Pressor response induced by clenbuterol treatment in immobilized normotensive rats

Short-term treatment with clenbuterol [0.6 mg/kg-1 subcutaneously (s.c.) daily] produces a pressor effect in stressed rats after a period of immobilization (40 min). The stress applied markedly increases the plasma concentrations of norepinephrine (NE) and epinephrine. After bilateral adrenal demedullation, the increased levels of catecholamines and the hypertensive response obtained after clenbuterol treatment in the stressed animals were reduced to the values of the control rats. Clenbuterol treatment produced desensitization of the beta 2-adrenoceptor-mediated effect and thus reduced the vasodilator response induced by isoproterenol and increased the vasoconstriction produced by epinephrine but not that caused by NE. This desensitization may be responsible for the hypertensive response after clenbuterol treatment in stressed animals which is attenuated after adrenal demedullation. In conclusion, our results provide evidence that clenbuterol treatment induces pressor effect in normotensive animals under stress.

The actin domain of Gardner-Rasheed feline sarcoma virus inhibits kinase and transforming activities

The transforming gene product, P70gag-actin-fgr, of Gardner-Rasheed feline sarcoma virus (GR-FeSV) is a single polypeptide composed of regions derived from cellular and viral genes. Gamma actin and c-fgr genes are the two known cellular components of the GR-FeSV genome. In the present study, sequences representing each cell-derived gene were deleted and the resulting constructs were tested for transforming activity by transfection of NIH 3T3 cells. Constructs lacking a portion of the c-fgr proto-oncogene failed to induce focus formation, demonstrating the essential nature of this component for GR-FeSV oncogenic activity. In contrast, the construct lacking the actin domain was more active than GR-FeSV DNA in transformation assays. Protein specified by the actin deletion mutant possessed a 2.4-fold greater specific protein-tyrosine kinase activity compared with that of the wild-type gene product. Furthermore, the actin domain had no detectable effect on the ability of the fgr kinase to associate with cytoskeleton or to phosphorylate unique cellular proteins on tyrosine. Our findings demonstrate that the actin domain inhibits focus formation and impairs protein-tyrosine kinase activity.

Effect of chronic administration of the converting enzyme inhibitor enalapril (MK 421) on brain atrial natriuretic peptide receptors in Wistar-Kyoto and spontaneously hypertensive rats

Spontaneously hypertensive rats (SHR) showed lower brain ANP binding density when compared with normotensive Wistar-Kyoto (WKY) rats. In the WKY, angiotensin converting enzyme inhibitor, enalapril (25 mg/kg, p.o. for 14 days), decreased the number of ANP binding sites selectively in the subfornical organ and area postrema. Conversely, enalapril increased ANP binding density in the SHR, but only in the area postrema. Enalapril has central effects on ANP binding sites, specific to the circumventricular organs.

Increased angiotensin II receptors in brain nuclei of DOCA-salt hypertensive rats

We analyzed angiotensin II (ANG II) receptors by in vitro autoradiography in selective brain nuclei of control, salt-treated (1% NaCl in drinking water), deoxycorticosterone acetate (DOCA)-treated (DOCA pivalate, 25 mg/kg sc weekly), and DOCA-salt-treated (DOCA + salt treatments) uninephrectomized male Wistar-Kyoto rats. After 4 wk of treatment, only the DOCA-salt group developed hypertension. ANG II binding increased in median preoptic nucleus and subfornical organ of salt- and DOCA-treated rats. DOCA-treated rats also showed increased ANG II binding in paraventricular nucleus. DOCA-salt-treated rats showed higher ANG II binding in nucleus of the solitary tract and area postrema, as well as in the areas mentioned before. Although salt and/or DOCA treatments alone increased ANG II receptors in some brain nuclei, after combined DOCA-salt treatment there was significantly higher ANG II binding in all areas, except the median preoptic nucleus. These results suggest that increased ANG II receptors in selected brain areas may play a role in the pathophysiology of mineralocorticoid-salt experimental hypertension.

