The concentration of renin substrate (RS) was measured in rat mesenteric artery tissue. The concentration of this substrate both in arterial tissue and in plasma was markedly higher in rats 1 day after bilateral nephrectomy than in sham-operated controls, the percentage difference being higher in plasma than in arterial RS. Conversely, the decrease apparently induced 3 days after adrenalectomy (i.e., the difference in RS concentration from sham-operated rats) was greater in arterial tissue than in plasma. This finding may be explained by changes in RS concentrations induced by the sham operation. Sham surgery itself increased plasma RS after 1 day (but not after 3 days) and arterial RS after 3 days (but not after 1 day). There was a positive correlation between arterial and plasma renin substrate concentration for the overall results but not within individual groups. As renin and angiotensin-converting enzyme activity are also present in arterial tissues, all the necessary components for local generation of angiotensin II have now been shown to be present within the wall of resistance vessels.

No abstract

A commercially available radioimmunoassay kit was modified to enable us to measure, in triplicate, the amikacin concentration in 1 ,ld of perilymph fluid. Amikacin levels in plasma and perilymph were measured in guinea pigs after continuous intravenous infusion at four different dosing rates. After a 4-h infusion, a good linear correlation was found between the amikacin concentration in plasma and the dosing rate. Likewise, a significant linear relationship was found between concentrations of amikacin In perilymph and plasma (y = 0.21x + 2.56; r = 0.67; n = 45) after 6 h of infusion. These results suggest nonsaturation kinetics at the concentrations used.The specific toxic effect of aminoglycoside antibiotics (AGs) on the inner ear is related to the fact that they penetrate the inner ear fluid compartments and cause hair cell damage (5,8). Although the rate of entry of AGs into the perilymph space is relatively slow (1, 2, 6, 11), drug levels are prolonged owing to their slow elimination from the inner ear. Different authors have reported widely variable rates of elimination (3 to 15 h) for different AGs (10). As discussed by Manuel et al. (6), intervals between AG doses of less than four half-lives will lead to a notable accumulation in the perilymnph until an equilibrium condition is established. Some authors have suggested the existence of a toxic threshold in the inner ear, meaning that beyond a certain dose the kinetics of AGs are modified and that the concentration increases suddenly in the perilymph (9). Others found a good linear correlation between the dose and perilymph concentrations (3, 4, 6, 10). Pharmacokinetic differences among AGs, as well as differences in the animal species investigated, dose, route of drug administration, time interval between doses, time of sampling, and the analytical technique used, may explain the conflicting conclusions.Amikacin is a semisynthetic derivative of kanamycin and is often used clinically for infections that are resistant to gentamicin or tobramycin. tion of the percentage of relative binding versus the log of the standard concentrations. Logit, probit, and arcsin transformations were tried, and the latter was found to give the best linearity. The rest of the procedure was done according to the protocol provided with the kit. The reproducibility of the assay was assessed by determining the intraassay and interassay coefficients of variation for triplicate determination. Intraassay coefficients of variation ranged from 7.1% for the lowest standard concentrations (5 ng/ml) (n = 10) to 1.8% for the highest standard concentration (320 ng/ml) (n = 10). Two piasma samples with amikacin concentrations of 26.5 and 111.0 ng/ml were measured in triplicate in 12 different assays. Coefficients of variation between assays were found to be 7.2 and 3.7%, respectively.The recovery of drug in 1-plA samples was tested for known concentrations of 4, 9.6, 12.5, and 32 pig/ml with dilution factors Of 1/90, 1/120, 1/250, and 1/500, respectively. The technique described below for per...

