A novel chloride conductance activated by extracellular ATP in mouse parotid acinar cells

Salivary gland fluid secretion is driven by transepithelial Cl- movement involving an apical Cl- channel whose molecular identity remains unknown. Extracellular ATP (ATP(o)) has been shown to activate a Cl- conductance (I-ATPCI) in secretory epithelia; to gain further insight into I-ATPCl in mouse parotid acinar cells, we investigated the effects of ATP(o) using the whole-cell patch-clamp technique. ATP(o) and 2'- and 3'-O-(4-benzoylbenzoyl)adenosine 5'-triphosphate triethylammonium salt (Bz-ATP) produced concentration-dependent, time-independent Cl- currents with an EC50 of 160 and 15 muM, respectively. I-ATPCl displayed a selectivity sequence of SCN- > I- = NO3- > Cl- > glutamate, similar to the Cl- channels activated by Ca2+, cAMP and cell swelling in acinar cells. In contrast,I-ATPCI was insensitive to pharmacological agents that are known to inhibit these latter Cl- channels, was independent of Ca2+ and was not regulated by cell volume. Moreover, the I-ATPCl magnitude from wild-type animals was comparable to that from mice with null mutations in the Cftr, Clcn3 and Clcn2 Cl- channel genes. Taken together, our results demonstrate that I-ATPCl is distinct from the channels described previously in acinar cells. The activation of I-ATPCl by Bz-ATP suggests that P-2 nucleotide receptors are involved. However, inhibition of G-protein activation with GDP-beta-S failed to block I-ATPCl, and Cibacron Blue 3GA and 4,4'-diisothyocyanostilbene-2,2'-disulphonic disodium salt selectively inhibited the Na+ currents (presumably through P2X receptors) without altering IATPCI,suggesting that neither P2Y nor P2X receptors are likely to be involved in I-ATPCl activation. We conclude that I-ATPCl is not associated with Cl- channels previously characterized in mouse parotid acinar cells, nor is it dependent on P-2 nucleotide receptor stimulation. I-ATPCl expressed in acinar cells reflects the activation of a novel ATP-gated Cl- channel that may play an important physiological role in salivary gland fluid secretion.