118 



O. F. HUTTER 



30 



60 



90 



Ti me C min) 



Fig. 2. Effect of acetylcholine on rate of loss of ^'"K from a quiescent tortoise 

 sinus venosus. Abscissa, time from beginning of efflux. Ordinate, fraction of 

 labelled K in tissue lost per min. The wash solution contained 21-6mM K 

 throughout the experiment and acetylcholine 2 .< 10"^ g/ml between 45-65 min 



control periods. When longer load periods were used the exchange of potas- 

 sium went further towards completion and the increase in the amount of 

 42K taken up in the presence of acetylcholine was then followed by a series 

 of diminishing increments in radioactivity as is to be expected in the later 

 stages of a load experiment. 



As an alternative procedure the sinus venosus of a tortoise was bisected 

 and the two halves used to follow the uptake of -I'-K from tracer-Ringer with 

 or without acetylchohne. This has the advantage that an inflow curve smooth- 

 ing out of the error of each assay may be constructed for each preparation, 

 but it involves assumptions about the relative sensitivity and composition 

 of the two halves and uncertainty about how persistent the action of acetyl- 

 choline is. In the event an increase in the rate of uptake was usually observed 

 also under these conditions, but the effect was striking only when the extra- 

 cellular potassium concentration was high (20 mM). How far the influx of 

 ^■-K from different external potassium concentrations is dependent on metabolic 

 activity has not yet been determined. 



