T he effect of endothelin-1 (ET-1) on the proximal tubule remains unclear. This may be due to a biphasic effect on transport in this segment. We hypothesized that ET-1 has a biphasic effect on fluid absorption (Jv) in the proximal straight tubule and that its inhibitory effect is superimposed on its stimulatory effect. ET-1 (10(-13) M) stimulated Jv from 0.68 +/- 0.07 to 1.11 +/- 0.20 nl/mm/min, a 60% increase (P < 0.04). 10(-12) and 10(-10) M ET-1 had no significant effect. 10(-9) M ET-1 reduced Jv from 0.81 +/- 0.19 to 0.44 +/- 0.15 nl/mm/min (P < 0.009). Staurosporine (STP, 10(-8) M) prevented both 10(-9) and 10(-13) M ET-1 from altering Jv significantly indicating that protein kinase C (PKC) is involved. Indomethacin (10(-5) M) blocked the inhibition produced by 10(-9) M ET-1. ETI (10(-6) M), a lipoxygenase inhibitor, also blocked ET-1 inhibition of Jv. Interestingly ET-1 (10(-9) M) stimulated Jv in the presence of both indomethacin and ETI. When 10(-9) M ET-1 was added in the presence of 10(-5) M quinacrine, a phospholipase (PL) inhibitor, Jv also increased from 1.02 +/- 0.20 to 1.23 +/- 0.22 nl/mm/min (P < 0.03). STP blocked this increase. We conclude that (a) 10(-13) M ET-1 stimulates fluid absorption by activating PKC; (b) 10(-9) M ET-1 decreases Jv by PKC-, PL-, cyclooxygenase-, and lipoxygenase-dependent mechanisms; and (c) the inhibitory effect of ET-1 on Jv is superimposed on the stimulatory effect.