The effect of intermittent hypoxia (IHx) on blood hemoglobin concentration ([Hb]) and the underlying mechanisms were studied in rats exposed to 10% O 2 , 1 h/day, for up to 5 wk. IHx protocols with longer daily hypoxic exposure show persistent polycythemia; however, it is unknown whether [Hb] increases transiently during hypoxia in protocols without polycythemia. Hypoxia produced a reversible [Hb] increase after 4 days of IHx but not in normoxic controls (NxC) or after shorter period of IHx.

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... enectomy abolished the phenomenon. Plasma epinephrine and norepinephrine levels during hypoxia were comparable in IHx and NxC groups, but the epinephrineinduced [Hb] increase was larger in IHx. The ␣ 1 -and ␣ 2adrenoreceptor blockade (phentolamine) and ␣ 2 -blockade (yohimbine) abolished the [Hb] increase of IHx rats. Conversely, ␣ 2 -receptor stimulation (oxymetazoline) increased [Hb] during normoxia in IHx but not in NxC. In conclusion, this IHx protocol results in reversible [Hb] increases during hypoxia via splenic contraction mediated by increased ␣ 2 -adrenoreceptor response. This may protect O 2 supply during hypoxia without the cardiovascular burden of polycythemia during normoxia. ␣ 2 -adrenoreceptor; Sprague-Dawley rat; acclimatization; oxygen delivery