Modulation by Mg²⁺ and ADP of ATP-sensitive potassium channels in frog skeletal muscle

Summary

The patch-clamp technique was used to examine the action of intracellular magnesium ions and ADP in the absence of ATP on skeletal muscle ATP-sensitive potassium channels (K-ATP channels). Inside-out patches were excised from the membrane of sarcolemmal blebs which arise spontaneously without enzymatic treatment after a frog muscle fiber is split in half.

In the absence of nucleotides, K-ATP channel open probability was not significantly affected by intracellular magnesium even at a concentration (20 mm) which fully blocks cardiac and pancreatic K-ATP channels. On the other hand, Mg²⁺ ions (10–20 mm) decreased both inward and outward unitary currents. The percent reduction in inward currents (about 8%) was independent of voltage while the reduction in outward currents was larger at higher voltages, suggesting that the former effect resulted from cancellation of surface charges and the latter from rapid channel block.

With or without Mg²⁺, intracellular ADP could either stimulate or inhibit K-ATP channel activity. Low concentrations (1–100 μ m) of ADP rapidly and reversibly increased average activity by a factor of 2 to 3. This activation was seen in half of the patches tested and was greater in the presence of mm Mg²⁺. High concentrations (>100 μ m) of ADP inhibited activity with a half-block concentration of 450 μ m in 0 Mg²⁺, i.e., more than an order of magnitude the value for ATP. ADP inhibition, like ATP inhibition, was partially relieved by mm Mg²⁺, suggesting that the Mg²⁺-bound ADP forms are less effective than free ADP forms.

During exercise, free ADP levels rise and ATP declines while remaining high. Since ADP inhibition appears to compete with ATP inhibition, the two distinct processes of ADP activation and ADP inhibition could therefore promote opening of K-ATP channels during intense muscle work.