Fig. 4
Determination of matrix phenformin concentrations in cells. a Oxygen consumption rate (OCR) as a percentage of the control for HepG2 (black), 143B (gray), and MDBK (white) cells, taken from Fig. 2 (left, 34 μM phenformin = IC50/10), is combined with the IC50 curve for phenformin inhibition of complex I in bovine mitochondrial membranes (middle), providing estimates for the matrix phenformin concentration (right). b Maximal rates of biguanide uptake into the matrix were calculated by linear regression from the data in Fig. 2a–c. Dark gray, 143B; light gray, HepG2; white, MDBK. c Biguanide concentration in the matrix relative to in the extracellular medium after 30 min in intact (white) MDBK and permeabilized (red) MDBK cells. For intact cells: n = 8 for control; n = 7 for compounds 1 and 5; and n = 6 for phenformin, proguanil, and compounds 2, 3, and 4. For permeabilized cells: n = 16 for control; n = 8 for phenformin and compounds 4 and 5; n = 7 for compound 1; n = 6 for compound 3; and n = 5 for proguanil and compound 2. The values for compounds 3 and 4 are underestimates as inhibition exceeded 90 %. Data show means ± standard error of the mean. Individual data points for panel c are provided in Additional file 1. phenf. phenformin, prog. proguanil