The role of divalent cations in the metabolic response of mouse blastocysts to serum

Uptake and incorporation of [3H]uridine by mouse blastocysts was measured in response to serum under various conditions of Ca2+ and Mg2+ availability. Previous studies (Fishel & Surani, 1978) showed that serum stimulated uptake and incorporation of uridine in blastocysts. Here it is shown that in Ca2+-deficient medium (< 20 microM-Ca2+) cell metabolism was also inhibited and increasing [Ca2+] resulted in increased uptake and incorporation. Maximum stimulation required an extracellular [Ca2+) of 0.25 mM. The effects of low Ca2+ were reversible and could also be alleviated by 15 and 20 mM-Mg2+. Magnesium greater than 20 mM was deleterious. Inorganic phosphate (Pi) was used to complex free Mg2+ in order to maximize the effects of Mg2+ deficiency. Inhibition was reversed by increasing [Mg2+] or by 15-20 mM-Ca2+. Calcium concentrations greater than 20 mM inhibited maximum stimulation. Inhibitors of Ca2+ influx D600 and papaverine, inhibited stimulation at concentrations above 5 and 20 microM respectively. Magnesium concentration of 15 mM alleviated the inhibitory effects of 50 microM D600. The effect of Ca2+-deficient medium was also alleviated by Sr2+. The results suggest that both Ca2+ and Mg2+ are required for blastocysts to respond maximally to serum; their initial role appeared to involve the binding of stimulatory serum molecules to the cells of the blastocyst followed by an influx of these cations.