Calcium distribution and exchange in the pregnant and post partum rabbit uterus

Calcium content and distribution of the 25-day pregnant (PR) and post partum (PP) rabbit uterus was studied by atomic absorption spectrophotometry and 45Ca determination. Total Ca content [2.28 +/- 0.28 (PR) and 2.19 +/- 0.12 (PP) mM/kg wet wt] extracellular [1.21 +/- 0.09 (PR) and 1.25 +/- 0.11 (PP) mM/kg wet wt] cellular [1.07 +/- 0.08 (PR) and 0.94 +/- 0.09 (PP) mM/kg wet et], total exchangeable [1.86 +/- 0.11 (PR) and 1.84 +/- 0.09 (PR) mM/kg wet wt] and inexchangeable [0.43 +/- 0.05 (PR) and 0.35 +/- 0.04 (PP) mM/kg wet wt] Ca fractions were identical in the two extreme endocrinological conditions. In contrast compartment size and rate constant of different exchangeable Ca fractions determined by kinetic analysis of 45Ca desaturation "urves (curve-peeling tecnique and computer method), revealed significant differences between PR and PP uteri. Two exchangeable phases could be identified in both endocrinological states. The rate constants of both phases of efflux were significantly higher in the PP (alpha 1 = 0.173 +/- 0.02 min-1; alpha 2 = 0.023 +/- 0.001 min-1) than in the PR uterus (alpha 1 = 0.099 +/- 0.01 min-1; alpha 2 = 0.018 +/- 0.01 min-1). Compartment size of phase 1 (fast component) was significantly higher in the PR (1.13 +/- 0.1 mM/kg wet wt) than in the PP uterus (0.77 +/- 0.06 mM/kg wet wt). In contrast, compartment size of phase 2 (slow component) was significantly smaller in PR than in PP uterine strips (0.74 +/- 0.06 and 1.08 +/- 0.11 mM/kg wet wt). The last portion of desaturation curves represents efflux from one homogenous compartment. The present results suggest that endocrinological control of the rabbit myometrium is linked to the regulation of the binding of a superficial exchangeable Ca fraction.