Effect of selected retinoids on carbohydrate metabolism in the freshwater monsoon prawn, Macrobrachium malcolmsonii

Retinoic acid isomers such as 9 cis retinoic acid (9CRA) and all trans retinoic acid (ATRA) have been discovered in crustaceans. However, their physiological significance in the biological framework of crustaceans is unclear. The present study evaluates the effect of retinoic acid on the hemolymph glucose levels in the monsoon prawn, Macrobrachium malcolmsonii. Injection of 9CRA into intact prawns significantly elevated the hemolymph glucose levels in a dose-dependent and time-dependent manner. However, such hyperglycemic response in 9CRA-injected eyestalk ablated (ESX) prawns was not observed. No changes in the hemolymph glucose levels were noticed in ATRA-injected intact or ESX prawns. Bilateral ESX showed significant elevation in the total carbohydrates and glycogen levels with a significant reduction in the activity levels of phosphorylase activity in the crustacean hyperglycaemic hormone (CHH) target tissues, hepatopancreas and muscle of prawns. Injection of 9CRA into intact prawns showed significant elevation in the activity levels of phosphorylase activity with a concomitant decrease in the total carbohydrates and glycogen levels in the CHH target tissues compared to vehicle-injected prawns. No significant differences were observed in the selected biochemical variables in 9CRA-injected prawns over 9CRA-injected ESX prawns. The expression levels of CHH in the eyestalks of 9CRA-injected prawns were significantly elevated over its respective control. It can be concluded that 9CRA-induced hyperglycemia, at least in part, mediates CHH in M. malcolmsonii.