**4. Conclusions**

Vascular homeostasis critically depends on the physiological response of endothelial cells to blood supply and the appropriate redox balance. The endothelium releases many factors to control vascular tone, the adhesion of circulating blood cells, the proliferation of smooth muscle cells, and inflammation.

Why should GH be considered a promising therapeutic agen<sup>t</sup> for neovascularization? The GH/PRL/PL family regulates the physiological growth and regression of blood vessels in female reproductive organs, and this fact strongly support its vascular role in neovascularization. There is no doubt about the fact that the GH/IGF-I axis has to play an important role in neovascularization, both in physiological and pathological states, as evidence here presented has underlined. This axis su ffers an important decline with aging, mainly a ffecting GH secretion. Considering that most patients with ischemic injuries are elderly, GH therapy could be considered of help in improving vascularization and mitigating symptoms.

However, information concerning the regulation of neovascularization by proangiogenic hormones such as GH is insu fficient, since few physiological or pathological conditions have been deeply studied, with some exceptions. This fact could be explained by the use of di fferent animal models of ischemia, types of tissue analyzed, disease status, hormone doses, or follow-up times. These e ffects also depend on the relative contribution of the local production of hormones or on the hormonal cleavage by proteases in cells or the clearance of these hormones by kidneys when they are exogenously administered. Surprisingly, data are also limited about endogenously produced antiangiogenic substances that might be overexpressed in chronic states such as ischemia and that could act with a harmful e ffect on GH actions.

The role of redox balance in arteriogenesis and how GH could aid in the mitigation of it were analyzed. We also proposed the possibility that GH and IGF-I could be parts of those mitogenic factors secreted by endothelial cells in response to shear stress forces. The large number of connections that both molecules have with cytokines, hormones, and cells involved in neovascularization reinforce their role in this process. Finally, in this review, it has been presented some molecular insights from the

GHAS trial in patients with critical limb ischemia that correlate perfectly with recent publications on arteriogenesis and that can help to understand the action of GH dealing with ischemia. Nevertheless, the molecular results of this initial clinical study still need to be confirmed in larger studies.

**Author Contributions:** Conceptualization, D.C., J.D., and P.D.; methodology, D.C. J.D. and C.V.A.; software, D.C.; validation, D.C. and J.D.; formal analysis, D.C. and J.D.; investigation, D.C., P.D. and C.V.A.; resources, D.C.; data curation, D.C., J.D. and C.V.A.; writing—original draft preparation, D.C. and J.D.; writing—review and editing, D.C. J.D. and C.V.A.; visualization, D.C. and J.C.; supervision, J.D.; project administration, D.C.; funding acquisition, D.C. All authors have read and agreed to the published version of the manuscript.

**Funding:** This review was funded by the Spanish Society of Angiology and Vascular Surgery (SEACV). Not this review, but the GHAS trial was funded by the Carlos III Health Institute and the European Regional Development Fund (ISCIII- FEDER), Madrid, Spain. Grant number PI 13-00790.

**Acknowledgments:** We thank Santiago Pérez Cachafeiro for providing an important stimulus to develop this review and the GHAS trial. Without his help and knowledge this article would not have been possible. We equally thank Sihara Pérez-Romero from the Center of Research in Molecular Medicine of the University of Santiago of Compostela (CIMUS), Spain, for her technical assistance in the molecular analysis in ischemic muscle samples from the GHAS study.

**Conflicts of Interest:** The authors declare no conflict of interest. The funders had no role in the design of the study; in the collection, analyses, or interpretation of data; in the writing of the manuscript, or in the decision to publish the results.
