Regulation of Angiotensin II Receptors in Medullary Thick Ascending Limbs
Angiotensin II (Ang II) is an important regulator of the function of the medullary thick ascending limb of loop of Henle (MTAL). Recent studies showed that changes in Ang II receptor expression occur and underlie changes in the function of proximal tubules during altered sodium intake. The present experiment was designed to determine 1) whether expression of the type 1 Ang II (AT1) receptor in MTAL is regulated by altered sodium intake, and 2) the specific pathway(s) mediating sodium-induced AT1 expression in MTAL. Wistar rats were fed a normal sodium (0.5%, NS), low sodium (0.07%, LS), or high sodium (4%, HS) diet for 2 weeks. Northern blot analysis and radioligand binding showed that in rats fed a normal sodium diet the rank of order for both AT1 mRNA expression and receptor density was outer medulla>cortex>inner medulla. Sodium restriction significantly increased both AT1 mRNA expression and receptor density in the outer medulla. In contrast, neither AT1 mRNA expression nor receptor density in the outer medulla was altered by sodium loading. Losartan treatment (3 mg/kg/per day by oral gavage for 2 weeks) prevented low sodium-induced upregulation of the AT1 receptor in the outer medulla, but it had no effect on AT1 expression in the outer medulla of rats fed a normal sodium diet. Highly purified suspensions of MTAL were isolated from rats fed a normal or low sodium diet. Low sodium intake significantly increased AT1 mRNA level by 184% and AT1 receptor density by 58% in MTAL. Primary culture of MTAL cells were treated with PBS, Ang II (10-8 M), and Ang II + 17 octadecynoic (17 ODYA, 10μM). Ang II caused about 2-fold increase in AT1 mRNA levels, and this increase was dinimished by about 30% by the addition of 17 ODYA. We conclude that 1) sodium restriction but not sodium loading increases AT1 receptor expression in MTAL, 2) low sodium-induced upregulation of the AT1 receptor in MTAL is Ang II-dependent, and 3) Ang II induced upregulation of the AT1 receptor in MTAL is mediated, at least in part, by cytochrome P450 pathways.