Differential effects of 1,4-dihydropyridine calcium channel blockers: therapeutic implications

Increasing recognition of the importance of calcium in the pathogenesis of cardiovascular disease has stimulated research into the use of calcium channel blocking agents for treatment of a variety of cardiovascular diseases. The favorable efficacy and tolerability profiles of these agents make them attractive therapeutic modalities. Clinical applications of calcium channel blockers parallel their tissue selectivity. In contrast to verapamil and diltiazem, which are roughly equipotent in their actions on the heart and vascular smooth muscle, the dihydropyridine calcium channel blockers are a group of potent peripheral vasodilator agents that exert minimal electrophysiologic effects on cardiac nodal or conduction tissue. As the first dihydropyridine available for use in the United States, nifedipine controls angina and hypertension with minimal depression of cardiac function. Additional members of this group of calcium channel blockers have been studied for a variety of indications for which they may offer advantages over current therapy. Once or twice daily dosage possible with nitrendipine and nisoldipine offers a convenient administration schedule, which encourages patient compliance in long-term therapy of hypertension. The coronary vasodilating properties of nisoldipine have led to the investigation of this agent for use in angina. Selectivity for the cerebrovascular bed makes nimodipine potentially useful in the treatment of subarachnoid hemorrhage, migraine headache, dementia, and stroke. In general, the dihydropyridine calcium channel blockers are usually well tolerated, with headache, facial flushing, palpitations, edema, nausea, anorexia, and dizziness being the more common adverse effects.     

Species differences in the negative inotropic response of 1,4-dihydropyridine calcium channel blockers in myocardium

To determine if species differences exist in myocardial response to 1,4-dihydropyridine (DHP) calcium channel blockers, the binding and pharmacologic responses of a series of DHP compounds were examined in both rat and rabbit myocardium. [3H]Nitrendipine was used to label specific binding sites in myocardial membrane particulates. The results of saturation binding experiments (n = 3) indicated no statistically significant difference in either Kd or Bmax between rat and rabbit myocardial membranes (0.19 +/- 0.02 nM and 157 +/- 29 fmol/mg protein in rat and 0.14 +/- 0.06 nM and 227 +/- 125 fmol/mg protein in rabbit). Furthermore, [3H]nitrendipine binding inhibition experiments using 12 unlabeled DHP analogues yielded Ki values for each compound that were almost identical in myocardium from rat and rabbit, resulting in an excellent 1:1 correlation when data for all of the compounds were compared (r = 0.997, p less than 0.001). The negative inotropic effect of five of these DHP compounds was studied in vitro in isolated right papillary muscles from rabbit and right ventricular strips from rat, and concentration required to displace 50% of ligand binding (IC50) values for inhibition of contraction were determined. The IC50 values were significantly greater in rat myocardium than in rabbit myocardium (p less than 0.003). Therefore, a significantly lower potency of DHP calcium channel blockers has been demonstrated in rat compared with rabbit myocardium, and this species difference cannot be explained by a difference in the DHP binding site. Rat myocardium differs from rabbit myocardium in a number of ways that may explain this lower potency.(ABSTRACT TRUNCATED AT 250 WORDS)     

4-(1,4-Benzochinonyl-2-)1,4-dihydropyridine als Nifedipin-Analoga

4-(1,4-Benzoquinonyl-2-)1,4-dihydropyridines as Analogues of Nifedipine 1,4-Dihydropyridines 7 and 12a , b are synthesized from 5a, b; 10a, b , and 11a , b . Oxidation yields 4-quinonyl-pyridines 6 and 14a , b . The bridged dihydropyridine 13Bb is isolated as by-product of 11 . Reduction of 14b yields 15 .     

