Chapter XIII 



BINDING OF HISTAMINE IN THE MAST CELL: 

 THE NATURE OF THE MAST-CELL GRANULE 



HEPARIN is an acid, histamine a base: is the mast-cell granule then a 

 salt-like compound, a ' heparinate of histamine ' ? The simple explanation 

 is attractive (Mota et ah 1953; Smith, 1958); yet if the two substances 

 can combine in the cell, why do they remain apart in the blood, as they appear 

 to do in the dog's blood during peptone shock? And how can we explain the 

 different effects of the same histamine-liberator on the mast cells of different 

 species (Feinberg and Sternberger, 1955), or at different sites in the same 

 individual (Nishiyama et al, 1957)? 



It was formerly believed — and until very recently was still believed by 

 Rocha e Silva (1955) — that histamine is firmly bound to some tissue component 

 as the amino-acid histidine, which is split off and decarboxylated when histamine 

 is released into the blood. The exact binding site was never precisely defined, 

 though it was suggested that the histidine might occupy a terminal position 

 in a protein or polypeptide chain. In this view, histamine-release involves 

 both a proteolysis and a decarboxylation. 



Now that we have tracked down the histamine to its depots in the tissue 

 mast cells, we are better placed for examining both the mode of its binding and 

 the mechanism of its release. Indeed, I believe that the experiments, previously 

 described, on the visible effects of the fluorescent diamidines on the tissue mast 

 cells of the rat represent the first real evidence for the intracellular location of 

 tissue histamine and its release by a chemical agent. 



Effect of pH 



However, it may be argued that if histamine and heparin do co-exist in 

 loose ionic linkage by virtue of their opposite electrical charges, it should be 

 possible to separate them by subjecting the combination to a greater potential 

 difference — an electrophoresis in vivo. The following experiment was therefore 

 devised (Fig. 43). 



A fresh loop of rat small intestine with its attached mesentery is spread on 

 a warm microscope slide and one of the avascular interspaces (a peritoneal 

 'window') is covered with a drop of normal saline at body temperature. Two 

 platinum wire electrodes, bent as shown, are placed about 1 cm. apart on either 

 side of a suitable microscope field containing numerous mast cells; the electrodes 

 are in turn connected through a switch to a 12- volt accumulator. A fragment 



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