Sources of calcium for ATP-induced contractions in rat urinary bladder smooth muscle

Adenosine triphosphate (ATP 10(-4) M) induced monophasic contractions in preparations of rat urinary bladder strips. These responses were reversibly abolished in calcium (Ca2+)-free, ethyleneglycol-bis-(beta-amino-ethyl ether)-N,N'-tetra-acetic acid (EGTA 0.1 mM)-containing medium, and were partially inhibited by 10(-6) M verapamil (43%) and 10(-6) M diltiazem (32%). However, the inhibition of ATP-stimulated 45Ca uptake by verapamil and diltiazem was more pronounced (109 and 75%, respectively). The results suggest that, in addition to the primary role of extracellular Ca2+, intracellular Ca2+ is involved in ATP-induced contractions.