Studies directed toward ascertaining the active conformation of 1,4-dihydropyridine calcium entry blockers

A series of unsymmetrically substituted 4-phenyl-1,4-dihydropyridine calcium entry blockers were investigated for their ability to relax potassium-contracted rabbit aortic smooth muscle and to competitively displace [3H]nitrendipine from its specific binding sites on guinea pig myocardial membranes in order to delineate the pharmacologically active conformer with respect to the position of the aromatic substituent (synperiplanar or antiperiplanar). The data show that the 1,4-dihydropyridine receptor distinguishes between 2',3'-disubstituted phenyldihydropyridines and 2',5'-disubstituted analogues as measured by changes of vasodilation and receptor affinity in vitro. The IC50 values for vasorelaxation by the analogues presented here correlate best with the Kd values for binding to the predominant receptor of two coexisting dihydropyridine binding sites in the guinea pig myocardium. We report the first observation of an antiperiplanar orientation of an o-phenyl substituent in the X-ray structure of 2-chlorophenyl analogue 3. Using nuclear Overhauser enhancement, we have developed a method that also demonstrates that an ortho (chloro or nitro) substituent on the phenyl ring does not preclude the presence of either synperiplanar or antiperiplanar phenyl rotamer in solution. These experimental findings contrast with the accepted belief that o-phenyl substituents essentially force these 1,4-dihydropyridines into the synperiplanar conformation exclusively.     

Correlation between the negative inotropic potency and binding parameters of 1,4-dihydropyridine and phenylalkylamine calcium channel blockers in cat heart

Summary Partially purified plasma membranes were prepared from cat ventricle. The purification factors for the calcium channel ligands (+)-³H-PN 200-110, ³H-nimodipine (1,4-dihydropyridines) and (–)-³-H-desmethoxyverapamil (a phenylalkylamine) were 3.1-, 3.4- and 2.9-fold, respectively, whilst -adrenoceptors labelled with (–)-³H-dihydroalprenolol were purified 3.0-fold.(+)-³H-PN 200-110 bound to 930±140 fmol/mg of membrane protein with a dissociation constant of 70 pmol/l at 25°C. Under the same conditions ³H-nimodipine bound to 490±24 fmol/mg of sites with a K _D of 120 pmol/l. (–)-³-H-desmethoxyverapamil bound to 530±55 fmol/mg of sites with a K _D of 2.47 nmol/l.Twelve 1,4-dihydropyridines were evaluated for binding inhibition constants (K _i) with (+)-³H-PN 200-110 and 13 phenylalkylamines with (–)-³-H-desmethoxyverapamil in radioligand binding assays. Of the twelve 1,4-dihydropyridines evaluated (±)-nitrendipine was the most potent with a K _i-value of 280 pmol/l, nifedipine had a K _i-value of 500 pmol/l and the weakest drug tested, (±)-Bay b4328, had a K _i-value of 14.3 nmol/l. The EC₅₀-values of the same 1,4-dihydropyridines to inhibit the electrically driven cat papillary muscle were 77- to 3,450-fold higher and little correlation existed between K _i and EC₅₀-values.Thirteen phenylakylamines were tested for their potency to inhibit (–)-³-H-desmethoxyverapamil binding. The most potent phenylalkylamine with respect to negative inotropy was (±)-D 595 with an EC₅₀-value of 794 nmol/l, the least potent substance was (±)-Sz 45 with an EC₅₀-value of 39.8 mol/l. The binding inhibition constants for the phenylalkylamines were 13-to 322-fold lower than the values for negative inotropy, but a significant positive correlation between the K _i and EC₅₀-values (n=12, r=0.84) was observed. The absolute differences may reflect the state-dependent binding of phenylalkylamines to the channel.QSAR analysis revealed nearly identical correlations between physicochemical substituent properties on the one hand and binding affinities or functional potency on the other hand. In both cases the electronic properties (F-constant) of ring substituents mainly determine the variance in potency.     

Regional distribution of calcium channel ligand (1,4-dihydropyridine) binding sites and 45Ca2+ uptake processes in rat brain

The binding of nimodipine, a 1,4-dihydropyridine Ca2+ channel antagonist, and of Bay K 8644, a Ca2+ channel activator, was measured in several regions of rat brain and compared to the distribution of K+ depolarization-induced 45Ca2+ uptake into synaptosomes. The maximum binding densities (Bmax) of [3H]nimodipine and [3H]Bay K 8644 were not significantly different one from the other, but differed according to brain region with binding being highest in the olfactory bulb and hippocampus, intermediate in the caudate nucleus and cerebral cortex (various regions), and lowest in the cerebellum [563 to 107 fmol/mg protein (mean)]. The KD values, [3H]nimodipine = 1.8 X 10(-10) M (mean) and [3H]Bay K 8644 = 1.4 X 10(-9) M (mean), did not differ according to region. Depolarization-induced 45Ca2+ uptake in synaptosomes occurred as fast (1 sec) and slow (10 sec) components distinguished by their selective occurrence in choline-containing and pre-depolarized preparations respectively. Distribution of the fast component of uptake paralleled that of [3H]nimodipine binding, being least in the cerebellum and greatest in the hippocampus and cortex, but the magnitude of the slow phase of 45Ca2+ uptake did not vary in the three brain regions studied.     

