SUMMARY1. ATP-activated inward current in PC12 pheochromocytoma cells was characterized using the whole-cell voltage-clamp technique.2. ATP (100 4aM) 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 > ATPyS > ADP; AMP, adenosine and a,,-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 * 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.
SUMMARY1. The adenosine triphosphate (ATP)-activated inward current was compared to the nicotine-activated inward current in nerve growth factor (NGF)-treated rat phaeochromocytoma PC12 cells.2. Both ATP and nicotine activated an inward current at negative holding potentials. The concentration of ATP necessary to activate the inward current was about 10-fold higher than that of nicotine; the EC50 was 20-5 /tM for ATP and 2-4 /tM for nicotine. The maximal responses induced by ATP and nicotine were almost identical in the same cells. The current-voltage relationship for the ATP-activated current was very similar to that for the nicotine-activated current, and both currents reversed around 0 mV in a physiological saline.3. The ATP-activated current and the nicotine-activated current were not additive; the current activated by a combined administration of ATP (100 ,UM) and nicotine (10 ftM) was only about 20% larger than the current activated by either ATP or nicotine alone. Nicotine (100 ,tM) did not increase the current activated by 1 #M-ATP. 4. ATP could activate an inward current in the cells even after desensitization to nicotine had developed.5. Hexamethonium (100,tM) selectively blocked the nicotine-activated current whereas suramin (100 ,tM), a purinoceptor antagonist, selectively blocked the ATPactivated current.6. Ionic selectivity was studied by changing compositions of extracellular solutions. When external Na+ was replaced with Cs', both ATP and nicotine activated inward currents. However, with an extracellular solution containing Tris or glucosamine as a major cation, only ATP, not nicotine, activated an inward current.7. ATP-and nicotine-activated currents were also recorded from cells bathed in a solution containing 1 8 mM-Ca2'as the only external cation, suggesting that both pathways are Ca2+ permeable. 8. The results suggest that the ATP-sensitive ionic pathway is not independent of the nicotine-sensitive pathway in these cells. Our working hypothesis is that ATP MS 8661 K. NAKAZA WA AND OTHERS and nicotine activate the same channels but the binding sites and the open-states of the channels are different between these two agonists.
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