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

Abstract

1. 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. 2. 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 microM) that caused equal prolongation of S-H interval (approximately 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). 3. 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. 4. 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.

Figure 1

Chemical structure N-(3(R)-tetrahydrofuranyl)-6-aminopurine riboside (CVT-510).

Figure 2

Concentration-response relationship for the A₁ receptor-mediated negative dromotropic (S–H interval prolongation) and the A_2A receptor-mediated coronary vasodilator (increase of coronary conductance) effects of CVT-510 in guinea-pig isolated perfused hearts. Symbols and error bars indicate means±s.e.mean of single determinations from each of four hearts. Second-degree AV block occurred in all hearts at a concentration of CVT-510⩾40 nM.

Figure 3

Concentration-response relationship for the calcium channel blocker diltiazem to prolong the S–H interval and increase coronary conductance in guinea-pig isolated perfused hearts. Symbols and error bars indicate means±s.e.mean of single determinations from each of 4–6 hearts.

Figure 4

The antagonism of the A₁-receptor-mediated increase in S–H interval (A) and the A_2A receptor-mediated increase in coronary conductance (B) by the A₁ receptor-selective antagonist CPX (60 nM) and by the A_2A receptor-selective antagonist ZM241,385 (100 nM), respectively, during infusion of CVT-510 (60 nM) in the guinea-pig isolated perfused heart. Bars represent the means±s.e.mean of single determinations from each of four hearts. ^*Denotes a significant difference from control (P<0.05).

Figure 5

Comparison of the effects of CVT-510 (40 nM) and diltiazem (1 μM) on left ventricular developed pressure (LVP, A) and coronary conductance (B) in guinea-pig isolated perfused heart for the same prolongation of the S–V interval (∼10 ms). Bars represent the means±s.e.mean of single determinations from each of four hearts. Numbers above bars are means±s.e.mean of dP dt⁻¹_max for each data set. ^*Denotes significant differences from both 2 : 1 A–V Block and Washout (P<0.05).

Figure 6

Differential effect of CVT-510 on atrial and ventricular monophasic action potential durations (MAPD). CVT-510 (1–300 nM) shortened the atrial MAPD in a concentration-dependent manner but had no effect on the ventricular MAPD. Values are means±s.e.mean of each of 5–11 hearts. Atrial and ventricular MAPD were measured at 90% of repolarization (MAPD₉₀).

Figure 7

Dose-response relationship for the effect of intravenous injections of CVT-510 or adenosine to prolong the P–R interval in atrial-paced (330–360 beats min⁻¹) anaesthetized guinea-pigs. Symbols and error bars represent means±s.e.mean of single determinations from each of 3–4 animals.

Figure 8

Representative electrocardiogram (ECG lead 2) and mean arterial blood pressure (MABP) from an anaesthetized guinea-pig heart recorded before (control, t=0), during, and after (washout, t=35 min) a continuous intravenous infusion of CVT-510. The heart was allowed to either beat spontaneously (‘Spontaneous') or was subjected to right ventricular pacing (V-pacing), at the same rate as the control atrial rate. At 10 min of infusion of CVT-510 (t=10 min), a stable 2 : 1 atrioventricular (AV) block occurred and this was associated with a decrease in MABP. Subsequently, (t=12 min and onward) during intermittent V-pacing, the MABP rose to control levels.

Figure 9

(A) Complete reversal by ventricular pacing of hypotension associated with 2 : 1 AV block caused by CVT-510 in anaesthetized guinea-pigs. Summary of data showing that 2 : 1 AV block caused by a continuous intravenous infusion of CVT-510 is associated with a significant reduction in mean arterial blood pressure (MABP). Pacing of the right ventricle (Ventricular Pacing) at the same rate as the control atrial rate, reversed the decrease in MABP observed during CVT-510-induced 2 : 1 AV block. At 30–40 min after stopping the infusion of CVT-510 (Washout), MABP was not different from baseline. Values are means±s.e.mean of single determinations from each of four guinea-pigs. (B) Partial reversal by ventricular pacing of hypotension associated with 2 : 1 AV block caused by the nonselective adenosine receptor agonist NECA in anaesthetized guinea-pigs. Summary of data showing that 2 : 1 AV block caused by continuous intravenous infusion of NECA is associated with significant reduction of mean arterial blood pressure (MABP). Pacing of the right ventricle (Ventricular Pacing) at the same rate as the control atrial rate only partially reversed the decrease in MABP observed during NECA-induced 2 : 1 AV block. At 30–40 min after stopping the infusion of NECA (Washout), MABP returned to baseline. Values are means±s.e.mean of single determinations from each of three guinea-pigs. ^*Denotes significant differences from both 2 : 1 A-V Block and Washout (P<0.05).

Figure 10

Summary of data comparing the effects of CVT-510 (0.75–1.0 μg kg⁻¹ min⁻¹) and diltiazem (200–400 μg kg⁻¹ min⁻¹) given as intravenous infusions on P–R interval, heart rate, and mean arterial blood pressure (MABP) in anaesthetized guinea-pigs. Hearts were allowed to beat spontaneously. Control values are given in the results section. Bars represent means±s.e.mean of changes from baseline (horizontal line at 100%) of single determinations from each of six hearts (three for CVT-510, three for diltiazem).