AJP - Heart and Circulatory Physiology, Vol 253, Issue 1 210-H214, Copyright © 1987 by American Physiological Society
Rectification of muscarinic K+ current by magnesium ion in guinea pig atrial cells
M. Horie and H. Irisawa
Rectifying properties of the acetylcholine (ACh)-sensitive K+ channels were
studied using a patch-clamp method in single atrial cells prepared
enzymatically from adult guinea pig hearts. In the presence of micromolar
concentration of nonhydrolyzable guanosine 5'-triphosphate (GTP) analogue
5'-guanylylimidodiphosphate (GppNHp) and the absence of Mg2+ at the inner
surface of patch membrane [( Mg2+]i), the channel activity recovered in
inside-out patch condition. The single channel conductance became ohmic
between -80 and +80 mV (symmetrical 150 mM K+ solutions). The rapid
relaxation of outward single channel currents was disclosed on a
depolarization. [Mg2+]i blocked the outward current through the channel
dose- and voltage-dependently and also induced a dose-dependent increase in
the channel activation. The apparent paradoxical role of [Mg2+]i is
important for the cholinergic control in the heart; voltage-dependent Mg
block ensures a low K+ conductance of cell membrane at the plateau of
action potentials during the exposure to ACh, thereby slowing the heart
rate without unfavorable shortening of the action potentials.
