PRESSURE-DIURESIS-NATRIURESIS RESPONSE IN HYPERTHYROID AND HYPOTHYROID RATS

1. Renal responses to changes in renal perfusion pressure were studied in anaesthetized hyperthyroid (thyroxine, 300 mu g day(-1) kg(-1)) an d hypothyroid (methimazole, 0.03% via drinking water) rats to determin e whether an abnormality in the pressure-diuresis-natriuresis phenomen on is involved in the resetting of kidney function in these disorders. 2. There were no significant differences between control and hypothyr oid rats with respect to the relationships between renal perfusion pre ssure and absolute or fractional water and sodium excretion. However, in hyperthyroid rats the pressure-diuresis-natriuresis mechanism was i mpaired. 3. Renal blood flow and glomerular filtration rate were well autoregulated and there were no differences between control and hypoth yroid rats at every level of renal perfusion pressure. A significantly lower glomerular filtration rate was observed in hyperthyroid rats wh en data were expressed per gram kidney weight, but glomerular filtrati on rate was similar to that of control rats when normalized by body we ight. 4. The shift in the pressure-diuresis-natriuresis response of hy perthyroid rats is mainly due to an increase in tubular reabsorption. Blunting of the renal pressure-diuresis-natriuresis mechanism in hyper thyroid rats may represent the functional resetting of the kidney nece ssary for sustained hypertension. However, a normal pressure-natriures is response was observed in hypothyroid rats, in which blood pressure was markedly reduced.