Myogenic and Flow-Induced Responses in Coronary Arterioles

Summary

The coronary vascular bed exhibits an intrinsic ability to autoregulate blood flow over a wide range of arterial perfusion pressures. Despite numerous studies suggesting the involvement of myogenic and flow-induced responses in the regulation of blood flow in many organ systems, the role of these two mechanisms in the control of vasomotor tone in the coronary microcirculation is not clear. The aim of this paper is to describe myogenic and flow-induced responses of isolated porcine subepicardial arterioles to physiological changes in intraluminal pressure and flow. Experiments are also described which quantitate the interaction of these two responses in vitro. Subepicardial arterioles, 40–100 μm in diameter, were isolated and cannulated with two glass micropipettes connected to independent reservoir systems. Initially, myogenic responses were studied by moving both reservoirs in the same direction to alter myogenic tone in the absence of flow. Flow-induced responses were studied by simultaneously moving the reservoirs in equal but opposite directions thus generating a pressure gradient (ΔP) to initiate flow without changing the mean intraluminal pressure (IP). Myogenic constrictions and dilations were observed when IP was increased (> 60cmH₂O) and decreased (< 60cmH₂O), respectively. The threshold and maximum flow-induced dilation occurred at ΔP = 4 cmH₂O (flow = 4nl/s) and 20cmH₂O (flow = 13nl/s), respectively. Flow-induced dilation competed with myogenic constriction when flow and pressure were elevated simultaneously. In addition, flow potentiated myogenic dilation when IP was decreased. The magnitude of the flow-induced dilation was greatest at intermediate levels of tone (IP = 60cmH₂O) but was attenuated at higher and lower levels of tone. In the presence of flow (ΔP = 4cmH₂O), pressure-diameter relationships were shifted upward, and the magnitude of myogenic responsiveness was attenuated. After mechanical removal of the endothelium, spontaneous tone and myogenic responses were preserved, but flow-induced dilation was abolished. Therefore, both pressure-dependent and flow-induced responses occur in isolated coronary arterioles, but only flow-induced responses require an intact endothelium. These two responses closely interact either competitively or additively, depending on the direction of local vascular pressure changes.