Plasma angiotensin in binephrectomised mice

Vascular smooth muscle Jak2 mediates angiotensin II-induced hypertension via increased levels of reactive oxygen species

AIMS

Angiotensin II (Ang II) type AT(1) receptors expressed on vascular smooth muscle cells (VSMCs) couple to the Jak2 signalling pathway. However, the importance of this tissue-specific coupling is poorly understood. The purpose of this investigation was to determine the importance of VSMC-derived Jak2 in angiotensin II-mediated hypertension.

METHODS AND RESULTS

The Cre-loxP system was used to conditionally eliminate Jak2 tyrosine kinase expression within the smooth muscle cells of mice. Following chronic Ang II infusion, the resulting increase in mean arterial pressure (MAP) was significantly attenuated in the Jak2 null mice when compared with littermate controls. The VSMC Jak2 null mice were also protected from the Ang II-induced vascular remodelling. Aortic rings from the VSMC Jak2 null mice exhibited reduced Ang II-induced contraction and enhanced endothelial-dependent relaxation via increased nitric oxide (NO) bioavailability. When compared with controls, the VSMC Jak2 nulls also had lower levels of hydrogen peroxide, Rho kinase activity, and intracellular Ca(2+) in response to Ang II.

CONCLUSIONS

The data indicate that VSMC Jak2 expression is involved in the pathogenesis of Ang II-dependent hypertension due to the increased presence of reactive oxygen species (ROS). As such, VSMC-derived Jak2 tyrosine kinase modulates overall vascular tone via multiple, non-redundant mechanisms.

Importance of the renin-angiotensin system in the regulation of arterial blood pressure in conscious mice and rats

AIM

The present experiments were designed to determine the mechanism(s) for increased sensitivity to blockade of the renin-angiotensin system in mice in comparison with rats.

METHODS

Mice and rats, with indwelling femoral arterial and venous catheters, were chronically administered angiotensin II or pharmacological inhibitors of the renin-angiotensin system as sodium intake was altered.

RESULTS

Increasing sodium intake led to suppression of circulating renin, angiotensin II, and aldosterone in rats and mice in the absence of alterations in arterial blood pressure. Additional experiments demonstrated that continuous intravenous infusion of angiotensin II (20 ng kg(-1) min(-1)) significantly increased arterial blood pressure by approximately 35 mmHg in conscious rats at all levels of sodium intake (n = 6). In contrast, arterial pressure was unaffected by angiotensin II infusion in conscious mice under conditions of low sodium intake, although arterial pressure was increased by 16 mmHg when mice were administered a high sodium intake while infused with angiotensin II (n = 6). In comparison, blockade of the endogenous renin-angiotensin system led to significantly greater effects on arterial pressure in mice than rats. Continuous infusion of captopril (30 microg kg(-1) min(-1)) or losartan (100 microg kg(-1) min(-1)) resulted in a 55-90% greater fall in blood pressure in conscious mice in comparison with conscious rats.

CONCLUSION

The present studies indicate that arterial pressure in mice is more dependent upon the endogenous renin-angiotensin system than it is in rats, but mice are more resistant to the hypertensive effects of exogenous angiotensin II.

Plasma renin in mice with one or two renin genes

AIM

In the present study we have investigated whether the presence of a second renin gene exerts an overriding influence on plasma renin such that mice with two renin genes have consistently higher renin levels than mice with only one renin gene.

METHODS

Plasma renin was determined as the rate of angiotensin I generation using a radioimmunoassay (RIA) kit with (plasma renin concentration, PRC) or without (plasma renin activity, PRA) the addition of purified rat angiotensinogen as substrate.

RESULTS

In male 129SvJ, DBA/2 and Swiss Webster mice, strains possessing both Ren-1 and Ren-2, PRC (ng Ang I mL(-1) h(-1)) averaged 178 +/- 36, 563 +/- 57 and 550 +/- 43 while PRA was 2.9 +/- 0.5, 3.6 +/- 0.8 and 7.8 +/- 1.2. In male C57BL/6, C3H and BALB/c mice that express only Ren-1, PRC averaged 426 +/- 133, 917 +/- 105 and 315 +/- 72, and PRA was 3.4 +/- 1.0, 6.9 +/- 1.7 and 4.5 +/- 1.2. In the two renin gene A1AR-/- mice compared with the one renin gene A1AR+/+, PRC averaged 538 +/- 321 and 415 +/- 159 while PRA averaged 3.2 +/- 1.1 and 4.4 +/- 1.4 ng Ang I mL(-1) h(-1). Aldosterone levels showed no significant differences between one renin (C57BL/6, C3H and BALB/c) and two renin (129SvJ, DBA/2 and Swiss Webster) gene mice. Furthermore, by quantitative real-time polymerase chain reaction (RT-PCR) we found no correlation between the number of renin genes and whole kidney renin mRNA levels from one and two renin gene mice.

CONCLUSION

Our data show that baseline plasma renin is not systematically higher in mice with two renin genes than in one renin gene mice. Thus, the presence of a second renin gene does not seem to be a major determinant of differences in PRC between different mouse strains.

