The characterization of a monovalent cation-selective channel of mammalian cardiac muscle sarcoplasmic reticulum.

Rabbit cardiac muscle sarcoplasmic reticulum (SR) was isolated and separated into ryanodine-sensitive and -insensitive fractions (L.R. Jones and S.E. Cala, J. Biol. Chem. 256:11809-11818, 1981). Vesicles of cardiac SR were incorporated into planar phospholipid bilayers by fusion and the channel activity of the membrane studied under voltage-clamp conditions (C. Miller, J. Membrane Biol. 40: 1-23, 1978). Both fractions contain a monovalent cation-selective three-state channel. In the presence of 75 mM K2SO4, the fully open state (beta) conductance of this channel is 157.2 +/- 30 pS and the sub-state (alpha) conductance is 100.7 +/- 21 pS. Both open states display the same selectivity sequence for monovalent cations, i.e. K+ greater than NH+4 greater than Rb+ greater than Na+ greater than Li+ and may be blocked by the skeletal muscle relaxants decamethonium and hexamethonium. Block occurs when the compounds are added to either side of the membrane. The properties of the cardiac SR cation channel are compared with those of the previously reported monovalent cation-selective channels of mammalian and amphibian skeletal muscle SR.