Effect of formamide-induced osmotic shock on di-8-ANEPPS staining and macroscopic ClC-1 currents in mouse FDB fibers. (A) Representative confocal x–y images of di-8-ANEPPS fluorescence in naive (left) and formamide-treated (right) FDB fibers obtained from a 15-d-old mouse. Arrows indicate regions of initial T-tubule invagination that are not fully uncoupled after formamide treatment. (B) Macroscopic whole cell patch clamp measurements of 9-AC–sensitive ClC-1 currents recorded from representative naive (left) and formamide-treated (right) FDB fibers isolated from a 15-d-old mouse.