β3-Adrenoceptors modulate left ventricular relaxation in the rat heart via the NO-cGMP-PKG pathway.

Aims: Using a model of isolated and Langendorff-perfused rat heart we analysed whether activation of β3-adrenergic receptors (β3-ARs) influences ventricular lusitropic performance. We also focused on the NOS/NO/cGMP/PKG cascade as the signal transduction mechanism. Methods: Hearts were treated with increasing concentrations (from 10−12 to 10−6 m) of BRL37344, a selective β3-AR agonist, and cardiac performance was evaluated by analysing both lusitropic parameters and coronary motility. Cardiac preparations were also perfused with BRL37344 in the presence of either isoproterenol (ISO) or nadolol, or pertussis toxin (PTx), or selective inhibitors of the NOS/NO/cGMP/PKG pathway. Results: BRL37344 caused a significant concentration-dependent reduction in (LVd P/d t)min, a decrease in half time relaxation significant starting from 10−12 m, and an increase in (LVd P/d t)max/(LVd P/d t)min ratio ( T/− t). BRL37344 abolished the ISO-mediated positive lusitropism. β3-AR-dependent effects on relaxation were insensitive to β1/β2-AR inhibition by nadolol (100 nm), and were abolished by Gi/o protein inhibition by PTx (0.01 nm). NO scavenging by haemoglobin (10 μm), and nitric oxide synthase (NOS) inhibition by NG-monomethyl-l-arginine (10 μm) revealed the involvement of NO signalling in BRL37344 response. Pre-treatment with inhibitors of either soluble guanylate cyclase (ODQ; 10 μm) or PKG (KT5823; 100 nm) abolished β3-AR-dependent negative lusitropism. In contrast, anantin (10 nm), an inhibitor of particulate guanylate cyclase, did not modify the effect of BRL37344 on relaxation. Conclusion: Taken together, our findings provide functional evidence for β3-AR modulation of ventricular relaxation in the rat heart which involves PTx-sensitive inhibitory Gi protein and occurs via an NO-cGMP-PKG cascade. Whether the effects of β3-AR stimulation on lusitropism are beneficial or detrimental remains to be established. [ABSTRACT FROM AUTHOR]