Fig. 1.

Representative phrenic neurograms from five rats receiving episodic phenylephrine (PE, an α₁ receptor agonist), acute intermittent hypoxia (AIH), or prazosin (an α₁ receptor antagonist) before PE. Intrathecal applications of low-dose PE (1 μM) (A) did not alter phrenic amplitude (Phr). Intrathecal applications of higher PE doses (10 μM) (B) and (100 μM) (C) separated by 5 min caused phrenic motor facilitation (pMF), similar to phrenic long-term facilitation (pLTF) induced by AIH [three 5-min periods of hypoxia (Hx1, Hx2, and Hx3, respectively) separated by normoxia, and 20% DMSO vehicle preapplication] (D). E: PE-induced pMF was blocked by preapplication of prazosin (1 mM) 20 min prior to episodic PE (10 μM).