; 10.1152/ajpendo. 00460.2002.-We found that, in the rabbit ear, the dermal protein contains 75.5% of cutaneous phenylalanine and 97.9% of cutaneous proline; the remaining 24.5% of phenylalanine and 2.1% of proline are in the epidermal protein. This finding led us to develop two novel models that use phenylalanine and proline tracers and the rabbit ear to quantify protein kinetics in the epidermis and dermis. The four-pool model calculates the absolute rates of protein kinetics and amino acid transport, and the two-pool model calculates the apparent rates of protein kinetics that are reflected in the blood. The results showed that both epidermis and dermis maintained their protein mass in the postabsorptive state. The rate of epidermal protein synthesis was 93.4 Ϯ 37.6 mg ⅐ 100 g Ϫ1 ⅐ h Ϫ1 , which was 10-fold greater than that of the dermal protein (9.3 Ϯ 5.8 mg ⅐ 100 g Ϫ1 ⅐ h Ϫ1 ). These synthetic rates were in agreement with those measured simultaneously by the tracer incorporation method. Comparison of the four-pool and two-pool models indicated that intracellular cycling of amino acids accounted for 75 and 90% of protein kinetics in the dermis and epidermis, respectively. We conclude that, in the skin, efficient reutilization of amino acids from proteolysis for synthesis enables the maintenance of protein mass in the postabsorptive state. stable isotopes; gas chromatograph and mass spectrometer; arteriovenous balance; fractional synthesis rate; rabbits THE SKIN IS ONE OF THE LARGEST TISSUES in the body. In normal circumstances, the skin maintains a constant protein mass, whereas skeletal muscle may have a significantly positive or negative protein balance (28). The homeostasis of skin protein mass plays a crucial role in preserving an intact barrier on the body surface. Once the integrity of the skin barrier is destroyed, several problems ensue immediately, such as loss of water and electrolytes, invasion of microorganisms, and impairment of immune functions (11,19,20). Thus knowledge of skin protein metabolism is important in understanding the maintenance and repair process of the skin barrier.In our previous experiments (28-30), we investigated protein metabolism in normal and scalded skin by use of a rabbit ear model. This model has been the only approach to simultaneously quantify the in vivo rates of protein synthesis and breakdown as well as amino acid (AA) transport in the skin. However, this model is limited by the consideration of the skin as a single pool. It is well known that the epidermis and dermis have different structure, function, and turnover rates (12,17,25). The dermis serves as a basis for supporting and nourishing the epidermis, whereas the epidermis exerts the barrier function of the skin. It is therefore important to distinguish between the metabolic regulation of the epidermis and that of the dermis under physiological and pathological conditions.The goal of this study was to establish an approach for quantitation of protein kinetics and AA transport in the epidermis and dermis. This...