Electrophysiological evidence for an ATP-gated ion channel in the principal cells of the frog skin epithelium.
In the present study we investigated the effects of adenosine 5'-triphosphate (ATP) on Na+ transport in frog skin epithelium. An experimental set-up was constructed to allow simultaneous measurement of Na+ transport, measured as the amiloride-sensitive short circuit current (Isc), and free cytosolic Ca2+ concentration ([Ca2+]i) measured with the Ca(2+)-sensitive dye fura-2. The cell potential (Vsc) was measured with microelectrodes. Addition of ATP (100 micrM) to the basolateral solution resulted in a fast transient decrease in Isc followed by a slower increase and a transient increase in [Ca2+]i. Microelectrode measurements showed that the primary response, i.e. the decline in Isc was accompanied by transient depolarisation, followed by a return to the control value. The decrease in current was Ca2+ independent; i.e. treatment with thapsigargin in Ca(2+)-free solutions abolished the Ca2+ transient but did not influence the current transient. The secondary response, i.e. the slow increase in current, was accompanied by slow depolarisation of the cell. Measurements of apical Na+ permeability showed that this was due to an opening or activation of apical Na+ channels. These data show that ATP causes a fast initial drop and a secondary, long-lasting increase in Na+ absorption. The ability of ATP to cause the initial decline in current is independent of Ca2+, i.e. it is not caused by secondary effects of the P2Y-type receptors present in the tissue. Measurements of intracellular potential indicate that the initial depolarisation is caused by opening of non-selective cation channels, suggesting that this decrease is due to a transient activation of P2X-type ATP receptors.
Brodin
B
B
Department of Pharmaceutics, Royal Danish School of Pharmacy, Copenhagen. bbr@mail.dfh.dk
Nielsen
R
R
D000255:Adenosine Triphosphate / Q000502:physiology*; D000818:Animals; D002118:Calcium / Q000378:metabolism; D004594:Electrophysiology; D004847:Epithelial Cells / Q000378:metabolism* / Q000502:physiology; D066298:In Vitro Techniques; D015640:Ion Channel Gating / Q000502:physiology*; D007473:Ion Channels / Q000502:physiology*; D008564:Membrane Potentials / Q000502:physiology; D011896:Rana temporaria; D018048:Receptors, Purinergic P2 / Q000187:drug effects / Q000502:physiology; D012867:Skin / Q000378:metabolism*; D012964:Sodium / Q000378:metabolism; D014544:Uridine Triphosphate / Q000494:pharmacology
D007473:Ion Channels; D018048:Receptors, Purinergic P2; D000255:Adenosine Triphosphate; D012964:Sodium; D002118:Calcium; D014544:Uridine Triphosphate