Rationale The regenerative potential of the heart is insufficient to fully restore functioning myocardium after injury, motivating the quest for a cell-based replacement strategy. Bone marrow derived mesenchymal stem cells (MSC) have the capacity for cardiac repair that appears to exceed their capacity for differentiation into cardiac myocytes. Objective Here we test the hypothesis that bone marrow derived MSCs stimulate the proliferation and differentiation of endogenous cardiac stem cells (CSCs) as part of their regenerative repertoire. Methods And Results Female Yorkshire pigs (n=31) underwent experimental myocardial Infarction (MI); and 3 days later received transendocardial injections of allogeneic male bone marrow-derived MSCs, MSC concentrated conditioned medium (CCM), or placebo (Plasmalyte). A no-injection control group was also studied. MSCs engrafted and differentiated into cardiomyocytes and vascular structures. In addition, endogenous c-kit+ CSCs increased 20-fold in MSC treated animals vs. controls (p<0.001), there was a 6-fold increase in GATA-4+ CSCs in MSC vs. control (p<0.001), and mitotic myocytes increased 4-fold. Porcine endomyocardial biopsies were harvested and plated as organotypic cultures in the presence or absence of MSC feeder layers. In vitro, MSCs stimulated c-kit+ CSCs proliferation into enriched populations of adult cardioblasts that expressed Nkx2-5 and troponin I. Conclusions MSCs stimulate host CSCs, a new mechanism of action underlying successful cell-based therapeutics.

When the length of the myocardium is increased, a biphasic response to stretch occurs involving an initial rapid increase in force followed by a delayed slow increase called the slow force response (SFR). Confirming previous findings involving angiotensin II in the SFR, it was blunted by AT1 receptor blockade (losartan). The SFR was accompanied by an increase in reactive oxygen species (ROS) of ∼30% and in intracellular Na + concentration ([Na + ] i ) of ∼2.5 mmol l −1 over basal detected by H 2 DCFDA and SBFI fluorescence, respectively. Abolition of ROS by 2-mercapto-propionyl-glycine (MPG) and EUK8 suppressed the increase in [Na + ] i and the SFR, which were also blunted by Na + /H + exchanger (NHE-1) inhibition (HOE642). NADPH oxidase inhibition (apocynin or DPI) or blockade of the ATP-sensitive mitochondrial potassium channels (5HD or glybenclamide) suppressed both the SFR and the increase in [Na + ] i after stretch, suggesting that endogenous angiotensin II activated NADPH oxidase leading to ROS release by the ATP-sensitive mitochondrial potassium channels, which promoted NHE-1 activation. Supporting the notion of ROS-mediated NHE-1 activation, stretch increased the ERK1/2 and p90rsk kinases phosphorylation, effect that was cancelled by losartan. In agreement, the SFR was cancelled by inhibiting the ERK1/2 signalling pathway with PD98059. Angiotensin II at a dose that mimics the SFR (1 nmol l −1 ) induced an increase in ·O 2 − production of ∼30-40% detected by lucigenin in cardiac slices, an effect that was blunted by losartan, MPG, apocynin, 5HD and glybenclamide. Taken together the data suggest a pivotal role of mitochondrial ROS in the genesis of the SFR to stretch.

Whether the growth hormone (GH)/insulin-like growth factor 1 (IGF-1) axis exerts cardioprotective effects remains controversial; and the underlying mechanism(s) for such actions are unclear.Here we tested the hypothesis that growth hormone-releasing hormone (GHRH) directly activates cellular reparative mechanisms within the injured heart, in a GH/IGF-1 independent fashion. After experimental myocardial infarction (MI), rats were randomly assigned to receive, during a 4-week period, either placebo (n = 14), rat recombinant GH (n = 8) or JI-38 (n = 8; 50 µg/kg per day), a potent GHRH agonist. JI-38 did not elevate serum levels of GH or IGF-1, but it markedly attenuated the degree of cardiac functional decline and remodeling after injury. In contrast, GH administration markedly elevated body weight, heart weight, and circulating GH and IGF-1, but it did not offset the decline in cardiac structure and function. Whereas both JI-38 and GH augmented levels of cardiac precursor cell proliferation, only JI-38 increased antiapoptotic gene expression. The receptor for GHRH was detectable on myocytes, supporting direct activation of cardiac signal transduction. Collectively, these findings demonstrate that within the heart, GHRH agonists can activate cardiac repair after MI, suggesting the existence of a potential signaling pathway based on GHRH in the heart. The phenotypic profile of the response to a potent GHRH agonist has therapeutic implications.cardiac stem cells | apoptosis | remodeling | heart failure C ongestive heart failure remains a leading cause of morbidity and mortality in developed countries. Despite major therapeutic advances, current therapies fail to fully reverse heart failure and/or left ventricular (LV) dysfunction. One major therapeutic avenue is that of cytokine and/or hormonal signaling pathways, and in this regard, various experimental and clinical studies have suggested an important role for the growth hormone (GH)/insulin-like growth factor 1 (IGF-1) axis in the regulation of cardiac growth and function (1, 2). Moreover, several clinical studies have tested the impact of GH replacement on the failing human heart, with controversial results (3, 4).In addition to GH itself and IGF-1, GH-releasing peptides such as ghrelin and synthetic GH secretagogues (GHS) are also suggested to have cardiac effects (5-8), and growth hormone-releasing hormone (GHRH) mRNA is detected in peripheral tissues, including the heart (9, 10), consistent with widespread biologic signaling potential beyond the hypothalamic-pituitary axis.Recently, Granata et al. (10) reported that rat GHRH (1-44) promoted survival of cardiomyocytes in vitro and protected rat hearts from ischemia-reperfusion injury. The detection of the GHRH receptor (GHRHR) on the cardiomyocyte sarcolemmal membrane supports the view that GHRH may elicit direct signal transduction within the heart, independent of the GH/IGF-1 axis per se (10). Ghrelin and other GHS may have pharmacologic potential (10) but also have pleiotropic actions with a high possibility...