beta-Adrenergic relaxation in mesenteric resistance arteries of spontaneously hypertensive and Wistar-Kyoto rats: the role of precontraction and intracellular Ca2+

BK Channels in Tail Artery Vascular Smooth Muscle Cells of Normotensive (WKY) and Hypertensive (SHR) Rats Possess Similar Calcium Sensitivity But Different Responses to the Vasodilator Iloprost

It has been reported that, in the spontaneously hypertensive rat (SHR) model of hypertension, different components of the G-protein/adenylate cyclase (AC)/Calcium-activated potassium channel of high conductance (BK) channel signaling pathway are altered differently. In the upstream part of the pathway (G-protein/AC), a comparatively low efficacy has been established, whereas downstream BK currents seem to be increased. Thus, the overall performance of this signaling pathway in SHR is elusive. For a better understanding, we focused on one aspect, the direct targeting of the BK channel by the G-protein/AC pathway and tested the hypothesis that the comparatively low AC pathway efficacy in SHR results in a reduced agonist-induced stimulation of BK currents. This hypothesis was investigated using freshly isolated smooth muscle cells from WKY and SHR rat tail artery and the patch-clamp technique. It was observed that: (1) single BK channels have similar current-voltage relationships, voltage-dependence and calcium sensitivity; (2) BK currents in cells with a strong buffering of the BK channel activator calcium have similar current-voltage relationships; (3) the iloprost-induced concentration-dependent increase of the BK current is larger in WKY compared to SHR; (4) the effects of activators of the PKA pathway, the catalytic subunit of PKA and the potent and selective cAMP-analogue Sp-5,6-DCl-cBIMPS on BK currents are similar. Thus, our data suggest that the lower iloprost-induced stimulation of the BK current in freshly isolated rat tail artery smooth muscle cells from SHR compared with WKY is due to the lower efficacy of upstream elements of the G-Protein/AC/BK channel pathway.

The renal vasodilatation from β-adrenergic activation in vivo in rats is not driven by KV7 and BKCa channels

The mechanisms behind renal vasodilatation elicited by stimulation of β-adrenergic receptors are not clarified. As several classes of K channels are potentially activated, we tested the hypothesis that KV7 and BKCa channels contribute to the decreased renal vascular tone in vivo and in vitro. Changes in renal blood flow (RBF) during β-adrenergic stimulation were measured in anaesthetized rats using an ultrasonic flow probe. The isometric tension of segmental arteries from normo- and hypertensive rats and segmental arteries from wild-type mice and mice lacking functional KV7.1 channels was examined in a wire-myograph. The β-adrenergic agonist isoprenaline increased RBF significantly in vivo. Neither activation nor inhibition of KV7 and BKCa channels affected the β-adrenergic RBF response. In segmental arteries from normo- and hypertensive rats, inhibition of KV7 channels significantly decreased the β-adrenergic vasorelaxation. However, inhibiting BKCa channels was equally effective in reducing the β-adrenergic vasorelaxation. The β-adrenergic vasorelaxation was not different between segmental arteries from wild-type mice and mice lacking KV7.1 channels. As opposed to rats, inhibition of KV7 channels did not affect the murine β-adrenergic vasorelaxation. Although inhibition and activation of KV7 channels or BKCa channels significantly changed baseline RBF in vivo, none of the treatments affected β-adrenergic vasodilatation. In isolated segmental arteries, however, inhibition of KV7 and BKCa channels significantly reduced the β-adrenergic vasorelaxation, indicating that the regulation of RBF in vivo is driven by several actors in order to maintain an adequate RBF. Our data illustrates the challenge in extrapolating results from in vitro to in vivo conditions.

