Voltage Dependence of ATP Secretion in Mammalian Taste Cells

Abstract

Mammalian type II taste cells release the afferent neurotransmitter adenosine triphosphate (ATP) through ATP-permeable ion channels, most likely to be connexin (Cx) and/or pannexin hemichannels. Here, we show that ion channels responsible for voltage-gated (VG) outward currents in type II cells are ATP permeable and demonstrate a strong correlation between the magnitude of the VG current and the intensity of ATP release. These findings suggest that slowly deactivating ion channels transporting the VG outward currents can also mediate ATP secretion in type II cells. In line with this inference, we studied a dependence of ATP secretion on membrane voltage with a cellular ATP sensor using different pulse protocols. These were designed on the basis of predictions of a model of voltage-dependent transient ATP efflux. Consistently with curves that were simulated for ATP release mediated by ATP-permeable channels deactivating slowly, the bell-like and Langmuir isotherm–like potential dependencies were characteristic of ATP secretion obtained for prolonged and short electrical stimulations of taste cells, respectively. These observations strongly support the idea that ATP is primarily released via slowly deactivating channels. Depolarizing voltage pulses produced negligible Ca2+ transients in the cytoplasm of cells releasing ATP, suggesting that ATP secretion is mainly governed by membrane voltage under our recording conditions. With the proviso that natural connexons and pannexons are kinetically similar to exogenously expressed hemichannels, our findings suggest that VG ATP release in type II cells is primarily mediated by Cx hemichannels.

Dispensable ATP permeability of Pannexin 1 channels in a heterologous system and in mammalian taste cells

“…We first assayed ATP secretion with ATP-biosensor at the level of taste buds. Since type II cells are capable of secreting ATP on both taste stimuli and sufficient depolarization (Huang et al, 2007;Romanov et al, 2007;Romanov et al, 2008;Huang and Roper, 2010) taste buds were stimulated by both KCl and tastants. In a typical experiment, we selected a poorly dissociated taste bud and placed it nearby a COS-1 cell responsive to ATP using a patch pipette (Fig.…”

“…It should be noted that WT taste cells can be identified electrophysiologically based on a family of VG currents they exhibited (Romanov and Kolesnikov, 2006;Romanov et al, 2007;Romanov et al, 2008). In particular, when dialyzed with a solution containing 140 mM CsCl, only taste cells of the type II were capable of generating large outward currents.…”

“…These Panx1 currents activated and deactivated rapidly (Gründken et al, 2011), as is the case with recombinant mouse Panx1 channels (Ma et al, 2009). Meanwhile, the biophysical analysis of ATP release, including the assay of its dependence on membrane voltage, implicates slowly deactivating ATP-permeable channels in mediating ATP secretion in taste cells of the type II (Romanov et al, 2008). Such a kinetic feature was never reported for recombinant Panx1 channels.…”

“…This entails activation of P2X2/P2X3 receptors on afferent taste nerves (Bo et al, 1999;Kataoka et al, 2006) and a variety of P2X and P2Y receptors on taste cells (Baryshnikov et al, 2003;Kataoka et al, 2004;Bystrova et al, 2006;Huang et al, 2009;Huang et al, 2011). Taste cells of the type II lack conventional synapses (Royer and Kinnamon, 1991;Tabata et al, 1995), but instead release ATP via voltage-gated channels (Huang et al, 2007;Huang et al, 2011;Romanov et al, 2007;Romanov et al, 2008).…”

Mechanical Stimulation-induced Calcium Wave Propagation in Cell Monolayers: The Example of Bovine Corneal Endothelial Cells

“…During mechanical deformation, hypoxia, inflammation or stimulation by various agents, ATP can be released from healthy cells [41][42][43][44] in response to shear stress, stretch, or osmotic swelling 44,45 . Different ATP-release mechanisms have been postulated, including vesicular exocytosis 44 and a plethora of transport mechanisms, such as ATP-binding cassette (ABC) transporters, plasmalemmal voltage-dependent anion channels 46 , P2X7 receptor channels 47,48 , as well as connexin hemichannels [49][50][51][52] and pannexin hemichannels 43,49,53 . Extracellular ATP can be rapidly hydrolyzed to ADP, AMP and adenosine 54,55 by ectonucleotidases that are present in the extracellular environment.…”