Edward E. Soltis

Influence of perivascular adipose tissue on rat aortic smooth muscle responsiveness.

Virtually every blood vessel in the body is surrounded to some degree by adipose tissue. A potential role for perivascular adipose tissue as a neurohumoral regulator of vascular responsiveness was studied. Thoracic aortae obtained from male Sprague-Dawley rats were cut into rings for use in a standard in vitro smooth muscle bath set-up. The vessels were either cleaned of surrounding adipose tissue or left intact. Contractile responses to KCl and phenylephrine as well as relaxation responses to acetylcholine, isoproterenol and sodium nitroprusside were not different between cleaned and intact tissues. However, a significant decrease in the sensitivity to norepinephrine was observed in intact vessels. This altered response was corrected by prior treatment with desipramine plus deoxycorticosterone. Contractile responses of aortic ring preparations to tyramine and a potassium-free physiological solution were significantly greater in intact tissues. Electrical stimulation resulted in no response in cleaned tissues, however, a frequency-dependent contraction was elicited in intact vessels. Phentolamine blocked the contractile responses generated by these manipulations which activate the sympathetic neuroeffector system. Responses evoked by electrical stimulation in intact vascular preparations were significantly attenuated by prior exposure to the selective angiotensin II (AII) antagonist SarI-Ile8-AII. These observations demonstrate that perivascular adipose tissue significantly influences vascular responsiveness in the in vitro setting.

α-Adrenoceptors and vascular regulation: Molecular, pharmacologic and clinical correlates

This manuscript is intended to provide a comprehensive review of the alpha-adrenoceptors (ARs) and their role in vascular regulation. The historical development of the concept of receptors and the division of the alpha-ARs into alpha 1 and alpha 2 subtypes is traced. Emphasis will be placed on current understanding of the specific contribution of discrete alpha 1- and alpha 2-AR subtypes in the regulation of the vasculature, selective agonists and antagonists for these receptors, the second messengers utilized by these receptors, the myoplasmic calcium pathways activated to initiate smooth muscle contraction, as well as the clinical uses of agonists and antagonists that work at these receptors. New information is presented that deals with the molecular aspects of ligand interactions with specific subdomains of these receptors, as well as mRNA distribution and the regulation of alpha 1- and alpha 2-AR gene transcription and translation.

Evidence for a complex interaction between the subtypes of the α1-adrenoceptor

Abstract Ligand binding studies with WB 4101 revealed that the rat aorta contains both the α 1a - and α 1b -adrenoceptor subtypes. Results obtained following treatment with the irreversible antagonists phenoxybenzamine, chlorethylclonidine or SZL-49 (4-amino-6,7-dimethoxy-2-quinazolinyl-4-(2-bicyclo[2,2,2]octa-2,5-dienylcarbonyl-2-piperazine) suggest that there is a complex interaction between the α 1 -adrenoceptor subtypes in the aorta. Chlorethylclonidine affects only the α 1b -adrenoceptor, whereas the predominant action of SZL-49 is on the α 1a -subtype. Chlorethylclonidine significantly inhibited the response to either methoxamine or phenylephrine, agents which are selective α 1a -adrenoceptor agonists. Following inactivation with either chlorethylclonidine or SZL-49, the response of the rat aorta to phenylephrine was only partially antagonized by either prazosin or WB 4101. SZL-49 also inhibited the response of the rat tail artery to electrical stimulation. The response of the tail artery obtained following inactivation with SZL-49 was effectively antagonized by prazosin. Phenylephrine, prazosin or WB 4101 afforded complete protection from chlorethylclonidine adrenoceptor inactivation, while these same ligands were only partially effective against SZL-49. Either SZL-49 or chlorethylclonidine significantly impaired the irreversible adrenoceptor blocking actions of phenoxybenzamine. These results suggest: (1) only the α 1a -adrenoceptor subtype appears to be associated with nerve terminals in the tail artery, (2) there may be a complex interaction between the α 1 -adrenoceptor subtypes such that both receptors must be iniact and functional to observe normal agonist and antagonist interactions, (3) there may be three sites of action for agonists associated with the rat aorta.

