Effect of extracellular adenosine triphosphate on electrical properties of subconfluent Madin‐Darby canine kidney cells.

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Abstract
1. The present study has been performed to test for an influence of extracellular ATP on the potential differences across the cell membrane (PD) in subconfluent MDCK cells utilizing conventional microelectrodes. 2. In the absence of ATP, the mean measured PD was ‐47.5 +/‐ 0.3 mV (+/‐ S.E.M., n = 320). Application of 10 mumol/l ATP leads to rapid (less than 2 s) hyperpolarization of the cell membrane by ‐18.5 +/‐ 0.4 mV (n = 221), reduction of input resistance by 14 +/‐ 1 M omega (n = 106) and increase of the sensitivity of PD to alterations of extracellular potassium. 3. The concentration needed for half‐maximal effect (K1/2) of ATP is approximately 0.5 mumol/l. ATP‐gamma‐S (K1/2 approximately 0.4 mumol/l) aand ADP (K1/2 approximately 0.9 similarly effective, whereas up to 1 mmol/l AMP or adenosine does not significantly alter PD. Application of 10 mumol/l theophylline, 1 mumol/l phentolamine and 10 mumol/l indomethacin does not blunt the hyperpolarizing effect of ATP. 4. The ATP‐induced hyperpolarization is completely abolished in the presence of 1 mmol/l quinidine but only incompletely by 0.1 mmol/l quinidine or 1 mmol/l barium. In calcium‐free extracellular fluid (1 mmol/l EDTA added) PD is 18.5 +/‐ 1.7 mV (n = 18). With reduced extracellular calcium, the hyperpolarizing effect of ATP is blunted (‐12.3 +/‐ 1.6 mV, n = 18) and only transient. 5. In conclusion, ATP hyperpolarizes MDCK cells by increasing the potassium conductance. The activation of potassium channels requires calcium.