Effects of the administration of thyroid hormone on the plasma levels of atrial natriuretic peptides and on atrial myoendocrine cells in the rat: an immunochemical, ultrastructural, and stereological study.

The effects of an altered thyroidal status on the levels of immunoreactive (ir-) atrial natriuretic peptide (ANP) in serum and in the right and left atria, as well as on the subcellular structures of atrial myoendocrine cells were explored in groups of male Sprague Dawley rats given the vehicle or triiodothyronine in the toxicological dose-range (50 micrograms/100 g bw/day) for 0,5, 1,2,4,7 or 14 days. Plasma levels of ir-ANP were 30% higher in T3-treated rats compared with controls at 0,5 and 1 day after hormone administration and then decreased to levels 30-40% lower than controls at days 2 and 4 to rise again above control values on day 7 and 14. Atrial ir-ANP levels decreased at first both in the right and in the left atria with different latencies (1 and 2 days, respectively) and rose back towards control levels by day 4. Changes in the numerical density of specific granules followed a parallel temporal pattern. An increased in the individual volume of the granules followed was also observed. Investigation into the circulatory effects of T3 administration showed that the heart rate was increased by hour 12 after hormone administration (simultaneously with the early rise in plasma ir-ANP levels) and that blood pressure was increased by day 2.(ABSTRACT TRUNCATED AT 250 WORDS)