ACTIVE TRANSPORT 701 



ACTIVE TRANSPORT 



A few examples of the effects of arsenicals on transport processes are 

 summarized in Table 6-9. In some cases the substance designated as trans- 

 ported actually may follow passively the substance truly moved, and in 

 other cases truly active transport may not be involved (e. g., propionate, 

 protamine, iodide, or water). The transport of glucose is difficult to investi- 

 gate because of the Pasteur-like effect produced by the arsenicals and 

 possibly the stimulation observed with rat intestine, and with other prep- 

 arations not included in the table, is related to this. In any event, it is 

 clear that active transports are quite sensitive to the arsenicals, and this 

 must be of great importance in interpreting the effects of these substances 

 on cellular metabolism, function, and growth. The problem as to whether 

 the primary action is on the transport system or membrane permeability 

 remains unsolved in most instances (see page 172 for a general discussion 

 of this), but permeability effects are probably not so likely with the arsen- 

 icals as with some other SH reagents, like the mercurials, although Maizels 

 (1951) interpreted the small effects on ion transport in erythrocytes as 

 due to permeability changes inasmuch as some hemolysis is induced. 

 Permeability changes may be caused not only by a reaction of inhibitors 

 with the membrane components but, in the case of the arsenicals, by a 

 depression of oxidative metabolism. 



Is the inhibition of transport by the arsenicals due to some specific 

 action on the transport system or merely to a general depression of energy 

 generation from the cycle? This is a difficult question to answer. Some 

 workers have attempted to obtain information on this by comparing the 

 degrees of depression of transport and respiration, apparently on the as- 

 sumption that a greater depression of the former would imply a selective 

 action. Actually, in every instance in which such measurements have been 

 made, the transport is inhibited more strongly (barley and mustard roots, 

 frog skin, lymphosarcoma, and rabbit kidney slices), although seldom is 

 the difference marked. Unfortunately, these results cannot establish either 

 a relationship or the lack of one, since there is no necessity for a parallel 

 inhibition of the transport and respiration. Only a fraction of the respiration 

 may be involved in providing energy for transport, and in addition the 

 rate of energy utilization for other activities will to some extent determine 

 the effect of energy reduction on transports.* Butler (1953) noted that 

 phosphate uptake in wheat roots can be depressed around 50% by 0.02 

 milf arsenite without altering protoplasmic streaming, indicating a selec- 

 tive effect on transport, a preferential requirement of transport for the 

 energy available, or an action on the uptake unrelated to active transport. 



* If transport were reduced without appreciable respiratory inhibition, some basis 

 for attributing a selective effect on transport would be provided, but such has not 

 been found experimentally. 



