552 Purinergic Signalling (2006) 2:545–557 Fig. 5 Loss of the inhibition of IATP by ACh in the presence of dTc and 5HT. A) Representative IATP in control conditions (applied alone), in the presence of ACh and in the presence of ACh and dTc. This sequence was routinely repeated twice, and the averaged response was obtained. Note the very transient nature of IACh in the presence of dTc, with the current returning to baseline levels within 0.5 s (insert). Note also the lack of occlusion under these conditions. Vh: j70 mV. B) Effect of dTc on IACh + ATP. The sequence of applications was repeated multiple times in each neuron. The peak amplitude of IACh + ATP + dTc was smaller than the predicted sum of IATP and IACh + dTc. C) Rapid recovery of IATP from inhibition by ACh in the presence of dTc. The amplitude of IACh + ATP + dTc 0.5 s following application was equal to the mean of two control IATP at an equivalent time. Inset shows the rapid decline of IACh + ATP + dTc on an expanded time scale. D) Representative IATP in control conditions, in the presence of ACh, and in the presence of both ACh and 5HT. In this example, the tau of IACh in response to the ﬁrst application was 2.7 s, while the tau of IACh+5HT was 0.9 s, showing the acceleration of the IACh decay by 5HT. Note the reduced occlusion under these conditions. Vh : j70 mV. ACh was smaller (to 92 T 2% of control, N = 6) in the presence of ACh and 5HT compared to that in the presence of ACh alone (to 73 T 3% of control, N = 6; P < 0.01). Next, we measured the current induced by the concomitant applications of ACh and ATP (IACh + ATP) at a Vh of j70 mV. As shown in Fig. 6A, IACh + ATP was 75 T 1% (N = 44; P < 0.01) of the predicted sum of IACh and IATP. The decay phase of IACh, IATP and IACh + ATP were ﬁt to a single exponential function, giving tau of 3.0 T 0.2, 7.5 T 0.6 and 3.7 T 0.2 s, respectively. When the decay of IACh was accelerated by the presence of 100 mM 5HT, the tau of IACh, IATP, and IACh + ATP were 0.8 T 0.3 s (N = 7), 7.4 T 1.4 s (N = 7) and 2.6 T 0.8 s (N = 7), respectively (the amplitude of IACh + ATP under these conditions was 76 T 2% of the predicted sum of IACh and IATP). Thus, IACh + ATP desensitization occurs with kinetics that cannot be