Myocardial adrenergic receptors and adenylate cyclase in the developing spontaneously hypertensive rat

Neurotransmitter Switching Coupled to β-Adrenergic Signaling in Sympathetic Neurons in Prehypertensive States

Single or combinatorial administration of β-blockers is a mainstay treatment strategy for conditions caused by sympathetic overactivity. Conventional wisdom suggests that the main beneficial effect of β-blockers includes resensitization and restoration of β1-adrenergic signaling pathways in the myocardium, improvements in cardiomyocyte contractility, and reversal of ventricular sensitization. However, emerging evidence indicates that another beneficial effect of β-blockers in disease may reside in sympathetic neurons. We investigated whether β-adrenoceptors are present on postganglionic sympathetic neurons and facilitate neurotransmission in a feed-forward manner. Using a combination of immunocytochemistry, RNA sequencing, Förster resonance energy transfer, and intracellular Ca2+ imaging, we demonstrate the presence of β-adrenoceptors on presynaptic sympathetic neurons in both human and rat stellate ganglia. In diseased neurons from the prehypertensive rat, there was enhanced β-adrenoceptor-mediated signaling predominantly via β2-adrenoceptor activation. Moreover, in human and rat neurons, we identified the presence of the epinephrine-synthesizing enzyme PNMT (phenylethanolamine-N-methyltransferase). Using high-pressure liquid chromatography with electrochemical detection, we measured greater epinephrine content and evoked release from the prehypertensive rat cardiac-stellate ganglia. We conclude that neurotransmitter switching resulting in enhanced epinephrine release, may provide presynaptic positive feedback on β-adrenoceptors to promote further release, that leads to greater postsynaptic excitability in disease, before increases in arterial blood pressure. Targeting neuronal β-adrenoceptor downstream signaling could provide therapeutic opportunity to minimize end-organ damage caused by sympathetic overactivity.

Sympathetic neurons are a powerful driver of myocyte function in cardiovascular disease

Many therapeutic interventions in disease states of heightened cardiac sympathetic activity are targeted to the myocytes. However, emerging clinical data highlights a dominant role in disease progression by the neurons themselves. Here we describe a novel experimental model of the peripheral neuro-cardiac axis to study the neuron’s ability to drive a myocyte cAMP phenotype. We employed a co-culture of neonatal ventricular myocytes and sympathetic stellate neurons from normal (WKY) and pro-hypertensive (SHR) rats that are sympathetically hyper-responsive and measured nicotine evoked cAMP responses in the myocytes using a fourth generation FRET cAMP sensor. We demonstrated the dominant role of neurons in driving the myocyte ß-adrenergic phenotype, where SHR cultures elicited heightened myocyte cAMP responses during neural activation. Moreover, cross-culturing healthy neurons onto diseased myocytes rescued the diseased cAMP response of the myocyte. Conversely, healthy myocytes developed a diseased cAMP response if diseased neurons were introduced. Our results provide evidence for a dominant role played by the neuron in driving the adrenergic phenotype seen in cardiovascular disease. We also highlight the potential of using healthy neurons to turn down the gain of neurotransmission, akin to a smart pre-synaptic ß-blocker.

Inhibition of angiotensin II Gq signaling augments beta-adrenergic receptor mediated effects in a renal artery stenosis model of high blood pressure

Chronic ventricular pressure overload states, such as hypertension, and elevated levels of neurohormones (norepinephrine, angiotensin II, endothelin-1) initiate cardiac hypertrophy and dysfunction and share the property of being able to bind to Gq-coupled 7-transmembrane receptors. The goal of the current study was to determine the role of endogenous cardiac myocyte Gq signaling and its role in cardiac hypertrophy and dysfunction during high blood pressure (BP). We induced renal artery stenosis for 8 weeks in control mice and mice expressing a peptide inhibitor of Gq signaling (GqI) using a 2 kidney, 1 clip renal artery stenosis model. 8 weeks following chronic high BP, control mice had cardiac hypertrophy and depressed function. Inhibition of cardiomyocyte Gq signaling did not reverse cardiac hypertrophy but attenuated increases in a profile of cardiac profibrotic genes and genes associated with remodeling. Inhibition of Gq signaling also attenuated the loss of cardiac function. We determined that Gq signaling downstream of angiotensin II receptor stimulation negatively impacted beta-adrenergic receptor (AR) responses and inhibition of Gq signaling was sufficient to restore betaAR-mediated responses. Therefore, in this study we found that Gq signaling negatively impacts cardiac function during high BP. Specifically, we found that inhibition of AT1-Gq signaling augmented betaAR mediated effects in a renal artery stenosis model of hypertension. These observations may underlie additional, beneficial effects of angiotensinogen converting enzyme (ACE) inhibitors and angiotensin receptor antagonists observed during times of hemodynamic stress.

