Chronotropic effect of angiotensin II via type 2 receptors in rat brain neurons

Previously, we determined that angiotensin II (Ang II) elicits an Ang II ty pe 2 (AT(2)) receptor-mediated increase of neuronal delayed rectifier K+ (I KV) current in neuronal cultures from newborn rat hypothalamus and brain s tem. This requires generation of lipoxygenase (LO) metabolites of arachidon ic acid (AA) and activation of serine/threonine phosphatase type 2A (PP-2A) . Enhancement of I-KV results in a decrease in net inward current during th e action potential (AP) upstroke as well as shortening of the refractory pe riod, which may lead to alterations in neuronal firing rate. Thus, in the p resent study, we used whole-cell current clamp recording methods to investi gate the AT(2) receptor-mediated effects of Ang II on the firing rate of cu ltured neurons from the hypothalamus and brain stem. At room temperature, t hese neurons exhibited spontaneous APs with an amplitude of 77.72 +/- 2.7 m V (n = 20) and they fired at a frequency of 0.8 +/- 0.1 Hz (n = 11). Most c ells had a prolonged early after-depolarization that followed an initial fu lly developed AP. Superfusion of Ang II (100 nM) plus losartan (LOS, 1 muM) to block Ang II type 1 receptors elicited a significant chronotropic effec t that was reversed by the AT(2) receptor inhibitor PD 123,319 (1 muM). LOS alone had no effect on any of the parameters measured. The chronotropic ef fect of Ang II was reversed by the general LO inhibitor 5,8,11,14-eicosatet raynoic acid (10 mM) or by the selective PP-2A inhibitor okadaic acid (1 nM ) and was mimicked by the 12-LO metabolite of AA 12-(S)-hydroxy-(5Z, 8Z, 10 E, 14Z)-eicosatetraynoic acid. These data indicate that Ang II elicits an A T(2) receptor-mediated increase in neuronal firing rate, an effect that inv olves generation of LO metabolites of AA and activation of PP-2A.