The influence of alpha, beta-methylene ATP on alpha 1-receptor-operated channels in guinea-pig taenia caeci

The influence of the ATP analog alpha, beta-methylene ATP on the action of adrenaline on alpha 1-receptors of smooth muscle cells of guinea-pig taenia caeci was studied by measuring potential changes. The preparation was superfused (1 ml/min) with Krebs solution or calcium-free solution containing atropine (10(-6) M) and propranolol (10(-6) M) at 22 degrees C, using the sucrose-gap method. The ATP analog (10(-5) to 4 X 10(-4) M) and adrenaline (10(-5) M) both caused a transient hyperpolarization in the absence of external calcium (20 min). The response (area under the 'curve') evoked under calcium-free conditions (20 min) increased with the concentration of the ATP analog. The response was diminished when preceded by the adrenaline response (10(-5) M) or when evoked after repeated addition of the analog to the superfusate (35 min). The adrenaline response was also diminished when preceded by the ATP analog. The responses to the ATP analog or adrenaline in the presence of apamin (3 X 10(-7) M) in the absence of external calcium were characterized by depolarization of the muscle cells. Repeated addition of the ATP analog or adrenaline to the preparation under these conditions did not cause any effect. The results suggest strongly that adrenaline and alpha, beta-methylene ATP both activate the same calcium-dependent process, producing calcium mobilization and the opening of apamin-sensitive potassium channels. Besides this action the ATP analog also activates apamin-sensitive potassium channels and this activation is independent of the availability of calcium in the adrenaline-sensitive pool.