Autoradiographic quantification of vasoactive intestinal peptide binding sites in sections from human blood mononuclear cell pellets

Quantitative autoradiographic methods were utilized to characterize specific, high-affinity vasoactive intestinal peptide binding sites (Kd = 310 +/- 60 pmol/L; Bmax = 93 +/- 11 fmol/mg protein) in frozen sections obtained from a mononuclear cell pellet derived from 20 ml of human blood. The method is at least one order of magnitude more sensitive than conventional membrane binding techniques, and it has the potential for wide applications in studies of neuropeptide, biogenic amine, and drug binding in clinical samples.

Alteration of atrial natriuretic peptide binding sites in spontaneously hypertensive rats

Atrial natriuretic peptide (ANP) binding sites were studied by quantitative autoradiography in young (4-week-old) and adult (14-week-old) spontaneously hypertensive rats (SHRs) and age-matched Wistar-Kyoto (WKY) control rats. Young SHRs had fewer ANP binding sites in the smooth muscle layer of thoracic aorta. Both young and adult SHR had undetectable ANP binding in superior cervical and stellate sympathetic ganglia, pituitary gland, thymus and spleen, and lower binding in subfornical organ and choroid plexus. Conversely, in young SHRs and WKY rats, ANP binding site density was similar in adrenal zona glomerulosa, kidney glomeruli, and papilla. The SHRs have notable tissue specific decrease in ANP binding site concentration. This alteration may relate to the pathophysiology of genetic hypertension.

Decreased angiotensin II binding affinity and binding capacity in the anterior pituitary gland of adult spontaneously hypertensive rats

Angiotensin II (ANG) binding sites were quantified in single pituitary glands from 4-week-old and 14-week-old male spontaneously hypertensive rats (SHR) and age-matched male normotensive Wistar-Kyoto (WKY) control rats after incubation with 125I-[Sar1]-ANG, autoradiography with computerized densitometry, and comparison to 125I-standards. The maximum binding capacity (Bmax) decreased while the dissociation constant (Kd) for ANG increased in 14-week-old SHR when compared to age-matched WKY control rats (Bmax: 265 +/- 9 and 224 +/- 4 fmol/mg protein; Kd: 0.79 +/- 0.04 and 1.14 +/- 0.08 10(-9) M in WKY and SHR, respectively). Conversely, no difference between rat strains was found in 4-week-old animals. Our results suggest that pituitary ANG binding sites may play a role in the pathophysiology of established genetic hypertension.

Increased concentration of angiotensin II binding sites in selected brain areas of spontaneously hypertensive rats

We studied the density of the angiotensin II (Ang II) binding site in discrete brain nuclei of 4-week-old and 14-week-old spontaneously hypertensive rats (SHR) and age-matched normotensive Wistar-Kyoto (WKY) control rats by autoradiographic binding techniques. Tissue sections were incubated in vitro with 3 nmol/l [125I]Sar1Ang and results were analysed by computerized microdensitometry and by comparison with 125I-standards. Both young and adult SHR (aged 4 and 14 weeks, respectively) had significantly higher Ang II binding site concentrations in the median preoptic nucleus (MPO), subfornical organ (SFO), paraventricular nucleus (PVN) and nucleus of the solitary tract (NTS) when compared to age-matched WKY control rats. No significant difference was found between strains in other brain areas such as the olfactory bulb, suprachiasmatic nucleus (SCh), inferior olive (IO) and area postrema (AP). It was observed that the concentration of Ang II binding sites increased with age in PVN of both SHR and WKY, while the number of binding sites in the MPO and IO decreased with age. In SHR, alteration in Ang II binding is restricted to brain nuclei involved in the central pressor action of Ang II and seems to be related to the development and maintenance of spontaneous hypertension.

Lower number of atrial natriuretic peptide receptors in thymocytes and spleen cells of spontaneously hypertensive rats

Spontaneously hypertensive rats (SHR) have a much lower number of atrial natriuretic peptide (ANP) receptors in thymus and spleen from young and adult animals than age-matched normotensive controls. In spite of this low receptor concentration, the ANP-stimulated cyclic GMP response in isolated thymocytes and spleen cells from SHR was similar to that of normotensive control rats. Alterations in ANP receptor concentration in thymus and spleen of SHR may be related to the immune abnormalities described in these animals, and to the pathophysiology of genetic hypertension.

