The cessation of breathing in the chicken embryo during cold-hypometabolism.

The avian embryo toward end-incubation combines gas exchange through the chorioallantoic membrane (CAM) and pulmonary ventilation (V˙E). The main experiments examined breathing activity during cold-hypometabolism. Chicken embryos close to hatching were prepared for simultaneous measurements of oxygen consumption ( [Formula: see text] ) and carbon dioxide production ( [Formula: see text] ; open-flow methodology) and breathing frequency (f; barometric technique). As ambient (Ta) and egg temperature (Tegg) dropped, breathing eventually ceased at ∼18°C, when [Formula: see text] and [Formula: see text] were 22-28% of the normothermic values. With the eggshell experimentally covered to reduce CAM gas exchange breathing ceased at slightly lower [Formula: see text] and [Formula: see text] (17-18% of normothermia). Once breathing had stopped, egg exposure to hypoxia (10% or 5% O2) or hypercapnia (3% or 8% CO2) did not resume breathing, which recovered with re-warming. In normothermia, 10% O2 caused hypometabolism and tachypnea; differently, in 5% O2 [Formula: see text] dropped as much as with hypothermia and breathing stopped, to recover upon return in air. Correlation analysis among Ta, Tegg, [Formula: see text] , [Formula: see text] and f during cooling and re-warming indicated that f followed more closely the changes in [Formula: see text] and, especially, in [Formula: see text] than the changes in Ta or Tegg. Some considerations suggest that in this experimental model the cessation of breathing in hypothermia or severe hypoxia may be due to hypometabolism, while the lack of chemo-responses may have a different mechanistic basis.

The contribution of heart rate to the oxygen consumption of the chicken embryo during cold- or hypoxia-hypometabolism.

In embryos, cooling and hypoxia cause a decrease in oxygen consumption ( [Formula: see text] ); we asked what was the relative contribution of heart rate (HR) and of the 'not-HR' factor (the product of stroke volume and arterial-venous O2 difference) to the drop in [Formula: see text] . Data of HR (with subcutaneous electrodes) and [Formula: see text] (by an open-flow methodology) were collected simultaneously on chicken embryos close to end-incubation. Over the last four days of incubation (E16-E20) differences in HR contributed about 30% of the differences in resting [Formula: see text] among embryos. At E20, progressive cooling from 38 to 8°C decreased [Formula: see text] entirely because of the decrease in HR, with minimal compensation of the 'not-HR' component. The same pattern during cooling occurred in younger embryos (age E16), in E20 embryos simultaneously exposed to hypoxia (15% O2) and in E20 normoxic embryos which were incubated in hypoxia (15% O2). Differently, in E20 embryos in normothermia, progressive hypoxia (15%, 10% or 5% O2) lowered [Formula: see text] largely because of the reduction in the 'not-HR' component. We conclude that at end incubation during hypometabolism the changes in HR contribute very differently to the decrease in [Formula: see text] , from about the totality of it during cold to only about 10-20% during hypoxia, depending on its severity. It follows that during cold-hypometabolism, but not during hypoxic hypometabolism, the changes in HR are a good index of the changes in [Formula: see text] . The close relationship between [Formula: see text] and HR during cold-hypometabolism may permit estimates of the changes in [Formula: see text] from the changes in HR in infants undergoing therapeutic hypothermia.

The hypometabolic response to repeated or prolonged hypoxic episodes in the chicken embryo.

Hypoxia (hx) in embryos causes a drop in oxygen consumption ( [Formula: see text] ) that rapidly recovers upon return to normoxia. We asked whether or not this pattern varies with the embryo's hypoxic history. The [Formula: see text] of chicken embryos in the middle (E12) or at end-incubation (E19) was measured by an open-flow methodology during 15-min epochs of moderate (15% O2) or severe hx (10% O2). Each hx-epoch was repeated or alternated with air by various modalities (air-hx-air-hx-air-hx-air, air-2·hx-air-2·hx-air, air-5·hx-air), in randomized sequences. The hx drop in [Formula: see text] was larger with severe than with moderate hx; however, in either case, its magnitude was essentially independent of the preceding hx history. E19 embryos had hx drops in [Formula: see text] of the same magnitude whether their incubation was in air or in moderate hx from E4 to E19. A different protocol (air-12·hx-air) gave variable results; with moderate hx, the [Formula: see text] response was similar to that of the other hx regimes. Differently, with severe hx most embryos progressively decreased [Formula: see text] and eventually died. We interpret these data on the basis of what is known on the 'compensatory partitioning' between costs of growth and maintenance. With moderate hx presumably each episode caused an energy shortfall absorbed entirely by the blunted growth. Hypoxic events of this type, therefore, should have no long-term functional effects other than those related to the small birth weight. Differently, the aerobic energy shortfall with severe hypoxia probably impinged on some maintenance functions and became incompatible with survival.