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HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



heart, that an opposing phenomenon frequently 

 operated simultaneously. This was first described by 

 Woodworth (340) in experiments on dog heart 

 muscle. He stimulated the muscle at a regular fast 

 frequency until a steady twitch tension was obtained. 

 When he shifted to a slow frequency the first one or 

 two twitches were greater than the previous ones, 

 rather than smaller as would have been expected on 

 the basis of the Bowditch staircase, and then progres- 

 sively declined as expected to a level characteristic 

 of a slow rate (fig. 12C). [This phenomenon is some- 

 times referred to as poststimulation potentiation 

 (252).] The increased amplitude of the first twitches 

 at the slow rate led him to believe that the twitch 

 tension was the result of two effects, "the stimulating 

 effect of a rapid succession of contractions and the 

 recuperative effect of a long pause." In certain 

 animals, such as the rat, the opposing phenomenon is 

 so dominant that even the steady state level of twitch 

 tension decreases with increasing frequency and vice 

 versa (14, 148, fig. 12D). Of the names used to refer 

 to this opposing phenomenon "reverse staircase" 

 will be employed in this chapter. The relative con- 

 tribution of reverse staircase and Bowditch staircase 

 for anv one species may change over tlie observed 

 frequency range (180). For example, even in the 

 case of the rat for \vhich the reverse staircase has 

 been so frequently described, the Bowditch staircase 

 becomes dominant at higher frequencies (unpublished 

 observations). The so-called "rest contraction" 

 (252) is another example of the reverse staircase 

 phenomenon (fig. i2£'), since the interval of rest can 

 be regarded as a temporary siiift to a lower frequency 

 which causes an increase in twitch tension. To this 

 already long list of phenomena must be added still 

 another one, first observed by Langendorff in 1885 

 (cf. 340), namely the effect of an extrasystole in 

 causing an increased amplitude of the following 

 contraction (fig. i'2F). These various force-frequency 

 relationships are discussed below. 



The Bowditch Staircase 



The central fact about the Bowditch staircase is 

 that for any frequency of stimulation there is a 

 characteristic twitch tension. Perturbations cau.sed 

 by extrasystoles or by shifts in the regular frequency 

 are followed, on the return to the control frequency, 

 by a return to the control twitch tension. The Bow- 

 ditch staircase in the frog heart has been studied In 

 Hajdu (110). He found that, when a previously 

 nonbeating heart was stimulated electricalh-, \\\xh 



FIG. 13. Correlation between tension and loss of internal 

 potassium. [From Hajdu (110).] 



the progressive rise in twitch tension which occurred 

 there was a progressive net potassium loss from the 

 muscle. This can be expressed in the diagram of 

 figure 13, which shows that over the range of i to 3 

 meq of potassium per liter of fiber water lost from a 

 muscle cell, the twitch tension rises from a very low 

 level to a maximum value. The higher the frequency 

 of stimulation the greater the loss of internal potas- 

 sium, but losses beyond 3 meq of potassium per liter 

 of fiber water were not associated with any further 

 increase in twitch tension. In summary, over the 

 rising portion of the curve shown in figure 13, for any 

 stimulation frequenc\' there is a unique value for 

 both twitch tension and cellular potassium content. 

 Over tlie flat portion of the ciuve there is no further 

 increase in twitch tension, but there continues to be 

 a correlation between stimulation frequency and 

 cellular potassium content. 



In an attempt to account for these findings the 

 following reasoning is presented. In the resting muscle 

 at the steady state, potassium influx and efflux are 

 of course equal. When the muscle is stimulated there 

 is a rise in potassium efflux, which probably occurs in 

 temporal relation to the repolarization phase of the 

 action potential (see section 11). Therefore, in the 

 shift from rest to activity, or in a shift from a lower to 

 a higher rate of stimulation, the increased potassium 

 efflux would lead to a progressive net loss of potassium 

 from the cell unless the influx was increased to a 

 corresponding level, or unless the efflux subsequently 

 decreased to the original level. One can conclude 

 from the recent work of Rayner & Weatherall (236) 

 on potassium movement in resting and stimulated 

 rabbit auricles, that potassium efflux remains elevated 

 during activity and that therefore a progressive net 

 potassium loss from the cell must be prevented by an 

 increase in the influx. The view may be taken, then, 

 tiiat on stimulation, potassiinn efflux increases, in- 



