Uptake and release of calcium by microsomes of nonvascular smooth muscle.

Microsomes prepared from guinea-pig ileum longitudinal smooth muscle and rat uterus continuously sequester calcium for a one hour period in the presence of Mg-ATP as an energy source and oxalate anion as a trapping agent. Dithiothreitol is essential for maximal calcium uptake activity of the rat uterus microsomes. On sucrose density gradients, calcium uptake of the smooth muscle microsomes appears to be associated with intracellular membrane (sarcoplasmic reticulum). Release of sequestered calcium from the longitudinal muscle microsomes is very slow (20% in 50 minutes). A small labile fraction (20%) is released by EGTA (1 mM) in 10 minutes. Rapid release of sequestered calcium (90% in 10 minutes) occurs in presence of the calcium ionophore A23187 (2 μM) or in the presence of chlorpromazine (1 mM).