EFFECTS OF STIMULATION AND DIPHENYLHYDANTOIN ON THE INTRACELLULAR SODIUM ACTIVITY IN PURKINJE-FIBERS OF SHEEP HEART

The effects of stimulus frequency on the intracellular Na+ activity .**GRAPHIC**. of Purkinje fibers from sheep hearts were investigated using ion-sensitive micro-electrodes. .**GRAPHIC**. increased by .apprx. 44% at 3 Hz and .apprx. 6.5% at 0.2 Hz. The size of the increase was very variable between preparations and did not appear to be related to the initial level of .**GRAPHIC**. before stimulation. During stimulation, the rise of .**GRAPHIC**. was exponential and gave a new steady-state level within about 15 1in. The time constant of this rise was independent of the frequency of stimulation. The recovery of .**GRAPHIC**. following a period of stimulation also appeared to be exponential but the time constant was dependent upon the extent to which .**GRAPHIC**. had been raised during stimulation, with shorter time constants following larger rises. The effects of stimulation on .**GRAPHIC**. were compared to inhibition of the Na+-K+ pump using high concentrations of cardioactive steroids. The influence of diphenylhydantoin (DPH) on the Na+ permeability of the cell membrane was assessed by measuring the rate of rise of .**GRAPHIC**. following Na+-K+-pump inhibition. DPH decreased the rate of rise of .**GRAPHIC**. by .apprx. 37% at a concentration of 10-4 M. The decrease of Na+ permeability produced by DPH is probably responsible for the observed reduction of .**GRAPHIC**. in quiescent preparations. DPH also produced substantial decreases of .**GRAPHIC**. in stimulated preparations. These results are discussed in relation to its known antiarrhythmic effects.