Stereoselective pharmacodynamic action of (+)-S-12967 and (-)-S-12968, isomers of a new slow acting 1,4-dihydropyridine derivative with calcium channel agonistic and antagonistic effects on rat aorta.

The effects of (+)-S-12967 and (-)-S12968, isomers of a new dihydropyridine (1,4-DHP) derivative [2(7-amino 2,5-dioxaheptyl) 3-ethoxycarbonyl 4-(2,3-dichlorophenyl) 5-methoxycarbonyl 6-methyl 1,4-dihydropyridine] were studied on contractile responses of isolated thoracic aortas from rats and compared to that of nifedipine. The maximal relaxant effect of both isomers was reached in about 2 hr whereas the maximal relaxant effect to nifedipine was obtained within 30 min. The two 1,4-DPH isomers and nifedipine had a far more potent inhibitory effect on potassium (K+) than on noradrenaline (NA) induced contractions. They shifted the K+, Ca2+ and NA-concentration response-curves to the right and depressed the maximal vessel response to these agonists. Nifedipine was about 10 times more potent than the (-)-isomer which again was about 100 times more potent that the (+)-isomer. In contrast to nifedipine (-)-S-12968 and (+)-S-12967 had a dual action on K+ and Ca(2+)-induced contractions as both isomers in low concentrations, 3 x 10(-9)M and 3 x 10(-7)M, respectively, shifted the K(+)-concentration response curves to the left and increased the maximal response. In K(+)-depolarized preparations they increased the response to low Ca(2+)-concentrations without affecting the maximal vessel response at the highest Ca(2+)-concentrations. The result indicates that (+)-S-12967 and (-)-S-12968 possess Ca(2+)-agonistic as well as Ca-antagonistic properties. Compared to nifedipine both isomers are slow acting vasodilators. Their action as regards their potency is stereoselective and the (-)-isomer is more potent than the (+)-isomer.     

(+)-S-12967 and (-)-S-12968: 1,4-dihydropyridine stereoisomers with calcium channel agonistic and antagonistic properties in rat resistance arteries.

1. The actions of (+)-S-12967 and (-)-S-12968 two isomers of a new 1,4-dihydropyridine (DHP) derivative, were studied on 125 mM K(+)-, Ca(2+)- and noradrenaline-induced contractions in rat isolated mesenteric resistance arteries and compared to those of nifedipine. 2. The action of (+)-S-12967 and (-)-S-12968 was slow in onset in contrast to nifedipine. Both isomers had a dual contractile and relaxant action in arteries contracted with 125 mM K+; however, the (-)-isomer was about 300 times more potent than the (+)-isomer. The response to 125 mM K+, being depressed by 70%, recovered within 20 to 30 min for all DHP derivatives. All vessels were treated with 1 x 10(-6) M phenoxybenzamine thus excluding the possibility that the contraction is mediated by activation of amine-receptors. 3. Both (+)-S-12967 and (-)-S-12968 at low concentrations potentiated responses induced by Ca2+ in arteries activated by 125 mM K+ and inhibited the responses at higher concentrations. (+)-S-12967 and (-)-S-12968 had no contractile action in arteries kept in normal buffer. Nifedipine had only an inhibitory action on vessel responses to 125 mM K+ and Ca2+. 4. Both isomers and nifedipine depressed the maximal vessel response to noradrenaline by about 20% and 44%, respectively. 5. The results confirm that DHP calcium antagonists selectively inhibit vascular smooth muscle responses induced by high potassium and that the potency of 1,4-DHP isomers may vary considerably.(ABSTRACT TRUNCATED AT 250 WORDS)     

Active conformation of 1,4-dihydropyridine calcium entry blockers. Effect of size of 2-aryl substituent on rotameric equilibria and receptor binding.

