Calcium transport and catabolism of adenosine triphosphate in the protozoan parasite Giardia lamblia

Abstract

1. 1. Calcium uptake by washed trophozoites of Giardia lamblia was dependent on inorganic orthophosphate and stimulated by glucose. Uptake was both rapid and substantial: 224 ± 73 nmoles Ca 2+/mg protein/min. 2. 2. Known inhibitors of Ca 2+ uptake in mammalian cells also impeded Ca 2+ influx into G. lamblia. 3. 3. The inhibitor studies indicated that Ca 2+ transport in G. lamblia was an active process. Energy for such a process could be provided by the action of ATPases. 4. 4. Two types of ATPases were found in the parasite; one, a membrane-associated enzyme activated by Ca 2+; the other, a soluble, cytosolic enzyme activated by Mg 2+. 5. 5. These enzymes differed not only in their intracellular distribution and divalent cation requirements, but also in their sensitivity to calmodulin antagonists. The particulate enzyme was sensitive to these inhibitors whereas the soluble ATPase was not. 6. 6. Our data indicate that Ca 2+ transport in G. lamblia is mediated by a membrane-bound, calmodulin-regulated, Ca 2+-ATPase.