The wall stress hypothesis for myogenic responses in the coronary circulation

Purpose: Coronary arterioles demonstrate myogenic reactivity, as the caliber of the arteriole is actively adjusted in response to the intraluminal distending pressure. The teleological purpose of myogenic reactivity remains unclear, but it has been proposed that myogenic responses act to mitigate pressure-induced changes in circumferential wall stress (σ). We investigated pressure-induced variations in σ with and without myogenic reactivity. Methods: Using previously published data, we calculated σ in coronary arterioles in both the longitudinal (branch order) and transmural (epicardial vs endocardial) directions with and without myogenic tone. Results: Active constriction from the myogenic response reduced σ in all arterioles examined. Arteriolar σ was higher in the endocardium in both the passive and active state, but the percent change from passive σ to active σ was similar when compared to epicardial arterioles of the same branch order. There was no major change in active arteriole σ in the longitudinal direction, except for branch order 5 epicardial arterioles, which had a larger reduction in σ and a larger change from passive when compared to other epicardial branch orders. Conclusions: The results indicate that the myogenic response reduces arteriole wall stress. The greater passive and active σ seen in endocardial arterioles compared to epicardial arterioles is due to transmural differences in wall thickness. Yet, the fact that the thinner endocardial arterioles were able to produce the same degree of reduction in active σ may indicate that these vessels have more myogenic activity.