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The conditional nature of the mammalian hypercapnic ventilatory response : effect of age, state, and temperature Sprenger, Ryan Joseph

Abstract

CO₂ is the primary respiratory stimulant under resting conditions in mammals and an elevation in internal or environmental CO₂ causes an increase in ventilation. My thesis focuses on two conditions in which the magnitude of the ventilatory response to excess CO₂ is altered within an animal: postnatal development and hibernation. I first examine the development of the hypercapnic ventilatory response (HCVR) in two fossorial species (golden-Syrian hamsters and 13-lined ground squirrels) to determine if the blunted adult fossorial HCVR is developed or inherited, then raised those species in burrow conditions to determine how peri- and postnatal exposure to burrow conditions affects the adult response. Both species developed their blunted adult HCVR over the first 15 days after birth, and peri- and postnatal exposure to the burrow environment sped up the development of the HCVR (5-10 days) without affecting the adult response. My data suggest that the HCVR is likely genetically determined and strongly influenced by neurochemical development which produces but not regulates the observed patterns in the development of the HCVR. I next examined the relatively elevated HCVR seen in adult hibernating mammals. I found that the HCVR diminishes during a period of CO₂ retention early in entrance into hibernation (where 7% CO₂ does not elicit a ventilatory response) that appears to contribute to metabolic suppression. After, it rises to the elevated HCVR in hibernation (~650% increase to 7% CO₂) tracking the reduction in metabolic rate. In steady state hibernation, the HCVR was elevated and plastic based on changes in metabolic rate, state, and body temperature. On arousal the HCVR is elevated further (~750% increase to 7% CO₂) when the animals are expelling excess CO₂ before falling as metabolic rate rises on arousal. These data suggest that chemoreceptor input, or changes to the integration of that input were altered but I confirm that the locus coeruleus is not involved in the changes in the HCVR during hibernation. Unlike development, CO₂ sensitivity appears to be tightly regulated to facilitate retention or expulsion of CO₂ in hibernation, thus the nature of the changes in CO₂ sensitivity are different between development and hibernation.

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Attribution-NonCommercial-NoDerivatives 4.0 International

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