Acute administration of loop diuretics like furosemide leads to a stimulation of renin secretion, an effect thought to result from inhibition of NKCC2-mediated salt transport at the macula densa. However, furosemide also inhibits NKCC1 with similar potency. In the present study we examined the influence of furosemide on renin secretion in NKCC1 knock-out and wild type mice and in isolated juxtaglomerular (JG) cells (all humanely killed animals) in order to distinguish between macula densa dependent and independent effects of furosemide. Baseline plasma renin concentration (PRC) was 259 ± 61 ng Ang I/ml h in NKCC1+/+ (n = 9) and 917 ± 144 ng Ang I/ml h in NKCC1-/- mice (n = 11). Acute administration of furosemide (50 mg/kg i.p.) increased PRC significantly (p=0.0001) to 1794 ±229 ng Ang I/ml h in NKCC1+/+, whereas it reached a value of 2130 ± 286 ng Ang I/ml h in NKCC1-/- mice (p<0.001). Wild type mouse JG cells were isolated by enzymatic digestion of the renal cortex, and the effect of increasing concentrations of furosemide (10-5 M-10-3 M) on renin secretion was measured by RIA. Forskolin (10-5 M) was used as control. Forskolin and furosemide (10-3 M) significantly increased renin release (renin in medium in % of renin in cell lysate + medium)from 20.5 ±1.6% to 35.9 ±2.4% and 31.7 ±3.4%, respectively (p<0.05, n = 12). Furthermore, we determined membrane capacitance as a measure of exocytosis and renin release by using patch clamp methods in isolated JG cells (n = 5). Furosemide (10-4 M) increased membrane capacitance significantly by 8.1 ± 0.9% compared with basal capacitance over a period of 1200 s. Finally, we determined the expression of NKCC1 mRNA in single JG cells by RT-PCR. Using mRNA from 2.5 JG cells as template and 36 cycles we found that NKCC1 is expressed in JG cells while no band was seen in the control minus RT. Our data suggest that besides modulating the macula densa pathway, furosemide also stimulates renin secretion directly on juxtaglomerular granular cells, and we suggest that this effect is mediated by inhibition of NKCC1.