. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 278 G. ESCALONA DE MOTTA ET AL. RESULTS Osmotic effects of high FMD concentrations. FMD is a highly permeant solute, as shown by the fact that the water content of ileal strips placed in 2 M FMD in Krebs-Ringer's solution show no appreciable change in weight after 20 min. In addition, as shown in Figure 1, no significant changes in water content could be detected when these ileal strips were transferred back to normal Krebs-Ringer's. The movement of FMD across the membrane systems of skeletal muscle is slightly more restrict
. The Biological bulletin. Biology; Zoology; Biology; Marine Biology. 278 G. ESCALONA DE MOTTA ET AL. RESULTS Osmotic effects of high FMD concentrations. FMD is a highly permeant solute, as shown by the fact that the water content of ileal strips placed in 2 M FMD in Krebs-Ringer's solution show no appreciable change in weight after 20 min. In addition, as shown in Figure 1, no significant changes in water content could be detected when these ileal strips were transferred back to normal Krebs-Ringer's. The movement of FMD across the membrane systems of skeletal muscle is slightly more restricted. Indeed, frog sartorius muscles exhibit a small (15%) decrease in total weight when immersed in 2 M FMD-Ringer's, and almost double their water content when transferred back to normal Ringer's (see Figure 2). This last obser- vation suggests that FMD does not leave the skeletal muscle fibers as easily as it goes into them, thus favoring the occurrence of an osmotic shock similar to that brought about in skeletal muscles exposed to hypertonic glycerol (Gage and Eisen- berg, 1969). Ultrastructure of muscles equilibrated in hypertonic FMD. To determine whether the block of contraction which occurs while muscles are still immersed in the FMD solution could be correlated with ultrastructural changes caused by hypertonicity, two muscles blocked after 18 min in M FMD were transferred to a fixing solution that also contained M FMD. Post-fixation and further processing of these spe- cimens was then done as described in Materials and Methods. Figures 3 and 4 show that the myofibrils and associated sarcoplasmic reticulum (SR), as well as the mem- branes of the tubular system (T-system), of these muscles are normal in appearance and do not present any obvious morphological features that may be associated with a loss of contractility. Structural changes in FMD-blocked muscles transferred to normal Ringer's. Two muscles blocked after 18 min in 2 M FMD-Ringer's were transferred to
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