Trade-offs between investment in carbohydrate reserves and growth occurred only among deciduous species, leading to differences in relative contribution made by the underlying components of RGR between the leaf habit groups. The results suggest that differences in drivers of RGR occur among savanna species because these have different selected strategies for coping with fire disturbance in savannas. It is expected that variation in the drivers of RGR will be found in other functional types that respond differently to particular disturbances.

Additional material and information about A J P -H eart a n d Circulatory Physiology can be found at:http://www.the-aps.org/publications/ajpheart This infomation is current as of May 11, 2011.A JP -H eart and Circulatory Physiology publishes original investigations on the physiology of the heart, blood vessels, and lymphatics, including experimental and theoretical studies of cardiovascular function at all levels of organization ranging from the intact animal to the cellular, subcellular, and molecular levels. (HUT) induces a re duction in preload, which is thought to be restored through sym pa thetic venoconstriction, reducing unstressed volume (Vu) and venous compliance (VeC). In this study, we assessed venous inflow and outflow responses and their reproducibility and determined the rela tion with autonomic function during HUT. Eight healthy non-pregnant women were subjected to 20° head-down tilt to 60° HUT at 20° intervals. A t each rotational step, we random ly assessed forearm pressure-volume (P-V) curves (venous occlusion plethysmography) during inflow (VeCIN) and outflow [venous emptying rate (VERoUT)]. V eCIN was defined as the ratio o f the slope of the volume-time curve and pressure-time curve, with direct intravenous pressure m easure ment. V EROUT was determined using the derivate of a quadratic regression m odel using cuff pressure. W e defined Vu as the y-intercept of the P-V curve. W e calculated, for both methods, the coefficients of reproducibility (CR) and variation (CV). V ascular sympathetic activ ity was determined by spectral analysis. V eCIN decreased at each rotational step compared with the supine position (P < 0.05), whereas VEROUT increased. CR of VeCIN was higher in the supine position than V EROUT but low er during HUT. CV varied between 19% and 25% (VeCIN) and between 12% and 21% (VEROUT). HUT decreased Vu. The change in V eCIN and V EROUT correlated with the change in vascular sympathetic activity (r = -0.36, P < 0.01, and r = 0.48, P < 0.01). This is the first study in which a reproducible reduction in V eCIN and V u and a rise in V EROUT during HUT are documented. The alterations in venous characteristics relate to changes in vascular sym pathetic activity.head-up tilt; venous compliance; venous emptying rate; venous ca pacity; autonomic system

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