INTRACELLULAR DIVALENT-CATIONS BLOCK SMOOTH-MUSCLE K+ CHANNELS

The patch-clamp technique was used to examine the sensitivity of delayed rectifier K+ channels to changes in intracellular divalent cations (Mg2+ and Ca2+). During voltage-step and ramp depolarizations, a delayed rectifier K+ current (I(K(dr))) was identified in renal, pulmonary, coronary, and colonic smooth muscle cells as a low-noise outward current that activated near -40 mV, was sensitive to 4-aminopyridine (4-AP), and was insensitive to charybdotoxin. During whole-cell voltage-clamp experiments in each of the cell types, the 4-AP-sensitiVe I(K(dr)) was significantly less in cells dialyzed with 10 mM Mg2+ as compared with cells in which no Mg2+ was added to the internal dialysis solution (P less-than-or-equal-to .05, n greater-than-or-equal-to 4). In coronary artery cells, 100 muM 2-(2-aminoethyl)pyridine (an H-1 receptor agonist) or 10 muM ryanodine, agents that cause an increase in [Ca2+]i, also caused a significant reduction of the 4-AP-sensitive I(K(dr)) similar to that produced by Mg2+. 4-AP (5 mM) significantly depolarized single renal arterial cells that were dialyzed with Mg2+-free solution but not those dialyzed with 10 mM Mg2+ (P<.01, n=4). In inside-out patches of renal arterial smooth muscle cells, with 200 nM charybdotoxin in the patch pipette to block large conductance Ca2+-activated K+ channels, a 59+/-10-picosiemen K+ channel that was sensitive to cytoplasmic Mg2+ was identified. In Mg2+-free solution, channel open probability was 0.028+/-0.012 (n=8) and 0.095+/-0.011 (n=8) at +40 and +80 mV, respectively. When the bath solution was changed to one containing 5 or 15 mM Mg2+, channel open probability was significantly reduced by 66% and 68% (+40 mV) or 93% and 96% (+80 mV), respectively. This decrease in the open probability of the delayed rectifier K+ channel resulted from a concentration- and voltage-dependent decrease in mean open time. At +40 mV, time constants for the open time distribution were significantly decreased from 5.5+/-0.52 to 1.2+/-0.14 milliseconds, whereas the closed time constant was significantly increased from 634+/-11.1 to 820+/-14.4 milliseconds (P<.01, n=4). It is concluded that a 4-AP-sensitive delayed rectifier K+ channel in both vascular and visceral smooth muscle cells is modulated by changes in intracellular Ca2+ and Mg2+ that may alter membrane potential and the contractile state of smooth muscle.