Tubular transport processes in proximal tubules of hypothyroid rats. Lack of relationship between thyroidal dependent rise of isotonic fluid reabsorption and Na+−K+-ATPase activity
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Hypothyroid rats reconstituted with 10 μg/kg b.w. per day of tri-iodothironine (T3) for 4 days resulted in normal free T3 and TSH levels. FT3 levels were: 0.53±0.3 pg/ml in hypothyroid rats; 2.78±1.21 pg/ml in hormone reconstituted rats and 2.90±0.90 pg/ml in euthyroid rats. TSH levels were 3,508±513 μg/ml in hypothyroid rats; 1,008±204 μg/ml in reconstituted rats and 270±184 ng/ml in euthyroid rats.
When hypothyroid rats were reconstituted with 50 μg T3/kg b.w. per day, TSH levels were nearly normal after 4 days (1,157±621 ng/ml). However FT3 levels after 1–4 days were always higher than in euthyroid rats.
Hypothyroid rats show a decrease in isotonic fluid reabsorption (Jv) in the proximal tubule (1.50±0.08 versus 4.96±0.23 10−2 nl·mm−1·s−1 in euthyroid animals). 1 day after T3 (10 μg/kg b.w./day) injectionJv was increased significantly to 2.05±0.20 10−2 nl·mm−1·s−1 and continued to increase during 4 days of T3 reconstitution.
When 50 μg T3/kg b.w./day was used,Jv increased to 2.75±0.07 after 1 day and to 3.10±0.42 10−2 nl·mm−1·s−1 after 4 days.Jv was never reaching a value close to that of euthyroid rats because the tubular radius in hypothyroid rats (14.7±1.8 μm) is less than that of euthyroid rats (19.2±0.5 μm). The radius in hypothyroid rats treated with T3 was unchanged over a 4 day course with either high or low doses of T3.
Na+−K+-ATPase activity was found to be 2.91±0.16 μM Pi/h×mg protein in homogenates of kidney cortex from hypothyroid rats. Treatment of hypothyroid rats with 10 μg or 50 μg of T3 resulted in an initial decrease in ATPase activity, followed by an increase to base level in hypothyroid rats with 10 μg and a significantly higher level with 50 μg. This decrease in ATPase activity was contrasted to the increase inJv.
These data indicate that there is a dissociation between the effects of physiological doses of thyroid hormones on proximal tubular reabsorption and the effects of T3 on Na+−K+-ATPase activity of kidney cortex. This leads to question the relationship between sodium transport and ATPase activity under physiological doses of thyroid hormones. An early effect of physiological doses of thyroid hormones on brush border Na+ permeability is suggested.
Key wordsThyroid gland Proximal tubular transport Na+−K+-ATPase
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