Regional modulation of cyclic nucleotides by endothelin-1 in rat pulmonary arteries: direct activation of G(i)2-protein in the main pulmonary artery

Endothelin(A)-endothelin(B) receptor cross talk in endothelin-1-induced contraction of smooth muscle

The efficacy of selective endothelin (ET) receptor antagonists may be limited by a functional interaction between the ET(A) and ET(B) receptors. This interaction, also termed "cross talk", is characterized by the dependency of the inhibition of an ET-1 response due to antagonism of one ET receptor subtype upon concomitant antagonism of the other ET receptor subtype. Although a reduction in ET(A)-ET(B) receptor cross talk would presumably increase the efficacy of selective ET receptor antagonists, an approach that accomplishes this aim is largely absent due to a lack of mechanistic understanding. Toward this goal, we evaluated the characteristics and potential dependencies of cross talk in smooth muscle. Smooth muscle was adopted as an exemplar not only because cross talk is widely reported in this tissue type, thereby allowing numerous comparisons, but also significant controversy surrounds the use of selective versus nonselective ET receptor antagonists in ET-1-related pathophysiologies involving smooth muscle. Based on this evaluation, we suggest that ET(A)-ET(B) receptor cross talk is a dynamic process directed by either or both ET receptor subtypes and expressed to varying magnitudes depending on the ET-1 and selective ET receptor antagonist concentrations, tone due to intraluminal pressure/stretch, agonists acting at receptors other than the ET(A)/ET(B) receptors, and endothelial/epithelial function. It is speculated that ET(A)-ET(B) receptor cross talk occurs through signal transduction pathways along with changes at the receptor level. Pharmacologic intervention of the signaling pathways could increase the therapeutic efficacy of ET receptor antagonists.

Muscular ETB receptors develop postnatally and are differentially distributed in specific segments of the rat vasculature

The endothelin/endothelin-receptor system is a key player in the regulation of vascular tone in mammals. We raised and characterized an antiserum against rat ETB receptor and investigated the distribution of ETB receptors in different vascular beds during postnatal development (day 0 through day 28) and in the adult rat. We report the tissue-specific and age-dependent presence of vasoconstrictor ETB receptors. At the time of birth, vascular smooth muscle cells from all tissues examined did not exhibit ETB receptor immunoreactivity. The occurrence of ETB receptor immunoreactivity in the postnatal development was time dependent and started in small coronary and meningeal arteries at day 5, followed by small mesenteric arteries as well as brachial artery and vein at day 14. At day 21, ETB receptors were present in the media of muscular segments of pulmonary artery, large coronary arteries, and intracerebral arterioles. At day 28, ETB receptor immunoreactivity was evident in interlobular renal arteries, vas afferens, and efferens. Large renal arteries, mesenteric artery, and elastic segments of pulmonary arteries, as well as coronary and mesenteric veins, did not exhibit ETB receptor immunoreactivity. These data demonstrate the age-dependent and tissue-specific presence of ETB receptors, mainly on arterial smooth muscle cells in the vascular system of the rat.

Inhaled nitric oxide increases endothelin-1 levels: a potential cause of rebound pulmonary hypertension

OBJECTIVE

Inhaled nitric oxide (iNO) is front-line therapy for pulmonary hypertension after repair of congenital heart disease. However, little clinical data exists regarding the effects of iNO on regulators of pulmonary vascular resistance. An imbalance between primary vasodilators, such as NO, and vasoconstrictors, such as endothelin-1 (ET-1), has been implicated in rebound pulmonary hypertension upon iNO withdrawal. The objective of this study was to determine whether iNO therapy alters plasma ET-1 levels.

DESIGN

This is a prospective study involving pediatric and adult patients at risk for pulmonary hypertension.

SETTING

Pediatric patients were in the cardiac intensive care unit and adult patients were in a tertiary-care hospital.

PATIENTS

Group 1 included children with congenital heart disease requiring iNO for treatment of pulmonary hypertension after cardiopulmonary bypass (n = 15), group 2 was adults receiving iNO (n = 10), and group 3 included children at risk for pulmonary hypertension after bypass that did not require iNO (n = 8).

INTERVENTIONS

Dosages of iNO were 2-60 ppm. The duration of therapy ranged from 23 to 188 hrs in group 1 and 29 to 108 hrs in group 2.

MEASUREMENTS AND MAIN RESULTS

Arterial blood was obtained for the measurement of ET-1 levels before and during iNO therapy and 24 hrs after iNO withdrawal. Group 1 mean ET-1 levels increased to 127% of baseline by 12 hrs of iNO, remained elevated at 48 hrs (p < .05), then decreased to 71% of iNO levels 24 hrs after withdrawal (p < .01). Group 2 ET-1 levels increased to 147%, and 137% of baseline at 12 and 24 hrs of iNO therapy, then fell to 68% of baseline within 24 hrs of discontinuing iNO. ET-1 levels in group 3 decreased after surgery (p < .05).

CONCLUSIONS

These data suggest that iNO increased plasma ET-1 levels, which subsequently decreased when iNO was discontinued. Increased circulating ET-1 levels might contribute to rebound pulmonary hypertension upon iNO withdrawal.

Endothelin-induced changes of secondary messengers in cultured corneal endothelial cells

The effect of endothelins on corneal endothelial cells is not well understood. We have investigated the effects of endothelin-1 (ET-1), endothelin-2 (ET-2) and endothelin-3 (ET-3) on bovine corneal endothelial cellular proliferation and the secondary messenger changes in cells in the presence of ET-1. It was found that the 3H-thymidine uptake was enhanced by ET-1 significantly, whereas ET-2 and ET-3 had no effect. ET-1 remarkably affects the increase of corneal endothelial cells on 3H-thymidine, 3H-leucine, and 3H-uridine uptakes in a dose-dependent manner. The 50% effective concentrations (EC50) for ET-1, as measured by 3H-thymidine uptake, 3H-uridine uptake, and 3H-leucine uptake were 10(-8.78) M, 10(-8.53) M and 10(-8.04) M, respectively. It was found that endothelin-1 increased intracellular calcium concentration by using the method of preloading with Fura-2-AM and assaying with spectrophotometry. The cellular IP1, IP2, and IP3 were also stimulated in the presence of ET-1. Moreover, ET-1 enhanced the basal cellular cAMP and cGMP concentrations in corneal endothelial cells in a dose-dependent manner. Immunofluorescent staining revealed that ET-1 increased the fibronectin protein concentration and changed protein distribution in corneal endothelial cells. These findings indicate that endothelin-1 increases in cell proliferation and biological changes may be involved in changing intracellular calcium mobility, increasing intracellular phosphoinositides, enhancing intracellular cGMP and cAMP accumulation, and fibronectin protein synthesis.