Circulation of Body Fluids 561 



molluscs He/fx,'"^ Venus,^^^' and AriolhnaxJ'^' Potentiation of the acceler- 

 ating effect of ACh occurs in the arthropods Astacus, ^^~- ■'•' Panidirus,^^''^ 

 Homarus,-^^ Limulus cardiac ganglion,**^ Cancer,^-^ the insect Stenopehna- 

 tus,^'^ and in the annelids Arenicola and Lumhricus.^^'^ The sensitivity of 

 hearts to acetylcholine is very great. A frog's heart (eserinized) is depressed by 

 1 part in 10^, a lobster's heart is accelerated by 1 part in 10**, and when eserin- 

 ized by 1 part in 10'-*; '^^ a Venus heart is sensitive to ACh 1 part in 10'", and at 

 some seasons to 1 part in 10'- (Fig. 208). '"'*'• ^'■"' 



Atropine antagonizes the action of ACh and vagal inhibition in the ver- 

 tebrate heart, not by preventing the hberation of ACh but by blocking its ac- 

 tion on the receiving cells. Similarly atropine antagonizes the acceleration of 

 ACh in the arthropods Astacus,^'-^- ^^ Melanoplus,^'^ Panulirus,-^'-^ Homa- 

 rus,-^"^ and Cancer.^^ Atropine has a stimulating effect on the Limulus heart 

 and may not antagonize ACh.^' In molluscs, however, atropine is toxic; its 

 effects are variable, and no antagonism of ACh has been found in Aplysia,^^'' 

 Osirea,^^^ Venus,^^*^ Sepia,^^^ Loligo,^^ Helix, ^'^^ and Ariolimax,^^ and no 

 antagonism of muscarine in Anodonta.^'^^ The receptor cells in which acetyl- 

 choline is blocked by atropine appear to be nerve cells in vertebrate hearts 

 (vagus secondary neurones) and in arthropods (pacemaker ganglia) whereas 

 in molluscs the site of ACh action is muscular. 



Pilocarpine depresses the vertebrate heart; it is also inhibitory to the hearts 

 of Anodonta^*^'^ and Anomia.^*''' Pilocarpine accelerates the hearts of Cancer, 

 Maja, and Carcinus,^-^ but has been reported as inhibitory in Limulus.^^ 



In the vertebrate heart acetylcholine resembles the drug muscarine in its 

 inhibition of the heart and its atropine antagonism. Muscarine also inhibits 

 hearts of the molluscs Awodonta,'^'* Mya (after initial stimulation),^" and 

 Octopus.^^^ The heart of Cancer is accelerated by muscarine,''-^ but the 

 Limidus heart has been reported to be inhibited. ^^^ 



Nicotine initially slows and may stop the vertebrate heart, and later and 

 in high concentration it accelerates and blocks the va^us. Inhibition of mol- 

 luscan hearts, much like ACh inhibition, has been reported for Anodonta,^^^ 

 Sepia,'^'^^ Anomia,^^^ Venus,^^*^ and Ostrea}'^'-^ 



In arthropod hearts, nicotine initially accelerates in Hoviarus,^^^ Cancer,^^ 

 Astacus,^^ Periplaneta,^^^ Melanophis,^'^ and Limidiis:^'^ After initial stimu- 

 lation nicotine has a paralyzing effect (Limidus, Periplaneta, Melanaplus'), 

 which has been seen in Limidus to consist of a blocking of the pacemaker 

 ganglion.'^'''* 



In its action on hearts of invertebrates acetylcholine cannot be character- 

 ized as being like muscarine or nicotine. 



Adrenin is a natural cardioaccelerator of the vertebrate heart. Adrenalin 

 is an accelerator of most other hearts: molluscs Aplysia,''- Loligo,^^ fresh- 

 water mussels,^-'-' Ostrea,^^''- '*^*"'' ^■'" Pecten,-''^ Anomia,^^^ and Venus,'^^^ and 

 arthropods Limulus,^^ Astacus,^'^ Carcinus, and Maja,^'-^' -"- Panulirus,'^'^^ and 

 Homarus,-'^^ and annelids Lumhricus and the leech. ^^ Slight species differ- 

 ences exist in the effects on tone and amplitude, but in general adrenalin ac- 

 celerates hearts, whether they are myogenic or neurogenic. Some hearts are 

 unaffected by adrenalin (Ciona,^^ Nereis,^^ and Artemia^^'^ ) . 



The effects on hearts of drugs which potentiate and antagonize adrenin 



