Uptake, flow, and digestion of casein and green fluorescent protein in the digestive system of Lygus hesperus Knight

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Abstract
Selected compounds were used to study physiological processes associated with digestion in the western tarnished plant bug, Lygus hesperus Knight. Durations of passage and rates of absorption, digestion, and excretion were determined for a digestible protein (casein), a non‐digestible protein (green fluorescent protein, GFP), and a non‐digestible carbohydrate (dextran). Dextran was used as a control to monitor the non‐absorptive flow rate of ingesta through the digestive system. Fluorescent tracking of FITC‐conjugates of casein and dextran, as well as immunoblotting and immunofluorescent staining of casein and GFP, were used to monitor the degradation (in vitro) and ingestion, digestion, and distribution (in vivo) of the respective compounds. Under our experimental conditions, L. hesperus took discrete meals, feeding and excreting at 2–3‐h intervals. Rate of food passage was variable. FITC‐dextran was found in the fecal material of most insects by 6–8 h after treatment initiation; by 12 h, 95% of ingested FITC‐dextran was recovered from all insects. FITC‐casein was digested extensively in in vitro homogenates of gut, hemolymph, and salivary gland. In vivo, FITC‐casein was ingested and partially absorbed as a holoprotein into the hemolymph. Ingested FITC‐casein was partially degraded in the gut and hemolymph within 2 h of ingestion, and no holoprotein was found after 12 h. In contrast, there was no detectable degradation of GFP in hemolymph, gut, and salivary gland homogenates after 24 h of incubation. Ingested GFP was not degraded in gut or hemolymph up to 8 h after treatment initiation, but did transfer to the hemolymph as a holoprotein. Analysis of immunohistological images confirmed that GFP bound to gut epithelial cell brush‐border membranes. However, the mechanism by which GFP and casein pass as holoproteins into the hemolymph remains unknown. Arch. Insect Biochem. Physiol. 50:62–74, 2002. Published 2002 Wiley‐Liss, Inc.†

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