Abstract
1. The effects of alternating current on pace-maker activity have been studied. in the isolated toad sinus venosus. The alternating current produced an immediate action followed by an after-inhibition and an after-augmentation.
2. The immediate action of stimulation by voltages less than 1.7V/cm was an acceleration of the rhythm and a depression of the contractile tension, while voltages of 1.7-5.0V/cm caused an irregularity in rhythm and tension. The voltage range in which the irregularity was obtained was lowered by acetylcholine and elevated by atropine.
3. A further increase in stimulation voltage produced, during current flow, a sustained contraction which increased linearly with voltage increase up to 150% of the peak tension of normal phasic contraction. The sustained contraction appeared even after the elimination of membrane excitability by procaine or by complete lack of external sodium ions, and hence it was thought to be a direct response of the intracellular elements.
4. The first after-effect was an inhibition of both pace-maker rhythm and contractile tension, which appeared prominent at higher voltage of stimulation. The inhibitory effect was strengthened by both acetylcholine and physostigmine, and blocked dramatically by atropine.
5. The second after-effect was an acceleration of the pace-maker rhythm and a potentiation of contractile tension. The potentiation appeared frequently separated into three phases, the fast, middle and slow phases. Dichloroisoproterenol and reserpine blocked the acceleration and eliminated the middle and slow potentiations. Cocain, on the other hand, abolished the fast and middle potentiations after a temporal enhancement of all phases.
6. The after-effects of mass stimulation, therefore, are attributable mainly to cholinergic and adrenergic transmitter actions, and it is postulated that three phases of the after-potentiation relate closely to the liberation of adrenergic transmitters from the different storage compartments.
