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* To whom correspondence should be addressed. E-mail: kkiyooka{at}cc.okayama-u.ac.jp.
We visualized the epicardial capillary network of the beating canine heart in vivo, using our high resolution intravital CCD videomicroscopy, in order to elucidate its functional role under control conditions, during reactive hyperemia (RH), and during intracoronary adenosine administration. The pencil-lens videomicroscope probe was placed over capillaries fed by the left anterior descending artery in A-V blocked hearts of open chest, anesthetized dogs paced at 60-90 beats per minute (n=17). In individual capillaries under control conditions red cell flow was predominant either during systole or during diastole, indicating that the watershed between diastolic arterial and systolic venous flows is located within capillaries. The capillary flow increased during RH and reached a peak flow-velocity (2.1±0.6 mm/sec), twice as high as the control (1.2±0.5 mm/sec), with enhancement of intercapillary cross-connection flow and enlargement of diamaeter (by 17%). With adenosine, capillary flow-velocity significantly increased (1.8±0.7 mm/sec). However, increase in volumetric capillary flow with adenosine estimated from red cell velocity and diameter was less than the degree of arterial flow augmentation, whereas that during RH was nearly equivalent to arterial flow increment. There was a time lag of about 1.5 s for refilling the capillaries during RH, indicating their function as capacitance vessels. In conclusion, the coronary capillary network functions as 1) the major watershed between diastolic-dominant arterial and systolic-dominant venous flows, 2) a capacitor, and 3) a significant local flow amplifier and homogenizer of blood supply during RH, but with adenosine, the increase in capillary flow-velocity was less than that in arterial flow.
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