Effect of hypoxia on neuromuscular transmission in the developing diaphragm

To study the effects of hypoxia on neuromuscular transmission in the developing diaphragm, phrenic nerve-hemidiaphragm preparations were obtained from newborn (4-9 days) and older (22-30 days) rats. Diaphragms were stimulated directly or indirectly (via the nerve) for 1 s at frequencies of 10-80 Hz. Force generated in response to stimulation was measured during perfusion of oxygenated Ringer solution (control) and Ringer solution bubbled with 95% N2-5% CO2 (hypoxia). After 45 min of hypoxia, the force response of the older diaphragms to direct stimulation had decreased to approximately 50% of control at > or = 40 Hz; however, when stimulation occurred via the nerve at these frequencies only 15-20% of control force was generated. In the newborn diaphragms, the force decrement after similar or longer periods of hypoxia (< or = 90 min) was 30- 40% irrespective of the route or frequency of stimulation. After 15 min of reoxygenation, the force response to both muscle and nerve stimulation recovered completely in the older diaphragms but only partially in the newborn diaphragms (range 77% of control at 50 Hz to 95% of control at 10 Hz). These data suggest that in the newborn diaphragm 1) neuromuscular transmission is more resistant to the effects of hypoxia than the older diaphragm and 2) the predominant effect of hypoxia is peripheral in the diaphragm muscle fibers, whereas in the older diaphragm the effect is before or at the neuromuscular junction.