Ultrasound intensity and contractile characteristics of rat isolated papillary muscle

The effect of graded intensities of continuous wave ultrasound on the contractile performance of isolated papillary muscle of rat was tested. Under isometric conditions rat left ventricular papillary muscles (n = 48) were electrically stimulated to contract at rates of 30, 60, 120 and 240 beats per minute. Muscles were perfused with a Tyrode solution at 30 degrees C under normoxic conditions. Ultrasound at intensities of 0.25, 0.50, 1.0 and 2.0 W/cm2 spatial average temporal average (SATA) at 963 kHz was applied to the muscles while recording muscle contractile characteristics. The analog data were digitized and stored on disk for analysis by computer. This revealed a significant (p less than 0.001) increase in peak developed force (F), peak rate of force development (+dF/dt) and peak rate of myocardial relaxation (-dF/dt) that was linearly related to ultrasound intensity. The muscles were more sensitive to ultrasound at 240 contractions per minute. Resting force was significantly decreased by ultrasound. Although bath temperature increased according to the ultrasound intensity, control studies in papillary muscles (n = 24) on the correlation between contractile parameters and temperature revealed that bulk heating could not account for the positive inotropic action with ultrasound. These data confirm the inotropic effect of continuous wave ultrasound on myocardial tissue and point to the possibility of applying this phenomenon therapeutically.