Spleen Contraction During Step‐Transition Supine Cycling Exercise: Preliminary findings

Abstract

The current literature suggests that the spleen plays a significant role during physiological stress like exercise or apnea, primarily due to its capacity to contract and release additional red blood cells (RBC) into the systemic circulation. Besides the RBC release, human spleen emptying is also associated with circulating platelets release. However, data on the circulating platelets release following acute exercise have not yet been examined thoroughly. This is especially important since the spleen platelet volume is ~20% larger compared to the volume of circulating platelets, and some authors suggest that increased spleen platelet concentration is associated with the greater risk of a cardiovascular or cerebrovascular event during exercise. The present study aims to explore the magnitude of spleen contraction during on-transient supine cycling exercise in healthy humans. We hypothesized that the spleen is capable of rapid contractions during on-transient cycling, thereby releasing an additional amount of both RBC and platelets into the systemic circulation. Such an increase in RBC count in particular may facilitate O2 transport, and result in faster oxygen kinetics during on transient supine cycling. Healthy, active volunteers (age= 22±3, V̇O2 max.= 48±5 mL·kg·min-1), completed three laboratory visits, including a detailed medical examination, resting ultrasound imaging of the spleen, V̇O2 max. test until voluntary exhaustion, and a supine step-transition test from 20 W to a moderate-intensity cycling power output (PO), corresponding to V̇O2 at 90% gas exchange threshold (GET). During the step-transitions breath-by-breath pulmonary oxygen uptake (V̇O2p), near-infrared spectroscopy derived muscle oxygenation, and beat-by-beat cardiovascular response (via Finapres) were continuously monitored. In parallel, minute by minute ultrasonic measurements of the spleen function were performed in successive, 3 sec. time frames. Blood samples were drawn from the antecubital vein before and immediately after the acute exercise intervention.Currently, six participants completed all study procedures. Compared to baseline readings, the largest spleen volume reduction (~40%) was observed during the 6th minute of cycling at 90% GET. Spleen volume recovered to baseline values after 20 minutes. Immediately after exercise cessation the mean RBC and platelet count increased significantly by 3.2% and 6.5%, respectively. In parallel, the V̇O2p was 46±10 sec, while the mean V̇O2 uptake was ~2.5 L·min-1. Secondary to these findings, transient rises in mean arterial pressure (MAP) and heart rate (HR), were observed during the step-transition exercise, with MAP and HR reaching the highest value of 110±4 mmHg and 149±2 bmp, respectively. In conclusion, these preliminary findings indicate that splenic contraction evoked by a series of on-transient cycling bouts at 90% GET results in a rapid increase in RBC and platelet count into the systemic circulation of healthy active individuals.