The ontogeny of renal alpha 1- and alpha 2-adrenoceptors in the Dahl rat model of experimental hypertension

Age-dependent salt hypertension in Dahl rats: fifty years of research

Fifty years ago, Lewis K. Dahl has presented a new model of salt hypertension - salt-sensitive and salt-resistant Dahl rats. Twenty years later, John P. Rapp has published the first and so far the only comprehensive review on this rat model covering numerous aspects of pathophysiology and genetics of salt hypertension. When we summarized 25 years of our own research on Dahl/Rapp rats, we have realized the need to outline principal abnormalities of this model, to show their interactions at different levels of the organism and to highlight the ontogenetic aspects of salt hypertension development. Our attention was focused on some cellular aspects (cell membrane function, ion transport, cell calcium handling), intra- and extrarenal factors affecting renal function and/or renal injury, local and systemic effects of renin-angiotensin-aldosterone system, endothelial and smooth muscle changes responsible for abnormal vascular contraction or relaxation, altered balance between various vasoconstrictor and vasodilator systems in blood pressure maintenance as well as on the central nervous and peripheral mechanisms involved in the regulation of circulatory homeostasis. We also searched for the age-dependent impact of environmental and pharmacological interventions, which modify the development of high blood pressure and/or organ damage, if they influence the salt-sensitive organism in particular critical periods of development (developmental windows). Thus, severe self-sustaining salt hypertension in young Dahl rats is characterized by pronounced dysbalance between augmented sympathetic hyperactivity and relative nitric oxide deficiency, attenuated baroreflex as well as by a major increase of residual blood pressure indicating profound remodeling of resistance vessels. Salt hypertension development in young but not in adult Dahl rats can be attenuated by preventive increase of potassium or calcium intake. On the contrary, moderate salt hypertension in adult Dahl rats is attenuated by superoxide scavenging or endothelin-A receptor blockade which do not affect salt hypertension development in young animals.

α- and β-Adrenoceptors in Hypertension: Molecular Biology and Pharmacological Studies

Recent years have witnessed astonishing progress in our understanding of the molecular basis of adrenoceptor structure, function and regulation and revealed an unexpected heterogeneity of adrenoceptors demonstrating the existence of at least 11 subtypes. This paper discusses the implications of these advances on studies regarding a specific role of adrenoceptors in the development of genetic hypertension. The available data indicate that among the alpha-adrenoceptor subtypes the alpha 2A-adrenoceptor is the most likely candidate for an alteration specifically linked to genetic hypertension in the animal model of the spontaneously hypertensive rat and possibly in some patients. Alterations of other alpha-adrenoceptor subtypes may be specific for some forms of genetic hypertension but are unlikely to play an important role for blood pressure regulation. Most beta-adrenoceptor alterations appear to occur secondary to blood pressure elevation independently of whether hypertension has occurred on a genetic basis or not. Moreover, the mechanisms regulating alpha- and beta-adrenoceptor responsiveness upon prolonged agonist exposure may be altered in hypertension and thereby contribute to the pathophysiology of this disease.

Renal α2-adrenoceptors in New Zealand genetically hypertensive rats

1. Renal alpha 1- and alpha 2-adrenoceptors were characterized in the New Zealand strain of genetically hypertensive (GH) rat and the Otago random-bred albino normotensive (NT) control rat at 4 and 12 weeks of age with [3H]-prazosin and [3H]-rauwolscine. 2. At 4 weeks of age, the density of alpha-adrenoceptors in NT and GH rats was similar to both the alpha 1- (193 +/- 11 vs 163 +/- 14 fmol mg-1 protein) and alpha 2- (347 +/- 34 vs 319 +/- 41 fmol mg-1 protein) adrenoceptor. At 12 weeks of age, GH rats had a greater density of renal alpha 1- (152 +/- 27 vs 238 +/- 17 fmol mg-1 protein) and alpha 2- (175 +/- 42 vs 350 +/- 23 fmol mg-1 protein) adrenoceptors compared to the NT rats. 3. Pre-incubation of kidneys from GH rats (12 weeks of age) with 1 and 10 microM clonidine decreased the density of receptors identified by unlabelled clonidine displacement of [3H]-rauwolscine to 77% and 56% of control. Pre-incubation with adrenaline, 2,6 dimethylclonidine or phenylephrine failed to alter binding. 4. Pre-incubation of kidneys from NT rats (12 weeks) or young GH rats (4 weeks) with 10 microM clonidine failed to alter displacement of [3H]-rauwolscine by unlabelled clonidine. 5. These studies demonstrate that in another strain of hypertensive rat, the GH rat, alpha 2-adrenoceptor density is increased as compared to the normotensive control at 12 but not 4 weeks of age.(ABSTRACT TRUNCATED AT 250 WORDS)

Peripheral adrenergic receptors in hypertension

Increased sympathoadrenal activity appears to play an important role in the development or maintenance of elevated blood pressure in hypertensive patients and various animal models of hypertension. Alterations of adrenergic receptor number or responsiveness might contribute to this increased activity. We therefore reviewed the data on adrenergic receptor alterations in hypertension with special emphasis on several key cardiovascular tissues (i.e., heart, vascular smooth muscle, and kidney) and on lymphocytes and platelets as human tissues available for such studies. The data suggest that the number of alpha-adrenergic receptors in hypertension is regulated by catecholamines, dietary salt intake, and genetic factors. Increases in renal alpha-adrenergic receptor number may be etiologic in genetic forms of essential hypertension. beta-Adrenergic receptor alterations in states of elevated blood pressure do not appear to be specific for genetic hypertension. Desensitization of beta-adrenergic receptor function in hypertensive animals and patients contrasts with reports of decreased, unchanged, and increased beta-adrenergic receptor number, suggesting that signal transduction of beta-adrenergic (and possibly other) receptors that stimulate adenylyl cyclase is disturbed in hypertension. The mechanisms of such heterologous desensitization in states of elevated blood pressure remain to be elucidated.

Differential ontogeny of alpha 1-adrenergic and cholinergic receptor sites in the atria and ventricles of the inbred Dahl hypertension-sensitive (S/JR) and -resistant (R/JR) rat

The ontogeny of atrial and ventricular alpha 1-adrenergic and muscarinic cholinergic receptor sites was investigated in inbred Dahl hypertension-sensitive (S/JR) and -resistant (R/JR) rats between 5 and 150 days of age. The density of sites in both cardiac regions was generally greater in the neonate than mature rat. A marked proliferation of sites was observed in neonatal and young adult rats that occurred in the following order: ventricular cholinoceptors----ventricular adrenoceptors----atrial cholinoceptors----atrial adrenoceptors. The density of ventricular adrenoceptors was greater in the S/JR rat than the R/JR rat at 5 days of age. At 150 days of age, the density of sites was less in the S/JR rat than the age-matched R/JR rat or the normotensive 50-day-old S/JR rat. The development of atrial adrenoceptors was similar between the strains, regardless of the blood pressure. The density of ventricular cholinergic receptors was greater in the S/JR strain at 5 and 15 days of age. However, the density of atrial cholinergic sites was consistently greater in the S/JR strain throughout development. The results of this study suggest that: (1) significant prenatal receptor development occurs in the heart; (2) receptor development may precede the functional maturation of postganglionic autonomic efferents; and (3) distinguishing differences in the regional density of alpha 1-adrenergic and muscarinic cholinergic binding sites are present between S/JR and R/JR rats at much earlier points in development than previously shown.