ATP release via anion channels

Abstract

ATP serves not only as an energy source for all cell types but as an 'extracellular messenger' for autocrine and paracrine signalling. It is released from the cell via several different purinergic signal efflux pathways. ATP and its Mg(2+) and/or H(+) salts exist in anionic forms at physiological pH and may exit cells via some anion channel if the pore physically permits this. In this review we survey experimental data providing evidence for and against the release of ATP through anion channels. CFTR has long been considered a probable pathway for ATP release in airway epithelium and other types of cells expressing this protein, although non-CFTR ATP currents have also been observed. Volume-sensitive outwardly rectifying (VSOR) chloride channels are found in virtually all cell types and can physically accommodate or even permeate ATP(4-) in certain experimental conditions. However, pharmacological studies are controversial and argue against the actual involvement of the VSOR channel in significant release of ATP. A large-conductance anion channel whose open probability exhibits a bell-shaped voltage dependence is also ubiquitously expressed and represents a putative pathway for ATP release. This channel, called a maxi-anion channel, has a wide nanoscopic pore suitable for nucleotide transport and possesses an ATP-binding site in the middle of the pore lumen to facilitate the passage of the nucleotide. The maxi-anion channel conducts ATP and displays a pharmacological profile similar to that of ATP release in response to osmotic, ischemic, hypoxic and salt stresses. The relation of some other channels and transporters to the regulated release of ATP is also discussed.

Figure 1

ATP in a purinergic world consisting of purine-generating reactions, purinergic signal efflux pathways, purino-converting enzymes and purinergic receptors. ENA, EAK and ENDK represent ectonucleotidase, ectoadenylate kinase and ectonucleoside diphosphate kinase, respectively. Arrows indicating ENA-mediated degradation of ATP to ADP and AMP as well as of ADP to AMP are missing because both ADP and AMP are rapidly degraded to adenosine by the same enzyme. Although the purino-converting enzymes are shown only on a cell different from the cell expressing purinergic signal efflux pathways and purinergic receptors, they may coexist on the same cell.

Figure 2

Representative current-voltage (I–V) curves composed of outward Cl⁻ currents and inward currents carried by ATP⁴⁻ (A), ADP³⁻ (B) and UTP⁴⁻ (C). The currents were recorded by ramp-clamp in macro-patches excised from C127 cells. The extracellular (pipette) solution was Cl⁻-rich Ringer solution, and the intracellular (bath) solution was Cl⁻-free solution containing 100 mM Na4ATP (A), Na3ADP (B) or Na4UTP (C). The reversal potentials are −16.2 ± 1.8 mV (n = 5: A), −20.3 ± 1.7 mV (n = 10: B) and −17.1 ± 0.8 mV (n = 10: C). The calculated values of P_ATP/P_Cl, P_ADP/P_Cl and P_UTP/P_Cl are 0.10 ± 0.014, 0.12 ± 0.010 and 0.09 ± 0.005, respectively.

Figure 3

Lack of effect on maxi-anion channel activity of octanol, atractyloside and bongkrekic acid added to the intracellular solution in inside-out patches excised from C127 cells. Dashed lines indicate the zero current level. The test pulse protocol applied is shown on the top. Traces are representative of 4, 3 and 3 experiments with octanol-1 (1 mM), atractyloside (10 µM) and bongkrekic acid (10 µM), respectively. Experimental conditions were same as in Sabirov et al. [115]. Maxi-anion channel activity was also not affected by the addition of these drugs to the extracellular (pipette) solution (data not shown, n = 2–4).

Figure 4

Putative non-exocytic pathways of regulated ATP release in response to hypoxia, ischemia, osmotic cell swelling or mechanical stimulation (see text for details).