Have you considered putting hearts into cardioplegic solution used for transportation of donor hearts like University of Wisconsin solution.

for small muscle biopsies we used BIOPS http://bioblast.at/images/3/3c/MiPNet03.02_Chemicals-Media.pdf

half concentration of calcium would be helpful. you can add calcium slowly to bring it up to the desired level.

I agree that a solution like Univ of Wisconsin might work, but you'll need to do more than just submerge the heart in solution, you'll need to perfuse some down the coronaries to get a good myocardial distribution.  It doesn't have to be UW however, you could try something as simple as 25mmol KCL in 1L of 0.9%NaCl.   I don't think using calcium is a good idea, as calcium will arrest the heart in an irreversible hypercontractile state.