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

Abstract

Salivary gland fluid secretion is driven by transepithelial Cl− movement involving an apical Cl− channel whose molecular identity remains unknown. Extracellular ATP (ATPo) has been shown to activate a Cl− conductance (IATPCl) in secretory epithelia; to gain further insight into IATPCl in mouse parotid acinar cells, we investigated the effects of ATPo using the whole-cell patch-clamp technique. ATPo 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 µm, respectively. IATPCl 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, IATPCl 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 IATPCl magnitude from wild-type animals was comparable to that from mice with null mutations in the Cftr, Clcn3 and Clcn2Cl− channel genes. Taken together, our results demonstrate that IATPCl is distinct from the channels described previously in acinar cells. The activation of IATPCl by Bz-ATP suggests that P2 nucleotide receptors are involved. However, inhibition of G-protein activation with GDP-β-S failed to block IATPCl, and Cibacron Blue 3GA and 4,4′-diisothyocyanostilbene-2,2′-disulphonic disodium salt selectively inhibited the Na+ currents (presumably through P2X receptors) without altering IATPCl, suggesting that neither P2Y nor P2X receptors are likely to be involved in IATPCl activation. We conclude that IATPCl is not associated with Cl− channels previously characterized in mouse parotid acinar cells, nor is it dependent on P2 nucleotide receptor stimulation. IATPCl 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.