2.4. Digestive gland (DG) and muscle metabolites
Before sampling, octopuses (24 h fasting animals) were anaesthetised in a cold seawater bath (15 °C) for 1 to 2 min (Moltschaniwskyj et al., 2007). Once anaesthetised the brain of octopus was cute and after samples of DG and arms were obtained. DG samples and two arms of each sampled animal were frozen in liquid nitrogen and then stored at − 80 °C until analysis. From these samples, soluble protein, acyglycerides, cholesterol, glucose and glycogen were measured in the DG. Muscle glycogen was evaluated in the octopus arms. Total soluble protein was evaluated with the Coomassie blue dye method according to the Bradford (1976) technique adapted to a microplate method using a commercial chromogen reagent (BioRad, Cat. 500–0006) and bovine serum albumin as the standard. Commercial kits were used for glucose (Bayer Sera Pak Plus B01 4509–01), acylglycerol (Bayer Sera Pak Plus B01 455101) and cholesterol (Bayer Sera Pak Plus B01 4507–01) measurements. Determinations were adapted to a microplate using 10 mL DG extract and gastric juice (GJ) (dilution 1:10) and 200 μL chromogen reagent. Absorbance was recorded in a microplate reader (BioRad 550) and the concentrations were calculated from a standard substrate solution.
Glycogen was determined using the method described by Carroll et al. (1956). Glycogen in the DG and arms were extracted with trichloroacetic acid 5% (TCA) and determined through the reaction with sulphuric acid and phenol. Sections of DG and arms were weighed (20–30 mg) and homogenized in TCA for 2 min at 4550 g. One hundred microlitres of supernatant was pipetted into a tube and mixed with 500 μL ethanol 95%. Tubes were placed in an oven at 37 °C for 3 h. After precipitation, the tubes were centrifuged at 4550 g for 15 min. The supernatant was discarded leaving the glycogen as a pellet. By adding 1 mL concentrated sulphuric acid and 200 μL phenol 5%, glycogen was dissolved. From the mix, 200 μL was transferred to a microplate and read at 490 nm in an ELISA reader (BioRad 550). Total weight of the digestive gland was also recorded.