A decrease in the amount and function of inhibitory GTP-binding protein in the resistance small artery from spontaneously hypertensive rats

The mechanism of distinct diurnal variations of renin-angiotensin system in aorta and heart of spontaneously hypertensive rats

Diurnal variations in plasminogen activator inhibitor-1 mRNA expression are different between the spontaneously hypertensive rats (SHRs) and the Wistar-Kyoto (WKY) rats, and between the aorta and the heart. To elucidate the mechanisms, we examined diurnal changes in the circulating renin-angiotensin system in the SHR and WKY rats. Diurnal variations in plasma renin activity (PRA), plasma angiotensin I, and aldosterone concentrations were similar between the SHR and WKY rats. On the other hand, plasma angiotensin II (Ang II) concentration in the SHR was lower than that in the WKY rats at most time points, but increased to the level of the WKY rats in the late light phase. Treatment with AT1 receptor antagonist candesartan increased plasma Ang II concentration except at ZT 8 and lessened its diurnal variation in the SHR. At the peak in plasma Ang II in the SHR, Ang II regulated genes such as transforming growth factor-beta1 and p22phox were upregulated in the aorta. On the other hand, these genes were upregulated throughout the day in the heart of SHR. Candesartan treatment increased AT1a receptor mRNA expression in the heart but not in the aorta of SHR. These findings suggest that an AT1 receptor-mediated mechanism might cause a surge in plasma Ang II concentration at the late light phase in the SHR. Homologous down-regulation of AT1a receptor by Ang II may dampen the effect of a surge in plasma Ang II concentration in the heart of SHR.

Mechanism of increased alpha-adrenoceptor-mediated contraction in small resistance arteries of rats with heart failure

1. Alterations in a(1)-adrenoceptor signalling that result in enhanced contraction in resistance arteries in heart failure are not well characterized. To clarify whether this enhanced constriction is due to Ca(2+)-dependent or -independent effects, we measured the phenylephrine-induced changes in [Ca(2+)](i) in the presence of a Rho kinase inhibitor or an inositol 1,4,5-trisphosphate (IP(3)) receptor inhibitor. 2. Heart failure was induced in rats by ligation of the left coronary artery. Changes in the internal diameter of pressurized small femoral arteries were examined using videomicroscopy. Phenylephrine concentration-response curves, constructed in the presence of the Rho kinase inhibitor Y27632 (0.3 micromol/L) or the IP(3) receptor inhibitor xestospongin C (0.3 micromol/L), were compared in heart failure rats and sham-operated (control) rats; fura-2 Ca(2+) signals were measured in the arteries of both groups. 3. The heart : bodyweight ratio, lung : bodyweight ratio, left ventricular end-diastolic pressure and plasma B-type natriuretic peptide were significantly higher in heart failure rats compared with control rats. Phenylephrine-induced contractile responses and increases in [Ca(2+)](i) were significantly greater in arteries from heart failure rats compared with arteries from control rats. At 0.3 micromol/L, Y27632 selectively inhibited phenylephrine-induced constrictions of heart failure arteries, but had no effect on the increase in [Ca(2+)](i). 4. Immunohistochemical staining for Rho kinase was greater in heart failure rats compared with control rats. 5. The degree of inhibition of both the phenylephrine-induced constriction and the increase in [Ca(2+)](i) by xestospongin C (0.3 micromol/L) was greater in arteries from heart failure rats than in those from control rats. 6. The increased contractile response to phenylephrine in arteries of heart failure rats results from IP(3)-dependent increases in [Ca(2+)](i) and from an enhanced Ca(2+) sensitivity via a Rho kinase-dependent mechanism.

Circadian gene expression of clock genes and plasminogen activator inhibitor-1 in heart and aorta of spontaneously hypertensive and Wistar-Kyoto rats

OBJECTIVE

Heart and aorta possess biologic clocks, but their involvement in genetic hypertension has been unknown. Plasminogen activator inhibitor-1 (PAI-1) expression is directly regulated by clock genes, while angiotensin II modulates both PAI-1 and clock gene expression. We therefore examined circadian expression of PAI-1 and clock genes, and effects of angiotensin type 1 (AT1) receptor antagonism, in heart and aorta of spontaneously hypertensive rats (SHR) and Wistar-Kyoto (WKY) rats.

