Title

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

Date of Award

1991

Availability

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

Link to Full Text

http://access.library.miami.edu/login?url=http://gateway.proquest.com/openurl?url_ver=Z39.88-2004&rft_val_fmt=info:ofi/fmt:kev:mtx:dissertation&res_dat=xri:pqdiss&rft_dat=xri:pqdiss:9205353