Hypertension in the transgenic rat TGR(mRen-2)27 may be due to enhanced kinetics of the reaction between mouse renin and rat angiotensinogen.

Abstract

The transgenic rat TGR(mRen-2)27, in which the Ren-2 mouse renin gene is transfected into the genome of the rat, develops severe hypertension with high adrenal renin and low kidney renin. These animals express both mouse and rat renin. To investigate the cause of hypertension in the TGR rat, we compared the kinetics of mouse renin acting on mouse and rat angiotensinogens. The optimum pH of the renin reaction in the Sprague-Dawley rat was 6.5, whereas the optimum pH of the reaction in the TGR rat was approximately 8.5. The optimum pH of the renin reaction in the DBA mouse was 6.0. Purified mouse Ren-2 renin acting on rat angiotensinogen showed a pH profile similar to that for the renin reaction in the TGR rat. The angiotensinogen concentration in pooled plasma from eight DBA mice was 104.5 ng angiotensin I/mL and was clearly lower than that in Sprague-Dawley rats (772.4±37.3 ng angiotensin I/mL, n=4). The reaction of purified mouse Ren-2 renin with rat angiotensinogen was 10 times faster than with mouse angiotensinogen. Plasma renin activity in DBA mice increased dramatically on addition of rat angiotensinogen (from T he renin-angiotensin system plays an important role in the development of hypertension. Recently, various transgenic animal models of hypertension have been developed 15 to elucidate the mechanisms of hypertension. The transgenic rat TGR-(mRen-2)27, in which the mouse Ren-2 renin gene that codes for mouse salivary gland and kidney renin is transfected into the genome of the Sprague-Dawley rat, was the first genetic hypertensive rat model. In this model an overexpression of the transgene is found in the adrenal glands, 2 and these rats show severe hypertension with remarkably high adrenal renin levels; however, the pathogenesis of the hypertension is not clear. We have reported that adrenal renin and circulating renin in TGR rats are probably derived primarily from the transfected mouse renin gene. 6 On the other hand, angiotensinogen in the TGR rat is solely rat angiotensinogen. It is known that mouse renin can generate angiotensin I (Ang I) by reacting with rat angiotensinogen.7 If the circulating renin in the TGR rat is mouse renin, then the kinetics of 253.4±66.7 to 225 000±48 000 ng angiotensin I/mL per hour). Intravenous injection of 2 or 10 £iL of DBA mouse plasma into the nephrectomized Sprague-Dawley rat increased the mean arterial pressure of the rat by 27.7±4.7 and 61.8±2.7 mm Hg, respectively, whereas injection of 200 JAL of Sprague-Dawley rat plasma did not change the mean arterial pressure of the rat. From these measurements and from the kinetic parameters measured by Poulsen and Jacobsen (/ Hypertens. 1986;4:175-180), we conclude that the molar concentration of mouse renin in the TGR rat is low compared with that of rat renin (1.5 versus 5.0 pmol/L) but that this low level of circulating mouse renin contributes significantly to plasma renin activity in the TGR rat. The elevation of renin activity and resulting hypertension in the TGR rat therefore can be attributed to the enhan...

Hypertension in the (mRen-2)27 Rat Is Not Explained by Enhanced Kinetics of Transgenic Ren-2 Renin

“…The rate of the heterologous reaction (Ren-2 renin on rat aogen) was considerably faster than the matched homologous mouse reaction, but the kinetic constants cannot be derived for mouse aogen, because maximum velocity was not approached. This experiment was conducted to test the hypothesis that the basis for the proposal of Tokita et al 13 was reproducible under the present conditions.…”

“…However, such extrapolation from kinetic data obtained by different authors who used various conditions and reactants could lead to erroneous conclusions. Neither Tokita et al 13 nor other authors have directly compared, under plasma physiologic conditions at pH 7.4, the mouse Ren-2 renin on rat aogen reaction with the rat renin on rat aogen reaction, which is the appropriate experiment to test this notion and which is reported in this article.…”

“…Another factor examined by Tokita et al 13 was the possibility that an enhanced reaction between the heterologous reactants, mouse Ren-2 renin and rat aogen, might explain why in some reports the adult animal displays low/normal plasma active renin but that the hypertension is nevertheless Ang II dependent. These investigators used high concentrations of mouse salivary gland Ren-2 renin acting on mouse and rat aogens and found that the heterologous reaction was 10 times faster than the mouse homologous reaction.…”

“…This confirmed the earlier conclusion by Poulsen and Jacobsen 14 of unusual renin kinetics in the normal mouse, wherein the turnover number was one quarter that of the homologous rat reaction. Tokita et al 13 then cited the kinetic data of Gutkowska et al 15 for the homologous rat reaction and of Poulsen and Jacobsen 14 for the heterologous mouse renin on rat aogen reaction and proposed that the hypertension could be Ang II dependent, despite low molar concentrations of active mouse renin in plasma due to enhanced kinetics. However, such extrapolation from kinetic data obtained by different authors who used various conditions and reactants could lead to erroneous conclusions.…”

Response to genetic manipulations of liver angiotensinogen in the physiological range

“…Animal have yielded strong proof of principle that a marked and global increase in AGT expression (Fukamizu et al 1993;Kimura et al 1992;Smithies and Kim 1994;Tokita et al 1994;Yang et al 1994) or in A-II formation (Cowley and McCaa 1976;Kobori et al 2001) could indeed lead to marked hypertension. The Smithies titration model has proved that a similar effect could be observed in a physiological range that closely approximates the situation that may prevail in humans.…”