PDE5 is commonly found in smooth muscle cells, cardiac cells, and skeletal muscle. Its release in serum must be related to cellular damage to those tissues in the patient. For example, in cases of AMI, creatine kinase-MB and troponin T and I are released in serum in few hours. I am not 100% sure about PDE5's release in serum and what tissue it is coming from. That can be found out based on patient's disease pathology. 

Thanks for the insight Tushar!.. I really like the idea that it is released due to cellular damage or stress. In my research that makes sense since there is significantly more PDE5 present in the sera of my diseased patients compared to normal control patient sera... However, I haven't found a lot of literature on the role of circulating PDEs? Why are they released into circulation? And from what tissues? And what are they doing?... I have seen only a couple of papers where PDE activity in sera was measured, but I think it was PDE3 and PDE1 not PDE5...  I also have not seen that they are a known biomarker for cardiac stress/disease (such as TnT, BNP, etc).....Maybe I have come across something interesting to look into :) 

If you find cardiac specific PDEs biomarker, that will be Nature paper for you!

I am wondering about the same Qs you have mentioned. PDEs biomarker role will be very interesting to look into. I am little worried about it, given the fact that PDEs are fond in all muscle groups, it would little difficult to specify which exact muscle group PDEs are coming out from? There might be the entirely new role of circulating PDEs which we don't know yet.

All I can say is best of luck, may the force be with you! :)