Influence of various drugs on the variations of blood pressure, hematocrit and plasma histamine caused by neurotensin and compound 48/80 in rats

Cardiac mast cells mediate left ventricular fibrosis in the hypertensive rat heart

Correlative data suggest that cardiac mast cells are a component of the inflammatory response that is important to hypertension-induced adverse myocardial remodeling. However, a causal relationship has not been established. We hypothesized that adverse myocardial remodeling would be inhibited by preventing the release of mast cell products that may interact with fibroblasts and other inflammatory cells. Eight-week-old male spontaneously hypertensive rats were treated for 12 weeks with the mast cell stabilizing compound nedocromil (30 mg/kg per day). Age-matched Wistar-Kyoto rats served as controls. Nedocromil prevented left ventricular fibrosis in the spontaneously hypertensive rat independent of hypertrophy and blood pressure, despite cardiac mast cell density being elevated. The mast cell protease tryptase was elevated in the spontaneously hypertensive rat myocardium and was normalized by nedocromil. Treatment of isolated adult spontaneously hypertensive rat cardiac fibroblasts with tryptase induced collagen synthesis and proliferation, suggesting this as a possible mechanism of mast cell-mediated fibrosis. In addition, nedocromil prevented macrophage infiltration into the ventricle. The inflammatory cytokines interferon-gamma and interleukin (IL)-4 were increased in the spontaneously hypertensive rat and normalized by nedocromil, whereas IL-6 and IL-10 were decreased in the spontaneously hypertensive rat, with nedocromil treatment normalizing IL-6 and increasing IL-10 above the control. These results demonstrate for the first time a causal relationship between mast cell activation and fibrosis in the hypertensive heart. Furthermore, these results identify several mechanisms, including tryptase, inflammatory cell recruitment, and cytokine regulation, by which mast cells may mediate hypertension-induced left ventricular fibrosis.

Evidence that neurotensin mediates postprandial intestinal hyperemia in the python, Python regius

Digestion of large meals in pythons produces substantial increases in heart rate and cardiac output, as well as a dilation of the mesenteric vascular bed leading to intestinal hyperemia, but the mediators of these effects are unknown. Bolus intra-arterial injections of python neurotensin ([His3, Val4, Ala7]NT) (1 − 1,000 pmol/kg) into the anesthetized ball python Python regius ( n = 7) produced a dose-dependent vasodilation that was associated with a decrease in systemic pressure (Psys) and increase in systemic blood flow (Qsys). There was no effect on pulmonary pressure and conductance. A significant ( P < 0.05) increase in heart rate ( fH) and total cardiac output (Qtot) was seen only at high doses (>30 pmol/kg). The systemic vasodilation and increase in Qtot persisted after β-adrenergic blockade with propranolol, but the rise in fH was abolished. Also, the systemic vasodilation persisted after histamine H2-receptor blockade. In unanesthetized pythons ( n = 4), bolus injection of python NT in a dose as low as 1 pmol/kg produced a significant increase in blood flow to the mesenteric artery (177% ± 54%; mean ± SE) and mesenteric conductance (219% ± 74%) without any increase in Qsys, systemic conductance, Psys, and fH. The data provide evidence that NT is an important hormonal mediator of postprandial intestinal hyperemia in the python, but its involvement in mediating the cardiac responses to digestion may be relatively minor.

Cold water swim stress increases the expression of neurotensin mRNA in the lateral hypothalamus and medial preoptic regions of the rat brain

Stress-induced analgesia is a well-documented phenomenon that occurs in all mammalian species. Forced cold water swim produces a type of stress-induced analgesia that is independent of mu opioid receptors. The neuropeptide neurotensin (NT) has been implicated in mu opioid-independent analgesia (MOIA), but the circuitry of this system is largely unknown. The medial preoptic area (MPO) and lateral hypothalamus (LH) are two regions that are known to modulate pain processing. These two regions also contain neurotensinergic projections to the periaqueductal gray, a region that has been shown to produce MOIA upon injection of NT. The goal of this study was to determine if cold water swim (CWS) stress, which produces MOIA, activates the NT-ergic systems in these two regions. In situ hybridization results indicate that CWS increases the level of NT mRNA within neurons in the MPO and LH, suggesting that these two regions are activated during this process.

