[125I]endothelin-1 binding to renal brush border and basolateral membranes

Endothelin A receptor-like immunoreactivity on the basal infoldings of rat renal tubules and collecting ducts

We investigated the distribution of endothelin A (ET(A)) receptor-like immunoreactivity in the rat kidney using affinity-purified antibodies against amino acid residues 403-417 of the rat ET(A) receptor modified by the multiple antigen peptide complex system. Western blot analysis using the affinity-purified anti-ET(A) antibody detected bands of approximately 47.3 and 64.5 kDa in the rat kidney. By light microscopy, ET(A) receptor-like immunoreactivity was seen in the basal side of the renal tubules and collecting ducts. The most intense immunoreactivity was present in the distal renal tubules and inner medullary collecting ducts. In addition to the basal infoldings, immunoreactive puncta were scattered in the epithelial cells of the renal tubules and collecting ducts. Specimens prepared using the pre-embedding method were examined by electron microscopy, and some immunopositive signals were seen on the basal infodings of the renal tubules and collecting ducts. The lengths of immunopositive cytoplasmic membrane were far longer in the distal tubules and inner medullary collecting ducts than in the proximal tubules and outer medullary collecting ducts. Immunopositive signals were also sometimes observed in the thick portion of Henle's loop, but never in the thin portion. We have not previously detected immunopositive signals on the renal vascular systems with the antibody used here. These results suggest that endothelin acts on the basal infoldings through the ET(A) receptor, particularly in the distal tubules and inner medullary collecting ducts, although involvement of the ET(B) receptor cannot be excluded.

A consensus sequence in the endothelin-B receptor second intracellular loop is required for NHE3 activation by endothelin-1

Endothelin-1 (ET-1) increases the activity of Na+/H+exchanger 3 (NHE3), the major proximal tubule apical membrane Na+/H+antiporter. This effect is seen in opossum kidney (OKP) cells expressing the endothelin-B (ETB) and not in cells expressing the endothelin-A (ETA) receptor. However, ET-1 causes similar patterns of protein tyrosine phosphorylation, adenylyl cyclase inhibition, and increases in cell [Ca2+] in ETA- and ETB-expressing OKP cells, implying that an additional mechanism is required for NHE3 stimulation by the ETBreceptor. The present studies used ETAand ETBreceptor chimeras and site-directed mutagenesis to identify the ET receptor domains that mediate ET-1 regulation of NHE3 activity. We found that binding of ET-1 to the ETAreceptor inhibits NHE3 activity, an effect for which the COOH-terminal tail is necessary and sufficient. ET-1 stimulation of NHE3 activity requires the COOH-terminal tail and the second intracellular loop of the ETBreceptor. Within the second intracellular loop, a consensus sequence was identified, KXXXVPKXXXV, that is required for ET-1 stimulation of NHE3 activity. This sequence suggests binding of a homodimeric protein that mediates NHE3 stimulation.

Contrasting effects of cPLA2 on epithelial Na+ transport

Activity of the epithelial Na+ channel (ENaC) is the limiting step for discretionary Na+ reabsorption in the cortical collecting duct. Xenopus laevis kidney A6 cells were used to investigate the effects of cytosolic phospholipase A2 (cPLA2) activity on Na+ transport. Application of aristolochic acid, a cPLA2 inhibitor, to the apical membrane of monolayers produced a decrease in apical [3H]arachidonic acid (AA) release and led to an approximate twofold increase in transepithelial Na+ current. Increased current was abolished by the nonmetabolized AA analog 5,8,11,14-eicosatetraynoic acid (ETYA), suggesting that AA, rather than one of its metabolic products, affected current. In single channel studies, ETYA produced a decrease in ENaC open probability. This suggests that cPLA2 is tonically active in A6 cells and that the end effect of liberated AA at the apical membrane is to reduce Na+ transport via actions on ENaC. In contrast, aristolochic acid applied basolaterally inhibited current, and the effect was not reversed by ETYA. Basolateral application of the cyclooxygenase inhibitor ibuprofen also inhibited current. Both effects were reversed by prostaglandin E2 (PGE2). This suggests that cPLA2 activity and free AA, which is metabolized to PGE2, are necessary to support transport. This study supports the fine-tuning of Na+ transport and reabsorption through the regulation of free AA and AA metabolism.

Endothelin and blood pressure regulation in the female rat: studies in normal pregnancy and with nitric oxide synthase inhibition-induced hypertension

OBJECTIVE

To evaluate the role of endothelin (ET) in blood pressure regulation in normal pregnant and nonpregnant rats and with nitric oxide synthase (NOS) inhibition.

METHODS

Pregnant and nonpregnant Sprague-Dawley rats were treated for 7 days with an ET(A)-selective (A-127722 or FR-139317), ET(B)-selective (A-192621), or ET(A)/ET(B) nonselective (A-182086) endothelin receptor antagonist, and/or with the NOS inhibitor, N(omega)-nitro-L-arginine methyl ester (L-NAME, 2. 5 mg/kg/h). In pregnant rats, the ET antagonists and L-NAME were administered from gestational day 14 through day 21 (term = 22 days). All rats received indwelling arterial catheters for blood pressure measurement. Mean arterial pressures were recorded on infusion days 1, 4, and 7 and these data were compared by analysis of variance among experimental groups with p < 0.05 considered significant.

RESULTS

The ET(A) receptor antagonism lowered blood pressure in both pregnant and nonpregnant rats (p < 0.05), whereas ET(B) antagonism resulted in hypertension (p < 0.001). ET(B) antagonism-induced hypertension was attenuated by pregnancy (p < 0. 001). Hypertension was induced in all rats treated with L-NAME (p < 0.001). Endothelin receptor antagonism, regardless of specificity, did not ameliorate L-NAME-induced hypertension in pregnant or nonpregnant female rats. The only observed effect of ET(A) antagonism on NOS inhibition-induced hypertension was the prevention of a continued rise at infusion day 7 in nonpregnant rats.

CONCLUSIONS

Endothelin, acting via both the ET(A) and ET(B) receptors, contributes to blood pressure homeostasis in pregnant and nonpregnant rats. Endothelin receptor antagonism does not ameliorate NOS-inhibition-induced hypertension in pregnant rats.