552 ■ Comparative Animal Physiology 



(or sympathin) have not been much studied. Such substances as tyramine, 

 sparteine, and various animal toxins have been insufficiently studied and may 

 be important as naturally occurring regulating agents. 



Effects of Salts on Hearts. Hearts continue to beat for many hours outside 

 the animal body in a solution of properly balanced salts. What specific anions 

 are in the solution seems to matter little unless they are toxic, and most hearts 

 beat actively in a neutral mixture of the chlorides of sodium, potassium, and 

 calcium (see Table 15, Chapter 3); a few hearts, particularly those of some 

 marine molluscs, require magnesium in addition. Deviations from the opti- 

 mum ratios of these cations have been used in analysis of the pacemaker, 

 and conductile and contractile processes. 



The effects of any single element are multiple, depending on the condi- 

 tion and previous history of the heart. The effects of an ion also vary with 

 the concentrations of other ions, e.g., calcium antagonizes some effects of 

 sodium, and potassium antagonizes some sodium action. Ionic ratios such as 

 (PsJa+ -\- K+)/Ca++ may be more significant than absolute concentrations 

 of each ion. The effects of excess or deficiency of an ion in vivo may differ 

 from the effects of such variations in vitro.'-*^ The cellular bases of cardiac 

 action of ions are not clear, but the gross effects show interesting comparative 

 differences. 



Table 69 summarizes in simplified manner some of the reports of salt ef- 

 fects on perfused hearts. 



Sodium chloride comprises the bulk of the solute in all body fluids and in 

 physiological salt solutions. Effects of a deficit in Na+ can be ascertained 

 only by maintaining the osmotic concentration of a solution by adding a non- 

 electrolyte such as a sugar; effects of excess Na+ are difficult to separate from 

 osmotic effects, and in pure NaCl solution there is a deficiency of K+ and 

 Ca+ + . Table 69 shows that in general sodium has a stimulating action on 

 heart pacemakers and favors contraction of heart muscle. Ringer found in 

 1882 that pure sodium chloride solution will not maintain the beating of 

 the frog heart; hearts of invertebrate animals also cease to beat in sodium 

 chloride solution. Sodium initiates a fast but irregular rhythm, and when 

 sugar is substituted for sodium the rate of most hearts declines. 



Potassium has been said to act oppositely on vertebrate and invertebrate 

 hearts; the explanation is that it may act predominantly on pacemaker, con- 

 ducting, or contracting mechanisms. In the perfused vertebrate heart, potas- 

 sium has a striking depressant action on the contracting and conducting sys- 

 tems. In excess potassium the heart relaxes and stops in diastole. Molluscan 

 hearts are not very sensitive to changes in potassium but may be slightly ac- 

 celerated by increase, and great excesses (6 to 7 times) may stop the heart in 

 systole; there is no such effect on contraction and conduction as in vertebrates. 

 In Pecten K+ seems not essential for maintaining the heart beat, and in Helix 

 an apparent slowing is really the result of dropping alternate beats.-"* In ar- 

 thropods also the pacemaker is stimulated (except in Camhariis'); the beat 

 may become fast and weak. The low-potassium effect in Limuliis is continu- 

 ous, high-frequency discharge of many neurones in the pacemaker, and in- 

 terruption of the normal rhythm; in the absence of K+ the ganglionic dis- 



