Effects of hypertonic perfusion on the ultrastructure of frog cardiac muscle

The ultrastructural changes induced by hypertonic perfusion were investigated using the ventricular muscle of the bullfrog. It was demonstrated that the fixative tonicity critically affects the ultrastructure. Effects of sucrose-, NaCl- and urea-hypertonicities were investigated and compared. In both sucrose- and NaCl-hypertonic media, although the cardiac muscle strongly shrank and the extracellular spaces markedly increased according to increasing tonicity the width of the intercalated disc cleft remained unchanged and the cleft was never separated even in 3 times hypertonicity. The sucrose-hypertonicity made the feature of the fine structure extremely obscure and the electron densities in both the Z-line and the intercalated disc region markedly decreased. When both the perfusate and fixative were made hypertonic by urea, which is known to easily penetrate the cell membrane, a shrinkage of the myocardial cells was observed but to a lesser extent as compared with sucrose or NaCl. The striation pattern was disordered in this condition though the intercalated discs were never affected.