Activation and Desensitization of the Recombinant P2X(1) Receptor at Nanomolar ATP Concentrations

Activation and desensitization kinetics of the rat P2X(1) receptor at nanomolar ATP concentrations were studied in Xenopus oocytes using two-electrode voltage-clamp recording. The solution exchange system used allowed complete and reproducible solution exchange in <0.5 s. Sustained exposure to 1–100 nM ATP led to a profound desensitization of P2X(1) receptors. At steady-state, desensitization could be described by the Hill equation with a K(1/2) value of 3.2 ± 0.1 nM. Also, the ATP dependence of peak currents could be described by a Hill equation with an EC(50) value of 0.7 μM. Accordingly, ATP dose-effect relationships of activation and desensitization practically do not overlap. Recovery from desensitization could be described by a monoexponential function with the time-constant τ = 11.6 ±1.0 min. Current transients at 10–100 nM ATP, which elicited 0.1–8.5% of the maximum response, were compatible with a linear three-state model, C-O-D (closed-open-desensitized), with an ATP concentration-dependent activation rate and an ATP concentration-independent (constant) desensitization rate. In the range of 18–300 nM ATP, the total areas under the elicited current transients were equal, suggesting that P2X(1) receptor desensitization occurs exclusively via the open conformation. Hence, our results are compatible with a model, according to which P2X(1) receptor activation and desensitization follow the same reaction pathway, i.e., without significant C to D transition. We assume that the K(1/2) of 3.2 nM for receptor desensitization reflects the nanomolar ATP affinity of the receptor found by others in agonist binding experiments. The high EC(50) value of 0.7 μM for receptor activation is a consequence of fast desensitization combined with nonsteady-state conditions during recording of peak currents, which are the basis of the dose-response curve. Our results imply that nanomolar extracellular ATP concentrations can obscure P2X(1) receptor responses by driving a significant fraction of the receptor pool into a long-lasting refractory closed state.