Ontogenetic aspects of hypertension development: analysis in the rat

In this review, we attempt to outline the age-dependent interactions of principal systems controlling the structure and function of the cardiovascular system in immature rats developing hypertension. We focus our attention on the cardiovascular effects of various pharmacological, nutritional, and behavioral interventions applied at different stages of ontogeny. Several distinct critical periods (developmental windows), in which particular stimuli affect the further development of the cardiovascular phenotype, are specified in the rat. It is evident that short-term transient treatment of genetically hypertensive rats with certain antihypertensive drugs in prepuberty and puberty (at the age of 4-10 wk) has long-term beneficial effects on further development of their cardiovascular apparatus. This juvenile critical period coincides with the period of high susceptibility to the hypertensive effects of increased salt intake. If the hypertensive process develops after this critical period (due to early antihypertensive treatment or late administration of certain hypertensive stimuli, e.g., high salt intake), blood pressure elevation, cardiovascular hypertrophy, connective tissue accumulation, and end-organ damage are considerably attenuated compared with rats developing hypertension during the juvenile critical period. As far as the role of various electrolytes in blood pressure modulation is concerned, prohypertensive effects of dietary Na+ and antihypertensive effects of dietary Ca2+ are enhanced in immature animals, whereas vascular protective and antihypertensive effects of dietary K+ are almost independent of age. At a given level of dietary electrolyte intake, the balance between dietary carbohydrate and fat intake can modify blood pressure even in rats with established hypertension, but dietary protein intake affects the blood pressure development in immature animals only. Dietary protein restriction during gestation, as well as altered mother-offspring interactions in the suckling period, might have important long-term hypertensive consequences. The critical periods (developmental windows) should be respected in the future pharmacological or gene therapy of human hypertension.

Adrenoceptors in SHR: alterations in binding characteristics and intracellular signal transduction pathways

There is much data on altered adrenoceptor function in the heart, blood vessel and kidney from spontaneously hypertensive rats (SHR). The enhancement of vascular and renal alpha-adrenoceptor function, i.e. vasoconstriction and retention of water and sodium, may contribute to the development and maintenance of the hypertension, whereas cardiac alpha1-adrenoceptor may be of minor physiological significance. Alpha1-adrenoceptor-mediated signal transduction as a whole is increased in SHR vascular tissues, but the intracellular signaling per receptor in the kidney seems to be decreased despite increased alpha1-adrenoceptor density. On the other hand, cardiac and vascular beta-adrenoceptor responsiveness is attenuated in SHR. Reduced vasorelaxation mediated by beta-adrenoceptors may also contribute to high blood pressure. The impaired cardiovascular beta-adrenoceptor function in SHR does not appear to be necessarily explained by alterations observed at receptor levels. Alterations in signal transduction should be also considered. Limited data on renal beta-adrenoceptor density and its signaling suggest decreased or unaltered cyclic AMP formation per receptor in SHR. We will review alterations in both binding characteristics and each component of intracellular signal transduction pathways in cardiovascular and renal adrenoceptors of SHR.

Effects of aging and drugs on myocardial beta-adrenergic receptors in M-SHRSP and SHRSP

Myocardial beta-adrenergic receptors were measured in membrane fractions from malignant SHRSP (M-SHRSP), SHRSP and WKY at different ages using [3H]-dihydroalprenolol (DHA) as a radioligand. The effects of isoproterenol (ISP) and chemical sympathectomy (6-hydroxydopamine treatment) on myocardial beta-receptors were also investigated in 10 and 24 week-old SHRS and WKY to examine the effects of hypertension and aging on receptor regulation. The number of myocardial beta-receptors in M-SHRSP at 4 weeks of age (W) and 10 W were significantly lower than those in age-matched SHRSP and WKY. In addition, the values in SHRSP at 4 and 10 W were significantly lower than those in age-matched WKY, but the numbers in SHRSP at 1, 24 and 48-54 W were not significantly different from age-matched WKY. The dissociation constant and activity of 5'-nucleotidase, which is a marker enzyme of cell membrane, were not significantly different among the three groups. ISP treatment significantly reduced the numbers of myocardial beta-receptors in 10 week-old SHRSP and 10 and 24 week-old WKY, but did not in 24 week-old SHRSP. The extent of this decrease of beta-receptors was lower in 10 week-old SHRSP than in 10 week-old WKY, and it was also lower in 24 week-old WKY than 10 week-old WKY. 6-Hydroxydopamine treatment significantly increased the number of myocardial beta-receptors in 10 and 24 week-old WKY, but did not in SHRSP. The extent of this increase of beta-receptors was lower in 24 week-old WKY than in 10 week-old WKY. These results suggest that the decrease of myocardial beta-receptor numbers in 4 and 10 week-old M-SHRSP and SHRSP does not appear to be genetically determined, but rather is caused by accelerated sympathetic activity, and that the regulation of myocardial beta-receptor is impaired in young and aged SHRSP and in aged WKY.

