Mechanical factors involved in the growth of the heart and its blood vessels.

Various proteins of cardiac myocytes are preprogrammed at a very early stage of heart development, but functional load (stretch, pressure) plays an important role in their expression under both physiological and pathological circumstances. Mechanical factors are also important in growth of vessels, particularly with respect to hypertrophy or hyperplasia of vascular smooth muscle. Their effect on growth of endothelial cells is less clear. Although they have been studied in cell culture, little is known about their involvement in capillary growth in vivo. Their possible role is considered in capillary growth in the normal adult heart where it was elicited by long-term administration of various vasodilators, by long-term bradycardia, or by increased inotropic action. Here the mechanical stimuli may act either by increased shear stress (resulting from increased velocity of flow in long-term dilatation) or by increasing vessel wall tension (in conjunction with increased diameters and/or stretch produced by increased inotropism). While the role of growth factors in the development of myocytes has been established, it is still questionable in capillary growth. It is also possible that various growth factors exert their effect on vessel growth by their vasoactive activity.