[³H]ATP was purified by HPLC, added to a 10 μM solution of ATP in Hanks’ buffered saline solution (1 μCi/ml), and then superfused for 30 sec (1.4 ml/min) over coverslips with or without cells (three experiments for each condition). ³H-labeled nucleotides in the effluent were resolved by HPLC, and radioactivity in the fractions corresponding to authentic standards was quantitated. Data are given as averages ± standard errors.