Tension-pCa relation and sarcoplasmic reticulum responses in chemically skinned smooth muscle fibers.

Properties of bundles of smooth muscle fibers in which the surface membrane has been rendered highly permeable by chemical treatment (saponin) are described. These preparations are termed chemically skinned smooth muscle fiber bundles. Thin bundles were isolated from guinea pig teniae ceci and skinned in a relaxing solution by treatment with 50 micrograms/ml saponin for 20-30 min. Because Ca2+ sensitivity progressively decreased on repeating contractions or on incubating in a relaxing solution, the tension-pCa (-log10 [free Ca2+]) relationship was determined soon after the skinning by a single contraction in which the [free Ca2+] was increased in a stepwise manner. Unlike skinned skeletal muscle fibers, in skinned smooth muscle fiber bundles the tension-pCa relationship was not affected appreciably by the changes in pH from 6.4 to 7.2, in ionic strength from 0.15 to 0.30 M, or in [MgATP] from 0.4 to 8 mM. The saponin-treated smooth muscle fiber bundles retained a functional sarcoplasmic reticulum (SR). Ca can be loaded in the SR if the [free Ca2+] is in the micromolar range and MgATP is present and can be released by an application of caffeine. Mitochondria also appear to be functional in saponin-treated fibers.