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Abstract

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 (J_v) 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) injectionJ v 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, J_v 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. J_v 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 P_i/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 in J_v.

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.