Melatonin did not significantly change heart rate in this study:
http://jap.physiology.org/cgi/content/abstract/91/3/1214
 
Mentioned because there was the earlier assumption elsewhere that
melatonin would slow the heart rate.
 
(There are a lot of studies on melatonin and the heart health and so
far I not encountered any negative ones.  Lupron is another matter on
that score so when in doubt it may pay to check heart health before
using Lupron.)
 
BTW, enlargement of the heart happens slowly over time in response to
heart disease, and is not a suddenly acquired condition.
 
-- Sukie (not a vet)
Ferret Health List co-moderator
http://www.smartgroups.com/groups/ferrethealth
FHL Archives fan
http://ferrethealth.org/archive/
replacing
http://fhl.sonic-weasel.org
International Ferret Congress advisor
http://www.ferretcongress.org
[Posted in FML issue 5033]