EFFECTS OF EXTRACTS OF PARS NEURALIS 



venously — especially if the doses are large or if they are given 

 too frequently — a "tolerance" appears so that successive 

 doses cause less pronounced effects and finally almost no 

 change (tachyphylaxis). However, some extracts, if injected 

 repeatedly, may later cause a depressor effect. This "inver- 

 sion" phenomenon appears to be due not to the vasopressor 

 principle but to depressor substances included in the ex- 

 tracts.' 



A typical tracing of the effects of the vasopressor principle 

 on the circulatory system varies considerably depending upon 

 the experimental conditions. The one selected for reproduc- 

 tion here (Fig. 54) illustrates the effect of an intravenous in- 

 jection of the vasopressor principle on the blood pressure, 

 heart-rate, and respiratory movements of an unanesthetized 

 dog.3 A preliminary rise in the blood pressure is followed by 

 a marked fall which lasts approximately 30 seconds; the 

 secondary rise which then appears (225 mm. Hg in compari- 

 son with 170 mm. Hg in the control period) persists through- 

 out the remainder of the tracing (longer than 8 minutes). 

 The primary fall in blood pressure is probably due to a 

 marked diminution in the volume of blood pumped by the 

 heart in each unit of time. As the minute-volume output in- 

 creases in the face of a peripheral vasoconstriction, the blood 

 pressure rises to a maximum of 225 mm. Hg. 



Both the early and all the later investigations of the effects 

 of posterior-lobe extract or of the vasopressor hormone on 

 the heart indicate that the marked transient impairment of 

 the heart's efficiency is due to a constriction of the coronary 

 arteries producing cardiac dilatation and even signs of as- 

 phyxia of the cardiac musculature. Anesthetics like "Chlore- 



^ References to only a few reports dealing with this controversy need be given: 

 Hogben and Schlapp (1924); Ceiling and Campbell (1926); Vincent and Curtis 

 (1926); and Stehle (1929). 



3 For experiments in man see Rosenow (1920); Sacks (1924); Csepai and Weiss 

 (i926);P6gany and Pinter-Kovats (i927);Hartl (1933); Moffat (1933); and Gonczy 

 and Kiss (1934). 



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