Caveolae intracellulares and sarcoplasmic reticulum in smooth muscle.

In the last 15 years several electron-microscope studies of visceral and vascular smooth muscle of vertebrates have been published (see reviews by Dewey & Barr, 1968, and Burnstock, 1970). The presence of bottle-shaped invaginations of the sarcolemmal membrane (micropinocvtotic vesicles, caveolae intracellulares) (Caesar, Edwards & Ruska, 1957) is well known. However, it is generally accepted that sarcoplasmic reticulum is poorly developed in smooth muscle, especially when compared with striated muscle (Mark, 1956; Caesar et al. 1957; Prosser, Burnstock & Kahn, i960; Dewey & Barr, 1968). In this paper the occurrence of well developed sarcoplasmic reticulum in the intestinal smooth muscle of guinea-pig is reported.