biopsies of the heart can be done, but only for compelling reasons. it carries non-trivial risk to the individual.

untilHellbanned described the better way -- ideally, there would be some sort of biomarker in the blood that serves as a proxy for the level in the heart. But how do you find it and validate it? It's challenging (at best...)

no. more practical to use proxies (this is the hard part) from more readily available tissues like the blood.