Tafline Fraser

L-arginine Partially Reverses Established Adrenocorticotrophin-induced Hypertension and Nitric Oxide Deficiency in the Rat

Background: L-arginine treatment prevents adrenocorticotrophin (ACTH) induced hypertension in the rat. This study examined whether L-arginine treatment could reverse established ACTH hypertension and its effects on markers of decreased NO activity. Methods: Sixty-four male Sprague-Dawley rats were randomly divided into 6 groups given 12 days of treatment: (1) sham (0.9% NaCl, 0.5 ml/kg, subcutaneously, sc, n = 16); (2) ACTH (0.5 mg/kg/day, sc, n = 16); (3) sham + L-arginine (0.6% in food, from treatment day 8 onwards, n = 10); (4) ACTH + L-arginine (n = 10); (5) sham + D-arginine (0.6% in food, from T 8 onwards) (n = 6); and (6) ACTH + D-arginine (n = 6). Systolic blood pressure, water intake, urine volume, and body weight were measured every second day. At the end of the experiments, plasma and urinary nitrate/nitrite (NOx), plasma amino acid concentrations (in groups 1-4), and urinary cyclic guanosine monophosphate (cGMP) concentrations were measured. Results

The role of corticosterone in corticotrophin (ACTH)-induced hypertension in the rat

Objective Corticotrophin (ACTH)-induced hypertension in the rat is prevented by L- but not D-arginine. We examined the effects of exogenous corticosterone in the male Sprague Dawley (SD) rat to determine whether ACTH- induced hypertension is mediated by corticosterone. Methods Exogenous corticosterone (10, 20 or 40 mg/kg per day) or sham (polyethylene glycol (PEG) 1 ml/kg per day) was injected subcutaneously in divided doses (s/c b.d.) over 15 treatment days to 40 SD rats (n = 10 each group). Subsequently, the effects of L-arginine, D-arginine or L-arginine + N-nitro-L-arginine (NOLA) on corticosterone-induced hypertension (corticosterone 20 mg/kg per day) were examined. Systolic blood pressure (SBP) and metabolic parameters were measured every two days. Results Twenty and 40 mg/kg per day of corticosterone increased SBP compared with sham (P < 0.01, P < 0.05 respectively, sham versus respective group). Forty mg/kg per day of corticosterone raised serum corticosterone concentration compared with sham (502 ± 20 versus 364 ± 25 ng/ml, P < 0.001). L-arginine prevented the rise in SBP produced by orticosterone (131 ± 3 to 131 ± 2 mmHg, control versus day 10) but D-arginine did not (129 ± 3 to 142 ± 4 mmHg on day 8, P < 0.01). NOLA blocked the effect of L-arginine and amplified the rise in blood pressure produced by corticosterone (130 ± 3 to 171 ± 6 mmHg on day 10, P < 0.001). Conclusions The haemodynamic features of ACTH-induced hypertension were reproduced by corticosterone excess, at concentrations of corticosterone similar to those in studies of exogenous ACTH administration. It is likely that ACTH-stimulated adrenal production of corticosterone accounts for the features of ACTH-induced hypertension in the rat.

Adrenocorticotrophin-induced hypertension: effects of mineralocorticoid and glucocorticoid receptor antagonism.