The conformational requisites at the receptor for unsymmetrically substituted phenyl-1,4-dihydropyridine calcium entry blockers are examined by screening a series of (2'-halophenyl)-1,4-dihydropyridines 1-4, with increasing bulk at the 2'-position of the phenyl ring, for their ability to relax potassium-contracted rabbit aortic smooth muscle and to competitively displace [3H]nitrendipine from its specific binding sites on guinea pig skeletal muscle. The fraction of synperiplanar rotamer in solution for these compounds, as determined by the nuclear Overhauser enhancement method, shows a positive correlation with vasorelaxant activity and receptor binding affinity. These findings are consistent with the synperiplanar rotamer of nonrigid unsymmetrically substituted phenyl 1,4-dihydropyridine calcium channel blockers being the receptor-bound conformation.     

Long-acting dihydropyridine calcium antagonists. 6. Structure-activity relationships around 4-(2,3-dichlorophenyl)-3-(ethoxycarbonyl)-2-[(2-hydroxyethoxy)methyl]-5 -(methoxycarbonyl)-6-methyl-1,4-dihydropyridine

The preparation of 4-(2,3-dichlorophenyl)-3-(ethoxycarbonyl)-2-[(2-hydroxyethoxy)methyl]-5- (methoxycarbonyl)-6-methyl-1,4-dihydropyridine (2) is described, and its potent calcium antagonist activity on rat aorta (IC50 = 4 x 10(-9) M) and marked tissue selectivity in vitro for vascular smooth muscle over cardiac smooth muscle are established. In order to exploit the excellent in vitro profile of compound 2, a range of analogues were prepared but none were found to have superior calcium antagonist potency and tissue selectivity. Compound 2 has excellent in vivo activity in the anesthetized dog (ED50 = 12 micrograms/kg for reduction of CVR) and a plasma half-life in the conscious dog of 7.2 h. The pharmacokinetic parameters of 2 are compared to those determined for the structurally related compounds amlodipine and felodipine. The plasma clearance for 2 (9.6 mL/min/kg) is similar to that of amlodipine and is consistent with the extended 2-substituent hindering approach to the cytochrome P-450 enzyme responsible for oxidation of the DHP ring to the corresponding pyridine.     

Negative inotropic and chronotropic activity of calcium channel ligands possessing a xanthone 1,4-dihydropyridine backbone.

A series of xanthone 1,4-dihydropyridine derivatives were prepared. The compounds were evaluated for inotropic, chronotropic, and calcium antagonist properties.     

Pharmacological studies of a newly synthetized 1,4-dihydropyridine derivative, 2,6-dimethyl-3,5-dimethoxycarbonyl-4- (o-difluoromethoxyphenyl)-1,4-dihydropyridine (PP-1466)

Hemodynamic effects of a new 1,4-dihydropyridine derivative, 2,6-dimethyl-3,5-dimethoxycarbonyl-4-(o-difluoromethoxyphenyl++ +)-1,4-dihydropyridine (PP-1466), were examined by measuring systemic blood pressure, various arterial blood flow such as femoral, vertebral, renal and superior mesenteric artery, heart rate and respiratory rate in anesthetized dogs. Cardiohemodynamic effects were examined by measuring cardiac output, coronary blood flow, maximal rate of rise of left ventricular pressure (dp/dt max.) and right atrial pressure. The changes in myocardial oxygen consumption associated with the parameters mentioned above were also examined by measuring coronary sinus outflow and oxygen content in blood samples. The results obtained are as follows: The intraarterial administration of PP-1466 to the femoral artery caused vasodilating response. The vasodilating effect of PP-1466 was approximately 200 times more potent than that of papaverine and comparable to that of nifedipine or nicardipine (YC-93). The effective dose to induce vasodilation on femoral, vertebral and superior mesenteric artery by the systemic use of PP-1466 was more than 0.3 microgram/kg i.v. and more than 0.03 mg/kg i.d. The vasodilation in vertebral artery was most prominent. After intravenous administration of PP-1466, renal blood flow decreased probably due to the fall in systemic blood pressure, whereas the flow progressively increased after intraduodenal administration. The effective dose to induce a fall in systemic blood pressure by PP-1466 was the range from 1 to 3 micrograms/kg i.v. and more than 0.1 mg/kg i.d., respectively. Diastolic hypotension was remarkable. Heart rate and respiratory rate were increased at the same time.(ABSTRACT TRUNCATED AT 250 WORDS)     

