Effect of Mg2+ on the binding of oxytocin to sheep myometrial cells.

Binding isotherms of oxytocin to sheep myometrial cells in a short-term cell culture were investigated in the presence of Mg2+ in the concentration range 0-10 mM. The occurrence of at least three binding sites had been demonstrated earlier. Mg2+ influences individual sites and individual features of the binding isotherm differently. Dissociation constants (Kd) of the high- and medium-affinity sites attain a minimum of 6.8 x 10(-10) and 4.2 x 10(-8) mol/l respectively at 2.75 mM-Mg2+. The two sites display the highest binding capacity (B) at 1 mM-Mg2+ (ratio of high affinity/medium affinity is 1:16). The B/Kd quotients reflecting relative binding (bound-to-free concentration) at the half-saturation of binding sites also have their maxima at 1 mM-Mg2+. The high-affinity site displays a strong positive co-operativity (Hill coefficient at 4 mM-Mg2+ of 2.4), which is amplified in the presence of Mg2+. Positive co-operativity of the medium-affinity site is markedly lower (Hill coefficient at 4 mM-Mg2+ of 1.5) and shows less dramatic Mg(2+)-dependence. Low-affinity sites are not co-operative at any Mg2+ concentration. It is concluded that Mg2+ may display its effect upon the oxytocin-receptor interaction predominantly by influencing positive co-operativity.