The immunosuppressor cyclosporin A inhibits the 
peptidyl-prolyl-cis/trans-isomerase activity of cyclophilins and the resulting 
complex inhibits the phosphatase activity of calcineurin. Both enzymes were 
detected in peripheral nerve endings isolated from the electric organ of Torpedo 
and shown to be affected by 10 micro m cyclosporin A. Among the cholinergic 
properties studied, choline uptake was specifically inhibited by cyclosporin A 
to a maximum of 40%. Cyclosporin A decreased the rate of choline transport but 
not the binding of the non-transportable choline analogue hemicholinium-3, 
indicating that the number of membrane transporters was not affected. Through 
the use of two other immunosuppressors, FK506, which also inhibits calcineurin, 
and rapamycin, which does not, two different mechanisms of choline uptake 
inhibition were uncovered. FK506 inhibited the rate of choline transport, 
whereas rapamycin diminished the affinity for choline. The Torpedo homologue of 
the high affinity choline transporter CHT1 was cloned and its activity was 
reconstituted in Xenopus oocytes. Choline uptake by oocytes expressing tCHT1 was 
inhibited by all three immunosuppressors and also by microinjection of the 
specific calcineurin autoinhibitory domain A457-481, indicating that the 
phosphatase calcineurin regulates CHT1 activity and could be the common target 
of cyclosporin and FK506. Rapamycin, which changed the affinity of the 
transporter, may have acted through an immunophilin on the isomerization of 
critical prolines that are found in the tCHT1 sequence.