After watching the video (link posted by Buster), it seems there may be a few people here who might want to contact Dr. Blair Grubb or Dr. Satish Raj, who Dr. Grubb mentions in the interview.  

  Dr. Grubb is at the University of Toledo Medical Center in the Cardiology Department.  Dr. Raj practices at Vanderbilt Heart Center in Nashville.  From the interview, it sounded as though Dr. Raj has published the only article on a variant POTS that includes mast cell activation.

MGC posted the following corrections to what I posted above....

  "The paper documenting the coexistence of POTS and Mast Cell problems in some patients was not done with Dr Raj as the lead researcher. The lead author was Dr Cyndya Shibao. Dr Grubb misspoke.  But yes, Dr Shibao is at Vanderbilt, and understands both disorders. She will, however, send patients to Dr Farenholz for the Mast Cell workup.

Link to paper:

http://hyper.ahajournals.org/content/45/3/385.full

Also meant to say that I contacted Dr Raj about who to see there, and he recommended Dr Shibao."

Joan... also meant to say that I contacted Dr Raj about who to see there, and he recommended Dr Shibao.

Interesting! I just saw Dr Fahrenholz on April 5th for a second opinion on the MCAS. he thinks I might also have POTS but he wants to work me down on my heart medication and then send me to the Autonomic Clinic there at Vanderbilt. He didn't mention a specific doctor. I'll keep you all posted when I know more.