (0.05). Practolo, a β1-antagonist showed a high β1-selectivity (KiL/KiH=15.3). In the presence of guanine nucleotide (GTP), the denopamine radioligand competition curve showed a rightward shift and its Hill coefficient increased like other agonists, although the degree of the shift was less than that observed with full agonists such as Iso. These results essentially correspond with the pharmacological and biochemical properties of denopamine and confirm the β1-selectivity and the agonist property of this compound.

Amiodarone. Biochemical aspects and haemodynamic effects.

The mechanisms underlying the non-competitive beta-antagonistic properties of amiodarone were investigated, and the haemodynamic responses to exercise following the administration of oral amiodarone or intravenous propranolol were compared in dogs with a healed myocardial infarction submitted to a graded treadmill exercise. In radioligand binding studies, amiodarone, up to 10 mumol/L did not compete with 125I-iodocyanopindolol for binding to rat heart beta-adrenoceptors. Exposure of cardiac membranes to greater concentrations of amiodarone induced a significant decrease in the number of beta-adrenoceptors without affecting their affinity for 125I-iodocyanopindolol. Similar results were observed ex vivo, in rats after single or multiple dose administration. When added in vitro to rat heart membranes, amiodarone non-competitively inhibited the activation of adenylate cyclase by isoprenaline, glucagon and secretin. Stimulation of adenylate cyclase by those agents which act at more internal sites in the sarcolemmal membrane such as GppNHp, sodium fluoride or forskolin, was much less affected by amiodarone. In dogs performing at a submaximal work level, amiodarone significantly reduced heart rate and tended to increase coronary flow and to reduce left ventricular end-diastolic pressure, but did not affect left ventricular dP/dt. During submaximal exercise, propranolol had similar effects on heart rate, but dramatically reduced myocardial contractility.

Effects of HIS 1 modifications on the ability of vasoactive intestinal peptide to stimulate adenylate cyclase from rat and human tissues

The importance of the N-terminal His residue of VIP for stimulating adenylate cyclase was appreciated by estimating the intrinsic activity and EC 50 of four VIP analogues on membranes from rat lung, liver, brain, anterior pituitary, and pancreas, and on human heart membranes. In all tissue preparations tested except one, the order of efficacy (and often potency) was: VIP > (AC-His 1)VIP > (Phe 1)VIP = (3-Me-His 1)VIP > (D-His 1)VIP. In rat heart membranes, the order of efficacy was somewhat different: VIP > (Ac-His 1)VIP = (Phe 1)VIP > (D-His 1)VIP > (3-Me-His 1)VIP. These data demonstrated the key role of His 1 in VIP in activating adenylate cyclase. They suggest that a given VIP analogue might act as full agonist in tightly coupled adenylate cyclase systems (such as those of rat lung and liver membranes) whereas the same analogue could not promote full activity in poorly coupled systems (such as that present in rat brain synaptic membranes).

Quinidine is a competitive antagonist at alpha 1- and alpha 2-adrenergic receptors.

Although quinidine is known to have antiadrenergic effects in the cardiovascular system, the precise mechanism by which it exerts these effects is not well defined. We asked whether quinidine binds directly to adrenergic receptors. Radioligand-binding assays were used to identify alpha 1-adrenergic receptors [( 3H]prazosin-binding sites) on membranes prepared from rat heart and kidney, alpha 2-adrenergic receptor [( 3H]yohimbine-binding sites) on human platelets and rat kidney membranes, and beta-adrenergic receptors [( 125I]iodocyanopindolol-binding sites) on rat heart and kidney membranes. Although it did not effectively compete for binding to beta-adrenergic receptors, quinidine competed for binding to alpha 1- and alpha 2-adrenergic receptors and yielded equilibrium dissociation constants of 0.3-3 microM. Two other antiarrhythmic agents, lidocaine and procainamide, did not compete for binding to alpha-adrenergic receptors. Further experiments demonstrated that the interactions of quinidine with the cardiac alpha 1- and platelet alpha 2-adrenergic receptors were competitive and reversible. We conclude that that antiadrenergic actions of quinidine can be explained by occupancy and competitive blockade of alpha 1- and alpha 2-adrenergic receptors.