The amplitude and frequency of oscillating currents induced by P2Y receptor agonists. (a) The mean amplitude of oscillating currents induced by ATP, UTP and UDP (10-4 M) was measured over successive 30 s intervals for 4 min in smooth muscle cells isolated from SPA and voltage-clamped at -50 mV. 0.5 = the first 30 s of agonist application, 1 = 30 – 60 s, and so on. (b) The frequency of the oscillations in the same cells was measured over successive 1 min intervals. 1 = the first min of agonist application, 2 = the second min, and so on. Vertical bars represent s.e.mean. The number of cells for each agonist is shown in parentheses. * P < 0.05 for amplitude of peak current at 30 s versus that at 2:30, 3:00, 3:30 and 4:00 min; ** P < 0.01 for 30 s versus 3:30 and 4:00 min.