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In the above-mentioned paper, Knabb et al. tried to give support to the hypothesis that adenosine mediates the coupling between myocardial oxygen consumption and coronary flow. As indicated in their introduction, myocardial interstitial adenosine concentration is assumed to determine the diameter of the resistance vessels and, therefore, coronary resistance. On these grounds, it seems logical to expect a correlation between adenosine concentration and resistance. However, as can be seen from their Table 1, this is not always the case. In two of the six conditions studied, i.e., during aortic constriction and during norepinephrine infusion, adenosine concentration is significantly increased while coronary vascular resistance is unchanged. The increase of flow in these two conditions is not caused by the larger adenosine concentration, but by the rise in perfusion pressure. Despite the parallel relationship between oxygen consumption, coronary blood flow, and adenosine concentration, adenosine is not proven to mediate the coupling between oxygen consumption and coronary flow by changes in peripheral resistance.