Volume changes in sarcoplasmic reticulum of rat hearts perfused with hypertonic solutions

To explore whether morphometry of intracellular membrane-limited subcompartments can be used to follow physiological volume changes in such subcompartments in hearts rapidly fixed by perfusion fixation, we have measured osmotically induced volume changes in electron micrographs of longitudinally oriented sarcoplasmic reticulum (LSR) and terminal cisterns (TC) of rat left ventricular myocardial cells. Vascular perfusion with solutions whose osmolality varied from 0.67 to 1.88 isomolal showed that in the hyperosmolal range LSR volume decreased linearly. Approximately 79% of LSR luminal volume participated in the osmotic rey unresponsive. By contrast, we found that the TC responded by dilation when hearts were perfused with hypersomolal NaCl, NaI, LiCl, or sucrose. Furthermore, with hyperosmolal NaCl the dilation developed within 1 minute; its rate and extent of development were concentration-dependent; it manifested an obligate association with prior or concomitant T-tubular dilation and was not readily reversible. We conclude that (1) the technique sensitively measures in situ changes of LSR volume; (2) most of LSR luminal water is osmotically responsive, but a significant fraction may not be; (3) exposure to hyperosomolal solutions may bring about (perhaps irreversible) structural changes in the diadic membrane complex, leading to changes in its solute permeability.