117chronic HF: >10 nM; Helle et al. 2007 ; see for details Tota and Cerra, Chap. 7 , present
volume), elicits significant negative inotropic and lusitropic effects without chang-
ing HR. Notably, rCgA1-64 elicits coronary activity, since it significantly reduces
CP. This effect, obtained on the intact coronary bed, supports the “vasostatin” (ad
litteram) behavior of this CgA domain, already proposed according to the vasodila-
tion observed on segments of bovine coronary resistance arteries, intrathoracic
artery and saphenous vein exposed to the hrCgA 1-78 active domain 1–40 (Brekke
et al. 2002 ).
An interesting characteristic of rCgA1-64 is its counteraction against the stimu-
latory effect on contractility induced by ISO and ET-1, as well as against the potent
ET-1-induced coronary constriction. Experiments on rat isolated papillary muscles,
an experimental model in which contractility is analyzed without the influence of
HR and coronary flow, confirmed the depressive activity of this CgA domain on
basal and ISO-elicited contractility (Cerra et al. 2008 ). Similar to hrCgA1-78
(Cerra et al. 2006 ), the rat CgA fragment antagonized β-adrenergic stimulation in a
non- competitive manner (Fig. 2 ). This is shown by the analysis of the percentage of
variations of LVP, which provides the EC50 values in the presence of either increas-
ing concentrations of ISO alone or of ISO plus rCgA1-64 (11, 33 and 65 nM)
(Fig. 2 ). Interestingly, as proposed on the bases of results obtained by measuring
chronotropism on in situ atria from anesthetized adult dog, the negative lusitropic
effect induced by the VSs peptides, together with their antiadrenergic effects, can
be associated to alterations of the sinus rate. This may also have clinical relevance
since it has been associated to a prolongation of refractoriness and a beneficial
stabilizing effect in the presence of arrhythmias (Stavrakis et al. 2012 ).
The similarity between hrCgA1-78 and rCgA1-76 is extended to the signal
transduction mechanism involved in the cardiac effects of the two peptides. In
fact, as in the case of hrCgA1-78 (Cappello et al. 2007 ), the rat fragment signals
via a Gi/o protein-PI3K-NO-cGMP-PKG-dependent pathway (Cerra et al. 2008 ).
As observed on isolated rat papillary muscles, the mechanism of action implicates
a calcium- independent/PI3K-dependent NO release by endothelial cells. In addi-
tion, on rat isolated ventricular cells, rCgA1-64 unaffects intracellular calcium
concentrations. At the same time, it induces NO release from cultured bovine
aortic endothelial cells (BAE-1) via a calcium-independent mechanism (Cerra
et al. 2008 ). Since the coronary dilation induced by rCgA1-64 is abolished by
inhibiting the NO-cGMP- PKG pathway, it is possible that the endothelium con-
tributes with the release of vasodilator autacoids, such as NO. It remains unex-
plored whether also the endocardial endothelium contributes to these NO-mediated
mechanisms.
It is extensively documented that myocardial contractility is under the tonic con-
trol of the NO/NOS system. On the rat myocardium, NO induced negative inotro-
pism via a sGC-PKG mechanism which decreases L-type Ca2+ current (Abi-Gerges
et al. 2001 ) and troponin I phosphorylation (Hove-Madsen et al. 1996 ). Activation
of the endothelial NOS isoform (eNOS) without calcium involvement takes place
after stimulation of the endothelium with several humoral mediators such as estro-
gens, insulin, and insulin-like growth factor-1 (IGF-1) (Hartell et al. 2005 ), possibly
Cardiac Physio-Pharmacological Aspects of Three Chromogranin A-Derived Peptides...