Three-dimensional observation on sarcoplasmic reticulum and caveolae in myocardium of spontaneously hypertensive rats

Three-dimensional observation of intracellular membranous structures in myocardium was carried out in spontaneously hypertensive rats (SHR; a model of hypertrophied myocardium) under a scanning electron microscope (SEM) and compared with those of age- and sex-matched normotensive Wistar-Kyoto rats (WKY). There was no apparent difference in the ultrastructures of the myocardium between 5-week-old SHR (before the onset of hypertension but with mild cardiac hypertrophy) and age- and sex-matched control rats. Ten-week-old SHR in an early stage of hypertension with significant cardiac hypertrophy, showed a significant increase in the number of caveolae compared with those of 10-week-old WKY, and numerous abnormal caveolae aggregates connected by filamentous structures. Moreover the density of the network of sarcoplasmic reticulum (SR) was also increased, and some SR had a giant, squamous appearance. Accumulation of mitochondria with dense cristae was also observed in 10-week-old SHR. In 20-week-old SHR with established hypertension and cardiac hypertrophy, the caveolae and SR observed in 10-week-old SHR were no longer present, but caveolae were distributed in bands with variable width, parallel to the long axis of the myocardium. A wavy arrangement of mitochondria, fragmentation and stacks of mitochondrial cristae were observed in 20-week-old SHR. The changes of caveolae and giant squamous SR in cardiac hypertrophy observed in the present study have never been reported in previous studies. Our results suggest that the changes in caveolae, SR and mitochondria observed in SHR may be important morphological signs during development of cardiac hypertrophy in SHR.