Fig. 2. Primary stimuli that may trigger the cascades that lead to angiogenesis and arteriogenesis in some models of cardiac hypertrophy. The hypertrophic response to work increases oxygen demand in cardiomyocytes (A). This relative hypoxia or increased metabolism triggers growth factors that facilitate angiogenesis. An increase in blood flow in response to increased O2 demand may also trigger growth factors that are important for arteriogenesis and angiogenesis. Stretch (B), which is increased in some models of cardiac hypertrophy (volume overload, exercise) is known to upregulate growth factors in the cardiomyocyte and receptors on endothelial cells since both cell types are subjected to stretch. This mechanical influence may also facilitate angiogenesis and arteriogenesis.

9. Modulators of Angiogenesis During Hypertrophy

As indicated in the previous sections, vascular growth during cardiac enlargement is, at least in part, model dependent. There is convincing evidence that some factors that stimulate compensatory vascular growth are operable in certain models. Figure 2 illustrates that both metabolic and mechanical factors are likely primary stimuli that trigger growth factor expression. Another important determinant of the extent of angiogenesis in cardiac hypertrophy is age, as documented in the next few paragraphs.

Embryonic myocardial vascularization is initiated by vasculogenesis, i.e. formation of vascular tubes from progenitor cells. Endothelial cells as well as vascular smooth muscle cells arise from the epicardium (see Ref. 115 for review). The rate and extent of vascularization appear

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Fig. 3. A comparison of capillary growth in humans with congenital aortic stenosis versus acquired (after birth) aortic stenosis. The data illustrated here are from Ref. 117. The data show that capillary growth in hearts with congenital aortic stenosis completely compensates for the marked cardiac hypertrophy. Note that despite a greater degree of cardiac hypertrophy in the hearts of the congenital aortic stenosis group, capillary density and domain area are normal. In contrast, hypertrophy that develops after birth is not characterized by a capillary growth that matches the magnitude of hypertrophy.

to be dictated, at least in part, by the increase in ventricular mass.116 The supposition that the developing heart in early life has a greater potential for angiogenesis is supported by data from both humans and animals. Rakusan and colleagues presented evidence that congenital aortic stenosis and coarctation are characterized by capillary growth that is proportional to the increase in heart mass (Fig. 3).117 This is in contrast to adults with acquired aortic stenosis who experience a marked decrease in capillary density. Data from this study show that capillary density in hearts hypertrophied by congenital pressure overload have capillary densities virtually identical to controls. In order to maintain normal capillary densities, angiogenesis must be 2.5- to 3.0- fold higher than the controls since heart weight exceeds that of controls by this magnitude during infancy, childhood and adulthood.

Animal studies support the data from humans. The degree of ventricular hypertrophy does not appear to be a determinant of compensatory capillary growth in neonatal rats subjected to a gradual pressure overload.118 When aortic constriction was imposed on days 2 and 6 of postnatal life, heart weight increased 24% and 55%, respectively, yet capillary densities in both groups were similar to the controls. Similar data were obtained from young lambs subjected to aortic banding. These data indicate that the extent of myocardial angiogenesis

during postnatal development is proportional to, rather than limited by, the magnitude of myocardial growth. Several studies have shown that the angiogenic response is stronger in young than in older animals during the development of hypertrophy evoked by various stimuli.41,42,45,80,119,120 However, the angiogenic response may be species dependent. For example, vascular growth was limited or non-existent in young dogs with pressure overload hypertrophy as indicated by decreased maximal coronary perfusion and increased minimal coronary vascular resistance.121,122

The embryonic/fetal heart is also characterized by a robust angiogenic response to the induction of cardiac hypertrophy. When we constricted the outflow track in chicks (in ovo) prior to coronary vascularization, ventricular mass increased by 64%.116 Since vascular volume and numerical densities were normal, the findings of these experiments revealed that the rate and magnitude of coronary vascular growth adapts to the rate and magnitude of myocardial growth.

Attempts to utilize exercise training to stimulate angiogenesis in the hearts of rats hypertrophied by hypertension have not been successful.15,69,74,123 Part of the failure may have been due to the strenuous nature of the training and a further increase in arterial pressure. However, when exercise training was begun in six-week- old spontaneously hypertensive rats, to correspond to a period of developing hypertension and cardiac hypertrophy, capillary growth paralleled the increase in cardiac mass.119 Thus, either age or the timing of the stimulus appear to be a determinant of the angiogenic response.

10. Stimuli of Angiogenesis During Hypertrophy

This review has noted that compensatory angiogenesis and arteriogenesis are characteristic of some models of cardiac hypertrophy, i.e. thyroxine, exercise and hypoxia. In volume overload, vascular growth occurs, though not necessarily uniformly, in the ventricular wall. Hypertrophy in response to pressure overload is problematic. In most circumstances vascular growth is non-existent or limited and maximal perfusion is

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compromised. However, there are exceptions, as noted earlier, when angiogenesis and arteriogenesis are stimulated in this model. As previously noted, both metabolic and mechanical stimuli affect growth factors and their receptors and thereby play key roles in the angiogenic and arteriogenic responses in the heart (Fig. 2).

Hypoxia is known to upregulate VEGF via activation of hypoxia inducible factor (HIF-1α)124 and also upregulates other growth factor genes, namely VEGF receptors, neuropilins, angiopoetin 2, PDGF-BB and IL-8 (reviewed in Ref. 125). As seen in Fig. 2A, the increased O2 demand in the heart undergoing hypertrophy necessitates an increase in blood flow. This is likely to be a consequence of adenosine which has also been found to stimulate VEGF mRNA.64 From these findings, we can conclude that hypoxia may serve as a direct metabolic factor for angiogenesis or an indirect factor by increasing blood flow which provides mechanical stimuli that trigger the angiogenic cascade. Increased flow, including elevated shear stress and wall tension, is a well-recognized stimulator of vascular growth.126 Shear stress activates a variety of molecules, e.g. transcription factors, proteins, in both vascular smooth muscle and endothelial cells.125 Laminar shear stress provides stimuli for endothelial cell proliferation by enhancement of NO and Ca2+ entry into cells, activation of phosphatidylinositol, and cell shape changes (reviewed in Ref. 126). There is a well-established link between increased blood flow and vessel growth, whether via growth factor enhancement or by direct mechanical effects on vascular cells. Thus, hypoxia may influence vascular growth directly via growth factor upregulation or indirectly by increasing blood flow. Chronic interventions that increase blood flow and cause coronary angiogenesis include dipyridamole/adenosine/xanthine derivatives, alcohol, thyroxine, exercise and hypoxia (reviewed in Ref. 126). Thus, the mechanical effects of enhanced blood flow may play a role in three of the models of hypertrophy discussed in this review, namely, hyperthyroidism, hypoxia, and exercise training.

Both shear stress and cyclic stretch have been implicated in vascular growth.126 An appropriate example is volume overload-induced cardiac hypertrophy.16,26,28,31,60,127 That stretch plays a role in vascular