POSTNATAL-GROWTH OF THE HEART AND ITS BLOOD-VESSELS

Although rapid growth of the heart during early postnatal development ceases with maturation of the organism, the potential for cardiomyocyt e growth is not lost and may be observed even in senescent hearts. Rap id developmental heart growth is accompanied by a proportional growth of capillaries but not always of larger vessels, and thus coronary vas cular resistance gradually increases. Growth of adult hearts can be en hanced by thyroid hormones, catecholamines and the renin-angiotensin s ystem hormones, but these do not always stimulate growth of coronary v essels. Likewise, chronic exposure to hypoxia leads to growth, mainly of the right ventricle and its vessels but without vascular growth els ewhere in the heart. On the other hand, ischaemia is a potent stimulus for the release of various growth factors involved in the development of collateral circulation. Heart hypertrophy develops in response to training, pressure or volume overload. Training usually leads to growt h of larger coronary vessels but little growth of capillaries, except in young animals. However, growth of the capillary bed, but not the re sistance vasculature capacity, can be induced by either increased coro nary blood flow, bradycardia (electrically or pharmacologically induce d) or increased inotropism, all of which are involved in the training stimulus. Thus, what actually promotes growth of larger vessels as opp osed to capillaries in training is unclear. Pressure overload hypertro phy is mediated by both the renin-angiotensin system and the response of cardiomyocytes to stretch; both lead to activation of early oncogen es (c-fos, c-jun, c-myc) and angiotensin II activates several protein kinases involved in cell growth. In this condition, growth of larger v essels is inadequate, although some capillary growth may occur. Volume overload leads to cardiomyocyte hypertrophy and hyperplasia and some increase in vascular supply. Deficits in capillary supply in pressure or volume overload hypertrophy can be reversed by chronic administrati on of ACE inhibitors, dipyridamole, the bradycardic drug alinidine or pacing-induced bradycardia respectively, but in neither case is traini ng effective. Mechanical and humoral factors are involved in growth of cardiomyocytes and vessels. For cardiomyocytes, stretch is most impor tant, activating oncogenes, protein kinases and possibly the inositol phosphate pathway, but not ion channels, with regulation by the balanc e of angiotensin II, TGF-beta 1 and IGF-1, but not FGFs. For vessels, growth is stimulated by stretch and shear stress, possibly with involv ement of VEGF. Increased shear stress disrupts the glycocalyx on the l uminal side of vessels and releases plasminogen activator and metallop roteinases which disrupt the basement membrane and enable endothelial cell migration and proliferation. It also causes rearrangement of the endothelial cytoskeleton and transmission of mechanical signals to the abluminal side disturbing extracellular matrix and causing distortion of capillary basement membrane. Stretch acting from the abluminal sid e has a similar effect resulting also in basement membrane disruption and endothelial cell proliferation.