The Role of Ouabain-Sensitive Adenosine Triphosphatase in the Stimulating Effect of Thyrotropin on the Iodide Pump of the Rat Thyroid

Abstract

Previous studies indicated that a (probably indirect) relationship existed between thyroidal ouabain-sensitive adenosine triphosphatase (ATPase) activity and iodide transport; they were also compatible with the idea that the known stimulating effect of thyrotropin (TSH) on the thyroidal iodide pump may be mediated by activation of ouabain-sensitive ATPase. In the thyroids of 23 normal rats fed Purina Laboratory Chow, ouabain-sensitive ATPase aptivity was found to be 70 [plus or minus] 3 (SE) [mu]moles inorganic phosphate (Pi) per g wet tissue per hr, or 6% of that of total ATPase. The thyroid serum radioiodide concentration ratio (T/S) in 12 of these rats averaged 44. Factors which increase the output of endogenous TSH [1% NaClO4 in the drinking water, low iodine intake, injections of propylthiouracil (PTU)] raised the activity of thyroidal ouabain-sensitive ATPase, both in absolute terms (up to over 500 [mu]moles Piper g per hr.) and as the percentage of that of total ATPase (up to 20% or more). Except for the NaClO4-treatment, the regimens elevated the T/S in a roughly parallel fashion (up to over 200). However, when thyroidal ouabain-sensitive ATPase activity was plotted against T/S in rats given no PTU or injected with 20 nag/day for 1, 2 or 3 days, the correlation was poor. Depression of the T/S (to 0.5-5) by chronic triiodothyronine-treatment or hypophysectomy was not accompained by a consistent change in the ouabain-sensitive ATPase activity of the thyroid. When the T/S of hypophysectomized rats was raised with TSH+PTU given for 1 or 3 days (by factors ranging from over 10 to over 100), thyroidal ouabain-sensitive ATPase activity either increased only slightly or even decreased. The results do not preclude the possiblity that a basal level of ouabain-sensitive ATPase activity is a prerequisite for iodide transport in the rat thyroid, but they show that stimulation of the thyroidal iodide pump by TSH is not secondary to an over-all increase of ouabain-sensitive ATPase action in the gland.