Reciprocal effects of Ca2+ and Mg-ATP on the ‘run-down’ of the K+ channels in opossum kidney cells

Using the patch clamp technique, we identified an inwardly rectifying K⁺ channel in the membrane of opossum kidney cells. The single channel conductance was about 90 pS for inward currents and 30 pS for outward currents under a symmetrical high-K⁺ condition. The activity of the channel was found to decrease with time during recording from inside-out patches. In the solution with submicromolar Ca²⁺, the activity disappeared within 4–20 min. Intracellular Ca²⁺ promoted the run-down of the channel activity at 0.1–1 mM, whereas millimolar Mg-ATP restored the activity after run-down. The run-down channels could never be reactivated by ATP in the absence of Mg²⁺, or by a nonhydrolyzable ATP analog, AMPPNP, even in the presence of Mg²⁺.