The Role of Nitric Oxide in Saline-Induced Natriuresis and Diuresis in Rats

This study was designed to determine to what extent nitric oxide (NO) mediates the natriuretic and diuretic responses to acute isotonic saline (0.9 gram % NaCl) volume expansion (SVE, 0.5 ml min−1 kg−1). Studies were performed on 49 pentobarbital anesthetized (65 mg/kg) female Sprague-Dawley rats with or without a NO synthase inhibitor, Nω-nitro-L-arginine (LNA). Group 1 received saline at 27 μl/min for 1 hr (baseline) and then SVE for 1 hr; Groups 2–4 received LNA at 10, 150, and 200 μg kg−1 min−1, respectively, for 1 hr followed by LNA + SVE. To determine to what extent inhibition of NOS would reverse an ongoing SVE-induced natriuresis and diuresis, Group 5 was saline-volume-expanded for hours 1 and 2 whereas Group 6 was administered SVE during the first hour and then SVE + 150 μg kg −1 min−1 LNA during the second hour. SVE caused a significant (P < 0.05) increase in the glomerular filtration rate (GFR) of Group 1 and the LNA-treated rats (Groups 2–4). This SVE-induced increase in the GFR occurred despite the fact that baseline GFR was significantly lower in the two groups of rats that were infused with the highest doses of LNA (Groups 3–4). SVE was also associated with similar increases in urine flow rate, sodium and potassium excretion, and total osmolar excretion in Groups 1–4. On the other hand, mean arterial pressure (MAP) was significantly higher in Group 2 during SVE + LNA and during the baseline as well as during the SVE periods in Groups 3–4; MAP was also significantly elevated in Group 6 during SVE + LNA. Thus, despite the fact that MAP was higher in LNA-treated rats, sodium and urine flow rates were the same as in Group 1 (i.e., there was no evidence of a pressure natriuresis or diuresis in these animals). Along these lines, there was a small but significant positive linear correlation coefficient (r = 0.41, P = 0.05) between sodium excretion values and corresponding MAP values in SVE control rats but not in Groups 3–4 during SVE (r = 0.28, P = 0.26). The current data demonstrate that 1) NO does not mediate SVE-induced hyperfiltration in the rat, 2) NO also does not mediate SVE-induced natriuresis or diuresis, and 3), consistent with other reports, NO appears to mediate pressure natriuresis and diuresis.