Sarcoplasmic reticulum release channels from frog skeletal muscle display two types of calcium dependence

Calcium channels derived from sarcoplasmic reticulum of frog skeletal muscle were fused with planar lipid bilayers. Fractional open times displayed two types of calcium dependence: (i) blockable channels showed a bell-shaped calcium dependence with an activation constant of 4.5 microM, a Hill coefficient for activation of 1.46 and a blocking constant of 226 microM, and (ii) non-blockable channels displayed a sigmoidal calcium dependence with an activation constant of 1.1 microM and a Hill coefficient of 1.42; no blocking effect was seen with calcium up to 0.5 mM. These two types of calcium dependence may underlie the coexistence of two different pathways for calcium release in frog skeletal muscle.