Ultrastructure and localization of calcium in uterine smooth muscle.

General ultrastructure and subcellular calcium localization was studied in the longitudinal muscle layer of the rabbit's post partum myometrium by means of the lead acetate and potassium pyroantimonate cytochemical techniques. The finding of abundant surface vesicles (SV) and intracellular tubules of smooth endoplasmic reticulum (SER), their close contacts in some parts of the fibre, and the isolated compartment between plasma membrane (PM) and basal membrane (BM) may be important in the understanding of characteristic features of excitation-contraction coupling in this tissue. In the resting fibre (Mn2+-induced relaxation) the lead acetate and pyroantimonate reaction product was localized in the PM, SV, SER and mitochondria (M), indicating the possible in situ Ca-sequestering properties of the above structures. Tissues fixed at the peak of K+-contraction revealed abundant precipitates in the cytoplasm, while little reaction product was seen at the periphery of the fibre. The specificity of the cytochemical reactions was verified by Ca-extraction EGTA in situ (lead acetate) and by electronprobe X-ray microanalysis (pyroantimonate). These findings indicate that redistribution of the precipitate during the contraction-relaxation cycle is the result of intracellular calcium translocation in the rabbit myometrium.