Key Points
- To determine the molecular mechanism of inward rectification in cardiac potassium channels by examining single-channel outward currents during cytoplasmic magnesium exposure.
- Conducted open cell-attached patch-clamp recordings on single guinea-pig cardiac myocytes at 15–16 °C with external K+ concentrations of 40 mM and 150 mM.
- Measured current-voltage relationships, single-channel conductance states, and voltage-step responses across internal Mg2+ concentrations ranging from 0 to 10 µM.
- Cytoplasmic Mg2+ selectively blocked outward currents with a half-saturation concentration of 1.7 µM at +70 mV, whereas Mg2+-free conditions exhibited linear I-V curves with conductances of 22 pS (150 mM K+) and 16 pS (40 mM K+).
- Internal Mg2+ at 2–10 µM induced transitions between four discrete current levels separated by ~7 pS intervals, matching a binomial distribution model of three identical conducting subunits blocking independently.
- Depolarization beyond the potassium equilibrium potential resulted in exponential current decay, with blocking rate mu increasing with voltage and Mg2+ concentration while unblocking rate lambda remained steady between 50 and 90 s⁻¹.
Structured PICO
PPopulationGuinea-pig heart cells (cardiac myocytes)
IInterventionIntracellular magnesium (Mg2+) at varying concentrations (2-10 microM)
CComparatorAbsence of intracellular Mg2+
OOutcomeSingle-channel outward currents and current-voltage (I-V) relations of inwardly rectifying K+ channelssurrogate
Inwardly rectifying K+ channels in cardiac myocytes are composed of three identical conducting subunits that are independently blocked by intracellular magnesium, explaining the mechanism of rectification.