The ionophore A23187 (10 micrograms/ml) did not affect the uptake of D-[U-14C]xylose by rat soleus muscle incubated under basal conditions. When muscles were incubated in a Ca2+/Mg2+-free (CMF) medium, A23187 promoted the efflux of intracellular Mg2+ and the efflux of 45Ca from preloaded muscles. Under these conditions, conditions, A23187 inhibited insulin-stimulated sugar transport without affecting 125I-insulin binding by the muscle. A23187 induced a slight fall in muscle ATP (16–18%); this does not appear to be responsible for the inhibitory effect of the ionophore on sugar transport. The inhibitory effect of A23187 was completely abolished when the CMF medium was supplemented with Mg2+ and partially reversed by Mn2+ or Zn2+; supplementation with Ca2+ did not reverse the inhibitory effect of the ionophore. These results suggest that insulin stimulates muscle sugar transport through a mechanism that involves intracellular Mg2+.

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