Disparate effects of antidiabetic drugs on arterial contraction

Type II diabetic patients and others with insulin resistance are at risk for development of hypertension characterized by elevated peripheral vascular resistance and loss of insulins normal vasodilating activity. Oral antidiabetic drugs have recently been recognized to have disparate effects on arterial pressure in such patients and in related rodent models. Sulfonylureas (e.g., glyburide), which stimulate insulin secretion, have been reported either to increase or not to affect arterial pressure, whereas nonsulfonylurea agents with insulin-sensitizing properties, the biguanide metformin and various thiazolidinediones (eg, pioglitazone), have been reported to decrease arterial pressure in humans and rodents. To help elucidate these disparate effects, we investigated these agents for direct actions on arterial vascular contractility and its sensitivity to insulin. Preincubation of intact rat tail arterial tissue rings for 2 hours with known therapeutically effective antidiabetic concentrations of metformin and pioglitazone significantly attenuated the force of contractions produced by either potassium (membrane depolarization) or norepinephrine ([NE] adrenergic receptor activation). Glyburide did not influence these contractions. Preincubation with metformin also induced an attenuating (vasodilating-like) action of insulin on arterial tissue rings contracted by potassium. Conversely, glyburide induced an accentuating action of insulin on potassium-mediated contractions. These results are consistent with measures of vascular function obtained in the past after oral administration of the drugs, which suggested but did not prove that they may exert direct effects on arterial vascular contractility. Thus, metformin and thiazolidinediones may decrease arterial pressure partly by direct vasorelaxant mechanisms, with metformin having an additional effect of inducing vasorelaxation by insulin. In contrast, sulfonylureas may directly induce a paradoxical vasoconstrictor response to insulin.

Polyamines, vascular smooth muscle, and deoxycorticosterone acetate-salt hypertension.

This study was performed to determine if an alteration in vascular polyamine contents is associated with the development of deoxycorticosterone acetate-salt hypertension. The effects of chronic administration of a-difluoromethylornithine, a specific irreversible inhibitor of ornithine decarboxylase and thus polyamine biosynthesis, on vascular polyamine contents, structure, and function as well as the development of hypertension was studied. Control and deoxycorticosterone acetate-salt rats received either tap water or a drinking solution containing or-difluoromethylornithine for 6 weeks, during which period systolic blood pressures were recorded. Vascular reactivity studies were performed on rings of aorta and tail artery. Medial thickness, vessel weight, and vascular polyamine contents were also assessed in these arteries. ar-Difluoromethylornithine treatment had no significant effect on either systolic blood pressure or vascular structure, function, and polyamine contents of control animals. The elevation in blood pressure and the increase in medial thickness, ring weight, and vascular polyamine contents as well as altered vascular reactivity observed in deoxycorticosterone acetate-salt rats was significantly attenuated by ar-difluoromethylornithine treatment These results are the first to demonstrate that vascular polyamine contents are elevated in the deoxycorticosterone acetate-salt rat and that chronic a-difluoromethylornithine treatment prevents the rise in vascular polyamines as well as the elevation in blood pressure and attendant changes in the vasculature. Thus, the increase in vascular polyamines may comprise a critical link between the initiating stimuli and the alterations in vascular structure and function implicated in the pathogenesis of deoxycorticosterone acetate-salt hypertension.

Acute and Chronic Effects of Losartan (Dup 753) on Blood Pressure and Vascular Reactivity in Normotensive Rats

The effects of in vivo treatment with the nonpeptide subtype 1 angiotensin II receptor antagonist, losartan, on blood pressure and vascular reactivity in normotensive male Sprague-Dawley rats were studied. Initial acute experiments demonstrated that blood pressure was significantly decreased six hours following a single injection of losartan (10 mg/kg, sc), but returned to control levels by 24 hours post-injection. Pressor responses to angiotensin II (0.1 ug/kg, iv) in these rats were significantly attenuated 2 and 24 hours following losartan injection. For chronic studies, rats were injected once daily for 21 days with either the same dose of losartan or saline vehicle. Blood pressure and pressor responses to angiotensin II were assessed at the end of the 21 day treatment period. A significant decrease in blood pressure was observed in chronic losartan treated rats 6 hours after the last injection on day 21; however, as in the acute studies, blood pressure had returned to control values by 24 hours post-injection. Although blood pressure had returned to normal, pressor responses to angiotensin II were significantly attenuated in chronic losartan treated rats 24 hours after the last injection. Following the in vivo studies, aortae and tail arteries were removed for experiments on vascular reactivity. Acute and chronic losartan treatment had no effect on KCl and norepinephrine reactivity. Endothelial-dependent and independent relaxation responses were also unaltered. A significant decrease in the maximal contractile response to angiotensin II was observed in aorta from acute and chronic losartan treated rats. Electrical stimulation-induced responses were unaltered in tail arteries from rats acutely treated with losartan but were potentiated in rats chronically treated with losartan. Exogenously applied angiotensin II, in concentrations which did not elicit contractile responses, potentiated electrical stimulation-induced responses of tail arteries from control rats but did not influence responses in arteries from acute and chronic losartan treated rats. These results demonstrate that losartan has significant blood pressure lowering effects in normotensive rats. Interestingly, although blood pressure returns to normal by 24 hours post-injection, pressor responses to angiotensin II remain attenuated in acute and chronic losartan treated rats. Finally, losartan treatment results in specific alterations in vascular reactivity associated with the actions of angiotensin II.

