Action of adenosine and adenine nucleotides on dogs' isolated veins

Adenosine depressed norepinephrine contractions of dogs' saphenous vein strips in both the presence and the absence of Ca2+ and after inhibition of calcium influx by verapamil. It antagonized noncompetitively contractions induced by Ca2+ in depolarized strips after alpha-adrenergic blockade. Contractions obtained with acetylcholine were also depressed by adenosine. This depression was not accompanied by an increase in cAMP or a decrease in the elevated cGMP level. Thus the depression of the smooth muscle cell reactivity still occurs in the absence of calcium influx and is not mediated by the cyclic 3',5'-nucleotide system. Adenosine diphosphate and triphosphate, but not adenosine (10(-6) to 10(-4) M), increased the basal tension of resting saphenous strips. This was prevented by removal of calcium from the bath. In contracted strips, lower concentrations of both nucleotides (10(-6) to 10(-5) M) caused relaxation whereas with high concentrations (10(-4) to 10(-3) M) further contraction occurred. Thus, unlike adenosine, the adenine nucleotides facilitate calcium influx.