Abstract
Ingestion of dietary nitrate ([Formula: see text]) is associated with improved exercise tolerance and reduced oxygen (O2) cost of exercise, ascribed to enhanced mitochondrial efficiency, muscle contractile function, or other factors. Nitrate ingestion has also been found to attenuate the reduction in arterial oxygen saturation ([Formula: see text]) during apnea and to prolong apneic duration. The spleen serves as a dynamic blood pool expelling erythrocytes into the circulation during apnea, and [Formula: see text] and nitric oxide donors may induce vasoactive effects in the mesenteric and splanchnic circulation. Our aim was to investigate the effect of ingestion of concentrated organic [Formula: see text]-rich beetroot juice (BR) on spleen volume and spleen contraction during apnea, and the resulting hemoglobin (Hb) concentration. Eight volunteers performed two apneas of submaximal and maximal duration during prone rest ∼2.5 h after ingesting 70 mL of BR (∼5 mmol [Formula: see text]) or placebo (PL; ∼0.003 mmol [Formula: see text]), on separate days in weighted order. Heart rate and [Formula: see text] were monitored continuously and spleen diameters were measured every minute for triaxial volume calculation. Capillary Hb samples were collected at baseline and after the maximal apnea. Baseline spleen volume was reduced by 66 mL after BR ingestion (22.9%; P = 0.026) and Hb was elevated (+3.0%; P = 0.015). During apneas, spleen contraction and Hb increase were similar between BR and PL conditions (NS). The study shows that dietary [Formula: see text]reduces spleen volume at rest, resulting in increased Hb. This spleen-induced Hb elevation following [Formula: see text] ingestions represents a novel mechanism that could enhance performance in conditions involving exercise, apnea, and hypoxia.NEW & NOTEWORTHY This is the first study to examine changes of spleen volume and circulating Hb following dietary [Formula: see text] supplementation. After dietary [Formula: see text] ingestion, the spleen volume at rest was reduced and Hb was elevated. The spleen contains a dynamic red blood cell reservoir, which can be mobilized and facilitate oxygen transport during various types of physiological stress. This study has revealed an additional, previously unexplored mechanism possibly contributing to the ergogenic effects of dietary [Formula: see text].
Highlights
During apnea, diving mammals exhibit several responses to maintain vital physiological functions based on the available oxygen (O2) stored in the lung, blood, and other tissues
Mean spleen volume was reduced by 22.9% with beetroot juice (BR) (222.5 ± 74.5 mL), compared with PL (288.1 ± 102.8 mL; P = 0.046; Table 1; Fig. 1), whereas spleen volume was not different between PL and BR for the other test conditions (NS)
The reduction was concomitant with an increase in Hb concentration, suggesting that stored erythrocytes in the spleen are ejected into the circulatgieosntsftohlalot wa isninggtlehedoNseOoÀ3f-dNeOriÀ3vehdasvtohleumpoeternetdiualcttiooenl.icTithsisplesuegn-induced Hb elevation, which depending on the exercise modality may contribute to beneficial effects during short-term exercise and hypoxic conditions
Summary
During apnea, diving mammals exhibit several responses to maintain vital physiological functions based on the available oxygen (O2) stored in the lung, blood, and other tissues. At least two such responses are evident: the cardiovascular diving response (diving response) [7, 23] and the hematological response resulting from spleen contraction [47]. The spleen is known to serve as a dynamic reservoir for erythrocytes in many mammals, including humans [26, 54]. Sympathetic nerve fibers densely innervate the spleen [45], and the organ can be recruited upon sympathoexcitation during various types of physiological stress to elevate blood O2 storage and transportation [54].
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