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 (IK(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 IK(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 < or = .05, n > or = 4). In coronary artery cells, 100 microM 2-(2-aminoethyl)pyridine (an H1 receptor agonist) or 10 microM ryanodine, agents that cause an increase in [Ca2+]i, also caused a significant reduction of the 4-AP-sensitive IK(dr) similar to that produced by Mg2+. 4-AP (5 mM) significantly depolarized single renal arterial cells that were dialyzed with Mg(2+)-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 Ca(2+)-activated K+ channels, a 59 +/- 10-picosiemen K+ channel that was sensitive to cytoplasmic Mg2+ was identified. In Mg(2+)-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.