Coupling of angiotensin II AT1receptors to neuronal NHE activity and carrier-mediated norepinephrine release in myocardial ischemia

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Abstract
In ischemia, cardiac sympathetic nerve endings (cSNE) release excessive amounts of norepinephrine (NE) via the nonexocytotic Na+-dependent NE transporter (NET). NET, normally responsible for NE reuptake into cSNE, reverses in myocardial ischemia, releasing pathological amounts of NE. This carrier-mediated NE release can be triggered by elevated intracellular Na+levels in the axoplasm. The fact that ischemia activates the intracellular pH regulatory Na+/H+exchanger (NHE) in cSNE is pivotal in increasing intraneuronal Na+and thus activating carrier-mediated NE release. Angiotensin (ANG) II levels are also significantly elevated in the ischemic heart. However, the effects of ANG II on cSNE, which express the ANG II receptor, AT1R, are poorly understood. We hypothesized that ANG II-induced AT1R activation in cSNE may be positively coupled to NHE activity and thereby facilitate the pathological release of NE associated with myocardial ischemia. We tested this hypothesis in a cSNE model, human neuroblastoma cells stably transfected with rat recombinant AT1Areceptor (SH-SY5Y-AT1A). SH-SY5Y-AT1Aconstitutively expresses amiloride-sensitive NHE and the NET. NHE activity was assayed in BCECF-loaded SH-SY5Y-AT1Aas the rate of the Na+-dependent alkalinization in response to an acute acidosis. ANG II activation of AT1R markedly increased NHE activity in SH-SY5Y-AT1Avia a Ca2+-dependent pathway and promoted carrier-mediated NE release. In addition, in guinea pig cSNE expressing native AT1R, ANG II elicited carrier-mediated NE release. In SH-SY5Y-AT1Aand cSNE, amiloride inhibited the ANG II-mediated release of NE. Our results provide a link between AT1R and NHE in cSNE, which can exacerbate carrier-mediated NE release during protracted myocardial ischemia.