Possible mediators of connecting tubule glomerular feedback

Abstract

In the renal cortex, the connecting tubule (CNT) returns to the glomerular hilum and contacts the afferent arteriole (Af-Art). Increasing Na delivery to the CNT dilates the Af-Art by activating epithelial Na channels, a process that we call connecting tubule glomerular feedback (CTGF). However, the mediator(s) of CTGF are unknown. We tested the hypothesis that Na reabsorption by the CNT induces release of arachidonic acid metabolites that diffuse to and dilate the Af-Art. Microdissected rabbit Af-Arts and adherent CNTs were simultaneously microperfused. CTGF was measured as the increase in diameter of norepinephrine-preconstricted Af-Arts in response to switching NaCl concentration in the lumen of the CNT from 10 to 80 mmol/L. Under control conditions, CTGF was repeatable and completely reversed norepinephrine-induced vasoconstriction. In the presence of 5,8,11,14-eicosatetraynoic acid, an inhibitor of arachidonic acid metabolism, CTGF was completely blocked (-0.7+/-0.3 versus 7.3+/-0.5 microm), suggesting that arachidonic acid metabolites mediate CTGF. Because both cyclooxygenase-derived prostaglandins and epoxygenase-derived epoxyeicosatrienoic acids are known vasodilatory arachidonic acid metabolites, we tested whether indomethacin or MS-PPOH (a cyclooxygenase and an epoxygenase inhibitor) could block CTGF. Both indomethacin and MS-PPOH partially blocked CTGF (2.3+/-0.8 versus 6.5+/-0.5 microm, and 2.9+/-0.8 versus 6.6+/-1.1 microm, respectively). When combined, they completely blocked CTGF (-0.4+/-0.3 versus 6.6+/-1.1 microm). We confirmed these findings by using the epoxyeicosatrienoic acid antagonist 14,15-EEZE. The combination of indomethacin plus 14,15-EEZE completely abolished CTGF (-0.3+/-0.2 versus 8.0+/-1.0 microm). We conclude that increasing Na concentrations in the CNT stimulate release of prostaglandins and epoxyeicosatrienoic acids, which mediate CTGF.

Figure 1

Left, Effect of adding ETYA to the CNT lumen (thereby inhibiting AA metabolism) on Af-Art dilatation induced by high NaCl in the CNT. Right, CTGF response measured as changes in Af-Art diameter in the absence (control) and presence of ETYA (12.5 µmol/L). ETYA completely blocked the preconstricted Af-Art dilation induced by high NaCl (n = 6).

Figure 2

Dose-response curve to Ach in the Af-Art during perfusion of ETYA (12.5 µmol/L) in the CNT. ETYA perfusion in the CNT did not prevent Af-Art dilatation induced by the CTGF-independent vasodilator Ach. P values are vs 0 Ach (n = 5).

Figure 3

Left, Effect of adding the COX inhibitor indomethacin to the CNT lumen on Af-Art dilatation induced by high NaCl in the CNT. Right, CTGF response measured as changes in Af-Art diameter in the absence (control) and presence of indomethacin (50 µmol/L). Indomethacin partially blocked the preconstricted Af-Art dilation induced by high NaCl (n = 6).

Figure 4

Left, Effect of adding the epoxygenase inhibitor MS-PPOH to the CNT lumen on Af-Art dilatation induced by high NaCl in the CNT. Right, CTGF response measured as changes in Af-Art diameter in the absence (control) and presence of MS-PPOH (1 µmol/L). MS-PPOH partially blocked the dilatation of preconstricted Af-Art induced by high NaCl (n = 6).

Figure 5

Left, Effect of adding both indomethacin and MS-PPOH to the CNT lumen on Af-Art dilatation induced by high NaCl in the CNT. Right, CTGF response measured as changes in Af-Art diameter in the absence (control) and presence of indomethacin and MS-PPOH. The combined treatment completely blocked the dilatation of preconstricted Af-Art induced by high NaCl (n = 6).

Figure 6

Left, Effect of adding the selective EET antagonist 14,15-EEZE to the bath in the presence of indomethacin in the CNT on Af-Art dilatation induced by high NaCl in the CNT. Right, CTGF response measured as changes in Af-Art diameter in the absence (control) and presence of indomethacin alone or with 14,15-EEZE. The combined treatment completely blocked the dilatation of preconstricted Af-Art induced by high NaCl (n = 6).