Autonomic control of blood pressure in mice: basic physiology and effects of genetic modification

Control of blood pressure and of blood flow is essential for maintenance of homeostasis. The hemodynamic state is adjusted by intrinsic, neural, and hormonal mechanisms to optimize adaptation to internal and environmental challenges. In the last decade, many studies showed that modification of the mouse genome may alter the capacity of cardiovascular control systems to respond to homeostatic challenges or even bring about a permanent pathophysiological state. This review discusses the progress that has been made in understanding of autonomic cardiovascular control mechanisms from studies in genetically modified mice. First, from a physiological perspective, we describe how basic hemodynamic function can be measured in conscious conditions in mice. Second, we focus on the integrative role of autonomic nerves in control of blood pressure in the mouse, and finally, we depict the opportunities and insights provided by genetic modification in this area.

Control of renin secretion from adrenal gland in transgenic Ren-2 and normal rats

In Ren-2 rats, plasma active renin and prorenin increase following binephrectomy (BNx) related to increasing plasma potassium. Adrenal is the source of the increasing prorenin but active renin comes mainly from thymus and gut. Trophic influences other than potassium were tested in the present work. Angiotensin did not influence the post-BNx increases in plasma active or prorenin but suppressed resting plasma prorenin from non-adrenal, non-renal sources virtually to zero. ACTH and histamine had no discernible effects. Hexamethonium decreased by 50% the post BNx increase in prorenin but not active renin. In Sprague-Dawley and spontaneously hypertensive rats, low levels of active renin secretion were detected from adrenal but no prorenin. Thus, in anesthetized Ren-2 rats, secreted prorenin is from two sources, i.e. extrarenal and extra-adrenal sites readily suppressible with angiotensin and the adrenal that is partly suppressible by autonomic blockage. This may assist in identifying the origin of extra-renal prorenin secreted in man.

Renal physiology of the mouse

As the transgenic and gene-targeting technology has become an invaluable experimental approach to study the function of gene products, the need has been expanded to assess the physiology in the mouse, which is virtually the only animal species to which that new genetic technology can apply. In this regard, renal physiologists have also received fruits of success from modern technology in several key areas, and areas are expanding in both depth and scope.

Renin in thymus, gut, hindlimb, and adrenal of (mRen-2)27 and normal rats: secretion and content studies

Thymic ablation and assay of organ renin revealed that one-third of the increasing plasma level of active renin after removal of kidneys and adrenals from Ren-2 rats originates from the thymus. Splanchnic arteriovenous difference and renin content indicate that gut can account for the remainder. Secretion of active renin from these sites correlated significantly with increasing plasma potassium. Prorenin was not secreted from these sites or from hindlimb in amounts sufficient to raise the plasma level, and yet plasma prorenin remained higher than active renin throughout the 12-h protocol. The source of prorenin that accounts for the high plasma prorenin phenotype of the intact conscious Ren-2 rat was not specifically identified. When sensitive assays were used, a low level of active renin secretion from thymus and gut was also apparent 12 h after removal of kidneys and adrenals in normal Sprague-Dawley rats, and plasma prorenin was at this time higher than active renin. A likely source of this extrarenal, extra-adrenal renin is the macrophage.

Regulation of human renin mRNA expression and protein release in transgenic mice

The renin-angiotensin system plays a major role in the regulation of blood pressure and electrolyte homeostasis in mammals. In this study, we subjected transgenic mice containing a human renin genomic construct to a variety of pharmacological and physiological manipulations to test whether expression of the human renin gene and release of active human renin in appropriately regulated in this model. These manipulations were designed to test major regulators of renin release, including angiotensin II, the macula densa, renal perfusion pressure, and beta-adrenergic receptors. We used human plasma renin concentration and human renal renin mRNA levels to document the response of the transgene to these stimuli. Human plasma renin concentration increased in response to both angiotensin-converting enzyme inhibition with captopril and isoproterenol and decreased after a high salt diet. A low salt or sodium-deficient diet did not stimulate renin release. Human renin mRNA levels in kidney increased after captopril but were unchanged in the other experimental groups. We also measured the levels of human renin mRNA in double transgenic mice containing the same human renin gene in addition to the human angiotensinogen gene. These mice are chronically hypertensive and have increased circulating levels of angiotensin II. Human renin mRNA levels in the kidney were paradoxically elevated compared with their single transgenic normotensive counterparts. These transgenic mice provide a model for examination of human renin regulation and may help elucidate the molecular mechanisms that regulate the gene in response to physiological cues.

Chronic hypertension and altered baroreflex responses in transgenic mice containing the human renin and human angiotensinogen genes

We have generated a transgenic model consisting of both the human renin and human angiotensinogen genes to study further the role played by the renin-angiotensin system in regulating arterial pressure. Transgenic mice containing either gene alone were normotensive, whereas mice containing both genes were chronically hypertensive. Plasma renin activity and plasma angiotensin II levels were both markedly elevated in the double transgenic mice compared with either single transgenic or nontransgenic controls. The elevation in blood pressure caused by the human transgenes was independent of the genotype at the endogenous renin locus and was equal in mice homozygous for the Ren-1c allele or in mice containing one copy each of Ren-1c, Ren-1d, or Ren-2. Chronic overproduction of angiotensin II in the double transgenic mice resulted in a resetting of the baroreflex control of heart rate to a higher pressure without significantly changing the gain or sensitivity of the reflex. Moreover, this change was not due to the effects of elevated pressure itself since angiotensin-converting enzyme inhibition had minimal effects on the baroreflex in spontaneously hypertensive BPH-2 control mice, which exhibit non-renin-dependent hypertension. This double transgenic model should provide an excellent tool for further studies on the mechanisms of hypertension initiated by the renin-angiotensin system.