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Reduced oxygen supply explains the negative force-frequency relation and the positive inotropic effect of adenosine in buffer-perfused hearts

Department of Internal Medicine, School of Medicine, State University of Campinas, Campinas SP, Brazil

Submitted 16 September 2003 ; accepted in final form 16 November 2004

In isolated rat hearts perfused with HEPES and red blood cell-enriched buffers, we examined changes in left ventricular pressure induced by increases in heart rate or infusion of adenosine to investigate whether the negative force-frequency relation and the positive inotropic effect of adenosine are related to an inadequate oxygen supply provided by crystalloid perfusates. Hearts perfused with HEPES buffer at a constant flow demonstrated a negative force-frequency relation, whereas hearts perfused with red blood cell-enriched buffer exhibited a positive force-frequency relation. In contrast, HEPES buffer-perfused hearts showed a concentration-dependent increase in left ventricular systolic pressure [EC₅₀ = 7.0 ± 1.2 nM, maximal effect (E_max) = 104 ± 2 and 84 ± 2 mmHg at 0.1 µM and baseline, respectively] in response to adenosine, whereas hearts perfused with red blood cell-enriched buffer showed no change in left ventricular pressure. The positive inotropic effect of adenosine correlated with the simultaneous reduction in heart rate (r = 0.67, P < 0.01; EC₅₀ = 3.8 ± 1.4 nM, baseline 228 ± 21 beats/min to a minimum of 183 ± 22 beats/min at 0.1 µM) and was abolished in isolated hearts paced to suppress the adenosine-induced bradycardia. In conclusion, these results indicate that the negative force-frequency relation and the positive inotropic effect of adenosine in the isolated rat heart are related to myocardial hypoxia, rather than functional peculiarities of the rat heart.

hypoxia; myocardium; myocardial contractility