Bloodstream forms of four populations of the livestock pathogen Trypanosoma congolense, isolated from different natural
infections, have been shown to exhibit a wide range of sensitivities to the trypanocide
isometamidium chloride (
Samorin(R)). In mice the 50% curative doses (CD50) for
Samorin range from 0.007 to 20 mg/kg
body weight. Uptake of
isometamidium chloride demonstrated Michaelis-Menten-type kinetics in all the populations, with Km values in the range 0.35-0.87 microM, and Vmax varied from 17 to 216 pmol/min per 10(8) cells. The magnitude of Vmax was correlated with sensitivity to the
drug. In contrast, no correlation was observed between Km values and
drug sensitivity. Pulse-chase experiments indicated two compartments for accumulation of
drug. The first consists of freely diffusible
drug that is invariant between populations; the other consists of retained
isometamidium, which is of variable magnitude between the populations and is correlated with
drug sensitivity. Autoradiography and fluorescence microscopy demonstrated initial, rapid accumulation of the
drug within the mitochondrion, specifically the kinetoplast. In a
drug-sensitive population of T. congolense, agents affecting mitochondrial function were shown to produce dose-dependent inhibition of mitochondrial membrane potential (DeltaPsimito), as measured by the accumulation of the lipophilic
cations [3H]
methyltriphenylphosphonium iodide or
rhodamine 123. The agents also produced parallel inhibition of
isometamidium uptake, suggesting an involvement of DeltaPsimito in the accumulation of the
drug. When characterized in each of the four populations, the spontaneous DeltaPsimito was shown to be characteristic of each population and was correlated with Vmax for
drug uptake and sensitivity to the
drug in vitro and in vivo. We therefore conclude that in T. congolense DeltaPsimito is an important determinant of the rate and accumulation of the trypanocide
isometamidium chloride. Populations of this trypanosome species vary with respect to DeltaPsimito, which is correlated with sensitivity to
isometamidium. We suggest that when exposed to
drug, the selection of such populations represents a novel mechanism of drug resistance in protozoan parasites.