Cardiac hypertrophy in response to an isometric training program in the cat

Cats were operantly conditioned to perform isometric exercise to determine the effect of this type of training program on heart morphology. The hearts of 11 cats trained for 2-9 months were removed and weighed. Heart weight-to-body weight ratios were increased significantly in the exercised animals over 15 controls (3.86 vs. 2.91 g/kg, P < 0.001). Body weights were not significantly different between the two groups. All chambers of the heart were larger in the exercising animals, compared to controls, as evidenced by a significant increase in both left (2.58 vs. 1.95 g/kg, P < 0.001) and right ventricle weight-to-body weight ratios (0.77 vs. 0.58 g/kg, P < 0.002), and increased atrial weight-to-body weight ratios (0.51 vs. 0.37 g/kg, P < 0.01). The same conclusions were reached when the heart weights were not normalized to body weight. To study the progression of cardiac hypertrophy throughout the exercise regimen, we implanted radioopaque markers in the left ventricle of cats and monitored wall thickness on a monthly basis in these animals, using biplane cinefluorography. Eleven cats implanted with markers were trained for 1-6 months. Left ventricular wall thickness increased in the trained animals after 1 month (13.15%) and continued to increase until, at 6 months, wall thickness was increased 32.5% from the initial measurement. The increase in wall thickness for the exercise period was correlated with the amount of isometric work that the cat performed, both total isometric work (r = 0.71, P < 0.02) and the mean daily isometric work (r = 0.76, P < 0.01). Males also exhibited larger increases in wall thickness than females (29.7% vs. 2.4%, P < 0.05). In addition, muscle fiber diameter was significantly larger in exercised animals than in controls (19.68 vs. 13.35 μm, P < 0.001). There was more variability in fiber size in the exercised animals, indicating that all fibers did not increase to the same extent.