Osmotic properties of the sealed tubular system of toad and rat skeletal muscle.
A method was developed that allows conversion of changes in maximum Ca(2+)-dependent fluorescence of a fixed amount of fluo-3 into volume changes of the fluo-3-containing solution. This method was then applied to investigate by confocal microscopy the osmotic properties of the sealed tubular (t-) system of toad and rat mechanically skinned fibers in which a certain amount of fluo-3 was trapped. When the osmolality of the myoplasmic environment was altered by simple dilution or addition of sucrose within the range 190-638 mosmol kg(-1), the sealed t-system of toad fibers behaved almost like an ideal osmometer, changing its volume inverse proportionally to osmolality. However, increasing the osmolality above 638 to 2,550 mosmol kg(-1) caused hardly any change in t-system volume. In myoplasmic solutions made hypotonic to 128 mosmol kg(-1), a loss of Ca(2+) from the sealed t-system of toad fibers occurred, presumably through either stretch-activated cationic channels or store-operated Ca(2+) channels. In contrast to the behavior of the t-system in toad fibers, the volume of the sealed t-system of rat fibers changed little (by <20%) when the osmolality of the myoplasmic environment changed between 210 and 2,800 mosmol kg(-1). Results were also validated with calcein. Clear differences between rat and toad fibers were also found with respect to the t-system permeability for glycerol. Thus, glycerol equilibrated across the rat t-system within seconds to minutes, but was not equilibrated across the t-system of toad fibers even after 20 min. These results have broad implications for understanding osmotic properties of the t-system and reversible vacuolation in muscle fibers. Furthermore, we observed for the first time in mammalian fibers an orderly lateral shift of the t-system networks whereby t-tubule networks to the left of the Z-line crossover to become t-tubule networks to the right of the Z-line in the adjacent sarcomere (and vice versa). This orderly rearrangement can provide a pathway for longitudinal continuity of the t-system along the fiber axis.
Launikonis
Bradley S
BS
Department of Zoology, La Trobe University, Bundoora, Victoria, Australia. zoobl@zoo.latrobe.edu.au
Stephenson
D George
DG
D000814:Aniline Compounds / Q000493:pharmacokinetics; D000818:Animals; D002024:Bufo marinus; D004305:Dose-Response Relationship, Drug; D066298:In Vitro Techniques; D008297:Male; D018482:Muscle, Skeletal / Q000166:cytology / Q000378:metabolism / Q000502:physiology*; D009994:Osmolar Concentration; D009995:Osmosis / Q000502:physiology; D011894:Rana pipiens; D051381:Rats; D020318:Rats, Long-Evans; D013395:Sucrose / Q000493:pharmacokinetics; D014966:Xanthenes / Q000493:pharmacokinetics
D000814:Aniline Compounds; D014966:Xanthenes; C059715:Fluo-3; D013395:Sucrose