KINETICS OF ATP-INDUCED CA2-MUSCLE CELLS DEPEND ON ATP CONCENTRATION AND STORED CA2+( TRANSIENTS IN CULTURED PIG AORTIC SMOOTH)

1. Single cultured pig aortic smooth muscle cells were studied using f ura-2 and dual excitation wavelength microfluometry. 2. Extracellular ATP in micromolar concentrations induced a transient increase of [Ca2]i due to Ca2+ release from internal stores. In the same concentration range application of ATP resulted in an increase of intracellular ino sitol phosphate level. 3. In a medium range of ATP concentrations (2-1 0 muM) the Ca2+ signal was oscillating, whereas at higher and lower co ncentrations only a Ca2+ transient with a single peak was elicited. 4. The rank order of potency for the tested purine and pyrimidine nucleo tides was: UTP > ATP > ADP much greater than AMP = adenosine = alpha,b eta-methylene ATP = 0. The response to the nucleotides could be abolis hed by the P2-purinoceptor antagonist suramin. 5. The latency between agonist application and onset of the Ca2+ transients as well as their amplitude and rate of rise are dependent on ATP concentration. 6. Remo val of Ca2+ from the extracellular solution led to a progressive decre ase of amplitude and prolonged latency of the Ca2+ transients. This sh ows that depletion of the Ca2+ stores affects kinetics of the ATP-indu ced Ca2+ release.7. The inorganic Ca2+-influx blockers Ni2+ and Co2+ a ffected amplitude and latency in a manner similar to Ca2+ removal, whi le the Ca2+ antagonist nifedipine was ineffective up to a concentratio n of 10(-6) M. 8. These results reveal a dual dependency of the InsP3- induced Ca2+ release on agonist concentration and filling state of the Ca2+ stores, which supports the hypothesis of a feedback amplificatio n between InsP3 and released Ca2+.