The effects of estradiol and mestranol on alpha-adrenoceptors in select regions of the rat brain

The role of estrogens in modulating the concentration of CNS alpha-adrenoceptors has not been elucidated nor has it been determined how different estrogenic compounds affect these receptors. In this study brain alpha-1 and alpha-2-adrenoceptor binding was measured in female rats treated with estradiol and/or the synthetic estrogen mestranol. Rats treated biweekly for 12 weeks with mestranol (50 micrograms/100 g b.wt.) had a significant reduction in the apparent number of alpha-2-adrenoceptors in the frontal cortex and nucleus tractus solitarius (NTS), while apparent numbers of both alpha-1 and alpha-2-adrenoceptors were depressed in the locus coeruleus. Estradiol treatment (50 micrograms/100 g b.wt.) caused a significant elevation in apparent alpha-1-adrenoceptor numbers in the NTS relative to control. Alpha-adrenoceptor numbers in the rostral and caudal hypothalamus were not affected by either steroid treatment. These results suggest that regulation of apparent numbers of alpha-1 and alpha-2-adrenoceptors in the CNS depends on the type of estrogen used for treatment.

Pre- and neonatal exposure of the Dahl rat to NaCl: development and regional distribution of myocardial alpha 1-adrenergic and cholinergic receptor sites

The prenatal and/or postweaning effects of a hypertensinogenic high NaCl-containing diet (8.0% NaCl, w/w) on (1) the regional distribution of alpha 1-adrenoceptors and muscarinic cholinergic receptor sites in the heart and (2) the predisposition/resistance to hypertension (HT) were assessed in the inbred Dahl HT-sensitive (S/JR) and HT-resistant (R/JR) rat. The density of alpha 1-adrenoceptors was reduced in the left ventricle but not consistently affected in the ventricular septum, right ventricle, or atria of S/JR offspring with NaCl-induced HT. Both normotensive and hypertensive S/JR rats also displayed a significantly greater density of cholinergic receptor sites in the atria but few consistent alterations in other regions of the heart, compared to R/JR rats. Maternal diet had no effect on the predisposition/resistance to salt-induced HT and little effect on the regional development of alpha 1-adrenoceptors and cholinergic receptor sites. The results of this study suggest that the reduced density of ventricular alpha 1-adrenoceptors in the S/JR strain is a consequence of HT while the elevated density of cholinergic receptors in the atria may be related to the genetic predisposition/resistance to HT.

Calcium-dependent proteolytic stimulation of adenylate cyclase in platelets from spontaneously hypertensive rats

Abnormalities of platelet aggregation and cyclic nucleotide metabolism are present in hypertension. We observed a greater increase in the level of cyclic adenosine monophosphate (AMP) after prostaglandin E1 (PGE1) stimulation and a lack of decrease of this cyclic nucleotide by epinephrine in platelets from spontaneously hypertensive rats (SHR) as compared to normotensive rats. The difference in cyclic AMP production between SHR and control rats in response to PGE1 is dependent upon platelet exposure to calcium. Since calcium and cyclic AMP are closely related and are both abnormally regulated in hypertension, we have studied the effect of calcium on adenylate cyclase activity. We show here that two forms of endogenous calcium-dependent proteases (membrane-bound and soluble) stimulate the basal activity and the hormonal responsiveness of adenylate cyclase. The sensitivity of calcium-dependent proteolytic control of adenylate cyclase to very-low concentrations of calcium indicates that the regulation may be physiologically important. Furthermore, calcium exerts a greater influence on platelet adenylate cyclase from SHR than on that from normotensive rats. The adenylate cyclase defect seems to be located in the membrane fraction and may, therefore, result from an increase in the activity of the membrane-bound calcium-protease or may be intrinsic to adenylate cyclase itself. The exact site that is sensitive to proteolysis remains to be established.