The effect of chronic bradycardial pacing on the oxidative capacity in rabbit hearts

Blood flow and oxygen consumption were estimated in isolated hearts from control rabbits or animals with chronic bradycardia achieved by transvenous atrial pacing for 4 weeks; these parameters were related to ultrastructural changes. Chronic reduction of heart frequency to about 50% of control values resulted in an increased capillary/fibre ratio (1.43 +/- 0.07 in paced, 1.26 +/- 0.07 in control hearts), increased capillary density (2339 +/- 148 vs 1897 +/- 95) and increased volume density of mitochondria (36.36 +/- 1.28% vs 31.38 +/- 1.7%), in paced and control hearts, respectively, with no signs of heart hypertrophy (fibre diameters 28.5 micron in both groups). Maximal blood flow (3 ml.g-1.min-1) was similar in paced and control hearts. Maximal oxygen consumption (achieved by a gradual increase in preload and increase in force of contraction by noradrenalin infusion) in paced hearts was significantly higher than in control hearts and was achieved by a higher oxygen extraction. This could be due both to the increased volume density of mitochondria and to a more homogeneous distribution of flow through an enlarged capillary bed. The increased oxidative capacity induced by chronic bradycardial pacing can explain improved maximal cardiac work found under these conditions previously in vivo.