Tussive challenge with ATP and AMP: does it reveal cough hypersensitivity?

Recent studies have demonstrated that blockade of P2X3 ATP receptors can profoundly inhibit chronic cough. We have considered whether inhaled ATP produces a tussive response and whether chronic cough patients are hypersensitive to inhaled ATP compared to healthy volunteers.A standardised inhalational cough challenge was performed with ATP and AMP. We randomised 20 healthy volunteers and 20 chronic cough patients as to the order of challenges. The concentration of challenge solution causing at least five coughs (C5) was compared for ATP and AMP.The study population consisted of six male and 14 female volunteers in each group. Two out of 19 healthy volunteers coughed with AMP (one volunteer could not take part in this challenge) and none reached C5. Eight out of 20 chronic cough patients coughed with AMP and two reached C5. Of the 20 healthy volunteers, 18 coughed with ATP, with 15 reaching C5. All 19 chronic cough patients completing the ATP challenge coughed with ATP and 18 reached C5. The chronic cough patients had a greater cough response at lower concentrations of ATP.The greater potency of ATP versus AMP in the inhalational challenge suggests that tussive responses are mediated through members of the P2X purinergic receptor family. This acute effect was, however, not sufficient to explain cough hypersensitivity syndrome.

Extracellular ATP Induces Calcium Signaling in Odontoblasts

Odontoblasts form dentin at the outermost surface of tooth pulp. An increasing level of evidence in recent years, along with their locational advantage, implicates odontoblasts as a secondary role as sensory or immune cells. Extracellular adenosine triphosphate (ATP) is a well-characterized signaling molecule in the neuronal and immune systems, and its potential involvement in interodontoblast communications was recently demonstrated. In an effort to elaborate the ATP-mediated signaling pathway in odontoblasts, the current study performed single-cell reverse transcription polymerase chain reaction (RT-PCR) and immunofluorescent detection to investigate the expression of ATP receptors related to calcium signal in odontoblasts from incisal teeth of 8- to 10-wk-old rats, and demonstrated an in vitro response to ATP application via calcium imaging experiments. While whole tissue RT-PCR analysis detected P2Y2, P2Y4, and all 7 subtypes (P2X1 to P2X7) in tooth pulp, single-cell RT-PCR analysis of acutely isolated rat odontoblasts revealed P2Y2, P2Y4, P2X2, P2X4, P2X6, and P2X7 expression in only a subset (23% to 47%) of cells tested, with no evidence for P2X1, P2X3, and P2X5 expression. An increase of intracellular Ca2+ concentration in response to 100μM ATP, which was repeated after pretreatment of thapsigargin or under the Ca2+-free condition, suggested function of both ionotropic and metabotropic ATP receptors in odontoblasts. The enhancement of ATP-induced calcium response by ivermectin and inhibition by 5-(3-bromophenyl)-1,3-dihydro-2H-benzofuro[3,2-e]-1,4-diazepin-2-one (5-BDBD) confirmed a functional P2X4 subtype in odontoblasts. Positive calcium response to 2′,3′-O-(benzoyl-4-benzoyl)-ATP (BzATP) and negative response to α,β-methylene ATP suggested P2X2, P2X4, and P2X7 as functional subunits in rat odontoblasts. Single-cell RT-PCR analysis of the cells with confirmed calcium response and immunofluorescent detection further corroborated the expression of P2X4 and P2X7 in odontoblasts. Overall, this study demonstrated heterogeneous expression of calcium-related ATP receptor subtypes in subsets of individual odontoblasts, suggesting extracellular ATP as a potential signal mediator for odontoblastic functions.