Angiotensin II inhibition of L-type Ca2+ current in sinoatrial node cells of rabbits

Abstract

The actions of angiotensin II (ANG II) were examined in the spontaneously active cells isolated from the rabbit sinoatrial node, using the nystatin-permeabilized, whole cell, patch-clamp method. At 30 nM, ANG II significantly lowered the spontaneous firing rate of the action potentials from 212 +/- 21 to 172 +/- 32 beats/min, with a concomitant reduction in the action potential amplitude. The voltage-clamp experiments showed that ANG II inhibited the L-type Ca2+ current (ICa) with a dissociation constant (Kd) of approximately 4 nM and a maximal inhibition of 30%. The inhibition was blocked by an AT1-receptor antagonist CV11974. Acetylcholine (ACh) at 10 microM reduced the ICa by 42 +/- 12%, and ANG II did not cause any further inhibition in the presence of ACh. At 100 nM, ANG II reduced the ICa by only 12% in the presence of 2 microM isoproterenol, and a similar inhibition was observed with 0.1 microM ACh. ANG II did not affect the dibutyryl adenosine 3',5'-cyclic monophosphate-stimulated ICa. Protein kinase C activator 12-O-tetra-decanoylphorbol-13-acetate did not mimic ANG II in the effects on ICa, and preincubation of the cells with calphostin C, a protein kinase C inhibitor, did not attenuate the ANG II effect. ANG II exerts a negative chronotropic effect in the pacemaker cells as its direct action through a pathway involving adenosine 3',5'-cyclic monophosphate-dependent protein kinase.