Impaired vascular β-adrenergic relaxation in spontaneously hypertensive rats: The differences between conduit and resistance arteries

It was suggested that impaired β-adrenergic relaxation in spontaneously hypertensive rats (SHR) might contribute to their high blood pressure (BP). Our study was focused on isoprenaline-induced dilatation of conduit femoral or resistance mesenteric arteries and on isoprenaline-induced BP reduction in SHR and Wistar-Kyoto rats (WKY). We confirmed decreased β-adrenergic relaxation of SHR femoral arteries due to the absence of its endothelium-independent component, whereas endothelium-dependent component of β-adrenergic smooth muscle relaxation was similar in both strains. Conversely, isoprenaline-induced relaxation of resistance mesenteric arteries was similar in both strains and this was true for endothelium-dependent and endothelium-independent components. We observed moderately reduced sensitivity of SHR mesenteric arteries to salmeterol (β2-adrenergic agonist) and this strain difference disappeared after endothelium removal. However, there was no difference in mesenteric arteries relaxation by dobutamine (β1-adrenergic agonist) which was independent of endothelium. The increasing isoprenaline doses elicited similar BP decrease in both rat strains, although BP sensitivity to isoprenaline was slightly decreased in SHR. The blockade of cyclooxygenase (indomethacin) and NO synthase (L-NAME) further reduced BP sensitivity to isoprenaline in SHR. On the other hand, salmeterol elicited similar BP decrease in both strains and the blockade of cyclooxygenase and NO synthase increased BP sensitivity to salmeterol in SHR as compared to WKY. In conclusion, attenuated β-adrenergic vasodilatation of conduit arteries of SHR but similar β-adrenergic relaxation of resistance mesenteric arteries from WKY and SHR and their similar BP response to β-adrenergic agonists do not support major role of altered β-adrenergic vasodilatation for high BP in genetic hypertension.

Dynein Coordinates β2-Adrenoceptor-Mediated Relaxation in Normotensive and Hypertensive Rat Mesenteric Arteries

BACKGROUND

The voltage-gated potassium channel (Kv)7.4 and Kv7.5 channels contribute to the β-adrenoceptor-mediated vasodilatation. In arteries from hypertensive rodents, the Kv7.4 channel is downregulated and function attenuated, which contributes to the reduced β-adrenoceptor-mediated vasodilatation observed in these arteries. Recently, we showed that disruption of the microtubule network, with colchicine, or inhibition of the microtubule motor protein, dynein, with ciliobrevin D, enhanced the membrane abundance and function of Kv7.4 channels in rat mesenteric arteries. This study aimed to determine whether these pharmacological compounds can improve Kv7.4 function in third-order mesenteric arteries from the spontaneously hypertensive rat, thereby restoring the β-adrenoceptor-mediated vasodilatation.

METHODS

Wire and intravital myography was performed on normotensive and hypertensive male rat mesenteric arteries and immunostaining was performed on isolated smooth muscle cells from the same arteries.

RESULTS

Using wire and intravital microscopy, we show that ciliobrevin D enhanced the β-adrenoceptor-mediated vasodilatation by isoprenaline. This effect was inhibited partially by the Kv7 channel blocker linopirdine and was dependent on an increased functional contribution of the β2-adrenoceptor to the isoprenaline-mediated relaxation. In mesenteric arteries from the spontaneously hypertensive rat, ciliobrevin D and colchicine both improved the isoprenaline-mediated vasorelaxation and relaxation to the Kv7.2 -7.5 activator, ML213. Immunostaining confirmed ciliobrevin D enhanced the membrane abundance of Kv7.4. As well as an increase in the function of Kv7.4, the functional changes were associated with an increase in the contribution of β2-adrenoceptor following isoprenaline treatment. Immunostaining experiments showed ciliobrevin D prevented isoprenaline-mediated internalizationof the β2-adrenoceptor.

CONCLUSIONS

Overall, these data show that colchicine and ciliobrevin D can induce a β2-adrenoceptor-mediated vasodilatation in arteries from the spontaneously hypertensive rat as well as reinstating Kv7.4 channel function.