METHODS

We examined cardiac and aortic mRNA expression for PAI-1 and clock genes (Per2, Bmal1, Clock, and Dbp) every 4 h throughout the day by quantitative reverse transcription-polymerase chain reaction, and intervention with the AT1 receptor antagonist candesartan and equihypotensive hydralazine.

RESULTS

Cardiac PAI-1 expression was high in the dark, while aortic PAI-1 expression was high in the light. Both cardiac and aortic PAI-1 expression were greater in SHR than in WKY rats. Candesartan treatment decreased cardiac PAI-1 expression only in the dark in WKY rats but throughout the day in SHR. Candesartan but not hydralazine strongly attenuated circadian fluctuation of aortic PAI-1 mRNA in SHR and WKY rats. Clock genes oscillated synchronously in heart and aorta of SHR and WKY rats. Clock gene expression was increased in heart but not aorta of SHR. Candesartan did not affect clock gene expression.

CONCLUSIONS

Enhanced expression of clock genes may increase PAI-1 expression in concert with activated renin-angiotensin system in SHR heart. Rather than clock genes, the renin-angiotensin system induces daily fluctuation and increased expression of aortic PAI-1 mRNA in SHR.

Augmented diurnal variations of the cardiac renin-angiotensin system in hypertensive rats

There are several controversies concerning the enhanced gene expression of cardiac renin-angiotensin system components in spontaneously hypertensive rats (SHR) compared with their normotensive control Wistar-Kyoto (WKY) rats. We hypothesized that these discrepancies arise from circadian fluctuations in gene expression. We examined the circadian mRNA expression of renin, angiotensinogen, ACE, and angiotensin type 1a (AT1a) and type 2 (AT2) receptors in the hearts of SHR and WKY rats by real-time quantitative reverse transcription-polymerase chain reaction. The cardiac mRNA expression of the renin-angiotensin system components showed circadian oscillations in both SHR and WKY rats. The amplitudes of these circadian fluctuations were greater in the SHR than in the WKY rats. The mRNA levels of the renin-angiotensin system components were also increased in the SHR compared with the WKY rats at many time points (especially during the dark phase). However, the levels of ACE, AT1a receptor, and AT2 receptor mRNA in the SHR and WKY rats were almost the same during the late light phase. In contrast to mRNA expression, ACE activity was similar both at the time of maximum and minimum mRNA expression. The AT1 receptor antagonist candesartan upregulated AT1a receptor mRNA and downregulated ACE mRNA at specific time points only in the SHR group. Our findings of differential diurnal expression of cardiac renin-angiotensin system genes in SHR and WKY rats appear to explain the discrepancies between prior studies. However, the physiological relevance of the differential circadian mRNA expression of the renin-angiotensin system components remains to be elucidated.

A decrease in the amount and function of the stimulatory GTP-binding protein in the small resistance arteries of spontaneously hypertensive rats

We investigated the concentration of stimulatory GTP-binding protein (Gs protein) in the peripheral resistance arteries of spontaneously hypertensive rats (SHR), normotensive Wistar-Kyoto rats (WKY), and renovascular hypertensive rats (RHR). Changes in the function of Gs protein in SHR and WKY were also investigated by microcannulation techniques. The localization and abundance of Gs protein were determined immunohistochemically in 4-, 10- and 20-week-old SHR and age-matched WKY (control), as well as in RHR. Sections of the cremaster artery were stained with polyclonal antibodies to Gs protein. The concentration of Gs protein-like immunoreactivity in the cremaster artery was significantly lower in SHR at 4, 10, and 20 weeks of age, relative to that in age-matched WKY. In contrast, no significant differences were detected in the abundance of Gs between RHR and control rats. The dilatory response by isoproterenol in the presence of beta1-adrenoceptor blocker was lower in 4- and 10-week-old SHR than in age-matched WKY. The dilatory response by cholera toxin was also lower in SHR than in WKY for these two age groups. These results indicated that the amount and function of Gs protein in the peripheral resistance vessels in SHR was reduced. Since this change occurred before the onset of hypertension and no changes were seen in the secondary hypertensive rats, this change was not a secondary change due to hypertension. The impaired receptor-Gs protein-mediated signal transduction in the peripheral resistance arteries may be one of the possible mechanisms responsible for the pathogenesis of hypertension in SHR.