Synthesis and calcium channel antagonist activity of 1,4-dihydropyridine derivatives containing 4-nitroimidazolyl substituents

Alkyl, cycloalkyl and arylakyl ester analogues of nifedipine (CAS 21829-25-4), in which the ortho-nitro phenyl group at position 4 is replaced by 1-methyl-4-nitro-5-imidazolyl substituent, were synthesized and evaluated as calcium channel antagonists using the high K+ contraction of guinea-pig ileal longitudinal smooth muscle. The results for the symmetrical esters showed that increasing the length of methylene chain in C-3 and C-5 ester substituents for more than two methylene units (n > 1) decreases activity. In the phenylalkyl ester series increasing the length of methylene chain increases activity. In the series of unsymmetrical phenylalkyl esters (R2 = Me or Et) increasing the length of the methylene chain decreases activity. The results demonstrate that most of the compounds had similar activity to the reference drug nifedipine. Among symmetrical diesters (methyl, ethyl and phenylpropyl derivatives) and among unsymmetrical series (benzyl methyl, benzyl ethyl and phenethyl ethyl derivatives) were more active than the reference drug nifedipine. The unsymmetrical phenethyl ethyl derivative was the most potent antagonist tested. The structure-activity data indicate that the 4-(1-methyl-4-nitro-5-imidazolyl) moiety, bioisoester of 2-nitro-phenyl moiety, is as good as other nitro-imidazolyl moieties.     

Synthesis and calcium channel antagonist activity of new 1,4-dihydropyridine derivatives containing dichloroimidazolyl substituents

A group of dialkyl, dicycloalkyl and diaryl ester analogues of nifedipine (CAS 21829-25-4), in which the ortho-nitrophenyl group at position 4 is replaced by a 4,5-dichloroimidazolyl substituent, were synthesized and evaluated as calcium channel antagonists using the high K+ contraction of guinea-pig ileal longitudinal smooth muscle. The results for the symmetrical esters in alkyl esters series showed that increasing the length of the methylene chain in C3 and C5 ester substituents (from n = 0 to n = 2) increased the activity. When increasing of the length was accompanied by increase of the hindrance, the activity decreased. In the unsymmetrical diester series, the results showed when R1 is a small substituent (R1 = Me), increasing of the lipophilic property in R2 substituent increases the activity if this high lipophilicity is not accompanied by steric hindrance. The results demonstrate that in the unsymmetrical series, several compounds (benzyl methyl, benzyl isopropyl and cyclohexyl ethyl) had activity similar to that of the reference drug nifedipine. In symmetrical diesters compounds, the most active compound was the diphenethyl ester derivative being more active than nifedipine. These structure-activity data indicate that the 4-(4,5-dichloroimidazolyl) moiety is the bioisoester of 3-nitrophenyl and 2,3-dichlorophenyl moieties.     

Synthesis of 4-(2,4-dioxo-5-pyrimidyl)-1,4-dihydropyridine derivatives

Hantzsch synthesis of 5-formyluracil (1), methyl acetoacetate (2) and methyl 3-aminocrotonate (3) gave 2,6-dimethyl-4-(2,4-dioxo-5-pyrimidyl)-1, 4-dihydropyridine-3, 5-dicarboxylic acid dimethylester (4a) in 54.6% yield. As the same procedure, 1,3-dimethyl-5-formyl-uracil (6) gave 2,6-dimethyl-4-(1,3-dimethyl-2,4-dioxo-5-pyrimidyl)-1, 4-dihydropyridine-3,5-dicarboxylic acid dimethyl ester (7a) in 52.2% yield.4a was methylated to afford7a also in 52% yield.     

Long-acting dihydropyridine calcium antagonists. 5. Synthesis and structure-activity relationships for a series of 2-[[(N-substituted-heterocyclyl)ethoxy]methyl]-1,4-dihydropyridine calcium antagonists

