Development of chemoreceptor response sensitivity: studies in fetuses, lambs, and ewes

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Abstract
Fetuses fail to respond to cyanide by respiratory excitation but show typical cardiovascular effects; these are modified by vagotomy. When sensory stimulation is given (pulling on tongue), respiratory responses to cyanide occur. Similarly, when artifical respiration is instituted, cyanide causes inspiratory effort that is abolished by vagotomy. When the central end of a cut vagus nerve is stimulated, expiratory effort to injected cyanide is seen. 1–2 and 5–6-day-old lambs exhibit adult-type respiratory responses to threshold doses of cyanide without accompanying hemodynamic changes. However, the threshold is tenfold higher than in adults. At 6–7 days of age the threshold equals that of nonpregnant ewes, and is five times higher than in pregnant ewes. Vagotomy abolishes inspiratory gasps when cyanide is injected into lambs and ewes; hyperpnea persists. The threshold for hyperpnea is raised by vagotomy in lambs; in ewes it is unchanged. Oxygen increases the sensitivity of vagotomized lambs to cyanide; in ewes, oxygen abolishes the response. Full chemoreceptor response sensitivity occurs by day 6–7 in sheep, although responses occur in fetuses if there is sensory input to the central nervous system. Regulation of breathing after birth is discussed.