Comparison of endothelin-1 and noradrenaline stimulated inositol phosphate formation in cultured aortic smooth muscle cells from spontaneously hypertensive and Wistar Kyoto rats

Expression and distribution of InsP(3) receptor subtypes in proliferating vascular smooth muscle cells

The expression and distribution of types 1, 2, and 3 inositol 1,4, 5-trisphosphate receptor (InsP(3)R) in proliferating, primary cultures of rat aortic smooth muscle were compared to fully developed and differentiated rat aortic smooth muscle. Subtype-specific InsP(3)R antibodies revealed that the expression of type 1 InsP(3)R was similar in cultured aortic cells and aorta homogenate but expression of type 2 and 3 InsP(3)R subtypes was increased 3-fold in cultured aortic cells. The distribution of the type 1 InsP(3)R was located throughout the cytoplasm; type 2 InsP(3)R was found closely associated with the nucleus and at the plasma membrane; type 3 InsP(3)R was distributed predominantly around the nucleus. Alterations in InsP(3)R subtype expression and localization may have important functions in regulating intracellular calcium release around the nucleus when vascular smooth muscle cells switch to a more proliferating phenotype.

Cardiac endothelin-1 content and receptor subtype in spontaneously hypertensive rats

Endothelin-1 (ET-1), a newly discovered peptide with potent vasoconstrictor and growth-promoting effects, has been implicated in high blood pressure and cardiac hypertrophy in the spontaneously hypertensive rat (SHR). In the present study, we measured plasma ET-1 levels and tissue ET-1 concentrations in the four cardiac chambers of 17- to 18-week-old SHR and their normotensive controls. Wistar Kyoto (WKY) rats. SHR had slightly but significantly higher plasma ET-1 levels than WKY. The ventricles had the highest ET-1 content and the atria in both strains had the highest ET-1 concentrations. ET receptor subtypes were analysed by radiogand binding with ET-1, BQ-123 and IRL 1620 in crude membrane preparations of the four cardiac chambers. No differences in receptor subtype densities or affinities were apparent between the two strains. ET(A) represented 75 to 85% of both ET receptors. Competition analysis revealed that in both strains left ventricular tissue had lower receptor densities and higher affinities than the atria. These results suggest that ET-1 and its receptor although contributing in the maintenance of high blood pressure may not be an important factor during stable cardiac hypertrophy in adult SHR. The differential distribution of ET-1 content and receptor densities favoring the atria in both strains suggest that this peptide may have a different physiological role in the atria from that in the ventricles.

Pertussis toxin sensitive endothelin-1 coupling to inositol phosphate formation via a GTP-binding protein: comparison in SHR and WKY cultured aortic smooth muscle cells

The effects of pertussis toxin on endothelin-1 and noradrenaline coupling to inositol phosphate (IP) formation was investigated in cultured aortic smooth muscle cells from 14 week SHR and WKY rats. Endothelin-1 (10(-6) M) stimulated IP formation was decreased in cells from SHR compared to WKY (WKY 1117 +/- 157, SHR 668 +/- 85% of basal). Pre-incubation with pertussis toxin produced a significant and similar reduction in endothelin stimulated IP production in both SHR (54% reduction) and WKY (55%). However, the observed reduction in endothelin-1 stimulated IP accumulation was still apparent in SHR when compared to WKY. Pertussis toxin preincubation followed by removal of extracellular calcium reduced further the endothelin responses by similar amounts in SHR and WKY cells, but SHR stimulated IP formation remained significantly decreased compared to WKY. The extent of pertussis toxin ADP-ribosylation of Gi alpha was similar in both SHR and WKY cells. Endothelin-1 produced a reduction in the extent of ADP-ribosylation of Gi alpha and this was of similar magnitude in both SHR and WKY cell membranes. In contrast, noradrenaline stimulated IP formation was unaffected by pertussis toxin pre-incubation. It was concluded that SHR cells do not appear to have an alteration in endothelin-1 activated, pertussis toxin sensitive G-protein coupling to IP formation or in the dependence of inositol phosphate formation on extracellular calcium.

Chronic exposure of bovine aortic endothelial cells to native and oxidized LDL modifies phosphatidylinositol metabolism

The aim of this study was to investigate the effects of chronic exposure to low density lipoprotein (LDL) and oxidised LDL (OXLDL) on phosphatidylinositol metabolism in bovine aortic endothelial cells. Basal levels of total inositol phosphates and inositol 1,4,5-trisphosphate were increased after both 18 and 66 h exposure to OXLDL 20 micrograms/ml. Levels also tended to be increased after exposure to LDL but this only reached significance for LDL 20 micrograms/ml after 18 h exposure. Absolute levels of inositol phosphates after stimulation with ATP were unaffected by incubation with LDL or OXLDL. However, when expressed as a percentage of basal levels, stimulated levels of inositol phosphates were reduced for ATP 10(-3) and 10(-4)M. Uptake of [3H]inositol into the phosphatidylinositol cycle was reduced after incubation with LDL and OXLDL for either 18 or 66 h. The effect of OXLDL was greater than that of LDL. The antioxidants EDTA and N-acetylcysteine attenuated the effects of LDL but not OXLDL. In addition, catalase but not mannitol or superoxide dismutase modified the effect of LDL on [3H]inositol uptake. These studies show that chronic exposure to OXLDL and to a lesser extent LDL can modify phosphatidylinositol metabolism in bovine aortic endothelial cells and that the effects of LDL may be attenuated by antioxidants and free radical scavengers. We hypothesise that the decreased uptake of [3H]inositol could be related to an alteration in membrane structure and integrity and may reflect alteration in transport of a number of ions and molecules.