Impact of low sodium intake on body sodium and systemic arterial pressure during reduced renal perfusion pressure
Research field:Cardiovascular physiology
Authors:Seeliger E, Safak E, Pfaff C, Reinhardt HW
Address of presenting
author:		Institute of Physiology, Charité, Humboldt University, Tucholskystr. 2, Berlin 10177, Germany
E-mail:erdmann.seeliger@charite.de
Phone:int+30 2802 6631
Fax:int+30 2802 6631
Text of abstract Introduction
Long-term 20% reduction of renal perfusion pressure (rRPP) in dogs on high Na intake (HSI, 5.5 mmol Na/kg bw per day) has been shown to induce Na retention on day 1, which increases total body sodium (TBS) by ~3 mmol/kg bw, thus increasing systemic mean arterial blood pressure (MABP) by ~25 mm Hg. On the following days, 24-h intake-output Na balances become equilibrated again, i.e., the surplus of TBS gained on day 1 is neither increased nor reduced, and MABP remains elevated. This resetting of 24-h balances on an elevated level of TBS has been termed Pressure Escape in analogy to mineralocorticoid escape [1]. Here we study 1) whether Pressure Escape would occur after achieving a smaller surplus of TBS and, thus, a smaller increase of MABP, when dogs on low Na intake are subjected to rRPP, and 2) whether the surplus of TBS and the elevation of MABP initially induced by rRPP during HSI would be reduced, when Na intake is changed to low Na intake during continued rRPP.

Methods
Balance studies were performed in freely moving Beagle dogs, using standardized conditions [1]. Each study lasted for 4 consecutive days. In protocol 1, continuous servo-controlled 20% reduction of RPP was performed in dogs maintained on low Na intake (LSI, 0.5 mmol Na/kg bw per day) throughout the study (rRPP+LSI). In protocol 2, the same reduction of RPP was combined with a change in Na intake (rRPP+CSI): after 2 days of HSI, Na intake was changed to LSI.

Results
During rRPP+LSI, Na was retained on all days, resulting in gradual increases of TBS and MABP. Pressure Escape did not occur within the 4 days studied. The surplus of TBS gained during 4 days amounted to ~1.2 mmol Na /kg BW, MABP rose by ~12 mm Hg. During day 1 of rRPP+CSI, Na was retained, thus TBS increased by ~2.7 mmol Na/kg bw, and MABP rose by ~20 mm Hg. On day 2, Pressure Escape occurred, i.e., the TBS surplus was neither increased nor reduced. On day 3, on which Na intake was changed to LSI, TBS surplus was reduced by only ~0.5 mmol Na/kg bw. The remainder of TBS surplus (~2.2 mmol Na/kg bw) was preserved on day 4. MABP remained on the elevated level gained on day 1.

Conclusions
Reduction of RPP apparently poses an error signal for the body's TBS control, which is suggesting, that TBS had been reduced. Consequently, Na would be retained until the surplus gained matches the assumed "deficit". Thus, Pressure Escape is accomplished within 24 hours if large amounts of Na are offered, yet would take several days to become completed if small amounts are offered. After accomplishing Escape, the actual surplus of TBS is defended by the body. Thus, reduction of Na intake does not suffice to reduce surplus of TBS and elevation of MABP induced by reduced renal perfusion pressure.

References
[1] Reinhardt, H.W. et al. 1994, Am. J. Physiol. 266: H650-H657

Keywords:sodium balance, renal perfusion pressure, blood pressure control

Created 2000-05-01