Regulatory Effect of Magnesium Ions on Interaction of Prolactin and Somatotropin with Receptors on Granulosum Cells of the Cow Bos taurus

A comparative study of effects of Mg2+ ions on specific and competitive binding of the bovine somatotropic hormone (STH) and bovine prolactin (PL) with cells of cow granulosum is carried out. It is found that Mg2+ increases the level of specific binding of 125I-PL with the cells at concentrations from 1 to 70 mM and decreases the level of specific binding of 125I-STH at a concentration of 70 mM. Analysis of the data by Scatchard's method has shown that the decrease of the level of specific binding of 125I-PL in the absence of MgCl2 and 125I-STH at a MgCl2 concentration of 70 mM is caused mainly by a decrease of the number of active binding sites on the cells. Oppositely directed effects of the studied divalent cations on the capability of unlabelled STH (25 µg/ml) and PL (25 µg/ml) for cross-suppression of specific binding of 125I-PL and 125I-STH, respectively, with the granulosum cells have been revealed. At the same time, with increase of Mg2+ ion concentration, the degree of replacement of 125I-STH and 125I-PL with unlabelled STH and PL, respectively, did not change. The obtained results are considered in connection with electrostatic model of participation of the divalent cations in interaction of PL and STH with receptors on cells.