Effects of the calcium ionophore A-23187 on the regulation of sugar transport in muscle

The distribution of (¹⁴C)-3-0-methyl-D-glucose and of (⁴⁵Ca) was followed in perifused left atria and intact hemidiaphragms of the rat. The carboxylic calcium ionophore A-23187 affected sugar and Ca²⁺ influx in parallel, with low concentrations inhibiting and higher ones stimulating influx under basal conditions. The stimulation of sugar transport by insulin, high concentrations of adrenaline or ouabain, or by K⁺-free medium was antagonized by the calcium ionophore. Likewise, A-23187 counteracted the depression of sugar transport caused by low concentrations of ouabain or adrenaline. These results support a role of Ca²⁺ in the regulation of sugar transport in muscle. However, increased influx of Ca²⁺ cannot explain all the effects of A-23187. It is suggested that the ionophore may also act by releasing Ca²⁺ from intracellular storage and binding sites.