VAEIATION OF INTRACELLULAR INHIBITION WITH pH 719 



colysis were important in this fall of pH^, one might predict that iodoacetate 

 would prevent this. Actually, Lundsgaard (1930 a) showed that muscle, 

 instead of becoming more acid when inhibited by iodoacetate, exhibited 

 a slight rise in pHj. Voegtlin et al. (193-1) found that injections of iodoace- 

 tate into rats raised the muscle pHj whereas cyanide lowered it, although 

 these experiments were complicated by the changes in respiratory function. 

 Dubuisson and Schulz (1938) reported that anaerobic tetanus in frog muscle 

 resulted in a pH, of 6.93 whereas in the presence of iodoacetate the pH^ 

 was 7.19. There has been understandably very little work on the changes 

 in j)H; brought about by inhibitors but it is an important aspect of the prob- 

 lem and it is to be hoped that the recent advances in techniques will soon 

 provide some information on this matter. Certainly, some of the actions of 

 inhibitors could be the result of intracellular changes in pH^ brought about 

 in either of the two ways mentioned. 



Heterogeneity of Intracellular pH 



The pHj is not uniform throughout the cell. There is very little experi- 

 mental evidence for this statement but our knowledge of the interior of the 

 cell makes it necessary to assume such a heterogeneity. The structural com- 

 plexity of the cytoplasm becomes more apparent each year and the pos- 

 sibility of many intracellular compartments becomes more likely. The 

 extrusion of cell contents in order to determine the pH leads to mixing and 

 a disruption of the structure that provides the bulk pH, values described 

 above. Even the insertion of a micro-pH-electrode, small enough not to 

 damage the cell irreversibly, must modify the cytoplasmic structure in the 

 region of the electrode tip and again can provide only an average cyto- 

 plasmic pH. The concept of a continuous bulk phase of cytoplasmic ground 

 substance, as assumed by the early cytologists, is no longer tenable. 



Where within cells might the pH be particularly different from the mean 

 or bulk pH? The following come to mind immediately: (a) mitochondria, 

 (6) vacuoles, (c) the region between double membranes, {d) the surfaces 

 and interiors of membranes, especially when an electric potential exists, 

 (e) within gel structures, such as myofibrils or spindle fibers, and (/) sites 

 where certain types of metabolism proceed at a high rate. The pH near a 

 charged surface, such as that of a protein or nucleic acid, is not that of the 

 medium, due to the attraction or repulsion of such surfaces for protons. 

 Furthermore, each region possessing a structurally fixed constellation of 

 charged groups will have a pH determined by Donnan equilibria. The oc- 

 currence of such multiple Donnan equilibria within cells has been treated 

 quantitatively by Caldwell (1956). The heterogeneity of the pH is expressed 

 not only in the microscopically observable cell components and structures, 

 but also on the molecular level throughout the cell. 



