Utilization of leucine by working rat heart.

Leucine provided substrate to partially support mechanical activity of working rat hearts; the beneficial effect on peak systolic pressure development was more marked as leucine concentration was increased to 10 mM. In Langendorff preparations that had been exposed to 5 mM [U-14C]leucine, the imposition of cardiac work was accompanied by a threefold increase in the rate of 14CO2 production within the first 30 s; however, the rate decreased 40% in the next 10 min. This transient acceleration of 14CO2 production was not observed when [1-14C]leucine was provided and appeared to be due to oxidation of a tissue pool of radioactive intermediates that was present after 10 min of Langendorff perfusion. Over a period of 1 h, oxidation of either [U-14C]- or [1-14C]leucine increased 25-40% in working compared to Langendorff preparations that were supplied 5 mM leucine and 11 mM glucose. In working hearts that were supplied a substrate and hormone mixture that simulated normal plasma and 1 mM leucine, a concentration found in the plasma of diabetic but not normal rats, leucine oxidation was accelerated 73% by work but amounted to only 3.3% of oxygen consumption.