CONTRACTILE RESPONSES OF ISOLATED CANINE BLOOD VESSELS TO ATP AND ADP

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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 hoth nucleotides. The response to the nucleotides and 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 μM 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.
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