## Dissertations from ProQuest

#### Title

The effects of intracellular magnesium on the large-conductance, calcium-activated potassium channel found in rat skeletal muscle

1991

Article

#### Degree Name

Doctor of Philosophy (Ph.D.)

#### Department

Physiology and Biophysics

#### First Committee Member

Karl L. Magleby, Committee Chair

#### Abstract

The patch-clamp technique was used to investigate the effect of intracellular Mg$\sp{2+}$ (Mg$\sb{\rm i}\sp{2+}$) on single-channel currents through the large-conductance, Ca$\sp{2+}$-activated K$\sp+$ channel in cultured rat skeletal muscle. Measurements of single-channel current amplitudes indicated that Mg$\sb{\rm i}\sp{2+}$ decreased the K$\sp+$ current in a concentration-dependent manner. Increasing Mg$\sb{\rm i}\sp{2+}$ from 0 to 5, 10, 20, and 50 mM decreased channel currents by 34%, 44%, 56%, and 73%, respectively, at +50 mV. The magnitude of the Mg$\sb{\rm i}\sp{2+}$ block increased with depolarization. For membrane potentials of $-$50, +50, and +90 mV, 20 mM Mg$\sb{\rm i}\sp{2+}$ reduced the currents 22%, 56%, and 70%, respectively. Mg$\sb{\rm i}\sp{2+}$ did not change the reversal potential, indicating that Mg$\sp{2+}$ does not permeate the channel. The magnitude of the Mg$\sb{\rm i}\sp{2+}$ block decreased as the intracellular concentration of K$\sp+$ was increased. At a membrane potential of +50 mV, 20 mM Mg$\sb{\rm i}\sp{2+}$ reduced the currents 71%, 56%, and 25% for K$\sb{\rm i}\sp+$ of 75, 150, and 500 mM. These effects of Mg$\sb{\rm i}\sp{2+}$, voltage, and K$\sp+$ were reversible. Although the Woodhull blocking model could approximate the voltage and concentration effects of the Mg$\sb{\rm i}\sp{2+}$ block (K$\sb{\rm d}$ $\sim$ 30 mM with 150 mM symmetrical K$\sp+$; electrical distance $\sim$0.22 from the inner surface), the Woodhull model could not account for the effects of changing intracellular K$\sp+$ concentration. Plots of 1/single-channel current versus 1/(K$\sp+$) (double reciprocal plots) in the presence and absence of Mg$\sb{\rm i}\sp{2+}$, indicated that the Mg$\sb{\rm i}\sp{2+}$ block is consistent with a competitive interaction between Mg$\sb{\rm i}\sp{2+}$ and K$\sb{\rm i}\sp+$. Ca$\sb{\rm i}\sp{2+}$, Ni$\sb{\rm i}\sp{2+}$, and Sr$\sb{\rm i}\sp{2+}$ were found to have concentration- and voltage-dependent blocking effects similar, but not identical, to those of Mg$\sb{\rm i}\sp{2+}$. These observations suggest the blocking of the large-conductance, Ca$\sp{2+}$-activated K$\sp+$ channel by Mg$\sb{\rm i}\sp{2+}$ is mainly nonspecific, competitive with K$\sp+$, and at least partially electrostatic in nature.

#### Keywords

Biology, Animal Physiology; Biophysics, Medical