I I46 HANDBOOK OF PHYSIOLOGY -~ CIRCULATION II 



300 



fig. 3. Tension changes in carotid artery strips on exposure to different concentrations of .-1, 

 K + and B, Cs + . Concentration of the cation in millimolars appears at the right of each line. [From 

 Laszt (135).] 



on an isolated aorta strip. He pointed out that the 

 addition of Li + relaxed the strip and that Na + pro- 

 duced no obvious change, while NH 4 +, Cs + , Rb + , and 

 K + in that ascending order caused increasing degrees 

 of contraction. He did, however, use fairly extreme 

 quantities, adding in each case 3 ml of an isotonic 

 concentration of the chloride to 10 ml of Tyrode's 

 solution. In effect he raised the K concentration of the 

 medium to about 50 meq per liter. 



Furchgott & Bhadrakom (88), in the course of 

 pioneer studies which established the rabbit aorta 

 strip as a standard test object, observed the effects of 

 more moderate increases in K„ . They noted that 

 contraction followed if the K level was raised to three- 

 or fourfold normal. The addition of smaller amounts 

 of K, sufficient only to double that of the normal 

 medium (Krebs), did not cause visible contraction 

 but did potentiate the response to low doses of epi- 

 nephrine. 



Bohr et a/. (13) confirmed and extended these 

 findings. Using the Furchgott procedure, they noted 

 a marked increase in the response of the aorta strip to 

 norepinephrine when the medium K. was doubled. 

 Additionally, they observed a marked and progressive 

 reduction of responsiveness in a K-free medium. More 

 recently, Barr et al. (10) studied the responsiveness of 

 clog carotid artery strips stored in the cold and then 

 rewarmed. Stored in the cold, 4 C, these strips, like 

 most tissues (165), gained Na, and lost K, , and with 

 this their responsiveness declined. The recovery of 

 maximal responsiveness was roughly parallel to the 

 steady-state K reaccumulation during the progression 

 of rewarming. Williamson & Moore (209) observed 

 that the sensitivity of the rabbit aorta strip varied 



with changes of K in the medium in a manner inverse 

 to Na. Thus, lowering Na or raising K, singly or in 

 combination, increased the sensitivity to norepineph- 

 rine. Conversely, raising Na or lowering K, singly 

 or in combination, decreased the norepinephrine- 

 induced contraction. Dodd & Daniel (49) also pointed 

 out that contractile responses increase in a high K 

 medium. They observed a transient increase of sensi- 

 tivity in a K-free medium as well, but this rapidly 

 yielded to the usual reduction in responsiveness 

 observed by most workers. 



Laszt (133-135) has recently produced unequivocal 

 evidence relating tension and the concentration of K. 

 in the medium (fig. 3). Using a carotid artery strip 

 (specified only as from "cattle") this worker has shown 

 that the stepwise elevation of K„ produces a stepwise 

 increase in tension. The threshold concentration of 

 cesium (5 ran) or rubidium (10 mill, which can simi- 

 larly induce a contraction when added to the medium, 

 is less than that required for potassium (30 mil), and 

 this Laszt considers to be direct evidence relating 

 the effect to ion size. Similarly, the tension induced In- 

 equal concentrations of cesium, rubidium, and potas- 

 sium chloride shows the same dependence on rank 

 order. The interpretation which this investigator 

 places upon these findings argues that the tension 

 increase is a function of the rate at which Cs + , Rb + , 

 or K + can move into cells. This neglects the fact that 

 the ability of ions to cross cell membranes is not 

 an exclusive function of their hydrated radius (see 

 table 3). 



One group of observations stands alone and con- 

 trasts sharply with the examples cited above. In these 

 a reduction of K„ is reported to be associated with 



