DIFFERENCES IN GASTROINTESTINAL GLUCOSE TRANSPORTERS BETWEEN CARNIVOROUS RAINBOW TROUT Oncorhynchus mykiss AND OMNIVOROUS NILE TILAPIA Oreochromis niloticus IN RESPONSE TO PULSE CROPS

Due to rising cost in fishfeed, fishmeal could be at least partially replaced with starch instead of proteins.  Our group has investigated the use of pulses which have a high content of slowly-digestible carbohydrates and protein.  High starch diets are commonly used in omnivorous species, but this may be detrimental in carnivorous species.  There is a paucity of studies to support this concern and none investigating basic mechanisms of carbohydrate absorption in any fish species.  Therefore, the present study was conducted to identify glucose transporters in different gastrointestinal regions in carnivorous Rainbow trout (Oncorhynchus mykiss) versus omnivorous Nile tilapia (Oreochromis niloticus).  Transporters were characterized in fish fed commercial diet (baseline study) or in fish after an 8-week growth trial comparing diets with pulse or corn starch inclusion (0-200, or 0-300 g/kg inclusion, respectively, for trout and tilapia).

Since pulse starches are digested primarily to glucose monomers, before absorption, we characterized glucose absorption through the sodium-dependent glucose transporters (SGLTs) using Ussing chamber techniques, qRT-PCR, and pharmacological inhibitors in pyloric caeca/proximal intestine, midgut/mid-intestine, and high-gut from both trout and tilapia.

In trout, pyloric caeca and midgut, baseline glucose transport kinetics were similar to mammalian SGLT1 (a high-affinity, low-capacity transporter) which was confirmed with inhibitors and qPCR.  In contrast, atypical kinetics combined with qPCR data suggests a novel SGLT transporter (low-affinity, high-capacity) in trout hind-gut.  In contrast to trout, the kinetics, inhibitor, and qPCR data in all three tilapia intestinal sections resembled a combination of SGLT1- and SGLT2-like transporters.  In the growth trial, both fish species grew equally well or better with high starch inclusion, but glucose transport physiology was dramatically altered or non-detectable throughout the gut.  This study confirms trout and tilapia grow well with high pulse starch inclusion.  However, further work is needed to better understand how high starch diets and glucose is handled in these two fish species.