Ability of insulin to increase calcium uptake by adipocyte endoplasmic reticulum.

The endoplasmic reticulum of adipocytes possess an ATPdependent calcium uptake system (Bruns, I). E., McDonald, J. M., and Jarett, L. (1976) J. Bid. Chem. 251, 7191-7197) which is shown in this report to be regulated by insulin treatment of intact adipocytes. Insulin (100 microunitslml) treatment of cells resulted in a 20% (p < 0.025 to 0.001 at various time points) increase in the steady state calcium filling capacity of endoplasmic reticulum. This insulin effect was specific as determined by the insulin-concentration dependency between 5 and 100 microunits of insulin/ml and by the lack of effect by desoctapeptide insulin. This increase resulted from a 28% (p < 0.025) increase in the apparent V,,,,, of calcium uptake. Insulin treatment had no affect on the K,,, (1.4 PM) of the reaction nor on the rate of calcium efflux from the vesicles when efflux was initiated by either ethylene glycol bis(/3-aminoethyl ether)N,N’-tetraacetic acid or the calcium ionophore A23187. Consistent with these changes, insulin treatment of cells resulted in a 33% (p < 0.001) increase in the atomic absorption measurements of the total calcium content of the microsomal fraction. The total calcium content of the microsomal fraction consisted of a highly stable pool and a less stable or labile pool. Only the labile pool was altered by insulin treatment, and the insulin-induced change represented a 50% increase of this pool. Insulin treatment had no effect on the magnesium content of the microsomal fraction. These findings support the concept that the calcium uptake system of the endoplasmic reticulum of adipocytes is likely to be physiologically important. Furthermore insulin regulation of intracellular calcium pools probably plays an essential role in the mechanism of insulin action.