Figure 1. Phrenic nerve activity is insensitive to pentobarbital application during pregnancy.

A, Representative, compressed phrenic neurograms from virgin female, G17 pregnant and 30 d post partum rats. After establishing baseline conditions, 10 mg/kg of pentobarbital was administered every 5 min up to a maximum total dose of 100 mg/kg. The protocol ended with 5 min of hypercapnic hypoxia to assess phrenic response to maximal chemosensory input under 100 mg/kg of pentobarbital. B, Survival plot of phrenic nerve activity by condition. 0% of post partum rats generated measurable phrenic nerve output above 50 mg/kg pentobarbital. No virgin female rats generated measureable phrenic nerve output above 60 mg/kg pentobarbital. 100% of pregnant animals generated phrenic nerve output at 60 mg/kg, and 17% generated activity at 90 mg/kg. C, Pentobarbital IC50 curve of phrenic nerve burst frequency demonstrates a significant rightward shift during pregnancy compared to virgin female and post partum animals. The hill slope of the IC50 curve during pregnancy is significantly increased as well. D, Pentobarbital IC50 curve of phrenic nerve amplitude demonstrates a significant rightward shift during pregnancy compared to virgin female and post partum animals. The hill slope of the amplitude IC50 during pregnancy is not significantly altered. E, Hypercapnic hypoxia produced a measurable response in the phrenic nerve in a greater number of pregnant animals (4/5) than virgin female (2/6), post partum (1/6), or male (0/6; not shown).