Antihypertensive metabolites of alpha-methyldopa.

alpha-Methyldopa acts through a metabolite in the central nervous system. Of the three metabolites most prominently considered as potential mediators of alpha-methyldopa hypotension--alpha-methyldopamine, alpha-methylnorepinephrine (MNE), and alpha-methylepinephrine (ME)--ME is the most potent depressor agent following intravenous infusion into rats, following injection into the lateral ventricle, and following injection into the solitary tract nucleus (NTS). The depressor effect of ME in the NTS is attenuated as effectively by timolol as by yohimbine, while the combination of both timolol and yohimbine completely abolishes the pharmacological activity of ME in the NTS. ME is approximately eightfold more potent than MNE in the NTS and also has a greater susceptibility to timolol attenuation.

Amphotericin B nephrotoxicity in humans decreased by salt repletion.

A major limitation in the use of amphotericin B is its potential to cause nephrotoxicity. In animals, increased dietary sodium reduces renal toxicity. Experience with five patients in whom impaired renal function developed early during amphotericin B therapy is reported. In four of the patients, there was evidence of sodium depletion due to low sodium intake, diuretic administration, or vomiting. In all five patients, sodium loading was associated with improved renal function, which permitted amphotericin B therapy to be continued in fully effective doses to the completion of elective courses of treatment without evidence of residual impaired renal function. These observations are consistent with the hypothesis that intrarenal regulatory mechanisms contribute to changes in renal function due to amphotericin B therapy.

Unclipping of two-kidney, one clip hypertensive rats produces endothelium-dependent inhibition of sympathetic vasoconstriction.

Removal of the renal artery-constricting clip in two-kidney, one clip (2-K,1C) Goldblatt-hypertensive rats produces a rapid fall in blood pressure which has been attributed to the release of vasodepressor lipids from the renal medulla. The aim of this study was to demonstrate the effect of unclipping 2-K,1C hypertensive rats, and of any hormone thereby released, on vasoconstrictor responses to sympathetic stimulation in the ex vivo blood-perfused tail artery, as well as the role of the vascular endothelium in mediating this effect. Blood was withdrawn at a rate of 2 ml/min from the carotid artery of an anaesthetized 2-K,1C rat and used to perfuse, via an extracorporeal circuit, a cannulated segment of tail artery which had been taken from a killed, normal rat and mounted in an organ bath. The blood was returned to the 2-K,1C rat via the jugular vein. Vasoconstriction and increases in perfusion pressure were produced by periarterial electrical stimulation of the tail artery. The tail artery was either intact or had had its endothelium removed. Sham operations which did not remove the renal artery clip did not change blood pressure or vasoconstrictor responses. Unclipping lowered blood pressure and reduced vasoconstrictor responses in ex vivo blood-perfused tail arteries in which the endothelium was intact, but did not decrease responses in tail arteries from which the endothelium had been removed. It is concluded that unclipping releases a hormone which lowers blood pressure and inhibits sympathetic vasoconstrictor responses via an endothelium-dependent mechanism.

Chemical renal medullectomy prevents frusemide-induced inhibition of sympathetic vasoconstriction in the in situ blood perfused rat mesentery.

The mesentery of anaesthetized rats was perfused in situ at a constant rate, via the cannulated superior mesenteric artery, with blood drawn from a cannulated carotid artery. Increases in perfusion pressure were produced by peri-arterial electrical stimulation of the mesenteric sympathetic nerves at 3, 6 and 10 Hz before and after the intravenous administration of frusemide 5 mg/kg. Loss of solutes and volume as a result of frusemide-induced diuresis was prevented by a urinary bladder-venous shunt. In control animals the vasoconstrictor responses to mesenteric nerve stimulation were significantly reduced after frusemide administration. However, in animals pretreated 4 weeks previously with bromethylamine (BEA) 200 mg/kg i.p., frusemide had no effect on vasoconstrictor responses. Chemical papillectomy with BEA has been shown by others to prevent the antihypertensive effects attributed to the non-prostanoid lipids produced by the interstitial cells of the renal medulla. Our data suggest that the vasoconstrictor-inhibiting effect of frusemide may be mediated by the release of renomedullary lipids.

Effects of dietary sodium and fish oil on blood pressure development in stroke-prone spontaneously hypertensive rats.

The postulated antihypertensive effect of dietary fish oil and the influence of dietary sodium on this effect were evaluated in young stroke-prone spontaneously hypertensive rats (SHRSP) by direct intra-arterial measurement of blood pressure. Weaning rats were fed synthetic diets containing olive oil or eicosapentaenoic acid-enriched fish oil (5% of dry weight) with normal (0.23%) or high (2.8%) sodium content. Catheters were implanted after 3 months for blood pressure measurement under resting conditions and to sample blood for catecholamine determinations. Effects of fish oil on vascular reactivity were assessed in the in situ blood-perfused mesentery. The overall observation, from a series of experiments, was that feeding diets containing 5% fish oil to young SHRSP resulted in a small but consistent suppression of the development of hypertension. This effect could be counteracted, however, by increasing dietary sodium intake. Observations after ganglion blockade indicate that the antihypertensive effect of fish oil is unlikely to result from a reduction in sympathetic vascular tone.

