CARDIAC TRANSMEMBRANE POTENTIALS— HOFFMAN 317 



tion of impulses across tlie specialized junction at the atrio-ventricular node.^^ The 

 change in transmembrane action potentials most directly related to this decrease in 

 conduction velocity has been studied recently in isolated Purkinje fibers. "^ In this 

 tissue it has been shown that a decrease in temperature results in a slower rising 

 velocity of the action potential (Qio=1.7 between 40-25° C). Similar results have 

 been obtained from studies of nerve^* and skeletal muscle.^' This decrease in up- 

 stroke velocity results in slower conduction because, in any given area of the mem- 

 brane, the critical level of membrane potential (threshold potential) is attained less 

 rapidly. Similarly, a smaller area of membrane is depolarized in advance of the 

 propagating action potential. At low^er temperatures, when the resting potential is 

 decreased, the diminished amplitude of the action potential will further decrease the 

 stimulating efficacy of the upstroke and result in greater slowing of impulse propa- 

 gation. In both auricular and ventricular muscle there is also a decrease in the rate 

 of depolarization of the membrane at low temperatures; quantitative data per- 

 mitting comparison with the Purkinje fibers, however, are not available. 



The nature of the changes in A-V conduction resulting from cooling have not 

 been directly studied by the microelectrode technique. However, the early occur- 

 rence of partial block may result from disproportionate changes in the duration of 

 refractoriness in auricular and specialized fibers. Complete block, on the other hand, 

 most likely reflects the inability of auricular muscle to conduct impulses at tempera- 

 tures below 23° C.-^ This failure of conduction in auricular muscle contrasts with 

 both ventricular and specialized fibers. In the former propagation is often main- 

 tained down to temperatures of 12-10° ; in Purkinje fibers spontaneous propagated 

 activity may persist until the temperature has reached similar low levels. 



Effects of pH and Pco,- Heart muscle is reported to be relatively insensitive to 

 changes in pH.^"- ^^ Studies of the transmembrane potentials of single ventricular 

 fibers tend to support this statement. In this work the pH was varied by changing 

 the concentration of bicarbonate buffer in Tyrode's solution aerated with a constant 

 mixture of CO2 and Oa-^^ Between pH values o.f 6.5 to 8.0 the membrane activity 

 of isolated ventricular muscle remains essentially unchanged. There are no signifi- 

 cant alterations in the magnitude of the action potential or resting potential, and 

 only minor changes in duration of the action potential. These changes in duration, 

 decreased by low and increased by high pH, are similar to those produced by al- 

 terations in the concentration of extracellular Ca (see below). 



The effects of changes in the partial pressure of CO2 are more dramatic. In 

 studies of isolated Purkinje fibers the Pco^ was varied by changing the gas mixture 

 employed to aerate a standard Tyrode's solution and transmembrane action poten- 

 tials were recorded in the usual manner.^' A gas mixture of 10 per cent CO2 and 

 90 per cent O^ resulted in marked slowing of spontaneous activity; the use of 

 higher concentrations of COo gave rise to repetitive firing, a decrease in amplitude 

 of both resting and action potential and complete disorganization of activity. Un- 

 fortunately measurements of pH were not made in this study. However, in view of 

 the minor effects of pH discussed above, it is likely that the changes noted are a 

 result of the high Pco^. Although the partial pressure of CO2 required to influence 

 membrane activity in these experiments was quite high it is possibly not outside the 



