The effect of heat stress on respiratory alkalosis, blood acid base balance and insulin sensitivity in cinnamon supplemented pigs

With increases in the frequency, intensity and duration of heat waves forecast, heat stress (HS) is both a current and emerging problem for pig producers. As insulin improves peripheral blood flow and radiant heat loss (Allwood et al. 1959; Cottrell et al. 2015), we hypothesised that cinnamon (Cinnamonium zeylancium) would improve insulin sensitivity and ameliorate the effects of HS in pigs. To test this, 36 female Large White × Landrace (ca. 41.4 kg) pigs were allocated to either Cinnamon (0 v. 1.5% in a standard grower diet) and HS (thermoneutral (TN) v. HS) conditions in a 2 × 2 factorial design (n = 9 treatments per group). Pigs were acclimatised to experimental diets for 14 days before being challenged with cyclic HS (35°C 9 am to 5 pm/28°C) or TN (20°C) for 7 days. Respiration rate (RR), rectal (RcT) and skin temperature (ST) were measured five times daily. On d 7 an intravenous glucose tolerance test (IVGTT) was performed and blood acid-base balance quantified. Data were analysed via an REML using Genstat v18 (VSN International, Hemel Hempstead, UK) with blocking on the experimental replicate.

Cinnamon did not ameliorate RR, RcT and ST or blood acid-base balance. Fasted glucose concentrations were lower in cinnamon supplemented pigs (6.55 v. 5.65 for Control v. Cinnamon, P = 0.050), but no interaction with HS was observed and no other influence of cinnamon on glucose and insulin kinetics were observed. Therefore the hypothesis that cinnamon would improve insulin sensitivity in HS pigs was not supported. The effects of HS on pig thermoregulation, blood biochemistry and insulin sensitivity were marked. Heat stress increased RR ~8-fold, from 22 to 172 breaths/min. The increase in respiration resulted in reduced blood CO₂ concentrations (pCO₂ 53.2 v. 47.3 mmHg for TN v. HS P < 0.001). As blood pH regulates the rate of biochemical reactions on a global basis it is very tightly regulated; however, HS pigs tended to have more alkaline blood than TN pigs (7.419 v. 7.432, P = 0.087). Blood HCO₃^– concentrations were reduced in HS pigs (34.4 v. 31.6 mmol/L, P = 0.002), indicating increased buffering by excretion of excess HCO₃^–. Compared to TN, urinary pH was lower in HS pigs (6.38 v. 5.41, P < 0.001), indicating that urinary bicarbonate excretion occurred before 7 days during HS. Heat stress reduced haematocrit (31.3 v. 26.1%, P < 0.001), indicating an expansion in plasma volume possibly due to reduced H₂CO₃ formation from CO₂ and H₂O. Heat stress reduced the maximal glucose (9.62 v. 8.18 mmol, P = 0.014) and insulin concentrations (1.06 v. 0.67 ng/mL) following the intravenous (IV) glucose bolus. Glucose area under the curve (AUC)_0–60 was not influenced by HS; however, the insulin AUC_0–60 was significantly lower in HS pigs (0.242 v. 0.169 ng/mL.h, P = 0.005). Collectively these results show that HS pigs had a lower insulin response to an IVGTT. This may be in part due to reduced glucose concentrations after the IV bolus due to an expansion in blood volume or an increase in insulin sensitivity. In summary HS alters blood acid-base balance, leading to a loss of minerals such as bicarbonate and an apparent expansion in blood volume. It is expected that the energetic cost of panting, buffering and loss of nutrients such as bicarbonate contribute to compromised production efficiency in HS pigs.