Effects of phrenicotomy and exercise on hypoxia-induced changes in phrenicmotor output

To investigate models of plasticity in respiratory motor output, we determi ned the effects of chronic unilateral phrenicotomy and/or exercise on time- dependent responses to episodic hypoxia in the contralateral phrenic nerve. Anesthetized (urethane), ventilated, and vagotomized rats were presented w ith three, 5-min episodes of isocapnic hypoxia (11% O-2), separated by 5 mi n of hyperoxia (50% O-2). Integrated phrenic land hypoglossal) nerve discha rge were recorded before and during each hypoxic episode, for the first 5 m in after the first hypoxic episode, and at 30 and 60 min after the final ep isode. Of 36 rats, one-half were sedentary while the other one-half had fre e access to a running wheel; each of these groups was split into three subg roups: 1) unoperated, 2) chronic left phrenicotomy (27-37 days), and 3) sha m operated. Neither unilateral phrenicotomy nor running wheel activity infl uenced the short-term hypoxic phrenic response (during hypoxia) or long-ter m facilitation (posthypoxia). Posthypoxia frequency decline was exaggerated in phrenicotomized-sedentary rats relative to unoperated-sedentary rats (c hange in burst frequency = -23 +/- 4 vs. -11 +/- 5 bursts/min, respectively ; 5 min posthypoxia; P < 0.05), an effect that was eliminated by spontaneou s exercise. The results indicate that neither voluntary running nor unilate ral phrenicotomy has major effects on time-dependent hypoxic phrenic respon ses, with the exception of an unexpected effect of phrenicotomy on posthypo xia frequency decline in sedentary rats.