the pharmacology of ca2+ channels and intracellular ca2+ release from the sarcoplasmic reticulum ( s.r . ) were compared by injecting ca2+ channel blockers into the cytoplasm and observing contraction under voltage clamp of frog skeletal muscle fibres , a preparation that contracts only in response to ca2+ release from the s.r .
C0036226
sarcoplasmic reticulum
C0010834
cytoplasm