Functional expression of the adenosine A1 receptor in rabbit lacrimal gland

It has become increasingly clear that purine compounds play a mediator role in exocrine secretion. Therefore, the present study was aimed at examining the presence of the adenosine A1 receptor in rabbit lacrimal gland and to evaluate the role of the A1 receptor in regulated secretion. The expression of the A1 receptor was investigated with reverse transcriptase PCR, cyto- and histochemistry as well as with pharmacological methods. Acinar cells were isolated, cultured in a serum-free medium for 2 days and thereafter treated with purinergic agonists/antagonists and/or carbachol and VIP. Secretory response was assessed by measuring secreted β-hexosaminidase enzymatic activity. Microscopical evaluation of the immunocyto- and histochemistry specimens indicated that the adenosine A1 receptor is expressed in the rabbit lacrimal gland, which was also supported by reverse transcriptase PCR resulting in a sequence 100% identical with the previously published sequence for the rabbit A1 receptor gene. Incubation of acinar cells with adenosine and the A1 specific agonist N6-cyclopentyladenosine (CPA) resulted in a fourfold increase of secretion at the determined optimal concentrations. Incubation with carbachol alone resulted in a 10- to 15-fold increase. Carbachol combined with either adenosine or CPA increased the secretion 20-fold or more, demonstrating a synergistic effect. Our data provides evidence for the presence of adenosine A1 receptors in both tissue and cultured cells. Even though adenosine and CPA alone had only a moderate effect on secretion, the observed synergistic effect with carbachol suggests a modulatory role for the adenosine A1 receptor in the lacrimal gland.