Blood pressure variability provokes vascular β-adrenoceptor desensitization in rats

Spontaneous variation in blood pressure is defined as 'blood pressure variability' (BPV). Sinoaortic denervation (SAD) is characterized by BPV without sustained hypertension. In the present study, we investigated whether BPV could be related to vascular β-adrenoceptor desensitization in rats. Three days after surgery (SAD and control), aortic rings were placed in an organ chamber and the relaxation stimulated by β-adrenoceptor agonists, isoprenaline, terbutaline, BRL37344 and cyanopindolol was verified. The participation of intracellular nucleotides signaling pathways was also verified using forskolin, sodium nitroprusside and acetylcholine to induce relaxation. The effects of BPV on the increase in endothelial cytosolic Ca(2+) concentration stimulated by the β2-adrenoceptor agonist was examined by confocal microscopy. In addition, the vascular expression of the β2-adrenoceptor was also examined by immunohistochemistry. The results show that isoprenaline and terbutaline-induced relaxation was lower in the aortas of rats with BPV. Relaxation responses to other vasorelaxant compounds were similar in both groups of rats. Histological analysis revealed a lower level of β2-adrenoceptor and confocal microscopy showed minor cytosolic Ca(2+) concentration in endothelial cells stimulated by the β2-adrenoceptor agonist in rats with BPV. In conclusion, BPV leads to desensitization of the β2-adrenoceptor, which could contribute to worse β-adrenoceptor agonist-induced relaxation in isolated aortas.

Impaired β-adrenoceptor-induced relaxation in small mesenteric arteries from DOCA-salt hypertensive rats is due to reduced K(Ca) channel activity

β-Adrenoceptor (β-AR)-mediated relaxation plays an important role in the regulation of vascular tone. β-AR-mediated vascular relaxation is reduced in various disease states and aging. We hypothesized that β-AR-mediated vasodilatation is impaired in DOCA-salt hypertension due to alterations in the cAMP pathway. β-AR-mediated relaxation was determined in small mesenteric arteries from DOCA-salt hypertensive and control uninephrectomized (Uni) rats. To exclude nitric oxide (NO) and cyclooxygenase (COX) pathways, relaxation responses were determined in the presence of l-NNA and indomethacin, NO synthase inhibitor and COX inhibitors, respectively. Isoprenaline (ISO)-induced relaxation was reduced in arteries from DOCA-salt compared to Uni rats. Protein kinase A (PKA) inhibitors (H89 or Rp-cAMPS) or adenylyl cyclase inhibitor (SQ22536) did not abolish the difference in ISO-induced relaxation between the groups. Forskolin (adenylyl cyclase activator)-induced relaxation was similar between the groups. The inhibition of IK(Ca)/SK(Ca) channels (TRAM-34 plus UCL1684) or BK(Ca) channels (iberiotoxin) reduced ISO-induced relaxation only in Uni rats and abolished the relaxation differences between the groups. The expression of SK(Ca) channel was decreased in DOCA-salt arteries. The expression of BK(Ca) channel α subunit was increased whereas the expression of BK(Ca) channel β subunit was decreased in DOCA-salt arteries. The expression of receptor for activated C kinase 1 (RACK1), which is a binding protein for BK(Ca) channel and negatively modulates its activity, was increased in DOCA-salt arteries. These results suggest that the impairment of β-AR-mediated relaxation in DOCA-salt mesenteric arteries may be attributable to altered IK(Ca)/SK(Ca) and/or BK(Ca) channels activities rather than cAMP/PKA pathway. Impaired β-AR-stimulated BK(Ca) channel activity may be due to the imbalance between its subunit expressions and RACK1 upregulation.

Hypotension in the chronically hypoxic chicken embryo is related to the β-adrenergic response of chorioallantoic and femoral arteries and not to bradycardia

Prolonged fetal hypoxia leads to growth restriction and can cause detrimental prenatal and postnatal alterations. The embryonic chicken is a valuable model to study the effects of prenatal hypoxia, but little is known about its long-term effects on cardiovascular regulation. We hypothesized that chicken embryos incubated under chronic hypoxia would be hypotensive due to bradycardia and βAR-mediated relaxation of the systemic and/or the chorioallantoic (CA) arteries. We investigated heart rate, blood pressure, and plasma catecholamine levels in 19-day chicken embryos (total incubation 21 days) incubated from day 0 in normoxia or hypoxia (14–15% O2). Additionally, we studied α-adrenoceptor (αAR)-mediated contraction, relaxation to the β-adrenoceptor (βAR) agonist isoproterenol, and relaxation to the adenylate cyclase activator forskolin in systemic (femoral) and CA arteries (by wire myography). Arterial pressure showed a trend toward hypotension in embryos incubated under chronic hypoxic conditions compared with the controls (mean arterial pressure 3.19 ± 0.18 vs. 2.59 ± 0.13 kPa, normoxia vs. hypoxia, respectively. P = 0.056), without an accompanied bradycardia and elevation in plasma norepinephrine and lactate levels. All vessels relaxed in response to βAR stimulation with isoproterenol, but the CA arteries completely lacked an αAR response. Furthermore, hypoxia increased the sensitivity of femoral arteries (but not CA arteries) to isoproterenol. Hypoxia also increased the responsiveness of femoral arteries to forskolin. In conclusion, we suggest that hypotension in chronic hypoxic chicken embryos is the consequence of elevated levels of circulating catecholamines acting in vascular beds with exclusive (CA arteries) or exacerbated (femoral arteries) βAR-mediated relaxation, and not a consequence of bradycardia.

