An ATP‐activated conductance in pheochromocytoma cells and its suppression by extracellular calcium.

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Abstract
1. ATP-activated inward current in PC12 pheochromocytoma cells was characterized using the whole-cell voltage-clamp technique. 2. ATP (100 .mu.M) applied extracellularly elicited an inward rectifying current with a reversal potential of about +7 mV. The current was desensitized in seconds in spite of continued presence of ATP. 3. A comparison was made of the ability of ATP and its analogues. The order of potency in activating the inward current was ATP > ATP.gamma.S > ADP; AMP, adenosine and .alpha.,.beta.-methylene ATP were inactive at concentrations up to 1 mM. 4. The ATP-activated current was also observed when external Na+ and Ca2+ were replaced with K+, TEA, Tris or glucosamine. The order of ion selectivity was Na+ > K+ > TEA .apprxeq. Tris > glucosamine. 5. The ATP-activated current was also recorded in extracellular solutions containing Ca2+ as the only external cation. The amplitude increased as the concentration of Ca2+ was increased in the range between 1.8 and 16.2 mM. However, the current amplitude decreased at higher Ca2+ concentrations and the current was not recorded in 110 mM-Ca2+ solution. 6. In the presence of 140 mM-Na+ in the external solution, the current amplitude also decreased as the external Ca2+ concentration was increased (from 1.8 to 16.2 mM). 7. The results indicate that Ca2+ as well as monovalent cations permeate through the ATP-sensitive pathway and that Ca2+ blocks ion permeation, including its own permeation through the pathway. This regulation by extracellular Ca2+ is different to the ATP-activated current in smooth muscle cells.