The effect of salt intake on cyclosporine-induced impairment of renal function in rats.

Three groups of rats were fed a low-sodium diet. Groups 1 drank water and was treated with cyclosporine, 100 mg kg-1 48 h-1 p.o. for 3 weeks (low-salt-treated group). Group 2 drank 0.15 M saline and was also treated with cyclosporine (high-salt-treated group). Group 3 drank water and was treated with the vehicle (low-salt-vehicle group). Measurements were made during a control period and weekly during the 3-week treatment period and a 3-week recovery period. Both cyclosporine-treated groups lost weight during treatment but the rises in serum creatinine and blood urea and decrease in creatinine clearance were greater in the low-salt group. The vehicle group gained weight and had no change in the other parameters over the three weeks. There was an increase in urine volume and sodium excretion in the high-salt group associated with cyclosporine treatment. Although the low-salt groups had a higher plasma renin concentration than the high-salt group there were no changes produced by cyclosporine treatment. Histopathological examination showed mild tubular lesions with vacuolar degeneration of proximal tubular cells. This was more prevalent in the low-salt-cyclosporine-treated group. The plasma concentrations of cyclosporine were not different after one-week treatment but were slightly greater after 2 week and 3-week treatment in the low-salt group. We have suggested that the greater impairment of renal function in the low-salt group produced by cyclosporine may be contributed to by an involvement of tubuloglomerular feedback.

Guanethidine and guanacline on the rat vas deferens.

1 In acute experiments guanethidine was considerably more potent than guanacline in reducing contractile responses of the rat vas deferens to electrical stimulation of intramural nerves.2 Chronic treatment of rats for 19 weeks with guanethidine (5mg/kg, daily) reduced responses to electrical stimulation to 25% of control, potentiated responses to exogenous noradrenaline and depleted endogenous noradrenaline.3 After cessation of guanethidine treatment responses to electrical stimulation increased to 60% of control (in one week) but showed no further increase. There was no decrease in the potentiation of exogenous noradrenaline (after 14 weeks) nor any increase in endogenous noradrenaline levels (after 7 weeks).4 Chronic treatment of rats for 19 weeks with guanacline (5mg/kg, daily) potentiated responses to exogenous noradrenaline and depleted endogenous noradrenaline as much as guanethidine treatment but did not reduce responses to electrical stimulation.5 On cessation of guanacline treatment there was some increase in noradrenaline content (after 2 to 3 weeks) and some decrease of potentiated responses to exogenous noradrenaline (after 2 weeks).6 The noradrenaline-depleting action of these drugs is distinct from blockade of nerve-mediated responses in the rat vas deferens and contractile function after guanethidine treatment can be partly restored despite persistence of noradrenaline depletion and supersensitivity.

Mechanism of the antagonism between guanethidine and dexamphetamine.

1. The effects of dexamphetamine were studied on the responses of rabbit ileum, rabbit ear artery and sheep spleen to sympathetic nerve stimulation after exposure to guanethidine and in the absence of guanethidine.2. In the absence of guanethidine, dexamphetamine enhanced the responses to sympathetic stimulation and, in the spleen, this was shown to be due to an increase in noradrenaline output. However, the increase in these responses was much less than the increase obtained in preparations treated with guanethidine.3. Cocaine, in a concentration which produced the same effect on noradrenaline uptake as the concentration of dexamphetamine used, was also effective in reversing the adrenergic neurone blocking actions of guanethidine.4. It is suggested that the antagonism between dexamphetamine and guanethidine is due to a reduction in the uptake of guanethidine by the nerve endings rather than to interaction of the two drugs at the receptor site for the adrenergic neurone blocking action of guanethidine.

Inhibition of vasoconstriction by frusemide in the rat.

