Differential effects of UTP and ATP on ion transport in porcine tracheal epithelium

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Isolated segments of porcine tracheal epithelium were mounted in Ussing chambers, current required to maintain transepithelial potential difference at 0 mV (short circuit current, I(SC)) was monitored and effects of nucleotides upon I(SC) were studied. Mucosal UTP (100 microM) evoked a transient rise in I(SC) that was followed by a sustained fall below basal I(SC) maintained for 30 min. Mucosal ATP (100 microM) also stimulated a transient rise in I(SC) but in contrast to UTP did not inhibit basal I(SC). Submucosal UTP and ATP both transiently increased I(SC). UTP-prestimulated epithelia were refractory to ATP but prestimulation with ATP did not abolish the response to UTP. The epithelia thus appear to express two populations of apical receptors allowing nucleotides to modulate I(SC). The UTP-induced rise was reduced by pretreatment with either bumetanide (100 microM), diphenylamin-2-carboxylic acid (DPC, 1 mM), or Cl(-) and HCO(3)(-)-free solution whilst the fall was abolished by amiloride pretreatment. Thapsigargin (0.3 microM) abolished the UTP-induced increase in I(SC) but not the subsequent decrease. Staurosporine (0.1 microM) inhibited basal I(SC) and blocked UTP-induced inhibition of I(SC). Inhibitors of either protein kinase C (PKC) (D-erythro sphingosine) or PKA (H89) had no effect. This study suggests that UTP stimulates Cl(-) secretion and inhibits basal Na(+) absorption. ATP has a similar stimulatory effect, which may be mediated by activation of P2Y(2) receptors and an increase in [Ca(2+)](in), but no inhibitory effect, which is likely mediated by activation of a pyrimidine receptor and possible inhibition of a protein kinase other than PKC or PKA.