EVIDENCE AGAINST β‐ADRENOCEPTOR BLOCKING ACTIVITY OF DILTIAZEM, A DRUG WITH CALCIUM ANTAGONIST PROPERTIES

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Abstract
1 In isolated spontaneously beating atria of the rat, diltiazem (0.01 to 0.1 μm) shifted the atrial rate concentration-response curves to isoprenaline to the right in a non-parallel manner and depressed their maxima. Under the same experimental conditions, (±)-propranolol (0.03 to 0.1 μm) behaved as a competitive β-adrenoceptor antagonist. 2 Whereas (±)-propranolol (IC50 = 12 nm) and isoprenaline (IC50 = 0.9 μm) inhibited (—)-[3H]-dihydroalprenolol binding to rat brain membrane preparations, diltiazem failed to do so in concentrations up to 10 μm. 3 Diltiazem but not (±)-propranolol, antagonized the positive chronotropic responses to calcium in spontaneously beating rat atria. 4 It is proposed that diltiazem inhibited the tachycardia induced by isoprenaline through an effect on calcium which may be an essential modulator of the sequence of events linking the β-adrenoceptor activation and heart rate response.
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