Caveolae or membrane vesicles are commonly observed in smooth and skeletal 
muscle as well as in working heart muscle. Using sections of fixed tissue and 
replicas of freeze-cleaved material, we show in this study that caveolae are 
also very numerous in sinus node cells of the rabbit, and to a lesser degree, in 
the atrial cells. Caveolae increase the plasma membrane surface area by 115% in 
the leading sinus node, and by 56% in the atrial cells. In these two cell types, 
the membrane of the caveolae contains four times fewer intramembranous particles 
than the rest of the plasma membrane, and this difference applies to both PF and 
EF faces. The role of the caveolae is still unclear, but it does not seem that 
they have a pinocytotic function.