Enriched inspired C02 stimulates respiration despite hypocapnia in hyperthermic exercising sheep

P. L. Entin, D. Robertshaw, R. E. Rawson

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Previously we have shown a negative correlation (R2-0.68) between rectal temperature (Tre) and arterial carbon dioxide tension (PaCO2) during exercise in sheep. We hypothesized that progressive hypocapnia during exercise in sheep reflects a decreasing set point for PaCO2 governed by mean body temperature, as opposed to failed adherence to a static set point. To test this hypothesis, we infused CO2 at 3-4 L/min for 3 minutes into the tracheae of 4 sheep at 6, 14.5, and 23.5 min of a 30 min moderate intensity exercise bout. As controls, we infused air in the same manner during separate exercise trials, and both air and CO2 at 1, 2, and 3 L/min into the sheep at rest. Tre and minute ventilation (VE) were measured continuously and PaCO2 was measured at 1.5 and 3 min of CO2 or air infusion and 5 min after the end of infusion. Air infusions caused no systematic changes in PaCO2 or VE at rest or during exercise. CO2 infusion at 1, 2, and 3 L/min at rest caused a variable increase in PaC02 and a proportionate increase in VE of 13±4, 19±2, and 29±6 L/min (n-4). CO2 infusion during exercise resulted in a transient increase in PaCO2, although often not to pre-exerclse levels, and an increase in VE despite hypocapnia. CO2 infusion did not alter the slope of the negative relationship between Tre and PaCO2 during exercise (p>0.1). The results support the hypothesis that the set point for PaCO2 decreases as hyperthermia increases in exercising sheep.

Original languageEnglish (US)
Pages (from-to)A641
JournalFASEB Journal
Issue number3
StatePublished - 1996

ASJC Scopus subject areas

  • Biotechnology
  • Biochemistry
  • Molecular Biology
  • Genetics


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