1,25(OH)2 vitamin D3 increases ionized plasma calcium concentrations in the immature Atlantic cod Gadus morhua

Intraperitoneal injections of 1,25(OH)2 vitamin D3 (1,25(OH)2D3) produced hypercalcemia in the marine teleost the Atlantic cod (Gadus morhua) with increased plasma concentrations of ionized calcium (CaI) while the total plasma calcium concentrations (CaT) were unaffected. A single injection of 10 micrograms kg bw-1 of 1,25(OH)2D3 increased CaI concentrations from 1.74 +/- 0.03 mM to 1.8 +/- 0.01 mM after 24 hr in Experiment 1 and from 1.67 +/- 0.03 mM to 1.82 +/- 0.06 mM after 72 hr in Experiment 2. This hypercalcemic effect was sustained by daily injections for 5 but not 7 days. Daily injections of a lower dose of 1,25(OH)2D3, 1 microgram kg bw-1, caused hypercalcemia after 5 days (CaI increased from 1.68 +/- 0.01 mM to 1.76 +/- 0.02 mM). Plasma calcium concentrations were not affected by any of the other seco-steroids (vitamin D3, (25(OH)), vitamin D3 (25(OH)D3), or 24,25(OH)2 vitamin D3 (24,25(OH)2D3), except for a decrease in CaT concentrations after 7 daily in injections of 25(OH)D3 (2 micrograms kg bw-1). Plasma phosphate concentrations were not changed by any of the seco-steroids. It is concluded that 1,25(OH)2D3 is hypercalcemic in the marine Atlantic cod.