THE INTERPLAY OF CENTRAL AND PERIPHERAL FACTORS IN LIMITING MAXIMAL O-2 CONSUMPTION IN MAN AFTER PROLONGED BED REST

1. The effects of bed rest on the cardiovascular and muscular paramete rs which affect maximal O-2 consumption ((V) over dot (O2,max)) were s tudied. The fractional limitation of (V) over dot(O2,max) imposed by t hese parameters after bed rest was analysed. 2. The (V) over dot(O2,ma x), by standard procedure, and the maximal cardiac output ((Q) over do t(max)), by the pulse contour method, were measured during graded cycl o-ergometric exercise on seven subjects before and after a 42-day head -down tilt bed rest. Blood haemoglobin concentration ([Hb]) and arteri alized blood gas analysis were determined at the highest work load. 3. Muscle fibre types, oxidative enzyme activities, and capillary and mi tochondrial densities were measured on biopsy samples from the vastus lateralis muscle before and at the end of bed rest. The measure of mus cle cross-sectional area (CSA) by NMR imaging at the level of biopsy s ite allowed computation of muscle oxidative capacity and capillary len gth. 4. The (V) over dot O-2,O-max was reduced after bed rest (-16.6%) . The concomitant decreases in (Q) over dot(max) (-30.8%), essentially due to a change in stroke volume, and in [Hb] led to a huge decrease in O-2 delivery (-39.7%). 5. Fibre type distribution was unaffected by bed rest. The decrease in fibre area corresponded to the significant reduction in muscle CSA (-17%). The volume density of mitochondria was reduced after bed rest (-16.6%), as were the oxidative enzyme activit ies (-11%). The total mitochondrial volume was reduced by 28.5%. Capil lary density was unchanged. Total capillary length was 22.2% lower aft er bed rest, due to muscle atrophy. 6. The interaction between these m uscular and cardiovascular changes led to a smaller reduction in (V) o ver dot(O2max) than in cardiovascular O-2 transport. Yet the latter ap pears to play the greatest role in limiting (V) over dot(O2,max) after bed rest (> 70% of overall limitation), the remaining fraction being shared between peripheral O-2 diffusion and utilization.