| Abstract: | Two types of divalent cation conducting channels from rabbit skeletal muscle sarcoplasmic reticulum (SR) were incorporated into planar lipid bilayers. A high conductance (100 pS in 53 mM trans Ca2+) Ca2+ channel was incorporated from heavy density SR fractions. The 100-pS channel was activated by adenine nucleotides and Ca2+ and inhibited by Mg2+ and ruthenium red. A 10-pS calcium and barium conducting channel could be incorporated into planar lipid bilayers from light, intermediate, and heavy density SR vesicles. 10-pS channel activity in bilayers was not dependent on cis Ca2+ and was only weakly dependent on adenine nucleotides. Ruthenium red at concentrations up to 1 mM had no effect and Mg2+ was only marginally effective in inhibiting macroscopic Ba2+ currents from this channel. Calcium releasing activity in intermediate and heavy density SR fractions was assayed according to a rapid quench protocol and compared with the results obtained in the bilayer. Results from this comparison indicate that the 10-pS channel is probably not involved in rapid Ca2+- and adenine nucleotide-induced Ca2+ release from isolated SR vesicles. |
