Effects of amiloride analogues on the production of prostacyclin by aortic endothelial cells

The acute blood pressure-lowering effect of amiloride is independent of endothelial ENaC and eNOS in humans and mice

AIMS

The epithelial sodium channel (ENaC) is expressed in cultured endothelial cells and inhibitory coupling to eNOS activity has been proposed. The present study tested the hypothesis that ENaC blockers increase systemic NO-products and lower blood pressure in patients and mice, depending on eNOS.

METHODS

NO-products and cGMP were measured in diabetes patient urine and plasma samples before and after amiloride treatment (20-40 mg for two days, plasma n = 22, urine n = 12 and 5-10 mg for eight weeks, plasma n = 52, urine n = 55). Indwelling catheters were implanted in the femoral artery and vein in mice for continuous arterial blood pressure and heart rate recordings and infusion.

RESULTS

Treatment with amiloride for two days increased plasma and urine NO-products, while plasma cGMP decreased and urinary cGMP was unchanged in patient samples. Eight weeks of treatment with amiloride did not alter NO-products and cGMP. In mice, amiloride boli of 5, 50, and 500 µg/kg lowered heart rate and arterial blood pressure significantly and acutely. Benzamil had no effect on pressure and raised heart rate. In hypertensive eNOS^-/- and L-NAME-treated mice, amiloride lowered blood pressure significantly. L-NAME infusion significantly decreased NO-products in plasma; amiloride and eNOS-deletion had no effect. An acetylcholine bolus resulted in acute blood pressure drop that was attenuated in eNOS^-/- and L-NAME mice. ENaC subunit expressions were not detected consistently in human and mouse arteries and endothelial cells.

CONCLUSION

Amiloride has an acute hypotensive action not dependent on ENaC and eNOS and likely related to the heart.

Is the bradykinin-induced Ca2+ influx and the formation of endothelium-derived relaxing factor mediated by a G protein?

In cultured porcine aortic endothelial cells bradykinin produced a long-lasting Ca2+ influx. In contrast to the G protein-independent Ca2+ entry evoked by ionomycin or digitonin, bradykinin-induced Ca2+ influx was antagonized by Ni2+ with an IC50 value of about 50 microM. Since identical IC50 values for Ni2+ were found when Ca2+ entry was induced by sodium fluoride or GTP gamma S, we suggest that stimulation of G protein(s) results in the activation of the same Ca2+ channels as stimulation by bradykinin. This conclusion is supported by our findings that inhibition of GTPase by mepacrine amplified bradykinin-stimulated Ca2+ influx, but did not interfere with the effect of the Ca2+ ionophore A23187. Similar to its effect on Ca2+ influx, mepacrine also potentiated endothelium-derived relaxing factor (EDRF) formation by bradykinin and sodium fluoride, but did not affect A23187-induced EDRF biosynthesis. We therefore suggest that in endothelial cells the bradykinin-induced Ca2+ influx and the resulting formation of EDRF are regulated by a G protein.

A role for Na+/H+ exchange in contraction of guinea pig airways by endothelin-1 in vitro

Endothelin-1-induced contractions of guinea pig tracheal and bronchial strips were dose-dependently attenuated by the amiloride analogues 5-(N-ethyl-N-isopropyl)amiloride (EIPA, 1-10 microM) and 5-(N,N-hexamethylene)amiloride (HMA, 1-10 microM). The calculated Ki values for EIPA and HMA were 0.11 +/- 0.02 microM and 0.06 +/- 0.02 microM in the trachea, and 0.28 +/- 0.11 microM and 0.70 +/- 0.25 microM in the bronchus, respectively. These values are in the same order of magnitude as those reported for inhibition of the Na+/H+ exchange in cells. Amiloride (1-10 microM) was ineffective. These data suggest that activation of the Na+/H+ exchange by ET-1 may be involved in mediating its myotropic action in guinea pig airway smooth muscle.

P2 purinoceptors on vascular endothelial cells: physiological significance and transduction mechanisms

The characterization of P2 gamma purinoceptors on vascular endothelial cells has progressed rapidly since their existence was first demonstrated in 1983. They transduce the actions of extracellular ATP and ADP--endothelium-dependent relaxation, prostacyclin synthesis, endothelial cell mitogenesis--which play a vital role in the interaction between platelets (a rich source of extracellular adenine nucleotides) and the vessel wall. Release of prostacyclin limits the extent of intravascular platelet aggregation following vascular damage and platelet stimulation, while the mitogenic effect may accelerate the repair of a lesion. P2 gamma receptors on endothelial cells are coupled to a phospholipase C by a GTP-binding protein. Jean-Marie Boeynaems and Jeremy Pearson explain how the increases in cytoplasmic Ca2+ and diacylglycerol resulting from this initial event mediate several further effects. In particular, activation of a Ca2(+)-sensitive phospholipase A2 explains the increased synthesis of prostacyclin, while the phosphorylation of several proteins by calmodulin-dependent kinases modulates other endothelial cell functions.