176 



HANDBOOK OF PHYSIOLOGY 



CIRCULATION I 



90/m,n 120/min 

 ..i I * 



TIME (SEC) 



2 SEC 



FIG. 14. Recovery of cardiac contractility following stimulation. Three recovery curves are shown 

 at basal stimulation frequencies of 60, 90, and 1 20 per min. See text for further description. [After 

 Braveny & Kruta (31).] 



FIG. 1 5. Superimposed records of isometric contraction curves of frog ventricle at different basal 

 stimulation frequencies, demonstrating the similarity of the rising phase of the four curves. [After 

 Niedergerke (218a).] 



Bowditch staircase will cause significant changes in 

 the maximum twitch tension). Referring again to 

 figure 14, it can be seen that for a heart beating at 60 

 per min (i sec interval between stimuli), each stimulus 

 interrupts the rising recovery curve of contractility, 

 the amplitude of contraction being indicated by the 

 dotted line in the figure. It is clear then that a pro- 

 longation of the interval between stimuli will find 

 the muscle fiber more fully recovered, and the twitch 

 tension will be correspondingly higher. The curve of 

 recovery of contractility is different in different ani- 

 mals, being greatly prolonged for example in the rat 

 heart (148, 180). The reverse staircase of rat heart 

 becomes less prominent after bathing in Krebs solu- 

 tion for long periods of time, and it is usually not 

 obvious in frog or guinea pig heart (unpublished ob- 

 servations). Many factors can change the rate of re- 

 covery of contractility. For example, the reverse 

 staircase is intensified (recovery prolonged) in hypoxic 

 frog heart (218a) and in rabbit atrium treated with 

 fluoroacetate (169). But the basic phenomenon of 

 interest, the slow recovery of contractility in cardiac 

 muscle, occurs under conditions in which hypoxia or 

 fatigue can hardly be invoked as explanation. Figure 

 15 shows contraction-relaxation curves in a range 

 where the reverse staircase is prominent. It can be 

 seen that the initial rate of rise of tension appears 

 similar at difTerent frequencies, and the smaller peak 

 tension at the higher frequencies appears to occur be- 

 cause relaxation sets in earlier (218a). 



None of the published studies on the reverse stair- 

 case provides any clue about the cause of the phe- 

 nomenon. Speculations have ranged from the idea 

 that during rest time is provided for certain metabolic 



reactions to achieve cellular recovery (168, 169, 340) 

 to the postulation of a potentiating substance (252). 

 Recently it has been suggested that the increased 

 contractile force observed after a rest is correlated 

 with the ability of the cell to maintain a low intra- 

 cellular sodium at low stimulation frequencies (176a). 

 It is possible that just as the increased tension of the 

 Bowditch staircase depends on the high frequency loss 

 of intracellular potassium, the increased tension of the 

 reverse staircase depends on a loss of intracellular 

 sodium due to increased extrusion at low frequencies. 



Postextrasystolic PotnUiation 



An example of postextrasystolic potentiation is 

 shown in figure 1 2F. Woodworth, exploring the earlier 

 findings of Langendorff found that two or more extra- 

 systoles had a greater stimulating effect than one, 

 that the effect persisted on the average for about eight 

 subsequent beats, and that the earlier in the contrac- 

 tion cycle the extrasystole occurred the greater was 

 the potentiating effect. These observations were later 

 confirmed by others (45, 46, 84, 147, 206, 276). 



Postextrasystolic potentiation has been thought by 

 many (45, 46, 231, 263) to be an example of the 

 Bowditch staircase. An analysis of the problem can be 

 facilitated by reference to an experiment on cat papil- 

 lary muscle, published by Hofmann et al. (147), which 

 is similar to an earlier one of Garb & Penna (84). 

 Figure 16 is a schematic diagram to show the essential 

 part of the experiment. It represents a continuous 

 tracing of contractions induced by stimulation at an 

 average frequency of i per sec. Whereas the stimula- 

 tion was regular in the first and last part of the tracing. 



