Spontaneous inhibition or inactivation of renin in rat plasma.

Renin appears to be rapidly inactivated in vitro. The present study was undertaken to clarify this observation and to establish the existence of substances involved in renin inactivation. The disappearance rate of renin (including pure renin) was measured in plasma incubated at 37 degrees C and in circulating blood. Pure renin added to plasmas disappears in vitro at the same rate (t1/2 approximately or equal to 40 min) that renin in plasma from normal rats and from rats submitted to a hemorrhage. This process appears not to be mediated by proteases. The disappearance rate of endogenous renin in the normal group (n = 18) was 39.7 min with rapid phase (R) of t1/2 = 14.2 min and a slow phase (S) of t1/2 = 94.3 min), whereas it was 32.1 min (t1/2 R = 13.1 min and t1/2 S = 69.1 min) in rats submitted to an hemorrhage (n = 6). The t1/2 of pure renin was 31.4 min (t1/2 R = 13.3 min and t1/2 S = 69.2 min). Incubation of plasma reveals that renin is inactivated or inhibited in vitro at a similar rate than in circulating plasma. These results suggest that inactivation and sequestration of renin could be two independent mechanisms in the maintenance of plasma renin activity.

Activation of renal renin by a protein plasma fraction: a novel enzymatic mechanism.

A 3-fold increase in active renin was found after a kidney cortex extract was incubated with plasma from either normal or nephrectomized rats (0.34 +/- 0.04 to 1.34 +/- 0.08 and 1.60 +/- 0.06 micrograms Angiotensin I/mg tissue/hr, respectively). A plasma protein that activates renal renin was purified 900-fold. Purification of the protein was achieved by a combination of ammonium sulfate fractionation, molecular filtration on Sephacryl S-200 HR and ion-exchange chromatography on Mono Q HR 5/5 associated to an fast performance liquid chromatography (FPLC) system. The protein shows a molecular weight of approximately 54,000 Da. Renin activation was not inhibited by serine protease inhibitors, such as phenylmethyl sulfonylfluoride, aprotinin, soybean trypsin inhibitor and N-tosyl-L-phenylalanine chloromethyl ketone or by the cystein protease inhibitors N-ethylmaleimide and leupeptin. By using enzyme inhibitors, it was found that the activation process is not mediated by kallikrein, plasmin, tonin, cathepsin B or trypsin-like enzymes. From these results, we conclude that there is in circulating plasma a previously unidentified enzyme capable of activating inactive kidney renin. However, the possibility that this protein acts by activating the renin-substrate reaction cannot be dismissed.

A renin-like enzyme in rat luteal tissue: evidence of local synthesis and its regulation during pregnancy.

Changes in the concentration of a renin-like enzyme were studied in androgenized rats in which a single luteal phase was induced by the administration of chorionic gonadotrophin. A significant increase in the luteal renin-like enzyme (RLE) concentration was found between the youngest corpora lutea (48 h old) and the oldest one studied (6 days old). The luteal RLE content varied independently of changes in plasma renin concentration. These results suggest that this enzyme was produced locally. The lack of correlation between the luteal RLE and plasma prolactin supports our previous observation that the changes in luteal renin concentration appear not to be prolactin-dependent. Furthermore, the suckling-associated hormones appear not to be related with the regulation of luteal RLE concentration, since the values were not modified in androgenized maternal rats which were suckling when compared with the controls. Changes in luteal renin concentration were also studied during pregnancy. A significant increase was found a few hours after a fertile mating which reached a peak on day 1 of pregnancy, followed by a rapid decrease to low levels throughout the remainder of the pregnancy. Because the renin-angiotensin facilitates angiogenesis, luteal renin may act as an angiogenic factor, stimulating blood vessel growth in the corpora lutea. An alternative hypothesis is that the increase in RLE could be a trigger for calcium flux redistribution and steroid biosynthesis.

Cyclical changes in plasma renin during the oestrous cycle in the rat: synchronized effect of oestrogen and progesterone.

The aim of the present work was to study the relationship between sex hormones and plasma renin levels during the oestrous cycle in a Wistar-derived rat strain. Plasma renin activity (PRA) as well as a plasma renin concentration (PRC) were increased during the day of oestrus in rats with controlled 4-day oestrous cycles. This increase in PRA and PRC was not found when rats were ovariectomized on dioestrus day 2 and samples measured on the expected day of oestrus. The increase in PRA and PRC was not found when normal cyclic rats were treated with either tamoxifen or the progesterone receptor blocker RU 38486. Treatment with progesterone at pro-oestrus after ovariectomy on dioestrus day 2 partially increased the PRA and PRC when compared with the values found during the day of oestrus in control rats. The combined treatment of ovariectomized rats on dioestrus day 2 with oestrogen and progesterone restored the normal increase in PRA and PRC values on the expected day of oestrus. We therefore postulate that the sodium diuresis promoted by progesterone may be modulated by the previous peak of oestrogen. However, stimulation of extrarenal sources of renin cannot be excluded nor can an involvement of inactive precursors of renin in the fluctuations of active renin that occur during the oestrous cycle. No important change in plasma renin substrate (PRC) was observed during the oestrous cycle. PRA, PRC and PRS were determined every 4 h during the 4-day oestrous cycle. Our results clearly show a rhythmic variation in PRA and PRC which increases during the day of oestrus with a peak at 06.00 h.(ABSTRACT TRUNCATED AT 250 WORDS)

Changes during lactation in the renin-like enzyme concentration in rat luteal tissue.

The activity of a renin-like enzyme (RLE) previously found in rat copora lutea was studied during lactation. Luteal RLE concentration significantly increased after delivery and reached a maximum on day 5 of lactation. Plasmatic levels of PRL and progesterone also increased through lactation. Treatment with 2 bromo-alpha-ergocryptine, which diminished plasma PRL and progesterone levels, enhanced luteal RLE activity. Therefore, the increase in luteal RLE during lactation seems to be independent of PRL and progesterone levels, but dopamine could be involved in its regulation. The increase in luteal RLE is not related to the intensity of the suckling stimulus, since RLE values were not modified in mothers suckling 2 to 10 pups. In conclusion, RLE activity in rat corpora lutea changes during lactation with a pattern similar to that of plasmatic PRL and progesterone, but seems not to be regulated by these hormones, nor by the intensity of suckling. On the contrary, luteal RLE may be regulated by dopamine.

A renin-like enzyme in luteal tissue.

The aim of this study was to identify immunologically and biologically a renin-like enzyme (RLE) in rat corpora lutea (CL). The biological activity of partially purified extracts of CL was tested in vivo by injection into anesthetized pentolinium-treated rats, obtaining a pressor response similar to renal renin. The enzyme activity in vitro was inhibited to about 50% by pretreatment with a specific antibody against renal renin. When the extracts were incubated with angiotensinogen, the product was inhibited mainly by angiotensin I antibody. The fact that there was no change in RLE content in 24 or 48 h nephrectomized rats, suggested the idea of a local production rather than an active blood renin sequestration.