The synthesis of a series of 1,4-dihydropyridines which have N-linked heterocycles at the terminus of an ethoxymethyl chain at the 2-position is described. The calcium antagonist activity on rat aorta of this class of DHPs is compared with their negative inotropic activity as determined by using a Langendorff-perfused guinea pig heart model. The compounds examined show a wide range of selectivity for vascular over cardiac tissue, with those analogues which possess an amide group at the terminus of the 2-substituent proving the most selective. From the in vitro data obtained for a series of 1,2,3-triazoles, it is possible to conclude that the SARs for binding to the calcium channels in vascular and cardiac tissue are different. One of the compounds, 2-amino-1-[2-[[4-(2,3-dichlorophenyl)-3-(ethoxycarbonyl)-5- (methoxycarbonyl)-6-methyl-1,4-dihydropyrid-2-yl]methoxy]ethyl]-4( 3H)- imidazolone (20b, UK-55,444), was identified as a potent (IC50 = 8 x 10(-9) M) calcium antagonist which is 40-fold selective for vascular over cardiac tissue and which has a significantly longer duration of action (greater than 3 h) than nifedipine in the anesthetized dog on intravenous administration.     

Stereoselectivity at the calcium channel: opposite action of the enantiomers of a 1,4-dihydropyridine

The stereoisomers of the new dihydropyridine derivative 202-791 [isopropyl 4-(2,1,3-benzoxadiazol-4-yl)-1,4-dihydro-2,6-dimethyl-5-nitro-3 -pyridinecarboxylate] were synthesized separately and tested on isolated rabbit aortic rings for effects on depolarization-induced contraction and depolarization-stimulated uptake of 45Ca2+. The racemic mixture enhanced contraction of rabbit aortic rings at low levels of depolarization but inhibited contraction and 45Ca2+ uptake at high levels of depolarization. The IC50 values were 2.0 X 10(-7) and 1.7 X 10(-7) M, respectively. The R enantiomer inhibited contraction and 45Ca2+ uptake with IC50 values of 3.2 X 10(-8) and 4.3 X 10(-8) M, respectively. This compound showed no stimulant activity. By contrast, the S enantiomer of 202-791 shifted the concentration-response curve for depolarization-induced contraction in an almost parallel fashion to the left, thus enhancing contraction. The EC50 value for this effect at a KCl concentration of 16 mM was 1.8 X 10(-7) M. This compound enhanced 45Ca2+ uptake concentration dependently at all levels of depolarization tested. Thus, the stereoisomers of a dihydropyridine derivative may behave as a calcium entry blocker or a calcium entry enhancer on vascular smooth muscle, depending only on the stereochemistry. If asymmetric compounds elicit effects suggesting a dualistic action at the calcium channel, then the stereoisomers should be prepared.     

Crystal structures of calcium channel antagonists: 2,6-dimethyl-3,5-dicarbomethoxy-4-[2-nitro-, 3-cyano-, 4-(dimethylamino)-, and 2,3,4,5,6-pentafluorophenyl]-1,4-dihydropyridine

The crystal structures of 2,6-dimethyl-3,5-dicarbomethoxy-4-(2-nitrophenyl)-1,4-dihydropyridine (Nifedipine) and the 3-cyano-, 4-(dimethylamino)- and 2,3,4,5,6-pentafluorophenyl derivatives were determined. The 1,4-dihydropyridine ring in all four compounds has a boat-type conformation with varying degrees of puckering at the C4 position. Increasing distortion from planarity at this position shows a limited correlation with decreasing biological activity, determined as the ability to inhibit the Ca2+-dependent muscarinic-induced mechanical responses of guinea pig ileal longitudinal smooth muscle.     

Substituted 1,2,3,4-tetrahydroaminonaphthols: antihypertensive agents, calcium channel blockers, and adrenergic receptor blockers with catecholamine-depleting effects

Substituted 1,2,3,4-tetrahydroaminonaphthols were found to be calcium channel blockers with antihypertensive properties. These compounds also possessed adrenergic beta-receptor blocking activity. From the structure-activity studies, no clear correlation emerged between the in vitro calcium channel blocking activity and the acute anti-hypertensive activity in cannulated spontaneously hypertensive rats. Extensive pharmacological testing of selected compounds indicated that aminonaphthols are antihypertensive agents with many pharmacological properties. The relative contribution of various pharmacological actions toward the observed antihypertensive activity is unclear. Since the clinically useful calcium channel blocker verapamil is structurally related to these compounds, one of the aminonaphthols, trans-3-[(3,3-diphenylpropyl)amino]-1,2,3,4-tetrahydro-6,7 -dimethoxy-2-naphthalenol (12), was compared with verapamil for calcium channel blocking activity, adrenergic blocking activity, and catecholamine-depleting activity. Both compounds were found to be equipotent in these test systems.