Jurg R. Pfister

A comparison of an A1 adenosine receptor agonist (CVT-510) with diltiazem for slowing of AV nodal conduction in guinea-pig.

The purpose of this study was to compare the pharmacological properties (i.e. the AV nodal depressant, vasodilator, and inotropic effects) of two AV nodal blocking agents belonging to different drug classes; a novel A1 adenosine receptor (A1 receptor) agonist, N‐(3(R)‐tetrahydrofuranyl)‐6‐aminopurine riboside (CVT‐510), and the prototypical calcium channel blocker diltiazem. In the atrial‐paced isolated heart, CVT‐510 was approximately 5 fold more potent to prolong the stimulus‐to‐His bundle (S–H interval), a measure of slowing AV nodal conduction (EC50=41 nM) than to increase coronary conductance (EC50=200 nM). At concentrations of CVT‐510 (40 nM) and diltiazem (1 μM) that caused equal prolongation of S–H interval (∼10 ms), diltiazem, but not CVT‐510, significantly reduced left ventricular developed pressure (LVP) and markedly increased coronary conductance. CVT‐510 shortened atrial (EC50=73 nM) but not the ventricular monophasic action potentials (MAP). In atrial‐paced anaesthetized guinea‐pigs, intravenous infusions of CVT‐510 and diltiazem caused nearly equal prolongations of P–R interval. However, diltiazem, but not CVT‐510, significantly reduced mean arterial blood pressure. Both CVT‐510 and diltiazem prolonged S–H interval, i.e., slowed AV nodal conduction. However, the A1 receptor‐selective agonist CVT‐510 did so without causing the negative inotropic, vasodilator, and hypotensive effects associated with diltiazem. Because CVT‐510 did not affect the ventricular action potential, it is unlikely that this agonist will have a proarrythmic action in ventricular myocardium.

Antagonism of ATP responses at P2X receptor subtypes by the pH indicator dye, Phenol red

1 Many types of culture media contain a pH‐sensitive dye. One commonly occurring dye, Phenol red sodium (Na+) salt, was tested for blocking activity at rat P2X1−4 receptors (P2X1−4Rs) expressed in Xenopus oocytes. 2 Phenol red Na+‐salt antagonised adenosine 5′‐triphosphate (ATP) responses at P2X1R (IC50, 3 μM) and, at higher concentrations, also blocked P2X2R and P2X3R. Phenol red Na+‐salt, purified of lipophilic contaminants, blocked P2X1R and P2X3R by acting as an insurmountable antagonist. 3 Two lipophilic extracts of Phenol red antagonised ATP responses at P2XRs. Extract A was a potent antagonist at P2X1R (IC50, 1.4 μM), whereas extract B was a potent antagonist at P2X3R (IC50, 4.1 μM). A bisphenolic compound (RS151030) found in these extracts was a potent antagonist at P2X1R (IC50, 0.3 μM) and at P2X3R (IC50, 2.4 μM). 4 Phenolphthalein base was a potent irreversible antagonist at P2X1R (IC50, 1 μM), whereas Phenolphthalein K+‐salt was 25‐fold less potent here. 5 Phenolphthalein base was a reversible antagonist of ATP responses at rat P2X4R (IC50, 26 μM), whereas Phenolphthalein K+‐salt was inactive. 6 Dimethyl sulphoxide (DMSO), used to dissolve lipophilic extracts, showed pharmacological activity by itself at rat P2X1R and P2X4R. 7 Thus, Phenol red and related compounds are antagonists at rat P2X1R, but are also active at other rat P2XRs. Phenolphthalein base is a newly identified, low potency antagonist of ATP responses at P2X4R. Culture media containing these red dyes should be used cautiously in future pharmacological studies of P2XRs. Also, wherever possible, the solvent DMSO should be used with caution.

RS 30026: a potent and effective calcium channel agonist

1 A series of dihydropyridine derivatives has been evaluated for calcium channel agonist activity using reversal of nisoldipine‐induced inhibition of beating of aggregates of embryonic chick myocytes. This test appears to be specific for calcium channel agonists since isoprenaline and cardiac glycosides are inactive. 2 RS 30026 was the most potent of the series, was significantly more potent than CGP 28392 and of similar potency to Bay K 8644 (pEC50 = 7.45, 6.16 and 7.20, respectively). RS 30026 increased edge movement of individual aggregates, in the absence of nisoldipine, by 50% at 2nm. 3 Compounds were also evaluated for their effects on guinea‐pig papillary muscle and porcine coronary artery rings. RS 30026 displayed positive inotropism at concentrations between 10−9 and 10−6 m (pEC200 = 8.21), but was a much more powerful inotrope than Bay K 8644, increasing contractility to 1300% of control at 10−6 m (compared to 350% of control for Bay K 8644). RS 30026 caused vasoconstriction at concentrations between 10−10 and 10−7 m. 4 Calcium channel currents in single embryonic chick myocytes were recorded by whole‐cell voltage clamp techniques. RS 30026 (100 nm‐500 nm) produced large increases in peak current amplitude and shifted the voltage for threshold and maximal currents to more negative values. RS 30026 (500 nm) also produced large increases in the inward tail currents evoked upon repolarization. The effects of Bay K 8644 (50 and 500 nm) were much less marked. 5 Analysis of the activation characteristics of currents showed parallel shifts in the activation curve to more negative potentials in the presence of 50 nm Bay K 8644, with a much smaller shift in the presence of 500 nm Bay K 8644. RS 30026 (100 and 500 nm) caused concentration‐dependent shifts in the activation of the calcium channel currents with an increase of the slope of the curve. 6 RS 30026 appears to be the most potent and effective calcium channel agonist described to date.