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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.

In the culture of epidermal keratinocytes, the cellular growth rate is reported to be accelerated by cholera toxin. The mechanism by which cholera toxin exerts biological effects is thought to result from changes in intracellular cyclic AMP concentrations. But in many reports cyclic AMP elevating agents appeared to inhibit growth of keratinocytes in culture. This study was done to clarify the discrepancy of this problem. Determination of cyclic AMP revealed that cholera toxin over a range of 10-14-10-8 M increased the intracellular concentration of cyclic AMP of cultured keratinocytes about 100-fold over the controls after incubation for 6 hr. When a small number (10(5)) of cells were inoculated in a 60 X 15 mm culture dish, cholera toxin strongly stimulated colony growth. When a relatively larger number (8 X 10(5)) of cells were inoculated in a dish, cholera toxin moderately accelerated cell division, and increased DNA and protein levels of the culture during early days of cultivation. But after about 20 days, of cultivation when the culture reached confluence, cholera toxin decreased both DNA and protein content in a culture dish. The cultures were pulse labeled with 3H-thyrmidine at 12 and 24 hr after the addition of 10-10 M cholera toxin, and its uptake into DNA was determined. In the early days of cultivation the uptake of 3H-thymidine increased after treatment with cholera toxin. But in the late days of cultivation, cholera toxin decreased the rate of 3H-thymidine incorporation into DNA. These results indicated that cholera toxin-cyclic AMP has effects on the proliferation of keratinocytes in culture biphasically according to cellular concentrations in culture.

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