Contractile responses of isolated canine blood vessels to ATP and ADP.

Effects of ATP and ADP on helically cut strips of canine different blood vessels were studied, in comparison with those of K+ and norepinephrine (N.E.). ATP and ADP were equally effective in causing transient contractions in contrast to sustained contractions induced by K+ and N.E.. The mesenteric, renal arteries and portal veins were sensitive to both nucleotides. The response to the nucleotides was K+ was more dependent on Ca++ and Mg++ concentrations in bathing media than that to N.E.. Treatment of the strips with EGTA in Ca++ -free media quickly abolished the contractile responses to ATP, ADP and K+. Application of 0.1 mM Cd++ attenuated the contractile responses to K+ more markedly than those to N.E., but did not affect ATP action. Verapamil at 0.5 micron inhibited the K+-induced contraction alone, and 2 mM procaine inhibited the response to N.E. alone, but rather intensified the ATP-induced contraction. It may be concluded that ATP and ADP produce a transient contraction of isolated canine blood vessels by a mechanism relating to an increased mobilization of loosely bound Ca++ in cell membranes.