Unexpected effect of insulin on glucose disposal explains glucose intolerance of rainbow trout

Abstract

The physiological reasons why salmonids show glucose intolerance are unclear. In mammals, rapid clearance of a glucose load is mainly achieved through insulin-mediated inhibition of hepatic glucose production ( Ra) and stimulation of glucose disposal ( Rd), but the effects of insulin on Ra and Rd glucose have never been measured in fish. The goal of this study was to characterize the impact of insulin on the glucose kinetics of rainbow trout in vivo. Glucose fluxes were measured by continuous infusion of [6-3H]glucose before and during 4 h of insulin administration. The phosphorylated form of the key signaling proteins Akt and S6 in the insulin cascade were also examined, confirming activation of this pathway in muscle but not liver. Results show that insulin inhibits trout Rd glucose from 8.6 ± 0.6 to 5.4 ± 0.5 µmol kg−1 min−1: the opposite effect than classically seen in mammals. Such a different response may be explained by the contrasting effects of insulin on gluco/hexokinases of trout versus mammals. Insulin also reduced trout Ra from 8.5 ± 0.7 to 4.8 ± 0.6 µmol·kg−1·min−1, whereas it can almost completely suppresses Ra in mammals. The partial inhibition of Ra glucose may be because insulin only affects gluconeogenesis but not glycogen breakdown in trout. The small mismatch between the responses to insulin for Rd (−37%) and Ra glucose (−43%) gives trout a very limited capacity to decrease glycemia. We conclude that the glucose intolerance of rainbow trout can be explained by the inhibiting effect of insulin on glucose disposal.

Effects of Adiponectin on Glucose Metabolism in the Hepatopancreas of Grass Carp (Ctenopharyngodon idella)

“…The variability in physiological function of some endocrine factors may contribute to glucose intolerance in fish. For example, insulin classically promotes glucose disposal in mammals, whereas it has the opposite effect in some fish species, e.g., it inhibits glucose disposal in rainbow trout (Oncorhynchus mykiss) [5].…”

Epigenetic and post-transcriptional repression support metabolic suppression in chronically hypoxic goldfish

“…Proteins were migrated in the gel at 100 V for ~ 2 h. After migration, they were blotted onto nitrocellulose 0.45-mm pore size membrane paper (Millipore, Etobicoke, ON, Canada) by wet transfer using the Mini TransBlot system (Bio-Rad) with blotting buffer (250 mM Tris base, 1920 mM glycine; all BioShop Canada) dissolved in dH2O, by applying 100 V for 2 h. Membranes were incubated with Odyssey blocking buffer (LI-COR Biosciences Lincoln, NE) for 1 h at room temperature using an orbital shaker. After the blocking step was completed, membranes were cut based on the molecular weight marker to allow separate development of post-translationally modified Akt-P Ser473 , S6-P Ser235/236 , 4e-bp1-P Thr 37 / 46 and Ampkα-P Thr172 proteins with specific primary antibodies validated in fish 82 , 94 . Partial membranes containing the relevant molecular weight range of proteins were incubated with rabbit raised primary Akt-P (no.…”

The mammalian insulin antagonist S961 does not exhibit insulin receptor antagonism in rainbow trout in vivo

“…(1.5 μg kg À1 min À1 ; exact infusion rates were determined individually for each fish to correct for differences in body mass) (Forbes et al, 2019). A blood sample was taken at 0 h, prior to S961 bolus injection and another following the 4 h insulin infusion ($400 μL each).…”

“…Insulin Caruso & Sheridan, 2012;Caruso & Sheridan, 2014). Exogenous insulin binds to rainbow trout insulin receptors (Gutiérrez et al, 1991) to stimulate rainbow trout insulin receptor signalling and gene expression (Mennigen et al, 2012;Plagnes-Juan et al, 2008) and regulate rainbow trout energy metabolism in vivo and in vitro (Forbes et al, 2019;Mennigen et al, 2014;Pereira et al, 1995). Major residues and structural components involved in binding insulin sites 1 and 2 (Supporting Information File S1) are generally highly conserved between mature trout and mammalian insulin, and between paralogues (Figure 8).…”

“…been investigated in great detail (Díaz et al, 2007;Forbes et al, 2019;Polakof et al, 2010), but progress has been hampered by the current unavailability of reliable assays to quantify circulating mature and thus bioactive insulin in rainbow trout and other fishes. Mammalian insulin is also bioactive in fish, but it is immunologically different, making it almost impossible to measure fish insulin with a mammalian homologous radioimmunoassay (RIA) (Plisetskaya, 1989).…”