Regulation of angiotensin II binding sites in the subfornical organ and other rat brain nuclei after water deprivation

1. Binding sites for angiotensin II have been localized in forebrain and brain-stem areas of water-deprived and control Sprague-Dawley rats, employing autoradiography with computerized microdensitometry. 2. Angiotensin II receptor sites were identified in the organum vasculosum of the lamina terminalis, subfornical organ, paraventricular nucleus, median preoptic nucleus, area postrema, nucleus of the solitary tract, and inferior olive. 3. After dehydration a significant increases in the concentration of angiotensin II receptors was detected only in the subfornical organ. Although there was an increased concentration of angiotensin II binding sites in the organum vasculosum of the lamina terminalis, the median preoptic nucleus, and the paraventricular nucleus after dehydration, these changes did not reach statistical significance. Other brain nuclei investigated did not show differences in angiotensin II binding sites in the dehydrated rats compared to controls. 4. These results indicate that angiotensin II receptors in the subfornical organ may play an important role in fluid homeostasis during dehydration.

Angiotensin II binding sites in the conduction system of rat hearts

Angiotensin II binding sites were localized and quantified in the conduction system of the rat heart by autoradiography in combination with computerized microdensitometry. Tissue sections (16-microns thick), containing cardiac vagus ganglia, sinus node, and atrioventricular node, were incubated with 125I-Sar1-angiotensin II to generate autoradiograms that were compared with other autoradiograms from 125I-labeled standards. Angiotensin II binding sites were highly localized in both the sinoatrial and atrioventricular nodes and in the cardiac vagus ganglia. In contrast, binding to the angiotensin converting enzyme, determined by incubation of adjacent tissue sections with the specific enzyme inhibitor 125I-351A, was very low in these areas but high in the cardiac endothelium. Our results suggest that angiotensin II may have direct chronotropic effects through stimulation of specific receptors in the conduction system of the heart and in intrinsic parasympathetic ganglia.

Atrial natriuretic peptide receptors in sympathetic ganglia: biochemical response and alterations in genetically hypertensive rats

High concentration of atrial natriuretic peptide (99-126) (ANP) receptors were localized by quantitative autoradiography in superior cervical and stellate ganglia from young and adult Wistar Kyoto (WKY) rats. ANP increased cyclic GMP formation in stellate ganglia from adult rats. Both young and adult spontaneously hypertensive rats (SHR) had a much lower number of ANP receptors in the sympathetic ganglia. In spite of low receptor concentration, the cyclic GMP response to ANP in SHR was unchanged. These results suggest the existence of physiologically active ANP receptors in the rat sympathetic ganglia. These receptors may also be involved in the pathophysiology of spontaneous hypertension.

Specific angiotensin II binding sites in the rat stellate and superior cervical ganglia

Angiotensin II binding sites were localized and quantified in rat stellate and superior cervical ganglia by quantitative autoradiography using 125I-Sar1-angiotensin II as a ligand. Both ganglia possess a single class of angiotensin II binding sites. High concentrations of binding sites were localized in the areas rich in principal ganglion cells. These binding sites might mediate the facilitatory action of angiotensin II on the ganglionic transmission.

Different pharmacological interaction of clonidine and guanabenz with antidepressive drugs

A study was made of the effect of several antidepressive drugs with different mechanisms of action on cardiovascular response induced by clonidine (CLO) or guanabenz (GUA) in anaesthetized normotensive rats. The acute pretreatment (1 mg.kg-1) with desipramine (DMI), maprotiline (MAP) and mianserin (MIA) reduced the hypotensive and bradycardiac responses induced by CLO (10 micrograms.kg-1), while only MIA prevented these central actions when elicited by GUA (30 micrograms.kg-1). Only chronic treatment with DMI reduced the hypotensive effect of CLO, but none of long-term antidepressant administrations decreased the fall in blood pressure induced by GUA. The long-term administration of DMI, MAP and MIA abolished the bradycardia from both agonists, CLO and GUA. The different pharmacological interaction of CLO and GUA with (acute or chronic) antidepressive treatments could be explained by a structural difference between the two agonists, and also by differences in localization and/or subtype of the alpha-adrenoceptors involved in their central cardiovascular effects.

