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University Laboratory of Physiology, Oxford OX1 3PT, United Kingdom
Nitric oxide (NO) decreases
norepinephrine (NE) release and the heart rate (HR) response to
sympathetic nerve stimulation (SNS). We tested the hypothesis that the
enhanced HR response to sympathetic activation following chronic
intermittent hypoxia (IH) results from a peripheral modulation of
pacemaking by NO. Isolated guinea pig double atrial/right stellate
ganglion preparations were studied from animals that had been exposed
to IH (n = 20) and control animals (n = 22). The HR response to SNS was significantly enhanced in the IH group
compared with the controls. However, the increase in HR with cumulative
doses (0.1-10 µM) of bath-applied NE was similar in both groups.
Western blot analysis showed less neuronal NO synthase in the right
atria from the IH group. In IH animals, the NO synthase inhibitor,
N
-nitro-L-arginine
(L-NNA; 100 µM) did not alter the increased HR response
to SNS, whereas in control animals L-NNA significantly increased the HR response to SNS; an effect that was reversed with
excess L-arginine. In conclusion, the enhanced HR response to SNS after IH may be related to a decreased inhibitory action of NO
on presynaptic NE release.
hypoxic training; nitric oxide; sympathetic nervous system
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