Interaction of age and thyroid hormone status on Na+-K+ ATPase in rat renal cortex and liver.

Na+-K+ ATPase was measured in euthyroid, hypothyroid, and hyperthyroid rats aged 6 weeks, 6 months, and 20-24 months. There was a small but significant decline in basal enzyme activity with increasing age in renal cortical tissue. Enzyme activity decreased with hypothyroidism and increased with hyperthyroidism. With increasing age, both the magnitude of the increase resulting from thyrotoxicosis and the absolute enzyme level were significantly less with each increase in the age of the experimental animals. Hypothyroid animals displayed no age-related decline in renal cortical Na+-K+ ATPase. The decline in Na+-K+ ATPase in renal cortical tissue is thyroid hormone dependent. In addition, age modifies the response of Na+-K+ ATPase to thyroid hormone.