Interaction between beta 2- and alpha 2-adrenoceptor responses in the vascular system: effect of clenbuterol

Subchronic treatment with the beta 2-adrenoceptor agonist, clenbuterol (0.3 mg X kg-1, twice daily for 14 days), significantly increased the median blood pressure in anaesthetized normotensive rats. The treatment produced a marked reduction in the vasodilator effect of isoproterenol. Acute clenbuterol administration (0.01 mg X kg-1 i.v.) reduced the contractile response induced by the alpha 2-adrenoceptor agonists, guanabenz or B-HT 920, and the alpha 1- and alpha 2-adrenoceptor agonist, clonidine, whereas it did not affect the vasoconstriction by the alpha 1-adrenoceptor agonist, methoxamine, in the pithed rat. Subchronic treatment with clenbuterol attenuated the effect of the beta 2-adrenoceptor agonist on the vascular alpha 2-adrenoceptor responses and enhanced the alpha 1-adrenoceptor response to phenylephrine in pithed rats. The effect of the beta 2-adrenoceptor agonist was reduced by 1- but not d-propranolol. These results suggest that subchronic treatment with clenbuterol produces a subsensitivity of the beta 2-adrenoceptors and reduced the interaction between beta 2- and alpha 2-adrenoceptors at the vascular wall.

Cardiovascular effects of alpha-adrenergic drugs: differences between clonidine and guanabenz

Guanabenz induced a pressor effect in pithed rats through postsynaptic alpha 2-adrenoceptors whereas clonidine activated both vascular alpha 1 and alpha 2-adrenoceptors. Previous treatment with prazosin, an alpha 1-antagonist, or depletion of the noradrenergic stores by reserpine produced supersensitivity to the pressor response to clonidine only, probably through postsynaptic alpha 1-adrenoceptors. The hypotension and bradycardia developed in normotensive rats after intravenous guanabenz administration were abolished by prazosin, whereas the central effects of clonidine were antagonized by both prazosin and yohimbine. Selective destruction of central noradrenergic neurons by [N-(2-chloroethyl)-N-ethyl-2-bromobenzylamine] (DSP 4) or reserpine plus blockade of catecholamine synthesis by alpha-methyl-p-tyrosine abolished the hypotension and bradycardia produced by guanabenz but merely reduced the bradycardia from clonidine. The present results suggest that, in rats, guanabenz is a selective stimulant of central alpha-autoadrenoceptors antagonized by prazosin whereas at a vascular level guanabenz preferentially activates alpha-adrenoceptors antagonized by yohimbine. The differences observed between the mechanisms by which guanabenz and clonidine produce their central cardiovascular responses might be attributed to their acting on different nuclei.

Treatment with clorgyline and pargyline differentially decreases clonidine-induced hypotension and bradycardia

A study was made of the effects of acute and chronic treatment with monoamine-oxidase (MAO) inhibitors on the peripheral and central cardiovascular response induced by clonidine in anaesthetized normotensive rats. Clonidine (30 nmoles X kg-1 i.v.) produced a biphasic change in mean blood pressure; an initial transient increase was followed by a prolonged hypotensive effect, coinciding with the maximal bradycardia. Twenty-four hours after acute (single) or chronic (daily for 7 days) administration of MAO inhibitors (pargyline 10 mg X kg-1 SC or clorgyline 0.3 mg X kg-1 SC) there was no effect either on the basal cardiovascular parameters or on the initial pressor response induced by clonidine. Chronic but not acute treatment with clorgyline, an inhibitor of type A MAO, greatly decreased the hypotension and bradycardia induced by clonidine for as long as 5 days after its discontinuation. On the other hand, after chronic administration of pargyline (10 mg X kg-1), a preferential type B MAO inhibitor, the hypotension and bradycardia caused by clonidine were differently affected. There was a reduction in the bradycardia up to the third day following the discontinuation of pargyline, whereas the hypotensive response induced by clonidine was only attenuated for 24 h and unaffected with a lower dose of pargyline (0.3 mg X kg-1). It is concluded that chronic administration of the type A MAO inhibitor, clorgyline, attenuates the central responses to clonidine through the reduction in sensitivity of brain alpha-adrenoceptors. Pargyline, that preferentially inhibits type B MAO, reduces only the bradycardia induced by clonidine. This result may indicate a different modulation of the receptors involved in this response to clonidine.