Figure 18.

Vasodilator responses in coronary circulation of dogs during exercise, under control conditions, and after triple blockade when nitric oxide, adenosine, and calcium ATP channels are all blocked using various pharmacological antagonists. Note that the rise in coronary blood flow in response to an increase in myocardial oxygen consumption generated by graded treadmill exercise is similar in both conditions. This is an example of vasodilator redundancy in the coronary circulation that is similar to the redundancy seen in many human and animal studies on the regulation of blood flow in contracting human and animal muscles. It is seen in both isolated preparations and during exercise in conscious animals. While there are key differences between the coronary and skeletal muscle circulations, the concept that redundant vasodilator mechanisms govern the blood flow responses to exercise is a key commonality, as are the many putative dilator substances and mechanisms that might be involved. [Adapted from Tune et al. (481).]