Insulin action in isolated fat cells: II. Effects of divalent cations on stimulation by insulin of protein synthesis,on inhibition of lipolysis by insulin,and on the binding of 125I-labelled insulin to isolated fat cells

The effects of omission of Ca²⁺ and Mg²⁺ from the incubation medium on three aspects of insulin action in isolated fat cells have been investigated. In the (Ca²⁺ + Mg²⁺)-free incubation medium incorporation of L-¹⁴C]leucine into fat cell protein was reduced in the absence of insulin. Insulin stimulated L-¹⁴C]-leucine incorporation only in the presence of added CaCl₂ or MgCl₂. Incubation of the cells in the (Ca²⁺ + Mg²⁺)-free medium reduced but did not abolish the ability of adrenaline to stimulate lipolysis or the ability of insulin to inhibit the adrenaline-stimulated lipolysis. Specific binding of ¹²⁵I-labelled insulin to the fat cells was reduced in the absence of Ca²⁺ and Mg²⁺ but was not abolished, even in the presence of EDTA. Ca²⁺ was routinely the most effective divalent cation in supporting these aspects of insulin action, but similar responses were obtained with Mg²⁺, Sr²⁺ and Ba²⁺.Since insulin still binds to the cells under conditions in which some of the cellular effects of the hormone are abolished, it is suggested that divalent cations may have a role, either direct or indirect, in the processes linking the insulin-insulin receptor complex to certain effector systems in the cells. It is tentatively suggested that this action occurs at the level of the fat cell plasma membrane.