The release of prostaglandin E2 (PGE2) and 6-ketoprostaglandin F1 alpha (6-keto-PGF1 alpha), the stable metabolite of prostacyclin (PGI2), by the perfused mesenteric arteries of renal and spontaneously hypertensive rats (SHR) have been measured. Unstimulated mesenteric arteries from two-kidney one-clip hypertensive rats (2K-1C) released 1.6 times as much PGE2 and 2.7 times as much 6-keto-PGF1 alpha as those of control rats. The release of PGE2 by mesenteric arteries from one-kidney one-clip hypertensive rats (1K-1C) was not significantly different from that of uninephrectomized normotensive rats, but the release of 6-keto-PGF1 alpha was 3.5 times higher in the former than in the latter. Norepinephrine (NE) induced a dose-related increase in perfusion pressure, in PGE2, and 6-keto-PGF1 alpha release in all four groups. However, its effect on the release of PGE2 was more pronounced in 2K-1C than in sham-operated rats. There was no difference between 1K-1C and the uninephrectomized group. The effect of NE on the release of 6-keto-PGF1 alpha was significantly higher for both renal hypertensive groups. These results indicate that the release of PGE2 is more dependent on the loss of renal mass than on hypertension, while the reverse applies to the release of 6-keto-PGF1 alpha. Unstimulated mesenteric arteries from SHR released less PGE2 and less 6-keto-PGF1 alpha than those of Wistar-Kyoto normotensive rats (WKY), but the release was not significantly different from Wistar rats. Under NE stimulation, WKY mesenteric arteries showed almost no increase in release of PGs. Compared with those of Wistar rats, SHR mesenteric arteries showed a greater pressor response to NE, a lower PGE2 release, and the same release of 6-keto-PGF1 alpha. These findings reveal the difficulty of selecting an appropriate control group in studies involving SHR.(ABSTRACT TRUNCATED AT 250 WORDS)

The purpose of this study was to determine whether a multiple-sampling procedure could be used in guinea pigs to study the kinetics of amikacin in perilymph. Amikacin was infused intravenously for 6 h into conscious anesthetized guinea pigs, and the concentrations of the drug in plasma and perilymph were measured. From each anesthetized guinea pig, five to six perilymph samples were collected from one ear, and one sample was collected from the other ear at 6 h. The concentrations of amikacin in perilymph were dose proportional and increased slowly during the 6-h infusion. However, after 6 h of intravenous infusion, the concentrations of amikacin in perilymph of the multiply sampled ears were significantly higher than those of the singly sampled ears, indicating that the multiple-sampling procedure should not be used as is to study the kinetics of amikacin in perilymph. Amikacin concentrations in perilymph were linearly related to amikacin concentrations in plasma in pentobarbital-anesthetized animals, as had previously been observed for conscious guinea pigs. However, the slope of the regression line was only 0.09 for anesthetized animals compared with 0.24 for conscious animals. Drug concentrations in plasma were found to be threefold higher in anesthetized animals, whereas drug levels in perilymph were the same in both groups at similar dosing rates. These results indicate that the amikacin concentration in perilymph is not solely dependent upon its concentration in plasma and that other factor(s) can affect the entry of amikacin into the inner ear.Ototoxicity is a well-known adverse effect of aminoglycoside antibiotics (AG). Many investigators believe that the inner ear damage is related to the concentration of AG in the perilymph (1,3,5,8,12,14). Although the rates of entry of AG into perilymph are much slower than those of AG into plasma, these drugs seem to accumulate in the perilymph after repetitive dosing because of their slow elimination rates from this body fluid.Investigators in our laboratory are presently trying to determine the relationship between ototoxicity and the concentrations of amikacin in the perilymph and plasma of guinea pigs. While trying to determine the kinetics of amikacin in perilymph, we wanted to reduce the number of animals required to establish the various rate constants. Since the perilymph compartment is both small and poorly accessible, generally only a single data point can be obtained from each animal (3, 15). We recently modified a radioimmunoassay technique so that only 1 ,ul of perilymph sample is required (4). This small size made multiple sampling of the perilymph feasible in an anesthetized animal. 350 g were used. Food and water were provided ad libitum. Room temperature (21 to 22°C), humidity (50 to 55%), and light cycle (12 h on, 12 h off) were kept constant throughout the experiment.The control group consisted of 19 conscious guinea pigs which had undergone jugular cannulation 48 h previously. Each animal received one of the following i.v. doses of amikacin for 6...