Lack of stimulation of renal (Na + + K +)-ATPase by thyroid hormones in the rabbit

Abstract

The effect of l-3,5,3′-triiodothyronine (T 3) and thyroxine (T 4) on (Na + + K +)-ATPase activities was examined in rabbit kidneys because in this tissue almost 80% of the metabolism is connected to active sodium transport. T 3-receptor concentrations were estimated as 0.62 and 0.80 pmol/mg per DNA in the cortex and outer medulla, respectively. A dose of 0.5 mg T 3/kg body weight for 3 days increased basal metabolic rate by almost 60%, and the mitochondrial 1-α-glycerophosphate dehydrogenase activity was increased by 50% in both the cortex and medulla. (Na + + K +)-ATPase activity in the liver was raised by almost 50%. However, no changes in (Na + + K +)-ATPase activities or binding sites for [ 3H]ouabain in either the kidney cortex or medulla could be observed. T 4 at 16 mg/kg daily for 14 days was also without effect on renal (Na + + K +)-ATPase activities. Furthermore, the response to T 3 was absent at high sodium excretion rates induced by unilateral nephrectomy and extracellular volume expansion. Thus, despite stimulation of basal metabolic rate and renal 1-α-glycerophosphate dehydrogenase activity by T 3 and T 4, the (Na + + K +)-ATPase activity in the rabbit kidney is identical in euthyroid and hyperthyroid states. However, thyroid hormones prevent the normal natriuretic response to extracellular volume expansion.