ioo Spontaneous Secretion of Saliva 



stimuli. This is, however, apparently not a property common to 

 all mucous cells. The mucous cells of the cat's submaxillary gland 

 which are very similar electrophysiologically (Lundberg, 1957) and 

 histologically to those of the sublingual gland, do not secrete 

 spontaneously. Furthermore, some serous cells of salivary glands 

 show a spontaneous secretion, for instance, those of the parotid 

 glands of ruminants, whereas others do not. Examples of spon- 

 taneous secretion can in fact be found in different species in all 

 types of salivary glands, as exemplified above. 



When a preparation of the cat or dog submaxillary and sublin- 

 gual glands is perfused with eserinized plasma, small amounts of 

 acetylcholine can be detected in the venous effluent even when 

 the parasympathetic nerve is not stimulated (Emmelin and Muren, 

 1950). This is the case even when the gland has been acutely 

 denervated and the perfusion fluid contains curarine (Fig. 3.2). 

 It might be suggested that the "spontaneous" secretion is due to 

 acetylcholine continuously released locally in the sublingual gland. 

 Such a mechanism, similar to that responsible for the "paroxys- 

 mal secretion" described in Chapter VI, does not seem likely, 

 however, since the spontaneous secretion is not abolished by 

 atropine. 



In order to support the view that the continuous sublingual flow 

 of the cat is a true spontaneous secretion, it may be pointed out 

 that this secretion differs in several respects from that caused by 

 agents acting on the secretory cell via some receptor mechanism 

 (Emmelin, 1953). In the isolated sublingual gland it continues even 

 when the gland has ceased to respond to secretory agents added to 

 the bath. A further difference may be illustrated in the following 

 way. If a continuous secretion is brought about in the submaxillary 

 gland by injection of pilocarpine or ephedrine, the threshold dose 

 of some other agent such as adrenaline is found to be very much 

 lowered and the gland is extremely sensitive to this agent. The 

 spontaneous sublingual secretion apparently lacks this effect, for 

 the threshold dose of adrenaline, required to accelerate the flow, 

 is high and of the same order of magnitude as that of a normal 

 submaxillary gland. Further, more or less continuous activity in 

 the submaxillary gland, caused by repeated injections of pilocar- 

 pine, prevents the development of a supersensitivity to chemical 

 agents which normally follows denervation ; the spontaneous sub- 

 lingual secretion, on the other hand, has not this effect. These 



