Calcium buffering properties of sarcoplasmic reticulum and calcium- induced Ca release during the quasi-steady level of release in twitch fibers from frog skeletal muscle

Contraction in skeletal muscle is initiated when Ca stored in the SR is released into the myoplasm in response to membrane depolarization. Studies of the mechanisms that control SR Ca release are aided by methods that permit estimating the Ca permeability of the SR membrane or, equivalently, the extent of activation of the SR Ca release channels. This quantity is proportional to the rate of Ca release divided by the driving force for release. Because the free calcium concentration in the myoplasm ([Ca]myo) is generally small relative to that in the SR ([Ca]SR), and because the SR membrane potential is expected to be near zero (Somlyo, et al., 1977; Labarca and Miller, 1981; Garcia and Miller, 1984; Pape et al., 1992; Gillespie and Fill, 2008), the driving force for Ca release should be proportional to [Ca]SR.