Interaction of angiotensin II with the angiotensin type 2 receptor inhibits the cardiac transient outward potassium current

Objective: The Ca2+-independent transient outward K+ current (I-to) plays a crucial role in shaping the cardiac action potential. In the present study, we examined whether angiotensin II (AngII) regulated the I-to as well as the putative intracellular cascade responsible for the effects. Methods: It. was recorded in rat ventricular myocytes using the nystatin-perforated patch-clamp configuration. Results: AngII (0.1 muM) inhibited I-to (21.9 +/- 4.8% at +40 mV), but not the I-KI, in a voltage- and time-independent manner. The inhibition decreased at concentrations higher than 1 muM resulting in a bell-shaped dose-response curve (IC50=3.1 +/- 1.5 muM). The blocking effects were abolished in the presence of the type 2 AngII receptor (AT2R) antagonist PD123319, but not in the presence of the selective type 1 AngII receptor (ATIR) antagonist candesartan. Moreover, the selective AT2R agonist CGP42112A completely reproduced the effects of AngII (20.5 +/- 2.4% of block at +40 mV), indicating that AngII-induced I-to block was mediated via stimulation of AT2R. Furthermore, selective stimulation of AT2R by CGP42112A significantly prolonged the rat atrial action potentials recorded using conventional microelectrode techniques. The AngII-induced inhibition of I-to was not modified by either N.-nitro-L-arginine-methyl ester (L-NAME) or eicosatetrayonic acid (ETYA), indicating that neither the nitric oxide (NO)-guanosine 3', 5'-cyclic monophosphate (cGMP) system nor the arachidonic acid cascade was implicated in the effects of AngII on I-to. However, the AngII-induced I-to inhibition was completely abolished by the serine/threonine phosphatase type 2A (PP2A) inhibitors, okadaic acid and cantharidin, but not by the inactive analog of okadaic acid, 1-norokadaone. Intracellular application of PP2A decreased Kv4.2 currents recorded in transiently transfected Chinese hamster ovary cells (CHO). Conclusion: These results indicate that AngII activates PP2A through the stimulation of the AT2R, resulting in a decrease of the I-to amplitude. (C) 2004 European Society of Cardiology. Published by Elsevier B.V. All rights reserved.