Exercício físico, receptores β-adrenérgicos e resposta vascular

O exercício aeróbio promove efeitos benéficos na prevenção e tratamento de doenças como hipertensão arterial, aterosclerose, insuficiência venosa e doença arterial periférica. Os receptores β-adrenérgicos estão presentes em várias células. No sistema cardiovascular, promovem inotropismo e cronotropismo positivo cardíaco e relaxamento vascular. Embora os efeitos do exercício tenham sido investigados em receptores cardíacos, estudos focados nos vasos são escassos e controversos. Esta revisão abordará os efeitos do exercício físico sobre os receptores β-adrenérgicos vasculares em modelos animais e humanos e os mecanismos celulares envolvidos na resposta relaxante. Em geral, os estudos mostram resultantes conflitantes, onde observam diminuição, aumento ou nenhum efeito do exercício físico sobre a resposta relaxante. Assim, os efeitos do exercício na sensibilidade β-adrenérgica vascular merecem maior atenção, e os resultados mostram que a área de fisiopatologia vascular é um campo aberto para a descoberta de novos compostos e avanços na prática clínica.

Changes in the vascular beta-adrenoceptor-activated signalling pathway in 2Kidney-1Clip hypertensive rats

1. beta-Adrenoceptor (beta-AR)-mediated vasodilation, which plays an important physiological role in the regulation of vascular tone, is decreased in two-kidney, one clip (2K-1C) renal hypertension. In this study, downstream pathways related to vascular beta-AR activation were evaluated in 2K-1C rats. 2. Relaxation responses to isoprenaline, forskolin and 8-Br-cAMP were diminished in aortas without endothelium from 2K-1C when compared to those in normotensive two kidney (2K). Basal adenosine-3',5'-monophosphate (cAMP), as well as isoprenaline-induced increase in cAMP levels, was not different between 2K and 2K-1C aortas. 3. Contractile responses to caffeine, after depletion and reloading of intracellular Ca(2+) stores, were greater in 2K-1C than in 2K. The presence of isoprenaline during the Ca(2+)-reloading period abolished the differences between groups by increasing caffeine contraction in 2K without changing this response in 2K-1C aortas. Inhibition of the sarcolemmal Ca(2+)ATPase with thapsigargin markedly attenuated isoprenaline vasodilation in both 2K and 2K-1C and abolished the differences between groups. 4. Blockade of ATP-sensitive K(+) channels (K(ATP)) channels with glibenclamide significantly decreased isoprenaline vasodilation in 2K-1C without affecting this response in 2K. Both vascular gene and protein expression of protein kinase A (PKA), as well as phosphoserine-containing proteins, were increased in 2K-1C vs 2K rats. 5. In conclusion, decreased isoprenaline vasodilation in 2K-1C hypertensive rats is related to impaired modulation of the sarcolemmal Ca(2+)ATPase activity. Moreover, K(ATP) channels may play a compensatory role on isoprenaline-induced relaxation in renal hypertension. Both Ca(2+)ATPase and K(ATP) channel functional alterations, associated with decreased beta-AR vasodilation, are paralleled by an upregulation of protein kinase A (PKA) and phosphoserine proteins expression.

