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hemodynamic function (Tota et al. 2012 ). Within the first 5 min of administration,
both Serp and pGlu-Serp dose-dependently (11–165 nM) enhance myocardial con-
tractility and relaxation. pGlu-Serp action starts at 1 nM, showing that this peptide
is more potent than Serp. These actions are accompanied by a slight, although non-
significant, coronary dilatation (Tota et al. 2012 ). A similar effect is evident also on
isolated rat papillary muscles. By measuring contractility as tension development
and muscle length, we found that, while Serp-Ala29Gly is unaffective, pGlu-Serp
and Serp induce positive inotropism by increasing the rate and extent of tension
development during systole (positive inotropy). This effect, in the whole heart aug-
ments stroke volume. In parallel, the peptides accelerate myocardial relaxation
(positive lusitropy), hence shortening the overall diastole duration. The analyses
carried out on isolated papillary muscle also showed that Serp actions are indepen-
dent from alterations in HR and coronary flow rate, as well as from norepinephrine
release from sympathetic nerve terminals (as demonstrated by tyramine treatment)
(Tota et al. 2012 ).
Interestingly, pGlu-Serp and Serp act as β1-AR-like agonists, mimicking, at
nanomolar range, the effects of intracardiac sympathetic neurotransmitters and/or
circulating Catecholamines. Consistent with this effect, and opposite to the depres-
sive and anti-adrenergic cascade activated by VS-1 and CST, pGlu-Serp and Serp
act via β1-AR/Adenylate Cyclase/cAMP/PKA (Tota et al. 2012 ) (Fig. 3 ). This is
shown by the results of physio-pharmacologic analyses carried out by using specific
inhibitors of the above cascade, by Western Blotting evaluations, and intracellular
cAMP evaluation. As in the case of other CgA fragments, no receptor or direct bind-
ing partner have been identified for Serp and pGlu-Serp. Thus, the earliest events
underpinning the transduction of the Serp signal are unknown. On the basis of phys-
iological and biomolecular evidences, it was presumed that, to activate the
β1-adrenergic-induced cascade Serp and pGlu-Serp function as allosteric modula-
tors of β1-AR independent from the ligand binding site (Tota et al. 2012 ). As other
cAMP elevating agonists (Koshimizu et al. 2010 ), both Serp peptides might bind a
G protein-coupled receptor (GPCR) to increase cardiac cAMP. This is indicated by
the lack of Serp-induced cardiostimulation after selective inhibition of adenylate
cyclase (AD) and PKA, major targets of the AD-cAMP signalling (Tota et al. 2012 ).
Once activated, PKA phosphorylates a number of proteins, including SERCA and
PLN, with consequent effects on inotropy and lusitropy. In fact, SERCA activation
promotes SR Ca2+ uptake, and thus cation removal during diastole. This affects
relaxation and the subsequent contraction (Satoh et al. 2011 ). Ca2+ sensitivity for
SERCA is enhanced also by PKA-induced phosphorylation of PLN, and this repre-
sents a crucial step of the β-adrenergic-PKA cascade (Mattiazzi et al. 2007 ). Of
note, positive inotropism and lusitropism induced by pGlu-Serp are accompanied
by an increased PNL phosphorylation at Ser16, and are abolished by SERCA inhi-
bition by thapsigargin (Tota et al. 2012 ). This suggests, in agreement with the sym-
pathomimetic function proposed for Serp peptides, that their action occurs by
modulating myocardial Ca2+ fluxes with the resulting stimulation of inotropy and
relaxation.
T. Angelone et al.