Heterogeneity of coronary vasomotion.

Apart from local heterogeneities of myocardial blood flow, the regulation of vascular tone is equally complex. Changes in vascular tone are essential for the adaptation of coronary blood flow to varying metabolic demands. The majority of coronary vascular resistance is found in the microcirculation, that is, in vessels with less than 150-200 microns in diameter; therefore, understanding the regulatory mechanisms that exercise control of the tone of these vessels is paramount to our understanding the control of myocardial perfusion. Recently, it became evident that different regulatory systems such as metabolic, neurohumoral, myogenic, and flow-mediated mechanisms have preferential effects on particular microvascular segments. However, the significance of specific vasoactive mediators is still under investigation. Growth factors, which are synthesized in cells in the surrounding of vessels such as mast cells and cardiac myocytes, have recently been suggested to play a role in the coordination of endothelium-dependent and endothelium-independent vasoactive mechanisms. The aim of this review is, first, to focus on the heterogeneity of the regulation of coronary vascular resistance in general and, second, to discuss recent studies showing a possible role of growth factors, especially fibroblast growth factor (FGF), heparin, and endothelin, as modulators of microvascular tone.