Figure 5.

The adenosine tissue content is markedly affected by hypothyroidism; under normal conditions, the metabolism of adenosine is very fast, thus the administration of exogenous adenosine within the μmolar range activates preferentially the high-affinity A1 receptors and induces vasoconstriction (a). In contrast, under hypothyroid conditions, since the metabolism of adenosine is abnormally low, exogenous adenosine reaches concentrations high enough to activate adenosine A2 receptors and induce vasodilation, (b).