MULTIPLE POLYAMINE REGULATORY PATHWAYS CONTROL COMPENSATORY CARDIOVASCULAR HYPERTROPHY IN COARCTATION HYPERTENSION

While a number of factors may initiate structural alterations within the cardiovascular system in response to hypertension, there are obligate cellular signaling mechanisms, such as the polyamines, through which they must operate. This study examined the effects of polyamine synthesis inhibition using eflornithine, a suicide inhibitor of ornithine decarboxylase on blood pressure, compensatory remodeling of the cardiovascular system, and cardiac and aortic polyamine contents using an aortic coarctation model in rats. Eflornithine treatment failed to reduce carotid arterial blood pressure and actually significantly elevated vascular pressure above and below the coarctation site by 14 days of hypertension. Eflornithine only transiently reduced aortic polyamine content of hypertensive rats while this agent reduced coarctation-induced aortic medial wall thickening and the synthesis/deposition of fibronectin and laminin in the hypertensive aorta. Increases in left ventricular mass and polyamine content were concomitantly reduced in hypertensive rats administered eflornithine. These results suggest that multiple polyamine regulatory pathways may maintain vascular polyamine content in response to aortic coarctation; however de novo polyamine synthesis is essential for select aspects of vascular remodeling, including matrix synthesis. Cardiac tissue, in contrast, may rely principally on de novo polyamine synthesis.

Eflornithine Treatment in SHR: Potential Role of Vascular Polyamines and Ornithine Decarboxylase in Hypertension

This study was performed to assess the potential role of polyamines in the alterations in vascular structure and function in spontaneously hypertensive rats (SHR). The effects of chronic administration of eflornithine (alpha-difluoromethylornithine; DFMO), a highly specific inhibitor of ornithine decarboxylase (the rate limiting enzyme in polyamine biosynthesis), on vascular polyamine contents, vascular structure and function, and blood pressure was studied. Male SHR (16-17 weeks of age) with an average systolic blood pressure (SBP) of 161 +/- 3 mmHg were used. The rats were divided into two groups and received either tap water or a 1% DFMO solution to drink for 6 weeks. SBP and body weight were recorded prior to and once-a-week during the experiment. Standard in vitro vascular reactivity studies on ring segments of aorta and tail artery were performed. Ring segment weight, arterial medial thickness, and vascular polyamine contents were also determined. Body weights were not significantly affected by the DFMO treatment. SBP in control SHR rose progressively to an average value of 185 +/- 5 mmHg by the sixth experimental week. Although DFMO treatment did not cause a significant decrease in SBP compared to pretreatment values, it did prevent a further increase in SBP. Aortic and tail artery responsiveness to norepinephrine and electrical stimulation, respectively, ring segment weight, arterial medial thickness, and vascular polyamine contents were all significantly less in SHR receiving the DFMO treatment. These data are the first to demonstrate the effectiveness of DFMO to lower polyamine contents in the vasculature of hypertensive SHR.(ABSTRACT TRUNCATED AT 250 WORDS)

Chronic hypertension and vascular alterations following short-term high dietary salt intake in postweaning rats.

The chronic effects of short-term administration of a high salt diet to postweaning rats on blood pressure (BP) was studied. BP was significantly elevated following 4 weeks of treatment. After stopping the high salt diet, BP dropped to control levels but then progressively rose again to hypertensive levels. A significant increase in arterial medial thickness as well as an increase in contractile sensitivity and a decrease in relaxation responsiveness were observed in aortic smooth muscle. These data show that short-term administration of a high salt diet to normal postweaning rats results in a chronic elevation in BP accompanied by significant alterations in vascular structure and function.

Age-Dependent Salt-Induced Hypertension in the Rat: Prevention with DSP-4, A Selective Noradrenergic Neurotoxin

The present study was performed to determine if chronic administration of a high salt diet induces hypertension similarly in young and adult rats and if treatment with DSP-4 alters the development of the hypertension. Three- (young) and ten- (adult) week-old male Sprague-Dawley rats were fed either a standard rat chow diet (0.71% NaCl), a 4% NaCl diet or an 8% NaCl diet for 12 weeks. Systolic blood pressure, using a standard tail-cuff technique, and body weight were recorded weekly during the dietary treatment period. Direct mean arterial pressure, heart rate, heart weight and kidney weight were determined after 12 weeks. Body weight was slightly reduced in young rats on the 8% NaCl diet. A significant increase in blood pressure as well as heart weight was observed only in young rats on the 8% NaCl diet. An increase in kidney weight was observed in both young and adult rats on the 8% NaCl diet. DSP-4 treatment prevented the development of hypertension as well as cardiac hypertrophy in rats fed the high salt diet but had no effect on rats receiving the normal diet. Body and kidney weights were similar in vehicle- and DSP-4-treated rats on the 8% NaCl diet. These results demonstrate that a critical developmental/maturational period exists during which the young rat is susceptible to the hypertensinogenic effects of a high salt diet. An intact central noradrenergic system appears to be necessary for the expression of this enhanced susceptibility and the subsequent development of hypertension.