Mechanisms contributing to the regional haemodynamic effects of neurotensin in conscious, unrestrained Long Evans rats

1. The regional haemodynamic effects of i.v. bolus doses of neurotensin (10-1000 ng) were assessed in conscious, unrestrained Long Evans rats chronically instrumented with miniaturized, pulsed Doppler probes. 2. Neurotensin caused increases in blood pressure, together with dose-related tachycardias and constrictions in the renal, superior mesenteric and hindquarters vascular beds. The tachycardia elicited by the 1000 ng dose of neurotensin was preceded by a transient bradycardia. 3. In the presence of phentolamine, the pressor effect of neurotensin (1000 ng) was converted into a hypotensive effect, accompanied by reduced tachycardic and constrictor responses in the renal, superior mesenteric and hindquarters vascular beds. The tachycardia was not preceded by a bradycardia. 4. In the presence of phentolamine and propranolol, the pressor and bradycardic responses to neurotensin were unaffected, whereas the tachycardia was abolished. The renal vasconstrictor effect was smaller, while the constrictions in the superior mesenteric and hindquarters vascular beds were not different from those in untreated rats. 5. In rats neonatally treated with capsaicin (50 mg kg-1, s.c.), the pressor effects elicited by neurotensin (300 and 1000 ng) were reduced as were the constrictor responses in the renal (at the dose of 300 ng), superior mesenteric (at the dose of 300 ng) and hindquarters (at both doses) vascular beds. The bradycardia elicited by neurotensin (1000 ng) was absent, whereas the tachycardia was potentiated. 6. The results indicate that in conscious, intact rats neurotensin appears to exert cardiovascular influences through activation of sympathoadrenal mechanisms and also through non-adrenergic effects on the heart, renal, superior mesenteric and hindquarters vascular beds. The latter effects appear to involve capsaicin-sensitive nerves.

Cardiovascular effects of neurotensin and some analogues on rats

When administered in vivo into the femoral vein of normotensive rats, neurotensin, neurotensin-(8-13), and [D-Lys8]neurotensin-(8-13) decreased diastolic blood pressure in a dose-dependent manner, without change in heart rate. All three peptides evoked tachyphylaxis and a triphasic depressor-pressor-depressor, response at higher doses. The rank order of potency was neurotensin greater than [D-Lys8]neurotensin-(8-13) greater than neurotensin-(8-13). In organ chamber experiments, both neurotensin and neurotensin-(8-13) at a range of concentrations which induced dose-dependent decreases in blood pressure, did not significantly change tension in rat aorta rings with or without endothelium. In contrast, [D-Lys8]neurotensin-(8-13) induced weak dose-dependent relaxation of both rings with or without endothelium. However, this effect was not obtained at concentrations able to decrease the blood pressure. Indomethacin did not change the vasodilator effect of [D-Lys8]neurotensin-(8-13). There was no correlation between the vasodilator effect of this peptide and its ability to decrease blood pressure. These experiments suggest that the hypotension was not due to a direct vasodilator effect on the smooth muscle. In addition, since the rank order of peptide potency was opposite of those found in previous studies of second messenger synthesis and binding to neural tissue, these data suggest that there is a second receptor for neurotensin or that neurotensin can also bind to a different unknown receptor.

Excitatory neurotensin receptors on the smooth muscle of the rat fundus: possible implications in gastric motility

Picomolar concentrations of neurotensin caused concentration-dependent contractions of the longitudinal musculature of the fundus of the rat stomach. The EC50 of neurotensin was approximately 1.5 nM. On a molar basis neurotensin was about 5-10 times more potent than 5-hydroxytryptamine (5-HT) and approximately 80 times as active as acetylcholine in producing similar contractions. Studies with structurally related peptides indicated that whereas the carboxy terminal portion of neurotensin was essential for biological activity, a substantial part of its amino terminus end could be removed without affecting its potency. The EC50 for the neurotensin fragment 8-13 was identical to that of neurotensin, however its 1-8 or 1-11 fragments were completely inactive. Tetrodotoxin did not modify the potency of neurotensin or structurally related analogues suggesting that the neurotensin receptor is probably located on the smooth muscle membrane. In addition, the potency of neurotensin in contracting the fundus was not modified by pretreatment with atropine, methysergide or diphenhydramine. Fade to the contractile response of neurotensin was followed by the development of tachyphylaxis; desensitization was concentration-dependent and characterized by a shift in the agonist concentration-response curve to the right and downwards. Desensitization with a priming concentration of neurotensin (approx. EC50) caused a substantial blockade of its excitability. There was cross-desensitization between neurotensin and the contractile activity of neurotensin 8-13 or xenopsin, but not with angiotensin II, bradykinin, substance P, acetylcholine, 5-HT or histamine. Pretreatment of the fundus strip with verapamil 0.3-1 microM antagonized in a concentration-dependent fashion the neurotensin-induced contractions but not the muscular contractions caused by acetylcholine. It is concluded that neurotensin activates a specific excitatory receptor probably located on the cell membrane of the smooth muscles of the rat fundus. In addition, we suggest that this receptor is somehow related to a voltage-dependent calcium channel, sensitive to verapamil.

