The neuronal cell line NG108-15 (180CC15) responds to extracellular stimuli of ATP or UTP with a transient increase in the level of cytosolic Ca2+. Desensitization was investigated by recording single-cell Ca2+ responses induced by consecutive, regularly spaced (100 s intervals), brief pulses of the nucleotides. The two natural ligands of the P2U receptor, ATP and UTP, were applied at a concentration that evoked responses of a comparable size. With two pulses of UTP (10 µM), a substantial decrease (of 43%) was observed in the size of the second response. The magnitude of response was determined by measuring either the maximal amplitude or the total response, represented by the area of the Ca2+ transient. The analogous studies with ATP pulses showed a much smaller decrease (of 12%). Comparable experiments performed to investigate the mutual interaction between ATP and UTP revealed that after stimulation with ATP the response to UTP was slightly (12%) diminished, whereas the response to ATP after UTP was greatly (52%) decreased. The different degree of desensitization by either UTP or ATP of P2U receptors could be due to (1) a difference in the mode of activation of the receptor by the two ligands or (2) recruitment of another effector mechanism besides elevated Ca2+. Our results indicate the existence of a novel mechanism of receptor control, involving different modes of the receptor, that are induced by two different, activating ligands. We also investigated the cross-talk between the bradykinin B2 receptor and the nucleotide receptor. ATP and UTP, even when eliciting responses of comparable size in the neuronal cell line, affect the desensitization of the bradykinin receptor differently. This suggests regulatory binding sites for the nucleotides on either the nucleotide receptor or the peptide receptor.

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