In Vivo Assessment of Resting Myocardial Function in High and Low Performing Genetic Models of Aerobic Running Capacity

Abstract

BACKGROUND: We previously identified two inbred rat strains divergent for treadmill aerobic running capacity (ARC); the low performing Buffalo (BUF) and the high performing DA rats. Subsequent cardiac performance assessment using the Langendorff-Neely working heart preparation demonstrated that isolated DA hearts exhibit increased stroke volume and cardiac output compared to the BUF hearts, with no difference in intrinsic heart rate (HR). In the present study, we investigated in vivo resting cardiac function in BUF and DA rats using echocardiography. METHODS: Male DA (n=11) and BUF (n=12) rats (aged 14-17 weeks) were lightly anesthetized with pentobarbital (40mg/kg, ip), the chest was shaved, and the animal was situated in the supine position on a warming pad. Two-dimensional guided M-mode studies were performed from the parasternal long axis for the determination of left ventricular end-diastolic (LVDd) and end-systolic (LVSd) diameters and fractional shortening (FS). The myocardial performance index (MPI), stroke volume (SV) and cardiac output (CO) were calculated using Doppler pulsed-wave traces of mitral and aortic flow measured at the level of the LV outflow tract from the apical four-chamber view. Cardiac index (CI) was calculated by dividing CO by body weight. RESULTS: BUF rats had significantly greater body weight and a larger SV than the DA rats, but had significantly lower HR and CI (Table). No differences in LVDd, LVSd, FS, MPI, or CO were observed (Table).TableCONCLUSION: After correcting for differences in HR and body weight, in vivo assessment of resting cardiac function revealed that DA rats exhibit greater cardiac performance than BUF rats. Our findings contrast with those of the working heart preparation in that in vivo assessment of cardiac function shows DA rats to have a smaller stroke volume and greater HR than the BUF rats, resulting in an identical CO between the two strains. Nevertheless, cardiac index is significantly greater in DA rats, possibly contributing to the superior ARC observed in this rat strain compared to that of the BUF strain.