The nature of transmembrane calcium exchange in the rabbit uterus

45Ca efflux was studied in uterine strips excised from 25-day pregnant and post partum rabbits in normal KRB (1.25 mM CaCl2), in Ca2+-free KRB and in the presence of 2 mM EGTA. The rate of 45Ca efflux decreased in the absence of extracellular Ca2+ and increased in the presence of EGTA. The significant difference between the rate of efflux of pregnant and post partum uteri disappeared both in Ca2+-free KRB and in the presence of EGTA. The results show that transmembrane Ca exchange exists during 45Ca efflux both in the pregnant and in the post partum rabbit uterus. This is explained by an accelerative exchange diffusion at a limited number of fixed negative binding sites along the pores of the cell membrane. The present data confirm earlier observations that superficial Ca-binding is augmented during pregnancy in the rabbit myometrium.