The depressed ventilatory response to hypoxia (HY) observed in hypothermic newborn piglets may be due to an increase in the CNS level of inhibitory amino acid neurotransmitters such as glycine. To evaluate the effects of strychnine(STR), a glycine antagonist, on the ventilatory response to HY during hypothermia, 32 anesthetized, paralyzed and mechanically ventilated newborn piglets (mean±SD; age, 5±1 days; wt, 2.0±0.4 kg) were studied at a brain temperature of 38.5±0.5°C (normothermia, NT, n=15) or 34.0±0.5°C (hypothermia, HT, n=17). Minute phrenic output(MPO), arterial blood pressure (ABP), heart rate (HR), oxygen consumption(VO2) and arterial blood gases were measured at room air (RA) and during 20 min of isocapnic HY (FiO2=0.10). After 10 min of HY, 50 μl of STR (90 μg) or Ringer solution (placebo, PL) was administered over a 1-min period into the cisterna magna, while the piglets remained in HY for an additional 10 min. Mean±SE values for MPO (au/min) for RA and 1′, 10′ and 20′ of HY, and% change (Δ) from RA (Pre-STR) or 10 min of HY (Post-STR) were: (see table).

Table 1 No caption available.

In the NT groups, the initial increase in MPO with HY was followed by a decrease to values above baseline at 10′ of HY. In contrast, in the HT groups a significant and gradual decrease in MPO to values below baseline was observed during HY. A significant increase in MPO during HY occurred in NT and HT groups after STR administration, but this increase was significantly greater in the HT group. The ventilatory response to HY was not modified by Ringer solution administration to either NT or HT group. Changes in VO2, ABP, HR, pH, PaO2 and BE with HY were not statistically different between NT and HT groups before and after STR. These data suggest that the marked depression of the neonatal ventilatory response to HY during hypothermia is in part modulated by an increase in CNS glycine concentration.

+p<0.05 (STR vs PL); *p<0.002 (HT vs NT).