Selective estrogen receptor modulators: mechanism of action and clinical experience. Focus on raloxifene

Selective estrogen receptor modulators (SERMs) are a diverse group of compounds that bind with specific, high-affinity binding to the estrogen receptor (ER). Depending on the tissue, SERMs can act as either ER agonists or antagonists. Recent advances in ER biology have provided insight into possible mechanisms by which SERMs elicit these tissue-specific estrogen agonist and estrogen antagonist activities. The estrogen response pathway has been shown to differ among target tissues depending on various tissue and cellular factors, such as the ER subtype, the structure of the bound receptor-ligand complex, and the tissue-specific cellular transcriptional machinery. Clinically available SERMs include clomiphene, tamoxifen and toremifene, which are triphenylethylenes, and raloxifene, a benzothiophene. Raloxifene has estrogen agonist effects on bone, serum lipids, and arterial vasculature, and estrogen antagonist effects in breast and uterus. Clinical trial data for raloxifene is used to illustrate some of the mechanisms by which SERMs exert their tissue-specific estrogen agonist and estrogen antagonist effects. The complex pharmacology surrounding the tissue selectivity of SERMs is discussed.

Role of endothelium in responses of isolated hepatic vessels to vasoactive agents

The role of the endothelium as a participant in the responses to vasoactive agents was evaluated in isolated canine hepatic artery (HA) and portal vein (PV) rings. Endothelial and smooth muscle integrity was determined by pharmacologic responses as well as by histologic examination. Smooth muscle relaxation was expressed as the percent of decrease of norepinephrine-induced isometric contraction. Acetylcholine (ACh)-induced relaxation of the HA was abolished by removing the endothelium or by the addition of either hemoglobin, methylene blue (MB) or Ng-mono-methyl-L-arginine. In addition, relaxation induced by nitroglycerin, but not that induced by prostaglandin E1, was attenuated by MB. These data suggest endothelium-dependency of the relaxation to ACh and mediation of the response by endothelium-derived relaxing factor through activation of guanylate cyclase. In contrast, ACh produced contraction of the PV which was unaffected by removing the endothelium. The calcium ionophore, A23187, on the other hand, produced relaxation of the PV, which was significantly decreased by removing the endothelium. Relaxation of both HA and PV, produced by 2-chloroadenosine (2-C-Ado) was partially attenuated by removing the endothelium. With the endothelium intact, neither hemoglobin, MB, Ng-monomethyl-L-arginine nor indomethacin affected the responses to 2-C-Ado in the HA and PV, suggesting that the responses were not mediated by endothelium-derived relaxing factor or products of guanylate cyclase or cyclooxygenase activity. Nitroglycerin relaxed both vessels in the presence or absence of endothelium, indicating that removal of the endothelium had not affected smooth muscle function.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypotensive properties of the protein kinase inhibitor, staurosporine, in normotensive and spontaneously hypertensive rats

The protein kinase inhibitor, staurosporine, was examined for hemodynamic actions in conscious rats and for its effects on isolated vascular smooth muscle. In conscious normotensive rats, staurosporine, 0.1 to 1.0 mg/kg intravenous, produced a dose-related decrease in blood pressure. Doses of 0.1 and 0.5 mg/kg produced reflex tachycardia, while heart rate decreased following 1.0 mg/kg. Higher doses of staurosporine (5 and 10 mg/kg), although not significantly altering heart rate, caused a precipitous drop in blood pressure and all rats died within 48 hours of treatment. Spontaneously hypertensive rats were significantly more sensitive than normotensive rats to the blood pressure lowering effects of staurosporine. The depressor activity of staurosporine was also observed in cord-stimulated pithed rats indicating that hypotension is a peripherally- rather than centrally-mediated effect of staurosporine. In ring preparations of rat thoracic aorta, staurosporine (3 nM to 0.1 mM) produced a concentration-dependent inhibition of serotonin-induced contraction, consistent with its ability to lower blood pressure by an effect at a peripheral vascular site. These results provide the first documentation of the potent cardiovascular activity of staurosporine in the conscious rat and support the contention that protein kinases, in particular protein kinase C, may be important in the regulation of vascular tone.

