Two distinct distribution patterns of sarcoplasmic reticulum in two functionally different giant smooth muscle cells of Beroe ovata.

The sarcoplasmic reticulum has been studied in radial and longitudinal giant smooth muscle fibres of the marine planktonic invertebrate Beroe. Impregnation with heavy metals has revealed that the smooth component is organised in a longitudinally oriented three-dimensional network of tubules running along the myofilaments. An ultrastructural morphometric analysis has shown that the relative volume of the sarcoplasmic reticulum is the same (1% of the myofilament volume) in both fibres but that the size, number and distribution of the sarcoplasmic reticulum tubules differ significantly. The longitudinal fibres are characterised physiologically by an action potential with a short calcium-dependent plateau that can trigger a short contraction; radial fibres produce action potentials without a plateau and their contraction requires a train of spikes. The sarcoplasmic reticulum tubules in longitudinal fibres are thinner (132 nm in diameter) and more numerous than those in radial fibres (160 nm in diameter). Moreover, the tubules are homogeneously distributed among the myofilaments in radial fibres, whereas they are more numerous in the centre of longitudinal muscles.