Effects of D-Asp on the synthesis of testosterone and cAMP in isolated rat Leydig cells. 1 ml of purified Leydig cells (1.0 × 106 cells) was incubated at 37°C for 60 min with Na-D-Asp 0.1 or 1.0 mM and control and then the testosterone and cAMP synthesis was determined. The left panel shows the total production of testosterone in the medium plus the Leydig cell homogenate, in the control sample and in the samples incubated with 0.1 mM or with 1.0 mM D-Asp: A, B and C, D (p < 0.001). The right panel shows the concentration of cAMP in the same above samples: A, B and C, D (p < 0.001). Values are the mean ± SEM obtained from 5 individual experiments. Asterisks indicate that the increased levels were significant compared with the controls at p < 0.001 as evaluated by one-way ANOVA.