This paper demonstrates the first measurement of the acoustoelectric (AE) interaction constant in cardiac tissue. Radiofrequency catheter ablation is performed in clinics as a standard treatment for cardiac arrhythmia with a high success rate. The procedure requires a detailed map of the heart's activation wave prior to treatment. Conventional electrical mapping techniques are slow, prone to registration errors and have limited spatial resolution. We have developed Ultrasound Current Source Density Imaging (UCSDI) as a new modality to map reentry currents in the heart. UCSDI is based on the AE effect and Ohm's Law. The AE effect states that ultrasound pressure can be converted to a change in resistivity. The conversion efficiency is determined by the AE interaction constant K, a fundamental property of all materials; it directly affects the magnitude of the detected signals in UCSDI. In this study, K was measured in rabbit heart tissue, NaCl and CuSO4 solution with UCSDI. A custom chamber was fabricated to control the geometry for estimating K. A 1 MHz transducer was pulsed at 200 Hz to induce a local and transient modulation of resistivity. K was calculated to be 0.0430.013%/MPa in the heart based on the AE signal recorded with UCSDI. The value of K was in range of 0.9% NaCl. This provides a baseline estimate of K for mapping reentry currents in the heart with UCSDI.