OBJECTIVE To examine whether the increase of blood pressure in adrenocorticotrophin-treated rats is mediated through mineralocorticoid or glucocorticoid receptors or corticosterone 6 beta-hydroxylation inhibition. DESIGN Rats were randomly allocated to 14 treatment groups for 10 days. The treatments included sham injection (n = 35), adrenocorticotrophin (5, 100, 500 micrograms/kg per day, subcutaneously, n = 5, 15 and 15, respectively), spironolactone (100 mg/kg per day, subcutaneously, n = 15), standard-dose or high-dose RU 486 (70 mg/kg every 3 days or 70 mg/kg per day, subcutaneously, n = 5 and 10, respectively), spironolactone + adrenocorticotrophin (100 micrograms/kg per day, n = 5, or 500 micrograms/kg per day, n = 10), standard-dose RU 486 + adrenocorticotrophin (500 micrograms/kg per day, n = 5), high-dose RU 486 + adrenocorticotrophin (100 micrograms/kg per day, n = 10), troleandomycin (40 mg/kg per day, subcutaneously, n = 5) and troleandomycin + adrenocorticotrophin (5 micrograms/kg per day, n = 5). Systolic blood pressure and metabolic parameters were measured every second day. RESULTS Adrenocorticotrophin treatment increased systolic blood pressure dose-dependently (5 micrograms/kg per day: +14 +/- 2 mmHg; 100 micrograms/kg per day: +20 +/- 2 mmHg; 500 micrograms/kg per day: +28 +/- 2 mmHg, all P < 0.001). Adrenocorticotrophin at 100 and 500 micrograms/kg per day increased plasma sodium and decreased plasma potassium concentrations. Spironolactone did not block adrenocorticotrophin-induced systolic blood pressure changes but did block changes in plasma sodium and potassium levels. Standard-dose RU 486 did not modify the adrenocorticotrophin-induced (500 micrograms/kg per day) systolic blood pressure rise but blocked the effect of adrenocorticotrophin on body weight. High-dose RU 486 partially blocked the adrenocorticotrophin-induced (100 micrograms/kg per day) systolic blood pressure increase (adrenocorticotrophin at 100 micrograms/kg per day: 143 +/- 3 mmHg; high-dose RU 486 + adrenocorticotrophin at 100 micrograms/kg per day: 128 +/- 5 mmHg, P < 0.001) and body-weight loss. Troleandomycin did not alter the development of adrenocorticotrophin-induced hypertension. CONCLUSIONS Spironolactone and standard-dose RU 486 did not modify adrenocorticotrophin-induced hypertension despite demonstrable antimineralocorticoid and antiglucocorticoid actions. High-dose RU 486 partially blocked adrenocorticotrophin-induced (100 micrograms/kg per day) hypertension, suggesting either a permissive effect of glucocorticoid on blood pressure or other antihypertensive actions of RU 486. Inhibition of glucocorticoid 6 beta-hydroxylation by troleandomycin did not modify adrenocorticotrophin-induced hypertension, suggesting that effects of corticosterone 6 beta-hydroxylation in adrenocorticotrophin-induced hypertension are negligible.

Adrenocorticotrophin dose-response relationships in the rat: haemodynamic, metabolic and hormonal effects.

Objective To determine adrenocorticotrophin dose–response relationships for increase of blood pressure and metabolic parameters of the Sprague–Dawley rat. Methods We injected 120 male Sprague–Dawley rats twice daily subcutaneously for 10 days with 0.5, 1, 5, 50, 100, 200 or 500 μg/kg synthetic adrenocorticotrophin per day (all n = 10) or subjected them to sham injection (0.9% NaCl; n = 50). Systolic blood pressure, 24 h food intake, water intake, urine volume and body weight were measured. Data from a further 45 rats treated with 500 μg/kg per day adrenocorticotrophin in previous studies were included in the blood pressure analyses. After we had killed these rats, their organ weights (kidney, heart, adrenal) and plasma electrolyte, adrenocorticotrophin and serum corticosterone concentrations were measured. Results On the final day of treatment systolic blood pressure of sham-injection control rats was 123 ± 1 mmHg (n = 50). Compared with sham treatment, a low dose of adrenocorticotrophin (1 μg/kg per day) increased systolic blood pressure from 122 ± 1 to 130 ± 2 mmHg (P < 0.001) without any metabolic effects, whereas a high dose of adrenocorticotrophin (500 μg/kg per day) increased systolic blood pressure from 121 ± 1 to 150 ± 2 mmHg (P < 0.001, n = 55) with increases in intake of water and urine volume (P < 0.001, n = 10) and a decrease in body weight (P < 0.001, n = 10). Plasma adrenocorticotrophin and serum corticosterone concentrations for the sham-injection control group were 162 ± 12 pg/ml (36 ± 3 pmol/l) and 376 ± 18 ng/ml (1038 ± 50 nmol/l), respectively. Plasma adrenocorticotrophin concentration was elevated by injections of 100 (P < 0.05), 200 (P < 0.01) and 500 μg/kg adrenocorticotrophin per day (P = 0.001). Serum corticosterone concentration was not significantly different from that of sham-injection rats with 0.5–5 μg/kg adrenocorticotrophin per day but was increased by injection of 50–500 μg/kg adrenocorticotrophin per day (P < 0.001). Conclusions These results define 1 μg/kg adrenocorticotrophin per day, administered subcutaneously, as the threshold dose for causing a rise in blood pressure in the rat. Thus administration of adrenocorticotrophin increases systolic blood pressure at doses that induce minimal adrenocorticotrophin metabolic effects. Administration of a low dose of adrenocorticotrophin to the rat is a suitable model for stress-induced hypertension. J Hypertens 16:593–600