Comparison of contractions to serotonin, carbamylcholine and prostaglandin F2 alpha in rat stomach fundus

Contractile effects of serotonin were compared to those of carbamylcholine and prostaglandin (PG) F2 alpha in an effort to characterize serotonergic receptor activation in rat stomach fundus. All three agents elicited concentration-dependent contractions of fundal strips with serotonin (EC50 = 2 X 10(-10) M) being approximately 100-fold more potent than carbamylcholine (EC50 = 2 X 10(-8) M) and PGF2, (EC50 = 10(-8) M). The calcium channel blockers, diltiazem (5 X 10(-7) to 5 X 10(-5) M) and nitrendipine (10(-8) to 10(-5) M), attenuated responses markedly to serotonin and PGF2 alpha whereas having only minimal effects on carbamylcholine-induced contractions. Neither serotonin (10(-11) to 10(-5) M) nor PGF2 alpha (10(-9) to 10(-5) M) altered [3H]inositol monophosphate generation in fundus whereas carbamylcholine (10(-6) to 10(-5) M) increased significantly [3H]inositol monophosphate with 10(-5) M eliciting an 8-fold increase. Strips of fundus contracted submaximally with either serotonin, PGF2 alpha or carbamylcholine were compared for sensitivity to relaxants. Pinacidil (10(-8) to 10(-5) M) was equipotent (EC50 = 0.1 microM) in relaxing serotonin- and PGF2 alpha-contracted tissues. In contrast, pinacidil was 10-fold less potent in relaxing contractions produced by carbamylcholine. Likewise, nitroglycerin (10(-8) to 10(-4) M) and isoproterenol (10(-12) to 10(-7) M) were at least 10-fold more potent in relaxing serotonin- and PGF2 alpha- than carbamylcholine-induced contractions. Thus, in rat fundus, contractions associated with increased phosphoinositide hydrolysis may be more resistant to relaxation than are contractions not associated with altered phosphoinositide hydrolysis.(ABSTRACT TRUNCATED AT 250 WORDS)

Endothelin: differential effects in vascular and nonvascular smooth muscle

Endothelin, a potent vasoconstrictor, produced concentration-dependent contractions in aorta, trachea and bladder body obtained from rat and rabbit. Contractions developed slowly, reaching maxiMum within 15-20 min. Although, in both rat and rabbit tissues, endothelin was 3- to 10-fold more potent in contracting vascular (approximate EC50, 1 nM) than nonvascular smooth muscle, rat trachea and rabbit bladder did contract in response to endothelin. Rat bladder body and rabbit trachea were the least sensitive tissues with only modest contractile responses to endothelin. To determine the role of calcium in these endothelin-induced contractions, the effects of diltiazem and nitrendipine were examined. Although diltiazem (5 x 10-5) M) or nitrendipine (10(-6) M) markedly attenuated contractions produced by KCl, neither agent significantly affected concentration response curves produced by endothelin in rabbit aorta or rat trachea. In rat aorta, nitrendipine had no effect on endothelin responses, whereas diltiazem modestly decreased the maximal contraction to endothelin. However, in rabbit bladder, both calcium channel blockers significantly decreased the maximum response to endothelin with no change in EC50. These results indicate that smooth muscle sensitivity to the contractile effects of endothelin may be both species and tissue specific.

Endothelial-dependent relaxation induced by leukotrienes C4, D4, and E4 in isolated canine arteries

