Increased blood oxygen affinity during digestion in the snake Python molurus

摘要

Many snakes exhibit large increases in metabolic rate during digestion that place extensive demands on efficient oxygen transport. In the present study, we describe blood oxygen affinity following three weeks of fasting and 48 h after feeding in the Burmese python (Python molurus). We also report simultaneous measurements of arterial blood gases and haematological parameters. Arterial blood was obtained from chronically implanted catheters, and blood oxygen-dissociation curves were constructed from oxygen-content measurements at known oxygen partial pressure (Po-2) values at 2% and 5% CO2. Arterial pH remained constant at approximately 7.6 after feeding, but digestion was associated with an approximately 6 mmol l(-1) increase in [HCO3-], while CO2 partial pressure (P-CO2) increased from 2.21+/-0.13kPa in fasted animals to 2.89+/-0.17kPa at 48h after feeding. Blood oxygen affinity in vivo was predicted on the basis of pH in vivo and the blood oxygen-dissociation curves obtained in vitro. The blood oxygen affinity in vivo increased during digestion, with P-50 values decreasing from 4.58+/-0.11 kPa to 3.53+/-0.24 kPa. This increase was associated with a significant decrease in the red blood cell [NTP]/[Hb(4)] ratio (relationship between the concentrations of organic phosphates and total haemoglobin) and a significant decrease in mean cellular haemoglobin content, which is indicative of swelling of the red blood cells. Our data for blood oxygen affinity and arterial oxygen levels, together with previously published values of oxygen uptake and blood flows, allow for a quantitative evaluation of oxygen transport during digestion. This analysis shows that a large part of the increased metabolism during digestion is supported by an increased venous extraction, while arterial P-O2 (Pa-O2) and haemoglobin saturation do not vary with digestive status. Thus, we predict that venous P-O2 (Pv(O2)) is reduced from a fasting value of 5.2 kPa to 1.6 kPa during digestion.