Selective induction of high-ouabain-affinity isoform of Na+-K+-ATPase by thyroid hormone.

The administration of thyroid hormone is known to result in an induction of the Na+-K+-adenosinetriphosphatase (Na+-K+-ATPase) in rat skeletal muscle and other thyroid hormone-responsive tissues. Since the Na+-K+-ATPase in a variety of mammalian tissues has recently been reported to exist in at least two forms distinguishable by differing affinities for the inhibitory cardiac glycoside ouabain, we have studied the effects of 3,3',5-triiodo-L-thyronine (T3) treatment on these two forms of the enzyme in rat diaphragm. The inhibition of Na+-K+-ATPase activity in a crude membrane fraction by varying concentrations of ouabain conformed to a biphasic pattern consistent with the presence of two distinct isoforms with inhibition constants (KIs) for ouabain of approximately 10(-7) and 10(-4) M, respectively. Treatment of hypothyroid rats with T3 (50 micrograms/100 g body wt on 3 alternate days) nearly tripled that portion of the Na+-K+-ATPase activity corresponding to the high-ouabain-affinity form (increased by 178 +/- 24%), whereas the enzyme activity corresponding to the low-ouabain-affinity form was only slightly changed (increased by 20 +/- 5%). Measurement of the specific binding of [3H]ouabain to these membranes confirmed the presence of a class of high-affinity ouabain binding sites with a dissociation constant (Kd) of slightly less than 10(-7) M, whose maximal binding capacity was increased by T3 treatment by 185%. The calculated catalytic turnover associated with the high-affinity site was 70-80 molecules ATP hydrolyzed.site-1.s-1 and was unchanged by T3 treatment.(ABSTRACT TRUNCATED AT 250 WORDS)