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Variability in interbeat duration influences myocardial contractility in rat cardiac trabeculae.

Torres CA, Varian KD, Janssen PM - Open Cardiovasc Med J (2008)

Bottom Line: Fixed rate response was measured before and after each variable period and average force was calculated.We observed no significant change in force at 4 Hz (n=17), and 6 Hz (n=6) between fixed and variable pacing but observed a significant, 10% increase in contractile strength at 8 Hz (from 15.1 to 16.5 mN/mm(2), p<0.05, n=6).Our results show that under certain conditions, by simply introducing variation in the beat-to-beat duration without affecting the number of beats per minute, a positive inotropic effect with corresponding changes in the calcium transients can be generated.

View Article: PubMed Central - PubMed

Affiliation: Department of Physiology and Cell Biology, and Department of Emergency Medicine, 304 Hamilton Hall, 1645 Neil Avenue, The Ohio State University, Columbus, OH, USA.

ABSTRACT
There is an intense search for positive inotropic strategies. It is well known that the interbeat duration is a critical determinant of cardiac contractility. Generally, when frequency increases, so does contractile strength. We hypothesize that the beat-to-beat variability at a given heart rate also modulates cardiac contractility. To test this hypothesis, thin, uniform rat cardiac trabeculae were isolated from the right ventricle and stimulated to isometrically contract, alternating between fixed steady state versus variable inter-beat intervals (same total number of beats in each period). Trabeculae were stimulated at 4 Hz with interbeat variation between 20 and 120% (n=17). In a second series of experiments trabeculae were stimulated at 3 different physiologic frequencies with a 40% interbeat variation. Fixed rate response was measured before and after each variable period and average force was calculated. In order to investigate the mechanism underlying the changes in contractility we used iontophoretically loaded bis-fura-2 salt to monitor intracellular calcium transients. We observed no significant change in force at 4 Hz (n=17), and 6 Hz (n=6) between fixed and variable pacing but observed a significant, 10% increase in contractile strength at 8 Hz (from 15.1 to 16.5 mN/mm(2), p<0.05, n=6). Our results show that under certain conditions, by simply introducing variation in the beat-to-beat duration without affecting the number of beats per minute, a positive inotropic effect with corresponding changes in the calcium transients can be generated.

No MeSH data available.


At a variability of 40%, average active developed force again was not significantly increased at an average cycle time of 4 and 6 Hz however at the 8 Hz, the 40% variability significantly increased the average developed force of the trabeculae, * (P<0.05, n = 7).
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Figure 4: At a variability of 40%, average active developed force again was not significantly increased at an average cycle time of 4 and 6 Hz however at the 8 Hz, the 40% variability significantly increased the average developed force of the trabeculae, * (P<0.05, n = 7).

Mentions: Trabeculae, while exposed to variable beat-to-beat stimulation protocol, showed a more substantial increase in Fdev from an average of 15.1 to 16.5 mN/mm2, an average percentile increase of 10.1% (P<0.05, n = 7, SEM 1.9) at 8 Hz (Fig. 4) when compared to their fixed-rate stimulation averages. This indicates that despite the fact that the number of beats per minute was the same, beat-to-beat variability increased contractility.


Variability in interbeat duration influences myocardial contractility in rat cardiac trabeculae.

Torres CA, Varian KD, Janssen PM - Open Cardiovasc Med J (2008)

At a variability of 40%, average active developed force again was not significantly increased at an average cycle time of 4 and 6 Hz however at the 8 Hz, the 40% variability significantly increased the average developed force of the trabeculae, * (P<0.05, n = 7).
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC2627530&req=5

Figure 4: At a variability of 40%, average active developed force again was not significantly increased at an average cycle time of 4 and 6 Hz however at the 8 Hz, the 40% variability significantly increased the average developed force of the trabeculae, * (P<0.05, n = 7).
Mentions: Trabeculae, while exposed to variable beat-to-beat stimulation protocol, showed a more substantial increase in Fdev from an average of 15.1 to 16.5 mN/mm2, an average percentile increase of 10.1% (P<0.05, n = 7, SEM 1.9) at 8 Hz (Fig. 4) when compared to their fixed-rate stimulation averages. This indicates that despite the fact that the number of beats per minute was the same, beat-to-beat variability increased contractility.

Bottom Line: Fixed rate response was measured before and after each variable period and average force was calculated.We observed no significant change in force at 4 Hz (n=17), and 6 Hz (n=6) between fixed and variable pacing but observed a significant, 10% increase in contractile strength at 8 Hz (from 15.1 to 16.5 mN/mm(2), p<0.05, n=6).Our results show that under certain conditions, by simply introducing variation in the beat-to-beat duration without affecting the number of beats per minute, a positive inotropic effect with corresponding changes in the calcium transients can be generated.

View Article: PubMed Central - PubMed

Affiliation: Department of Physiology and Cell Biology, and Department of Emergency Medicine, 304 Hamilton Hall, 1645 Neil Avenue, The Ohio State University, Columbus, OH, USA.

ABSTRACT
There is an intense search for positive inotropic strategies. It is well known that the interbeat duration is a critical determinant of cardiac contractility. Generally, when frequency increases, so does contractile strength. We hypothesize that the beat-to-beat variability at a given heart rate also modulates cardiac contractility. To test this hypothesis, thin, uniform rat cardiac trabeculae were isolated from the right ventricle and stimulated to isometrically contract, alternating between fixed steady state versus variable inter-beat intervals (same total number of beats in each period). Trabeculae were stimulated at 4 Hz with interbeat variation between 20 and 120% (n=17). In a second series of experiments trabeculae were stimulated at 3 different physiologic frequencies with a 40% interbeat variation. Fixed rate response was measured before and after each variable period and average force was calculated. In order to investigate the mechanism underlying the changes in contractility we used iontophoretically loaded bis-fura-2 salt to monitor intracellular calcium transients. We observed no significant change in force at 4 Hz (n=17), and 6 Hz (n=6) between fixed and variable pacing but observed a significant, 10% increase in contractile strength at 8 Hz (from 15.1 to 16.5 mN/mm(2), p<0.05, n=6). Our results show that under certain conditions, by simply introducing variation in the beat-to-beat duration without affecting the number of beats per minute, a positive inotropic effect with corresponding changes in the calcium transients can be generated.

No MeSH data available.