The effect of altered thyroidal status on levels of immunoreactive (ir)- atrial natriuretic
peptide (ANP) in serum and the four cardiac chambers, and of tissue ANP mRNA, was
determined in groups of rats given vehicle, thyroxine (T4), propylthiouracil (PTU)
or T4 plus PTU for 3 weeks. Serum levels of ir-ANP were approximately 3-fold higher
in T4-treated animals compared with control; levels in PTU or PTU/T4 groups were not
different from control. Right ventricular ANP mRNA was below detection; in other chamber,
levels rose with T4, alone or plus PTU, and fell after PTU compared with control.
Atrial ir-ANP levels were unchanged by T4, but increased (left atrium, LA) or decreased
(right atrium, RA) after PTU alone. After PTU/T4, some indices (e.g. tissue weight)
remained at control levels, others (e.g. ANP mRNA levels) were equivalent to levels
in the T4-alone group, and others (e.g. LA ir-ANP) were equivalent to those seen with
PTU alone. We conclude that the role of thyroid hormones on ANP synthesis may be similar
between chambers but their effects on release appear to differ widely. The extent
to which this represents secondary rather than direct effects, or possible T3-versus
T4-specific events, awaits elucidation.