Leukotriene D4 has been shown to possess the capacity to relax canine superior mesenteric and renal arterial rings in an endothelial-dependent manner. The present study was designed to determine if the remaining peptidoleukotrienes, leukotrienes C4 and E4, share this property. In addition, influences of atropine and of inhibitors of cyclooxygenase and lipoxygenase activities on relaxation produced by leukotriene D4 and acetylcholine were determined to characterize further leukotriene D4-induced relaxation and to compare these properties with those of acetylcholine. Vasomotor tone was measured with isometric force transducers. Following induction of tone with norepinephrine, leukotriene C4 and acetylcholine produced concentration-dependent relaxation of renal and superior mesenteric arterial rings in which the endothelium was intact. Only minimal decreases in tone were produced in response to leukotriene E4. Neither acetylcholine nor leukotriene C4 altered tone after endothelium had been intentionally disrupted. Nitroglycerin relaxed rings both before and after rubbing the endothelium. These results demonstrate that, similar to leukotriene D4, leukotriene C4 possesses the capacity to produce endothelial-dependent relaxation in canine renal and superior mesenteric arteries. Relaxation of the superior mesenteric artery produced in response to acetylcholine, but not leukotriene D4, was inhibited in presence of atropine. Incubation of the rings with meclofenamate had no effect on relaxation induced by either acetylcholine or leukotriene D4. Thus, it appears that endothelial-dependent relaxation induced by leukotriene D4 is neither dependent on muscarinic receptor activation nor related to generation of cyclooxygenase metabolites of arachidonic acid. In contrast, 5,8,11,14-eicosatetraynoic acid and nordihydroguaiaretic acid attenuated relaxation in response to leukotriene D4 and acetylcholine, suggesting that lipoxygenase-derived products may participate in leukotriene D4-induced as well as acetylcholine-induced relaxation.

Relationship between LTD4-induced endothelium-dependent vasomotor relaxation and cGMP

The objective of the present investigation was to determine whether cyclic GMP (cGMP) participated in endothelium-dependent leukotriene (LT) D4-induced relaxation of canine superior mesenteric artery. All experiments were performed with endothelium-intact arterial rings, and tone was measured with isometric force displacement transducers. After tone had been induced with norepinephrine, LTD4 acetylcholine (ACh), nitroglycerin (GTN) and isoproterenol (ISOP), each produced concentration-dependent vasomotor relaxation. Because contractile responses produced by norepinephrine were enhanced in presence of methylene blue, the concentration of norepinephrine was reduced in order to induce tone that was equivalent to control. After this adjustment, incubation of the rings with methylene blue attenuated relaxation produced by LTD4, ACh and GTN, whereas ISOP-induced decreases in tone were unaltered. In addition, results of a separate series of experiments showed that cGMP levels in the arterial rings were enhanced at the time of peak relaxation produced in response to LTD4, ACh and GTN. In contrast to these observations, ISOP-induced relaxation was not associated with a change in cGMP concentration. When taken together, these results suggest that cGMP serves as a participant in endothelium-dependent relaxation produced by both LTD4 and ACh, as well as endothelium-independent relaxation produced by GTN.

Leukotriene D4 relaxes canine renal and superior mesenteric arteries

To characterize the influences of leukotriene D4 on regional vascular smooth muscle, effects of leukotriene D4 on vasomotor tone of canine renal and superior mesenteric arterial rings were determined. Vascular smooth muscle tone was measured with isometric force transducers. After tone had been induced with norepinephrine, leukotriene D4, in concentrations of 10(-8) M to 10(-7) M, produced dose-dependent relaxation of renal and superior mesenteric arterial rings. Leukotriene D4-induced relaxation was observed only in those ring preparations in which care had been taken to avoid damaging the luminal surface. Acetylcholine (10(-7) M) also decreased tone in these same ring segments. Neither acetylcholine nor leukotriene D4 altered tone of arterial rings after the endothelium had been intentionally disrupted by rubbing with a cotton-tipped applicator. Nitroglycerin (10(-6) M) relaxed rings both before and after rubbing the intimal surface. These results demonstrate that leukotriene D4 possesses the capacity to relax canine superior mesenteric and renal arterial rings in an endothelial-dependent manner. Because relaxation of renal and superior mesenteric arterial rings in response to leukotriene D4 was not altered after incubation with indomethacin (10(-5) M), the observed endothelial-dependent relaxation induced by leukotriene D4 did not appear to be related to release of a cyclooxygenase metabolite(s). In contrast, FPL 55712 (10(-5) M) attenuated the relaxation produced by leukotriene D4, suggesting that this response was a receptor-linked consequence.