Endothelin stimulates inositol phosphate accumulation in rat right and left atria: a comparison with noradrenaline

Effects of ischaemia, reperfusion and alpha 1-adrenergic receptor stimulation on the inositoltrisphosphate receptor population in rat heart atria and ventricles

Binding sites specific for inositol 1,4,5-trisphosphate (InsP3) have been demonstrated in sarcoplasmic reticulum vesicles isolated from heart muscle. Scatchard analysis of a binding isotherm indicated a high as well as a low affinity binding site [1]. In this study a comparison was made between InsP3, binding to crude microsomal membranes prepared from rat heart atria and ventricles respectively. Results obtained showed a four-fold higher incidence of binding to atrial membranes. Furthermore, the receptor populations of the atria and ventricles behaved differently during conditions causing fluctuations in tissue InsP3, levels, viz. ischaemia, reperfusion and alpha 1-adrenergic stimulation. Reperfusion, as well as phenylephrine stimulation, caused an increase in InsP3 levels associated with down-regulation of the ventricular InsP3 receptor population while binding to atrial binding sites was elevated. In the ventricular population this down-regulation was the result of a reduction in Bmax alone with no changes in the Kd values of the high- or the low-affinity binding sites. The reason(s) for the differential response of the atrial and ventricular InsP3 receptor populations to changes in InsP3 levels, remains to be established.

Diminished vascular response to inhibition of endothelium-derived nitric oxide and enhanced vasoconstriction to exogenously administered endothelin-1 in clinically healthy smokers

BACKGROUND

Smoking is a major risk factor for the development of atherosclerosis. Because endothelial dysfunction may be a marker for future atherosclerosis, we investigated the effects of smoking on endothelium-dependent control of vascular tone.

METHODS AND RESULTS

The effects of brachial arterial infusions of NG-monomethyl-L-arginine (L-NMMA), a nitric oxide synthesis inhibitor; sodium nitroprusside; endothelin-1; and norepinephrine on forearm blood flow (strain-gauge plethysmography) were compared in 29 long-term smokers and 16 nonsmokers. The acute effects of smoking on systemic hemodynamics, plasma catecholamines, and forearm vascular responses to these compounds were investigated in smokers only. Smokers did not differ from nonsmokers (n = 16) regarding the vascular effects of sodium nitroprusside (n = 13) or vasoconstriction due to norepinephrine and endothelin-1 (n = 16). Low-dose endothelin-1-induced vasodilation, believed to reflect endothelial prostacyclin or nitric oxide release, was absent in smokers (n = 16), and their increase of forearm vascular resistance (FVR) after L-NMMA (n = 13) was impaired (35.6 +/- 27.9% versus 118.8 +/- 43.2%, P < .001). Short-term smoking (n = 11) increased blood pressure, heart rate, and plasma epinephrine concentrations (P < .05 or less); enhanced endothelin-1-induced vasoconstriction (delta FVR, 457 +/- 192% versus 254 +/- 143%, P < .01); and decreased norepinephrine-induced vasoconstriction (P < .05), but had no effect on the other interventions.

CONCLUSIONS

Long-term smoking is associated with a diminished nitric oxide-dependent component of basal vascular tone and an impaired endothelium-dependent vasodilator response to low-dose endothelin-1 and short-term smoking enhances endothelin-1-induced vasoconstriction. Impaired endothelial control of vascular tone might reflect impairment of normal antiatherosclerotic endothelial functions in smokers, but the relevance of smoking-induced enhancement of endothelin-1 vasoconstriction remains to be determined.

The effect of ischaemia-reperfusion on [3H]inositol phosphates and ins(1,4,5)P3 levels in cardiac atria and ventricles--a comparative study

In this study incorporation of [3H]inositol into inositol phosphates and phosphoinositides as well as tissue Ins(1,4,5)P3 levels of the atria and ventricles of isolated, perfused rat hearts were compared. Although the incorporation of [3H]inositol into the phosphoinositides of atria and ventricles was similar, significantly higher (2-3 fold) incorporation rates into inositol phosphates were observed in atrial tissue. Using a D-myo-[3H]Ins(1,4,5)P3 assay system, the Ins(1,4,5)P3 levels observed in atria from perfused rat hearts were also significantly higher than those obtained under the same experimental circumstances in the ventricles. Since previous studies on whole hearts showed inhibition of the phosphatidylinositol (PI) pathway during ischaemia with an immediate significant stimulation upon reperfusion [12, 20], the effects of ischaemia and 1 min postischaemic reperfusion were also examined separately in atria and ventricles. The results showed that 20 min of global ischaemia significantly depressed Ins(1,4,5)P3 levels as well as incorporation of [3H]inositol into ventricular InsP2 and InsP3. Reperfusion caused an immediate (within 1 min) increase in Ins(1,4,5)P3 levels and also [3H]inositol incorporation into all three cytosolic inositol phosphates in the ventricles. However, the effect of ischaemia and reperfusion on Ins(1,4,5)P3 levels as well as the incorporation of [3H]inositol into the inositol phosphates were less prominent in the atria. It therefore appears that the differential responses of the atria and the ventricles to an oxygen deficiency [41] are also reflected in the differences in PI metabolism during ischaemia-reperfusion.

