Filipin and digitonin studies of membrane cholesterol in frog atrial fibers with unusual gap junction configurations.

The gap junctions of frog myocardium present, in freeze-fracture, an atypical organisation of their junctional particles. Freeze-fracture cytochemistry with the cholesterol probes filipin and digitonin has been used to investigate whether the particular arrangement of the particles involves a lipid segregation in the plane of the membrane. Both probes labeled uniformly the non-junctional membrane, but no deformations were ever found inside the smooth membrane area circumscribed by the circle of junctional particles. At the level of junction formation zones almost no sterol probe complexes were found in the intramembranous particle free area which surrounded small clusters of junctional particles. These results suggest a regional variation in cholesterol related to the necessity of membrane fluidity during junction morphogenesis.