Uptake and energy-dependent extrusion of calcium in the rat uterus

The ability of lanthanum (La) to displace extracellular calcium or/and block calcium efflux from smooth muscle cells of the rat uterus was examined. Presence of 10 mM La in the medium used for tissue wash following 45Ca uptake (La method) increased 45Ca efflux particularly in the early phase (60 min) indicating displacement of extracellular calcium. Lowering of temperature from 37 degrees C to 2 degrees C of the La wash resulted in about three-fold increase in 45Ca retention by the tissue. Depolarization of tissue induced by high KCl during La wash at 37 degrees C had no effect on the retention of 45Ca. However, depolarization during 45Ca uptake significantly increased 45Ca uptake by the tissue which was counteracted by the presence of Nifedipine in the medium. The inhibition of tissue metabolic activity by FCCP had different effects on 45Ca retention depending on whether the tissues were washed at 37 degrees C or 2 degrees C. When active 45Ca extrusion during tissue washing was blocked (2 degrees C), 45Ca content of the tissues exposed to FCCP was reduced by about 35%. When tissues were allowed to actively pump out 45Ca (37 degrees C), 45Ca content of FCCP exposed tissues was increased by about 50%. It is concluded that about two thirds of the Ca extruded by the tissue is resistant to La blockage, and is dependent on the energy provided by tissue metabolic activity. During the use of the standard La method for measuring cellular calcium in the uterus, there is an underestimation due to unblocked 45Ca loss.