Coronary arteriolar myogenic response in regulation of coronary circulation

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Coronary blood flow has a sophisticated control system including three major mechanisms : (1) "autoregulation" which keeps blood flow rate constant when perfusion pressure is changed, (2) "reactive hypermia" in reperfusion after short-time coronary artery occlusion, and (3) "an increase in bolld flow rate due to load" when myocardial oxygen consumption rate increases. This coronary blood flow control is considered "concerted control" of various mechanisms such as metabolic factors (adenosine, Pco2,and Po2) and control factors via endothelium. In this study, we focus "myocardial control" which has recently been a popular research topic. Our main goals are to study the effects of this control mechanism under the coronary blood flow controlling condition including "autoregulation," "reactive hypermia,) and "an increase in blood flow rate due to load," and then to clarity the significance of these effects during pathological conditions such as myocardial ischemia.In this report, we present … More the results for in vivo experiments in which we have made great advancement. A needle-probe CCD microscope was used to observe the microvessels in subendo- and subepi-cardial sides. The microvessels in the epi-cardial side was observed by gently contacting the probe to observation spots. Those in the subendo-cardial side was observed by introducing the probe through left arterial appendage into left ventricle and contacting it to observation spots. The blood around the probe was flushed with a Krebs-Ringer solution (37ﾟC). The diameter of the arteriole in the subepicardial side increased during systole while that in the subendocardial side decreased about 20 % during systole. as for the reaction of the arteriole at reactive hypermia by 20-sec blood flow hold, the diameter was maximum about 20 sec and returned to the initial value after about one min. The blood vessels in the subendocardial side needed longer time to return to the initial value than in those in the subepicardial side. When the prolonged diastole was induced by an AV block, the diameter of coronary arteriole decreased with decreasing aortic pressure. In contrast, that of coronary venule increased first and then reached a steady value. Nitroglycerine increased the diameter of both coronary arteriole and venule, and also increased the extent of the change in the diameter over a heart cycle. Less