Mesenteric blood flow was measured in anaesthetized rats with a non-cannulating electromagnetic flow probe around the superior mesenteric artery. Reductions in blood flow were produced by intravenous bolus injections of angiotensin II (1-300 ng) and noradrenaline (3-300 ng) before and after the administration of frusemide (5 mg kg-1, i.v.). Loss of volume after frusemide was prevented by either a urinary bladder-intravenous shunt or replacement of urinary output by intravenous saline. Frusemide administration caused a small increase in baseline blood pressure of 3.2 +/- 1.3 mmHg (P less than 0.05) but did not change mesenteric blood flow. This dose of frusemide inhibited the vasoconstrictor responses to both angiotensin II and noradrenaline (P less than 0.01, two way analysis of variance). Responses to angiotensin II were inhibited to a greater extent. Acute bilateral nephrectomy or treatment with indomethacin (2 mg kg-1, i.v.) completely prevented the inhibitory effect of frusemide on the responses to angiotensin II and noradrenaline. To test whether frusemide-induced increased endogenous levels of angiotensin II may be responsible for the effects of frusemide on the vasoconstrictor responses, a separate group of rats were not given frusemide but were infused with exogenous angiotensin II (12.5-25 ng kg-1 min-1). This produced a small increase in mean blood pressure (4.0 +/- 1.4 mmHg, P less than 0.05) but did not change baseline mesenteric blood flow. Unlike frusemide, the responses to bolus injections of angiotensin II and noradrenaline were not changed by the infusion of angiotensin II. 5 It is suggested that frusemide may release directly or indirectly a prostanoid from the kidney (or a substance from the kidney which leads to the formation of a prostanoid) which inhibits constrictor responses in the peripheral vasculature.

Barbiturate inhibition of endothelium-dependent dilatation of blood- and Krebs-perfused rat tail arteries.

Rat tail artery segments were cannulated both ends, immersed in an organ bath filled with Krebs solution, and perfused at a constant 3 ml/min with Krebs solution from a reservoir or arterial blood from a donor conscious rat. Donor rats had chronic indwelling exteriorized silastic cannulas in a carotid artery and jugular vein. Blood withdrawn from the carotid artery returned to the donor rat via the jugular vein cannula after perfusing the tail artery segment. Arteries were constricted and perfusion pressure increased by infusing noradrenaline into the perfusate. In Krebs perfused arteries vasodilator responses were obtained to acetylcholine (ACh) and sodium nitroprusside (NP) infused for 1 min at 10(-6) M. The addition of the barbiturates, 'Inactin' (5-ethyl-5-(1-methylpropyl)-2-thiobarbiturate) and pentobarbitone, at 10, 30 and 100 micrograms/ml progressively inhibited responses to ACh but not to NP. Pentobarbitone 30 micrograms/ml also significantly inhibited responses to 3 X 10(-7) M of the calcium ionophore A23187, shifted the dose-response curve for ACh to the right and suppressed the maximum ACh response. Vasoconstrictor responses were also obtained by periarterial stimulation of the sympathetic nerves at 5 Hz for 5 s every 2 min. These responses were markedly inhibited by ACh (10(-5) M) and this inhibition was not affected by Inactin 100 micrograms/ml. In arterial segments perfused with blood from a donor conscious rat, ACh infused for 1 min at a rate calculated to produce a concentration of 10(-4) M in the blood perfusate at the site of infusion, decreased tail artery perfusion pressure.(ABSTRACT TRUNCATED AT 250 WORDS)

Reproducible vasodilatation by platelet-activating factor in blood- and Krebs-perfused rat kidneys is albumin-dependent.

The vasodilator effect of platelet-activating factor (PAF) and its reproducibility was determined in rat kidneys perfused in situ with blood or in isolated kidneys perfused with Krebs solution or Krebs plus 0.25% bovine serum albumin (BSA) at 3 ml/min. Dilatation was measured as inhibition of the increase in perfusion pressure produced by stimulation of the renal nerves or by infusion of noradrenaline. PAF, in saline and 0.25% BSA, was infused into the perfusate at 0.05 ml/min to produce eight incremental consecutive concentrations from 3 x 10(-11) to 10(-7). Two sets of PAF infusions were made during nerve-stimulated responses followed by one set during noradrenaline infusion. With blood perfusion, PAF consistently produced dose-dependent dilatation and 3 x 10(-9) M reduced nerve-stimulated responses to 52% of control. This effect was reproduced by a second infusion. However in Krebs-perfused kidneys the effect of the first PAF infusion was not consistent, with responses either not affected or reduced only at the lower doses so that the mean maximum decrease was only 10% and the vasodilatation was not dose-dependent. The second PAF infusion invariably had no effect. On the other hand perfusion with Krebs and 0.25% BSA produced consistent and dose-dependent inhibition of vasoconstriction which was reproduced by a second infusion. PAF was effective at 10-fold lower doses in Krebs-albumin perfused compared to blood-perfused kidneys. These results indicate that exogenous PAF is a potent and reproducible dilator of the rat renal vasculature perfused with blood or Krebs-albumin solution but not albumin-free Krebs solution.

alpha-Methylepinephrine, a methyldopa metabolite that binds to alpha-receptors in rat brain.