Chronic antioxidant treatment improves sympathetic functions and beta-adrenergic pathway in the spontaneously hypertensive rats

BACKGROUND

Correlations have been found between oxidative stress and hypertension.

OBJECTIVE

To determine whether antioxidants can normalize sympathetic dysfunction at pre- or postsynaptic levels in spontaneously hypertensive rats (SHRs).

METHODS AND RESULTS

Untreated SHRs and Wistar-Kyoto (WKY) rats were compared with rats treated with melatonin (30 mg/kg per day) or N-acetylcysteine (NAC) (4 g/kg per day) given in drinking water for 4 weeks. At the presynaptic level, SHRs had greater plasma noradrenaline concentrations (P <0.01) and an enhanced release of [3H]noradrenaline from isolated atria (P <0.001). At the postsynaptic level, they exhibited an increased proportion of beta2-adrenoceptors in the heart (P <0.001)and a decrease in the chronotropic and mean arterial pressure (MAP) responses to isoproterenol (P <0.001). Melatonin and NAC decreased MAP (P <0.001) and heart rate (P <0.05), and restored the plasma noradrenaline concentrations (P<0.01 and P <0.001, respectively), the chronotropic response to isoproterenol (P <0.05) and the proportions of beta(1)/beta(2)-adrenoceptors in the heart (P <0.05) in SHRs to the levels found in WKY rats. The same treatments decreased the release of [3H]noradrenaline from isolated atria (P <0.05), and melatonin slightly improved the relaxation in the aorta in SHRs only (P <0.05). Plasma concentrations of adrenaline, the isoproterenol-induced relaxation in mesenteric arteries, the total density and affinity of beta-adrenoceptors in the heart, and the adenylate cyclase reactivity of cardiac membranes to isoproterenol, forskolin, sodium fluoride and guanylylimidophosphate were not altered by the treatments.

CONCLUSION

These results suggest that NAC and melatonin decreased the MAP and heart rate and improved the chronotropic response to isoproterenol in SHRs, in association with an inhibition of sympathetic activity and the restoration of cardiac beta-adrenoceptor function.

Responsiveness, affinity constants and beta-adrenoceptor reserves for isoprenaline on aortae from normo-, pre- and hypertensive rats

The objective of this study was to determine the responsiveness, affinity constants and beta-adrenoceptor reserves for isoprenaline on the isolated aorta in the maturation of normotensive and hypertensive rats. The effects of a very slowly reversible antagonist, bromoacetylal-prenololmenthane (BAAM), on the relaxant responses of the aortae of 5- and 14-week-old Wistar Kyoto normotensive rats (WKY) and spontaneously hypertensive rats (SHRs) to isoprenaline were determined. Five-week-old SHRs are pre-hypertensive and the aortic rings are less responsive to isoprenalinethan age-matched WKY (pD2 values: WKY, 8.40; SHRs, 8.03). Similar relaxant responses to forskol in were obtained on the aortae of 5- and 14-week-old WKY and SHRs. The K(A) value for isoprenaline at the aortic beta2-adrenoceptors of the 5-week-old WKY was 2.1 x 10(-7) M, and similar values were obtained on the aortae of 5-week-old SHR and 14-week-old WKY and SHRs. In the maturation of the WKY aortae from 5 to 14 weeks, there was a reduction in the maximum response, a major loss of sensitivity and a loss of beta2-adrenoceptor reserve for isoprenaline. On 5-week-old SHR aorta, the sensitivity to isoprenaline was 2.5-fold lower, and the beta2-adrenoceptor reserve was less than on age-matched WKY. In the development of hypertension on the SHR aorta from 5 to 14 weeks, there was a reduction in the maximum response to isoprenaline. At 14 weeks, the sensitivity and the beta-adrenoceptor reserve to isoprenaline were similar, but the maximum responses were lower on the SHR than WKY. As there are differences in pre-hypertensive SHR and age-matched WKY aortic responses to isoprenaline, it is no longer valid to consider that the loss of responsiveness to isoprenaline in hypertension is solely owing to the hypertension. There are no changes in affinity, but major changes in the sensitivity, maximum responses and aortic beta2-adrenoceptor reserves to isoprenaline in the maturation of normotensive and pre-hypertensive aortae.