Effects of extracellular calcium concentrations and calcium antagonists on vitellogenin induction by estradiol-17 beta in primary hepatocyte culture in the rainbow trout Oncorhynchus mykiss

Abstract

The involvement of calcium in vitellogenin (Vg) synthesis in response to estradiol-17 beta (E2, 2 x 10(-6) M) was electrophoretically examined in primary hepatocyte culture in the rainbow trout Oncorhynchus mykiss. The relative rate of Vg synthesis was evaluated by measuring the integrated optical density of the Vg band after SDS-PAGE and expressed as a percentage of Vg to total proteins. The induction of Vg by E2 did not occur at 0 meq Ca/liter, while some proteins other than Vg (main subunit) were weakly synthesized. Vg synthesis increased in a calcium concentration-dependent way and reached the maximum at 5 meq Ca/liter in the incubation medium. The addition of bovine growth hormone (50 ng/ml) and/or ovine prolactin (100 ng/ml) had no effect on stimulating Vg synthesis by E2, regardless of the calcium concentrations in the medium. Lanthanum and verapamil (10(-4) M) markedly inhibited Vg synthesis, while diltiazem (10(-4) M) was insignificant. Reactive blue (10(-4) was also effective in inhibiting Vg synthesis to about 60% of the control. These results suggest that the synthesis of Vg, a calcium-binding protein, is more susceptible to calcium than are other proteins. Receptor-operated and verapamil-sensitive calcium channels have been proposed to be involved in calcium entry into hepatocytes in rainbow trout.