The presence of phenylethanolamine-N-methyl transferase in those brain-stem nuclei where methyldopa is thought to act suggested to us that methylepinephrine could be an active metabolite of methyldopa. We assessed the potential of methylepinephrine to be an active metabolite by determining its capacity to compete for ligands which bind to alpha-receptors in rat brain. Thus, the abilities of epinephrine, norepinephrine, methyldopa, (+/-)-methylnorepinephrine and (+/-)-methylepinephrine to compete for alpha 1-([3H]WB-4101) and alpha 2-([3H]clonidine) binding sites in rat brain wee compared. Order of potency at each binding site was: [3H]WB-4101 -- (-)epinephrine = (-)-norepinephrine greater than (+/-)-methylnorepinephrine greater than (+/-)-methylepinephrine much greater than (-)-methyldopa; [3H]clonidine -- (-)-epinephrine greater than (+/-)-methylnorepinephrine greater than (-)-norepinephrine greater than (+/-)-methylepinephrine much greater than (-)-methyldopa. Since it can compete with a specific ligand for alpha 2-receptors, methylepinephrine could be a centrally active metabolite of methyldopa.

Comparison of the pulmonary, hepatic and renal extraction of PGI2 and 6-keto-PGE1.

Prostaglandin (PG) I2 and 6-keto-PGE1 were infused into the aortic arch, femoral vein, renal artery and portal vein in anesthetized dogs over a dose range to produce a stable decrease in systemic blood pressure after 7 min of infusion. Mesenteric artery blood flow was measured continuously with a non-cannulating electromagnetic flow probe. Parallel log dose-response relationship were seen for decreases in blood pressure and increases in mesenteric blood flow with both prostaglandins when infused into the aortic arch. The dose responses for intra-aortic and intravenous infusion of PGI2 were not different but were shifted to the right with intrarenal and intraportal infusions. With 6-keto-PGE1 the dose responses after intravenous, intrarenal and intraportal infusions were all shifted to the right. By comparing doses which reduced blood pressure by 10 mm Hg, the extraction of the PGs by the lungs, kidney and liver was calculated PGI2 had no significant pulmonary extraction. 31% renal and 75% hepatic extraction. In contrast, 6-keto-PGE1 had extractions of 40, 66 and 51% for pulmonary, renal, and hepatic vascular beds, respectively. We conclude that 6-keto-PGE1 is reasonably well extracted by the lungs, kidney, and liver.

Effect of aminophylline on renal vasoconstriction produced by amphotericin B in the rat.

Administration of the antifungal agent amphotericin B causes a pronounced reduction in renal blood flow (RBF). Since amphotericin B induced renal vasoconstriction may contribute to the clinical nephrotoxicity of this drug, the purpose of these studies was to investigate the mechanism of amphotericin B induced renal vasoconstriction. To determine if the vascular response to amphotericin B is linked to the intrarenal release of either adenosine or angiotensin II, the effects of aminophylline (5 mumol/kg/min for 10 min followed by 0.5 mumol/kg/min) and saralasin (6 micrograms/min) on the renal vascular response produced by two 10 min intravenous amphotericin B (0.35 mg/kg) infusions were examined. In the control group, amphotericin B decreased RBF 1.7 ml/min (22%, P less than 0.01) and 3.5 ml/min (44%, P less than 0.001) during the 1st and 2nd amphotericin B infusions, respectively. In animals pretreated with aminophylline the decrease in RBF produced by amphotericin B was only 0.4 ml/min (5.5%; N.S.) and 1.3 ml/min (15%, P less than 0.05) during the 1st and 2nd amphotericin B infusions, respectively. In contrast, neither saralasin nor the direct vasodilator sodium nitroprusside (0.4-2 micrograms/min) influenced the renal vascular response to amphotericin B. These data suggest that the renal vascular response to amphotericin B is not linked to the formation of angiotensin II, but rather might be mediated by increases in renal adenosine levels.

Attenuation of cisplatinum-induced nephrotoxicity in the rat by high salt diet, furosemide and acetazolamide.

The influence of variations in sodium chloride diet, furosemide and acetazolamide on nephrotoxicity induced by cisplatinum have been investigated in the rat by measuring serum creatinine concentrations 5 days after cisplatinum (5 mg/kg, ip) administration. Sodium chloride depletion enhanced, while sodium chloride loading minimized changes in renal function. Increases in urine flow rate following a dextrose water load failed to alter the nephrotoxic response. Both furosemide and acetazolamide, given 30 min before cisplatinum, attenuated the nephrotoxic response. In contrast, neither sodium chloride loading, furosemide, nor acetazolamide influenced the change in renal function when given 30 min after cisplatinum. These observations indicate that renal damage due to cisplatinum can be modified by alterations in dietary salt and by diuretics and that the extent of ultimate renal damage is dependent on factors occurring immediately after cisplatinum administration.