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Background-The phase difference of coronary arterial and venous flows indicates the importance of intramyocardial capacitance vessels in storing diastolic flow and in discharging volume in systole. However, the anatomic and functional characteristics of the capacitance vessels are unclear. We aimed to clarify those characteristics with their transmural difference by 3D visualization of transmural microvessels under diastole and systole. Methods and Results-We performed complete intracoronary filling of a contrast medium into Langendorff's Wistar rat hearts under (1) St Thomas-perfused diastolic arrest (D-mode) and (2) BaCl 2 -induced systolic arrest (S-mode). Precise transmural 3D architectures of capillaries and of pre-and post-capillary microvessels (ie, microvessels larger than capillaries) were visualized clearly with a confocal laser scanning microscope and x-ray microcomputed tomography (microCT), respectively. Vascular volume fraction (VF) and systolic-induced VF reduction rate from D-to S-mode were analyzed. The net capillary VF in D-mode (20.4Ϯ0.9%) was 10 times that of larger microvessels and was reduced in S-mode by 32% without capillary collapse. Systolic-induced VF reduction rate was smaller in capillaries than in larger microvessels (48%; PϽ0.05). The larger microvessel VF in D-mode (2.2Ϯ0.2%) was reduced in S-mode, accompanied by complicated 3D deformation. Conclusions-Capillaries were relatively resistant to the systolic extravascular compression compared with pre-and post-capillary microvessels, conveniently beneficial for the myocardial oxygen delivery throughout a cardiac cycle. Nevertheless, a larger change in the absolute volume of capillaries may function as effective capacitance. On one hand, the pre-and post-capillary microvessels showed a larger phasic change in resistance, which may function to maintain the capillary patency during systole. (Circulation. 2002;105:621-626.)

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Gap junctions are membranous channels that directly join the cytoplasms of the pericyte and endothelial cell and imply some substantial role for the pericyte on the endothelial proliferation. It is postulated that the function of the pericyte in the prenatal mammals are assigned to the regulation of the development of cerebral microcirculation.