Background

Although natriuretic peptides are considered cardioprotective, clinical heart failure trials with recombinant BNP (nesiritide) failed to prove it. Unsuspected proadrenergic effects might oppose the anticipated benefits of natriuretic peptides.

Methods and Results

We investigated whether natriuretic peptides induce catecholamine release in isolated hearts, sympathetic nerve endings (cardiac synaptosomes) and PC12 cells bearing a sympathetic neuron phenotype. Perfusion of isolated guinea pig hearts with BNP elicited a 3-fold increase in norepinephrine release which doubled in ischemia/reperfusion conditions. BNP and ANP also released norepinephrine from cardiac synaptosomes and dopamine from nerve-growth-factor-differentiated PC12 cells in a concentration-dependent manner. These catecholamine-releasing effects were associated with an increase in intracellular calcium, and abolished by blockade of calcium channels and calcium transients, demonstrating a calcium-dependent exocytotic process. Activation of the guanylyl cyclase-cGMP-protein-kinase-G system with nitroprusside or membrane-permeant cGMP analogs mimicked the proexocytotic effect of natriuretic peptides, an action associated with an increase in intracellular cAMP and protein-kinase-A activity. cAMP enhancement resulted from an inhibition of phosphodiesterase-type-3-induced cAMP hydrolysis. Collectively, these findings indicate that, by inhibiting phosphodiesterase-type-3, natriuretic peptides sequentially enhance intracellular cAMP levels, protein-kinase-A activity, intracellular calcium and catecholamine exocytosis.

Conclusions

Our results show that natriuretic peptides, at concentrations likely to be reached at cardiac sympathetic nerve endings in advanced congestive heart failure, promote norepinephrine release via a protein-kinase-G-induced inhibition of phosphodiesterase-type-3-mediated cAMP hydrolysis. We propose that this proadrenergic action may counteract the beneficial cardiac and hemodynamic effects of natriuretic peptides, and thus explain the ineffectiveness of nesiritide as a cardiac failure medication.

Background

Renin released by ischemia/reperfusion (I/R) from cardiac mast cells activates a local renin-angiotensin system (RAS). This exacerbates norepinephrine release and reperfusion arrhythmias (VT/VF), making RAS a new therapeutic target in myocardial ischemia.

Methods and Results

We investigated whether ischemic preconditioning (IPC) prevents cardiac RAS activation in guinea-pig hearts ex-vivo. When I/R (20-min ischemia/30-min reperfusion) was preceded by IPC (2×5-min I/R cycles), renin and norepinephrine release and VT/VF duration were markedly decreased, a cardioprotective anti-RAS effect. Activation and blockade of adenosine A2b/A3-receptors, and activation and inhibition of PKCε, mimicked and prevented, respectively, the anti-RAS effects of IPC. Moreover, activation of A2b/A3-receptors, or activation of PKCε, prevented degranulation and renin release elicited by peroxide in cultured mast cells (HMC-1). Activation and inhibition of mitochondrial aldehyde dehydrogenase type-2 (ALDH2) also mimicked and prevented, respectively, the cardioprotective anti-RAS effects of IPC. Furthermore, ALDH2 activation inhibited degranulation and renin release by reactive aldehydes in HMC-1. Notably, PKCε and ALDH2 were both activated by A2b/A3-receptor stimulation in HMC-1, and PKCε inhibition prevented ALDH2 activation.

Conclusions

The results uncover a signaling cascade initiated by A2b/A3-receptors, which triggers PKCε-mediated ALDH2 activation in cardiac mast cells, contributing to IPC-induced cardioprotection by preventing mast-cell renin release and the dysfunctional consequences of local RAS activation. Thus, unlike classical IPC where cardiac myocytes are the main target, cardiac mast cells are the critical site at which the cardioprotective anti-RAS effects of IPC develop.