European Journal of Pharmacology, 164 (1989) 163-166 163 Elsevier EJP 20368 Short communication Sources of calcium for ATP-induced contractions in rat urinary bladder smooth muscle Manjunatha B. Bhat, Santosh K. Mishra and Vellanki Raviprakash * Division of Pharmacology and Toxicology, Indian Veterinary Research Institute, Izatnagar-243 122 (U. P.), India Received 21 February 1989, accepted 14 March 1989 Adenosine triphosphate (ATP 10 -4 M) induced monophasic contractions in preparations of rat urinary bladder strips. These responses were reversibly abolished in calcium (CaZ+)-free, ethyleneglycol-bis-(/~-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 Ca 2+, intracellular Ca 2+ is involved in ATP-induced contractions. ATP; Verapamil; Diltiazem; 45Ca uptake; Tyrode solution (Ca2+-free) 1. Introduction The contraction of smooth muscle in response to various agonists is critically dependent upon a rise in the intracellular free calcium (Ca 2÷) con- centration. This can be achieved through the mobilization of extracellular Ca 2÷ and/or through the release of intracellularly bound Ca 2÷ from a mobile pool by a specific drug or a neurotrans- mitter (Bolton, 1979). The contractile responses of the urinary bladder to potassium (K ÷) depolariza- tion and to various agonists/neurotransmitters have been reported to be dependent upon an extracellular source of Ca 2+ (Andersson and For- man, 1986; Batra et al., 1987). However, there is little information in the literature about the source of Ca 2+ and the Ca 2+ entry pathways involved in the adenosine triphosphate (ATP)-induced con- traction of urinary bladder smooth muscle (Husted et al., 1983). Keeping in view the physiological significance of ATP as a putative neurotransmitter * To whom all correspondence should be addressed. mediating non-cholinergic contractile responses in the mammalian urinary bladder (Kasakov and Burnstock, 1983), we undertook the present study to elucidate the source of the Ca 2+ participating in the contractions induced by this agonist in the rat urinary bladder. We used verapamil and diltiazem to characterize the calcium channels involved in the ATP-induced contractile responses and 45Ca uptake in this smooth muscle. 2. Materials and methods 2.1. Handling of tissues and solutions Male albino rats weighing 100-130 g (Labora- tory Animal Resource Section, Indian Veterinary Research Institute) were stunned by a blow on the head and exsanguinated. The urinary bladder was immediately removed and placed in Tyrode solu- tion at room temperature. Smooth muscle strips (3 × 10 ram) were dissected from the bladder and were suspended in a 20 ml organ bath containing Tyrode solution which was continuously bubbled 0014-2999/89/$03.50 © 1989 Elsevier Science Publishers B.V. (Biomedical Division)