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Effects of heat stress on pancreatic insulin content and β-cell distribution in growing pigs
Heat stress (HS) induces postabsorptive metabolism changes, independent of inadequate nutrient intake. Despite marked hypophagia, HS inexplicably increases plasma insulin kinetics in a variety of animal models, and we have previously demonstrated that the HS pigs included in this study had increased plasma C-peptide (10%; P<0.05), an indicator of insulin secretion. Whether higher plasma insulin is due to increased pancreatic insulin production or secretion is unknown. Seventeen crossbred gilts (57±5 kg BW) were subjected to one of two environmental treatments: 1) constant HS conditions (32°C, 23% relative humidity) and ad libitum feeding (n=7), or 2) pair-feeding in thermoneutral conditions (20°C, 36% relative humidity; PFTN; n=10) to eliminate the confounding effects of dissimilar feed intake. After 8 d of environmental exposure, pigs were sacrificed and the pancreas was immediately collected. Insulin content was determined after acid-ethanol protein extraction from frozen samples using a commercially available ELISA kit. Fixed samples were sectioned and stained for insulin using an indirect immunoperoxidase immunohistochemical technique. Heat stress tended to decrease (32%; P=0.08) insulin stained area compared to PFTN conditions. In agreement, pancreatic insulin protein content was numerically reduced (39%; P=0.18) in HS pigs. There were no differences (P>0.10) in the quantity of β-cell groups, determined as the number of insulin positive cells clusters. However, when classifying β-cell groups by size, HS decreased (66%; P<0.01) the percentage of larger β-cell clusters (diameter>100μm), while increasing (4%; P≤0.01) the percentage of smaller clusters (diameter<50μm). In conclusion, the similar amount of β-cell groups between treatments, coupled with the decrease in pancreatic insulin content in the HS pigs indicates that HS-induced changes on plasma insulin concentration might be due to increased pancreatic insulin secretion, rather than production.
Keywords:
Heat stress, pig, insulin, pancreas