Activation of Nucleotide Receptors Inhibits High-threshold Calcium Currents in NG108-15 Neuronal Hybrid Cells

A P_2U (UTP-sensitive) nucleotide receptor has previously been cloned from NG108-15 neuroblastoma × glioma hybrid cells and it has been shown that activation of this receptor inhibits the M-type K⁺-current. We now report that UTP also inhibits Ca²⁺-currents in differentiated NG 108-15 cells, but probably through a different nucleotide receptor. UTP (100 μM) inhibited the peak of the high-threshold current by 28.4 ± 3.1% ( n = 38) with no effect on the low-threshold current. Two components of high-threshold current were identified: one inhibited by 100 nM ω-conotoxin (CgTx) and one inhibited by 2 μM nifedipine and enhanced by 1 μM BAY K8644. UTP inhibited the former by 31.0 ± 3.1%, with an IC₅₀ of 2.8 ± 1.1 μM, and the latter by 34.2 ± 6.1% with an IC₅₀ of 1.7 ± 1.3 μM. Pertussis toxin pretreatment prevented inhibition of the CgTx-sensitive, nifedipine-resistant but not CgTx-resistant current. Inhibition was not prevented by intracellular BAPTA (20 mM) or CAMP (1 mM). Effects of UTP on both currents were imitated by UDP, ATP, ADP, AP₄A and ATPγS but weakly or not at all by 2-MeSATP, GTP, AMP-CPP or ITP. Since the receptors which inhibit Ca²⁺-currents are activated by ATP, it is suggested that they might mediate auto-inhibition of transmitter release by ATP if present on purinergic nerve terminals.