Influence of histamine and prostaglandin on desensitization to neurotensin in rat blood pressure

Triphasic depressor-pressor-depressor blood pressure responses to neurotensin (NT: 1.67 micrograms/kg i.v.) in anesthetized rats were not elicited when the second dose of NT was administered 20 min after the first injection. Pretreatment of animals with histamine markedly reduced the depressor response to NT, and vice versa. The triphasic blood pressure pattern remained unaffected with acetylcholine and serotonin treatment, and hypotensive effects of acetylcholine and serotonin were not modified by NT. Attenuation of depressor response induced by the second injection of NT was antagonized by pretreatment with prostaglandin synthesis inhibitors such as indomethacin, mefenamic acid and acetylsalicylic acid. These results suggest that histamine and prostaglandins play a role in the development of desensitization to NT in rat blood pressure.

Neurotensin stimulates histamine release from the isolated, spontaneously beating heart of rats

Neurotensin (NT) evoked a transient, dose-dependent histamine release (ED50 170 ng ml-1) from the rat perfused heart. Histamine release by NT occurred within seconds and lasted less than 2 min. The histamine releasing effect of NT was followed by a dose-dependent increase of the perfusion pressure and a slight tachycardia. The histamine releasing effect of NT was completely abolished in hearts derived from rats pretreated for 3 days with high doses of compound 48/80. The coronary vasoconstrictor effect of NT was increased in hearts derived from compound 48/80-pretreated rats. The mast cell inhibitor cromoglycate markedly inhibited NT-induced histamine release without affecting the coronary vasoconstrictor effect of NT. The histamine releasing effect of NT was inhibited, while its coronary vasoconstrictor effect was markedly potentiated, in hearts derived from rats pretreated with the antiallergic and antiinflammatory steroid dexamethasone. The increase of perfusion pressure evoked by NT was not modified by antihistamine drugs. Infusions of exogenous histamine (10(-6)-10(-5) g ml-1) caused a dose-dependent coronary vasodilation in the rat perfused heart. The results suggest that NT stimulates histamine release from cardiac mast cells. These results together with those obtained in previous studies suggest that mast cell mediators (particularly histamine and serotonin) are unlikely to be responsible for the coronary vasoconstrictor effect of NT in the rat perfused heart.

Histamine release by neurotensin in the rat hindquarter: structure-activity studies

Histamine releasing effects of neurotensin (NT) and several NT fragments and structural analogues were measured in the rat perfused hindquarter. The results show that the chemical groups responsible for histamine release are located in the C-terminal sequence Arg9-Pro10-Tyr11-Ile12-Leu13-OH. Both the spatial configuration and positive charge of Arg8 and Arg9 appear to contribute to the histamine releasing effect of NT. Optimization of the histamine releasing effect of NT requires both a free C-terminal carboxyl group and the presence in position 11 of NT of an aromatic residue, with the L-configuration, bearing an heteroatom capable of hydrogen bonding with the receptor. The results indicate that the structural requirements of NT to induce histamine release from the rat perfused hindquarter are similar to those involved in other peripheral biological actions of NT.

Characterization of the inhibitory effect of [D-Trp11]-NT toward some biological actions of neurotensin in rats

We assessed the influence of various doses of [D-Trp11]-NT on the increase of histaminemia and hematocrit, and decrease of blood pressure, caused by intravenous injections of neurotensin (NT), substance P (SP) and compound 48/80 (C48/80) in anesthetized rats. [D-Trp11]-NT was found to inhibit dose-dependently and selectively the changes of histaminemia, hematocrit and blood pressure caused by NT. Since the highest dose of [D-Trp11]-NT utilized exhibited slight NT-like activity, we tested the possibility that desensitization rather than true pharmacological antagonism was responsible for the inhibitory action of [D-Trp11]-NT toward NT. This hypothesis was verified by evaluating the influence of intravenous infusions of sub-stimulatory and slightly active doses of NT on NT-induced effects. The sub-stimulatory dose (0.1 nmoles kg-1 min-1) as well as a higher dose rate (0.2 nmole kg-1 min-1) of NT were found to inhibit markedly the changes of histaminemia, hematocrit and blood pressure evoked by bolus doses of NT, without altering the effects of C48/80 on the same parameters. These results suggest that the inhibitory action of [D-Trp11]-NT toward NT-induced changes of histaminemia, hematocrit and blood pressure could be the result of receptor desensitization rather than to a true pharmacological antagonism. The results also suggest that the sensitivity of target tissues to exogenous NT could be modulated to some extent by endogenous circulating levels of NT.