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

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, AT(1)R, are poorly understood. We hypothesized that ANG II-induced AT(1)R 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 AT(1A) receptor (SH-SY5Y-AT(1A)). SH-SY5Y-AT(1A) constitutively expresses amiloride-sensitive NHE and the NET. NHE activity was assayed in BCECF-loaded SH-SY5Y-AT(1A) as the rate of the Na(+)-dependent alkalinization in response to an acute acidosis. ANG II activation of AT(1)R markedly increased NHE activity in SH-SY5Y-AT(1A) via a Ca(2+)-dependent pathway and promoted carrier-mediated NE release. In addition, in guinea pig cSNE expressing native AT(1)R, ANG II elicited carrier-mediated NE release. In SH-SY5Y-AT(1A) and cSNE, amiloride inhibited the ANG II-mediated release of NE. Our results provide a link between AT(1)R and NHE in cSNE, which can exacerbate carrier-mediated NE release during protracted myocardial ischemia.

Cardiac mast cell-derived renin promotes local angiotensin formation, norepinephrine release, and arrhythmias in ischemia/reperfusion

“…Based on our present and previous results (11,16), we postulate that mast cell degranulation, as occurs in myocardial ischemia, releases active renin into the interstitium, where renin comes in contact with angiotensinogen (43, 44), cleaving it to Ang I. Ang I is then converted to Ang II by interstitial ACE (44). The resultant Ang II then interacts with AT 1 receptors present on the nerves.…”

“…endings expressing AT 1 receptors (15,16). This then leads to augmented NE release and arrhythmic dysfunction.…”

“…The resultant Ang II then interacts with AT 1 receptors present on the nerves. Ang II activation of AT 1 receptors potentiates Na + /H + exchanger activity (16), and this serves to increase intraneuronal Na + to a level that triggers excessive carrier-mediated NE release (45). According to our model, mast cell degranulation is a pivotal event in the activation of a cardiac RAS, leading to excessive release of NE (see Figure 9).…”

“…These findings suggest that mast cells are a source of extrarenal renin, which, when liberated, could initiate the local formation of Ang II. By activating angiotensin II receptor, type 1 (AT 1 ) receptors on sympathetic nerve terminals, Ang II enhances norepinephrine (NE) exocytosis (15) and elicits carrier-mediated NE release by stimulating the neuronal Na + /H + exchanger (16). Inasmuch as mast cells are known to degranulate in myocardial ischemia (17,18), we have now investigated and determined that release of mast cell renin activates a local cardiac RAS, thereby promoting NE release and arrhythmic cardiac dysfunction in ischemia/reperfusion.…”

Mast cells: A unique source of renin

“…Inasmuch as mast cells are known to degranulate in response to myocardial ischemia͞reperfusion (19), and ANG II is known to enhance norepinephrine release from cardiac nerve endings (20,21), we analyzed the spatial relationship between cardiac mast cells and nerves in heart. Fig.…”