Activation of Phosphorylase by Anoxia and Dinitrophenol in Rabbit Colon Smooth Muscle: Relation to Release of Calcium from Mitochondria

Abstract: The effect of anoxia or 2,4-dinitrophenol (DNP) on the phosphorylase a activity and the calcium content in subcellular fractions from rabbit colon smooth muscle was studied. Anoxia for 15 min. as well as DNP (6.6 × 10^?5 M) for 5 min. increased the phosphorylase a activity. The calcium content in the mitochondrial subfraction, prepared from the anoxic- or DNP-treated intact muscle and determined by atomic absorption spectroscopy, was reduced. The calcium content in the nuclear and the microsomal fractions was not changed in preparations with a normal Ca-content. When the muscle was incubated for 60 min. in a Ca²⁺-free medium containing 2.0 mM EGTA, the calcium content in the mitochondrial fraction was reduced to 38 % of the control. This calcium level was still further reduced and the phosphorylase a activity was increased by DNP in this “Ca-poor” muscle. In these preparations the Ca-content of the microsomal + supernatant fraction increased. Only when the muscle was incubated, initially, in an anoxic medium containing 0.1 mM Ca²⁺ for 120 min. and, subsequently, in an oxygenated medium containing 0.1 mM Ca²⁺ for 20 min., DNP failed to activate phosphorylase and to decrease the calcium content in the mitochondrial fraction. These results indicate that mitochondrial Ca²⁺ release is one of the regulatory factors of the anoxic-induced glycogenolysis.