Distinct biophysical signature of the low-affinity ATP response in spermatogonia. (A) Representative whole-cell voltage-clamp recording (V_hold = −80 mV; S₁, S₆) comparing the rapidly desensitizing response to relatively low [ATP]_ex (10 µM) with the gradually increasing current induced by prolonged exposure to high [ATP]_ex (1 mM). (B–E) Current–voltage relationships (B and D) and current density time course (C and E) in response to 100 µM (n = 16) and 1 mM ATP (n = 7), respectively. (B, inset) Command voltage ramp, repeated at 2 Hz. I–V curves depict mean traces (black) and SEM (gray shadows). Data were corrected by digital offline subtraction of averaged leak controls recorded before stimulation. (C and E) Representative plots of current density measurements at −80 mV (black) and 80 mV (gray), respectively, over time. (F) Bar graph illustrating the difference in rectification as a function of stimulus concentration (100 µM vs. 1 mM ATP; mean ± SEM). Current rectification is calculated from individual ramp recordings (B and D) as the ratio of each cell’s peak currents at 80 and −80 mV (I₊₈₀/I₋₈₀). Asterisk (*) denotes statistical significance, P < 0.005 (unpaired t test). (G) Bar diagrams comparing different current properties at 80 and −80 mV when activated by either 100 µM (gray) or 1 mM ATP (black). Plotted are peak current densities (I_max), rise time (time-to-peak), and desensitization rate during (%/s) ATP stimulation (mean ± SEM). Asterisks (*) indicate statistical significance, P < 0.005 (unpaired t tests).