Fig. 2. The electrophysiological response of Paramecium to 25 µmol l^-1 ß,-methylene ATP was a transient depolarization. This is the ß,-methylene ATP-receptor potential. As the response was somewhat variable, two different cells are shown. The very fast spikes seen in the first trace (Cell 1) are the Ca²⁺-based, graded action potentials common to these cells but they are less evident in the second trace (Cell 2). In both traces, the cells were exposed to 25 µmol l^-1 ß,-methylene ATP during the entire time. The recording medium contained 1 mmol l^-1 CaCl₂, 1 mmol l^-1 MOPS, 0.5 mmol l^-1 MgCl₂ and pH 7.2 with Tris-base. The recording electrode contained 500 mmol l^-1 KCl.