Renal autoregulation is complex and challenging, with numerous complementary mechanisms maintaining renal blood flow and glomerular filtration rate over a wide range of pressures. As if this is not enough, transmural variations occur. The major factor controlling the resistance of the coronary microcirculation appears to be local production of vasodilatory metabolites. This phenomenon antagonises autoregulatory controls. Local myocardial adenosine production appears important as a regulator of coronary flow.
Ischaemic myocardium releases increased amounts of adenosine and inosine, for example.
The coronary endothelium also appears to play an enormous part in regulating coronary arterial tone. Endothelial NO production in the coronary circulation seems to mediate coronary vasodilatation in response to adenosine infusion. Circulatory reserve' in the heart is very important because of the near-maximal extraction of oxygen from coronary blood at rest. An important feature of resting skeletal muscle is that it appears to be substantially vasoconstricted. This has been attributed to high baseline sympathetic tone - 20 HETE may also play a role.