Dynamics of acid–base metabolic compensation and hematological regulation interactions in response to CO₂ challenges in embryos of the chicken (Gallus gallus)

Abstract

CO₂ exposure elicits multiple changes in the acid–base balance and hematology of avian embryos, but the time-specific, dose-dependent effects of graded increases in extrinsic CO₂ in a normoxic environment are poorly understood. Consequently, we exposed day 15 chicken embryos to 1, 3, 5, 6 or 10 % CO₂ in 20 % O₂. We hypothesized that both the magnitude of hypercapnic respiratory acidosis and the resultant metabolic compensation within 24 h of exposure to <10 % CO₂ are proportional to ambient CO₂ concentration ([CO₂]). We also predicted that regulation of hematological respiratory variables is graded according to [CO₂]. Time-course (2, 6 and 24 h) changes were determined for acid–base disturbances and hematological respiratory variables; hematocrit (Hct), red blood cell concentration ([RBC]), hemoglobin concentration, mean corpuscular volume (MCV) and other mean corpuscular indices. Both the decrease in uncompensated pH, which indicates uncompensated respiratory acidosis, and the compensatory pH increase, a sign of metabolic compensation, increased with [CO₂]. The partial metabolic compensation across all CO₂ gas mixtures was ~17, 46 and 53 % compensation at 2, 6 and 24 h, respectively. Hematological responses were nearly identical across the entire range of [CO₂], with Hct decreasing across the time course of CO₂ exposure due to a decrease in MCV from 2 to 24 h and a decrease in [RBC] at 24 h. Even though hematological regulation was not graded, chicken embryos were able to compensate and survive exposure to <10 % CO₂.