Diadenosine tetraphosphate contributes to carbachol-induced tear secretion

The purpose of this study is to investigate if the cholinergic stimulation by carbachol on tear secretion is a direct process or if it is also mediated by purinergic mechanisms. Experiments were performed in New Zealand male rabbits. The amount of tear secretion was measured with Schirmer’s test and then analyzed by a HPLC protocol in order to study the nucleotide levels. Animal eyes were instilled with carbachol (a cholinergic agonist), pirenzepine, gallamine and 4-DAMP (muscarinic antagonists), PPADS, suramin and reactive blue 2 (purinergic antagonists), and a P2Y₂ receptor small interfering RNA (siRNA). Tear secretion increased with the instillation of carbachol, approximately 84 % over control values 20 min after the instillation and so did Ap₄A and ATP release. When we applied carbachol in the presence of muscarinic antagonists, tear volume only increased to 4 % with atropine, 12 % in the case of pirenzepine, 3 % with gallamine, and 8 % with 4-DAMP. In the presence of carbachol and purinergic antagonists, tear secretion was increased to 12 % (all values compared to basal tear secretion). By analyzing tear secretion induced with carbachol in presence of a P2Y₂ receptor siRNA, we found that tear secretion was diminished to 60 %. The inhibition of tear secretion in the presence of carbachol and purinergic antagonists or P2Y₂ siRNA occurred with no apparent change in the tear amount of Ap₄A. These experiments demonstrated the participation of Ap₄A in lacrimal secretion process.