Effect of luminal nucleotides on Cl- secretion and Na+ absorption in distal bronchi

P2Y receptor agonists stimulate Cl- secretion across both normal and cystic fibrosis (CF) airway epithelia, and therefore have potential for use in th e treatment of CF. Although CF pathology is manifest primarily in the dista l airways, most studies of P2Y-receptor-mediated airway epithelial ion tran sport have used cells cultured from proximal regions. Here we report the re sults of studies of P2Y-receptor-mediated ion transport in distal bronchi i solated from porcine lungs, cannulated and perfused. A luminal microelectro de was used to record transepithelial potential difference (PD) and cable a nalysis was applied to determine resistance (R-t) and equivalent short-circ uit current (I-SC). Luminal UTP (100 mu mol/l) transiently hyperpolarized P D (from -5.8+/-0.3 to -6.5+/-0.4 mV) and increased I-SC (from 47+/-6 to 55/-8 mu A cm(-2)) before inhibiting PD to below the pre-UTP level (-5.0+/-0. 4 mV). The decline was attenuated by pretreatment with amiloride, and addit ional treatment with bumetanide inhibited the initial hyperpolarization, su ggesting that UTP stimulates Cl- secretion and inhibits Na+ absorption acro ss distal bronchi. Luminal addition of P2Y, [ADP, 2-methylthio-ATP (2MeSATP )] and P2Y(6) (UDP) receptor agonists had no effect on ion transport. Pretr eatment with thapsigargin (0.3 mu mol/l) did not pre vent the UTP-induced i ncrease in PD and I-SC but attenuated the secondary fall in PD. Pretreatmen t with BAPTA/AM (50 mu mol/l), however, had no effect on the response to UT P. Additional studies of isolated bronchial epithelial cells using Fura-2 s howed that UTP increases [Ca2+](in), and this increase is blocked by pretre atment with thapsigargin. These results suggest that in intact distal bronc hi luminal UTP stimulates Cl- secretion by a Ca2+-independent mechanism and inhibits Na+ absorption by a Ca2+-dependent mechanism. Both effects are li kely to favour increased hydration of the airway surface, and may therefore be beneficial in CF.