A comparative study of endothelin- and platelet-activating-factor-mediated signal transduction and prostaglandin synthesis in rat Kupffer cells

Endothelin-3 (ET-3) stimulated phosphoinositide metabolism and synthesis of prostaglandins in cultured rat Kupffer cells. ET-3-induced hydrolysis of phosphoinositides was characterized by the production of various inositol phosphates and of glycerophosphoinositol. The mechanism of ET-3-stimulated metabolism of phosphoinositides and synthesis of prostaglandins appeared to be distinct from the effect of platelet-activating factor (PAF) on these processes described previously [Gandhi, Hanahan & Olson (1990) J. Biol. Chem. 265, 18234-18241]. On a molar basis ET-3 was significantly more potent than PAF in stimulating phosphoinositide metabolism, e.g. ET-3-induced hydrolysis of phosphoinositides occurred at 1 pM, whereas PAF was ineffective at concentrations less than 1 nM. Upon challenging Kupffer cells with both ET-3 and PAF, an additive stimulation of phosphoinositide metabolism was observed, suggesting that the actions of these factors may be exerted on separate phosphoinositide pools. Treatment of Kupffer cells with pertussis toxin resulted in an inhibition of ET-3-induced phospholipase C activation; in contrast, cholera toxin treatment caused potentiation of ET-3-stimulated phospholipase C activity. Both toxins, however, inhibited PAF-stimulated phospholipase C activity. The present results suggest that the stimulatory effects of ET-3 and PAF on the phosphodiesteric metabolism of phosphoinositides in Kupffer cells require different guanine-nucleotide-binding proteins. Furthermore, the effects of bacterial toxins on ET-3- and PAF-induced phosphoinositide metabolism were not mediated by cyclic AMP. ET-3-induced metabolism of phosphoinositides was inhibited completely in Kupffer cells pretreated with ET-3, suggesting homologous ligand-induced desensitization of the ET-3 receptors. In contrast, similar experiments using PAF showed only a partial desensitization of subsequent PAF-induced phosphoinositide metabolism. In contrast to the increased production of prostaglandins E2 and D2 observed upon stimulation of Kupffer cells with PAF, ET-3 stimulated the biosynthesis of prostaglandin E2 only. Consistent with their additive effects on phosphoinositide metabolism, PAF and ET-3 exhibited an additive stimulation of the synthesis of prostaglandin E2.

The role of alpha 1-adrenergic stimulation in inositol phosphate metabolism during post-ischaemic reperfusion

The aim of this study was to elucidate the mechanism of enhanced inositol phosphate metabolism during reperfusion. Inositol phosphate stores were prelabelled by perfusing isolated rat hearts for 1 h with [3H]inositol (1.5 microCi/ml). LiCl (10 mM) and prazosin (0.3 microM) were subsequently added 15 min before (i) 20 min control perfusion; (ii) 20 min normothermic ischaemic cardiac arrest (NICA); (iii) 20 min NICA followed by 1 min reperfusion. The ventricles were freeze-clamped before determination of isotopical incorporation of [3H]inositol into the inositol phosphates (Dowex anion exchange chromatography) and InsP3 levels (Amersham InsP3 assay system). In addition, noradrenaline release into the perfusate was also assessed (HPLC and electrochemical detection). The results showed: (i) increased noradrenaline release into the perfusate immediately after the onset of reperfusion; (ii) significant depression of [3H]inositol incorporation into inositol phosphates and InsP3 levels after 20 min NICA; (iii) reperfusion caused an immediate significant increase in isotopical incorporation of [3H]inositol into inositol phosphates as well as InsP3 levels; (iv) the alpha 1-adrenergic blocker, prazosin (0.3 microM), completely inhibited the reperfusion-induced increase in inositol phosphate metabolism. These observations suggested that increased alpha 1-adrenergic receptor stimulation by noradrenaline might be responsible for the stimulation of ventricular inositol phosphate metabolism during postischaemic reperfusion.