Cystic fibrosis transmembrane conductance regulator facilitates ATP release by stimulating a separate ATP release channel for autocrine control of cell volume regulation.

These studies provide evidence that cystic fibrosis transmembrane conductance regulator (CFTR) potentiates and accelerates regulatory volume decrease (RVD) following hypotonic challenge by an autocrine mechanism involving ATP release and signaling. In wild-type CFTR-expressing cells, CFTR augments constitutive ATP release and enhances ATP release stimulated by hypotonic challenge. CFTR itself does not appear to conduct ATP. Instead, ATP is released by a separate channel, whose activity is potentiated by CFTR. Blockade of ATP release by ion channel blocking drugs, gadolinium chloride (Gd(3+)) and 4,4'-diisothiocyanatostilbene-2,2'disulfonic acid (DIDS), attenuated the effects of CFTR on acceleration and potentiation of RVD. These results support a key role for extracellular ATP and autocrine and paracrine purinergic signaling in the regulation of membrane ion permeability and suggest that CFTR potentiates ATP release by stimulating a separate ATP channel to strengthen autocrine control of cell volume regulation.

Nucleotides regulate NaCl transport in mIMCD-K2 cells via P2X and P2Y purinergic receptors.

Extracellular nucleotides regulate NaCl transport in some epithelia. However, the effects of nucleotide agonists on NaCl transport in the renal inner medullary collecting duct (IMCD) are not known. The objective of this study was to determine whether ATP and related nucleotides regulate NaCl transport across mouse IMCD cell line (mIMCD-K2) epithelial monolayers and, if so, via what purinergic receptor subtypes. ATP and UTP inhibited Na(+) absorption [measured via Na(+) short-circuit current (I(Na)(sc))] and stimulated Cl(-) secretion [measured via Cl(-) short-circuit current (I(Cl)(sc))]. Using selective P2 agonists, we report that P2X and P2Y purinoceptors regulate I(Na)(sc) and I(Cl)(sc). By RT-PCR, two P2X receptor channels (P2X(3), P2X(4)) and two P2Y G protein-coupled receptors (P2Y(1), P2Y(2)) were identified. Functional localization of P2 purinoceptors suggest that I(Cl)(sc) is stimulated by apical membrane-resident P2Y purinoceptors and P2X receptor channels, whereas I(Na)(sc) is inhibited by apical membrane-resident P2Y purinoceptors and P2X receptor channels. Together, we conclude that nucleotide agonists inhibit I(Na)(sc) across mIMCD-K2 monolayers through interactions with P2X and P2Y purinoceptors expressed on the apical plasma membrane, whereas extracellular nucleotides stimulate I(Cl)(sc) through interactions with P2X and P2Y purinoceptors expressed on the apical plasma membrane.

Bioluminescence detection of ATP release mechanisms in epithelia.

Autocrine and paracrine release of and extracellular signaling by ATP is a ubiquitous cell biological and physiological process. Despite this knowledge, the mechanisms and physiological roles of cellular ATP release are unknown. We tested the hypothesis that epithelia release ATP under basal and stimulated conditions by using a newly designed and highly sensitive assay for bioluminescence detection of ATP released from polarized epithelial monolayers. This bioluminescence assay measures ATP released from cystic fibrosis (CF) and non-CF human epithelial monolayers in a reduced serum medium through catalysis of the luciferase-luciferin reaction, yielding a photon of light collected by a luminometer. This novel assay measures ATP released into the apical or basolateral medium surrounding epithelia. Of relevance to CF, CF epithelia fail to release ATP across the apical membrane under basal conditions. Moreover, hypotonicity is an extracellular signal that stimulates ATP release into both compartments of non-CF epithelia in a reversible manner; the response to hypotonicity is also lost in CF epithelia. The bioluminescence detection assay for ATP released from epithelia and other cells will be useful in the study of extracellular nucleotide signaling in physiological and pathophysiological paradigms. Taken together, these results suggest that extracellular ATP may be a constant regulator of epithelial cell function under basal conditions and an autocrine regulator of cell volume under hypotonic conditions, two functions that may be lost in CF and contribute to CF pathophysiology.