Relative vasodilator potency of ATP, ADP, AMP and adenosine (Ado) in the cat hind limb

The relative vasodilator potencies of ATP, ADP, AMP and Ado were determined by bioassay using the skinned and unskinned cat hind limb and comparing the increase in blood flow induced by intra-arterial injection of the adenyl compounds. The relative vasodilator potencies of these agents were similar in either hind limb preparation and were as follows: ADP > ATP >> Ado ≥ AMP. ADP was 2-4 times more potent than ATP and about 200 times more potent than Ado or AMP. When the adenyl compounds were incubated in blood for 2 min before injection, their potencies were not changed in the bioassay. Furthermore, in a chemical assay, after Ado and ADP were incubated for 2 min in cat blood (at the concentrations used in the bioassay) only 3% of Ado and 29% of ADP were metabolized. The results indicate that ATP and ADP exert a vasodilator effect that is not dependent upon their conversion to Ado. This deduction cannot be made for AMP because its relative potency was quite similar to Ado. Since a similar relative potency was found in the skinned and the unskinned hind limb, it is concluded that the vasodilator effect due to ATP and ADP is occurring in skeletal muscle but this does not exclude simultaneous cutaneous vasodilation. The vasodilator effect of adenyl compounds may be related to purinergic receptor activation.