CELLULAR ELECTROPHYSIOLOGY OF THE HEART 



259 



/) There is evidence that the rising phase of the car- 

 diac action potential is due to a specific increase in 

 gNa and that there is an activation-inactivation 

 process. 2) The evidence concerning the nature of the 

 repolarization process is presented at some length. 

 j) Excitation-contraction coupling, the passive elec- 

 trical properties of cardiac cells, and the problem of 

 intercellular transmission will be discussed. 



In the absence of voltage-clamping techniques ap- 

 plicaljle to cardiac tissue, intracellular recording has 

 been used to obtain most data on the nature of the 

 cardiac action potential. Nevertheless, prior to the 

 development of intracellular recording techniques in 

 1947 (87) a considerable body of information about the 

 action potentials of heart muscle had been accumu- 

 lated (cf. 13, 105). Certainly overshoot is evident in 

 records oljtained with suction electrodes (cf. 25, 26). 

 It was, however, the advent of the Na+ hypothesis 

 (63) and the possibility of measuring transmembrane 

 potentials in cardiac cells directly and quantitatively 

 with intracellular microelectrodes that led to the 

 recent rapid growth of knowledge of the ionic basis of 

 cardiac excitability. .S. VVeidmann, in particular, has 

 made important contributions using this technique. 



Intracellular Recording 



The first clear evidence of overshoot in squid axons 

 was obtained simultaneously by Curtis & Cole (29) 

 in the United States and Hodgkin & Huxley (56) 

 in England just before World War II. Thev used 

 glass pipettes filled with salt solution and inserted 

 down the axis of the axon in estimating values of the 

 transmembrane resting and action potentials. 



Shortly after the war, Graham & Gerard (50) 

 dexeloped and Ling & Gerard (87) perfected the 

 ultramicroscopic microelectrode or ultramicroelec- 

 trode. It is a tapering hollow glass tube drawn down 

 to a tip diameter of about 0.2 /x and filled with an 

 electrolyte, usually 3 m KC.l (95). This electrode can 

 be inserted transversely through the membrane in 

 such a manner that there is little or no current leak- 

 age around the point of insertion. The "success" of an 

 impalement depends critically on the tip diameter of 

 the electrode. The tip diameter dividing usable from 

 unusable microelectrodes is less than i /x, but depends 

 on the tissue under study. Details of the manufacture 

 and electrical properties of these electrodes can be 

 found in the chapter by Frank (42). 



If such a microelectrode is mounted on a micro- 

 manipulator and advanced until it touches the surface 

 of an excised muscle, a large, steady, potential dif- 



ference suddenly appears, presumably when the elec- 

 trode penetrates a cell membrane. If the membrane 

 is not damaged by the electrode insertion, i.e., if the 

 membrane seals around the electrode, then this po- 

 tential is the transmembrane potential plus an un- 

 avoidable junction potential. Nastuk & Hodgkin (95) 

 showed that some impalements produce little mem- 

 brane damage. By inserting two electrodes at two 

 points in a cell, they found that the entry of the second 

 electrode frequently caused only a small reduction in 

 the voltage recorded at the first electrode. Woodbury 

 & Brady (136) greatly improved the practicability of 

 recording from moving tissues by mounting the tip 

 of a microelectrode on a 25-^ tungsten wire. This tech- 

 nique was used to record from a human heart 

 at surgery (137). 



Considerably more information concerning the 

 electrical properties of the cell membrane can be ob- 

 tained if two electrodes can be inserted into the same 

 cell. The passive electrical properties, the capacity 

 (Cm) and the resistance (Rn,) of i cm- of membrane 

 and the specific resistivity of the internal medium 

 (pi) can be simply measured using this technique. The 

 procedure is to apply a constant current through one 

 electrode and to measure the resulting changes in 

 membrane potential at a point near the current source 

 and at least one other point about a space constant 

 away. Weidmann (126) has used this technique to 

 measure the electrical properties of Purkinje fibers 

 during diastole. He also estimated the changes in 

 membrane slope conductance during repolarization 

 (125) and voltage-clamped a small region of mem- 

 brane in a study of the inactivation of g^;, (127, 128). 



Depolarization 



Since the overshoot of the action potential in squid 

 axons results from a specific increase in gx;,, the find- 

 ing of an overshooting action potential in other tissues 

 is presumptive evidence for the Na+ meclianism. The 

 first intracellular recordings of transmembrane po- 

 tentials from cardiac cells were reported almost simul- 

 taneously by Coraboeuf & Weidmann (22), who 

 worked with Purkinje fibers of dog, and by Woodbury 

 and co-workers (140), who worked with frog ventri- 

 cle. Overshoot was found in both tissues. A year later 

 Draper & Weidmann (35) reported that the over- 

 shoot and maximal rate of rise of the action potential 

 (£d) in mammalian Purkinje tissue were directly de- 

 pendent on [Na+]o. At the same time Cranefield et al. 

 (25) reported that reduction of [Na+]„ reduced the 

 size of the action potential recorded with a suction 



