The uptake and intestinal permeability of the seven alkaloids strychnine (Str), brucine (Bru), beta-colubrine (Col), strychnine N-oxide (S-N), brucine N-oxide (B-N), pseudostrychnine (Psd), and icajine (Ica), which were isolated from the processed seeds of Strychnos nux-vomica L., were investigated in the human intestinal Caco-2 model. Determination of compounds was carried out by HPLC. The apparent permeability coefficients (P-app) for Str, Bru, Col, S-N, B-N, Psd, and Ica in the apical-to-basolateral direction were (3.11 +/- 0.17) X 10(-5), (1.67 +/- 0.65) x 10(-5), (2.67 +/- 0.30) X 10(-5), (0.17 +/- 0.01) x 10(-5), (0.35 +/- 0.02) X 10(-5), (2.51 +/- 0.33) x 10(-5), and (2.61 +/- 0.34) x 10(-5) cm/s, respectively. In the concentration range of 10-200 mu M, Str, Bru, Col, and Psd showed substantial concentration-dependent transport across the monolayers. The transports of all seven alkaloids were linear with time and showed moderate to high permeabilities. In the presence of 2,4-dinitrophenol or sodium azide, the P-app of Ica was reduced significantly in both the apical-to-basolateral and basolateral-to-apical directions. The dominant mechanism of the intestinal absorption for Str, Bru, Col, S-N, B-N, and Psd was passive diffusion, while it was partially ATP dependent for Ica.