Hi I do not use jabref, however in bibtex you may use strings{} for this. The best way is to use a shorthand form of the journal when you enter the citataion (like AJP) and then make a new bibfile with: @string{AJP = "Am. J. Physiol."} and call that bibfile before your bibfile with citations. in LaTeX: \bibliography{abbreviations,citations}
You may make several @strings to format into "Am. J. Physiol." if your citation.bib has many different formats for each journal. Ingar