Regulation of a truncated isoform of AMP-activated protein kinase α (AMPKα) in response to hypoxia in the muscle of Pacific oyster Crassostrea gigas

AMP-activated protein kinase alpha (AMPK alpha) is a key regulator of energy balance in many model species during hypoxia. In a marine bivalve, the Pacific oyster Crassostrea gigas, we analyzed the protein content of adductor muscle in response to hypoxia during 6 h. In both smooth and striated muscles, the amount of full-length AMP-activated protein kinase alpha (AMPK alpha) remained unchanged during hypoxia. However, hypoxia induced a rapid and muscle-specific response concerning truncated isoforms of AMPK alpha. In the smooth muscle, a truncated isoform of AMPK alpha was increased from 1 to 6 h of hypoxia, and was linked with accumulation of AKT kinase, a key enzyme of the insulin signaling pathway which controls intracellular glucose metabolism. In this muscle, aerobic metabolism was maintained over the 6 h of hypoxia, as mitochondrial citrate synthase activity remained constant. In contrast, in striated muscle, hypoxia did not induce any significant modification of neither truncated AMPK alpha nor AKT protein content, and citrate synthase activity was altered after 6 h of hypoxia. Together, our results demonstrate that hypoxia response is specific to muscle type in Pacific oyster, and that truncated AMPK alpha and AKT proteins might be involved in maintaining aerobic metabolism in smooth muscle. Such regulation might occur in vivo during tidal intervals that cause up to 6 h of hypoxia.