Ming Li, Tafline Fraser, Jian Wang* and Judith A Whitworth

1. The effects of L‐arginine treatment on dexamethasone‐induced hypertension were examined in the Sprague‐Dawley rat Seventy rats were randomly divided into the following eight groups: sham, dexamethasone (5 and 10 fig/day), L‐arginine (100 and 500 mg/kg per day), L‐arginine (100 or 500 mg/kg per day) + dexamethasone (10 ug/day), L‐arginine (520‐797 mg/kg per day in food) + dexamethasone (5 fig/day). Systolic blood pressure (SBP), bodyweight and plasma nitrate/nitrite concentration were measured.

Haemodynamic mechanisms of corticotropin (ACTH)-induced hypertension in the rat.

OBJECTIVES To investigate the roles of cardiac output and systemic and regional resistances in corticotropin (ACTH)-induced hypertension in the rat METHODS This study consisted of three series of experiments with eight groups of male Sprague-Dawley rats (n = 132). Series 1 comprised groups 1-4, where group 1 = sham (0.9% NaCl, subcutaneous (s.c.) injection); group 2 = ACTH (0.5 mg/kg per day, s.c.); group 3 = atenolol + sham; group 4 = atenolol + ACTH treatments. Series 2 comprised groups 5 and 6, where group 5 = minoxidil + sham and group 6 = minoxidil + ACTH treatments. Series 3 comprised groups 7 and 8, where group 7 = ramipril + sham and group 8 = ramipril + ACTH treatments. Systolic blood pressure, water and food intakes, urine volume, and body weight were measured every second day. After 10 days of treatment, mean arterial blood pressure was measured by intra-arterial cannulation, and cardiac output (CO), and renal, mesenteric and hindquarter blood flows (RBF, MBF and HBF) determined using transonic small animal flowmeters. RESULTS ACTH treatment increased blood pressure (P < 0.001) with a rise in CO (P < 0.01) and renal vascular resistance (RVR, P < 0.05), but did not affect total peripheral resistance (TPR). Atenolol blocked the rise in CO without affecting the rise in blood pressure produced by ACTH treatment Minoxidil lowered TPR, but did not prevent the rise in blood pressure or renal vascular resistance. Ramipril blunted the rise in RVR and blood pressure without significantly affecting TPR. CONCLUSION Neither preventing rise in CO nor lowering TPR altered the ACTH-induced rise in blood pressure in the rat However, both the hypertension and rise in RVR were prevented by ramipril. These data suggest that increase in RVR may play a role in the pathogenesis of ACTH-induced hypertension in the rat.

Comparison Of Telemetric And Tail-Cuff Blood Pressure Monitoring In Adrenocorticotrophic Hormone-Treated Rats

1. The aim of the present study was to validate a telemetric blood pressure (BP) monitoring system against tail‐cuff blood pressure in both adrenocorticotrophic hormone (ACTH)‐ and sham‐treated rats. In the statistical analyses, we first tested whether there was a detectable effect on systolic blood pressure (SBP) of 10 days treatment with ACTH compared with saline. Second, we compared results of telemetered and tail‐cuff measurements and, third, we developed a novel method for estimating the relative power of the two techniques.

Hemodynamic profile of corticotropin-induced hypertension in the rat

Objectives To examine hemodynamic variables in corticotropin-induced hypertension in rats and the effects of reversal of the hypertension by L-arginine on the hemodynamic profile. Methods Sixty male Sprague–Dawley rats were randomly divided into four groups: sham treatment (0.9% NaCl, injected subcutaneously); 0.5 mg/kg corticotropin per day, subcutaneously; 0.6% L-arginine in food plus sham; and L-arginine plus corticotropin. Systolic blood pressure, water and food intakes, urine volume, and body weight were measured every second day. After 10 days mean arterial blood pressure was measured by intra-arterial cannulation, and cardiac output, and renal, mesenteric, and hindquarter blood flows were determined using transonic small animal flowmeters. Results Injection of corticotropin increased blood pressure, water intake, urine volume, and plasma sodium concentration, and decreased body weight and plasma potassium concentration. It increased cardiac output (P < 0.01), mesenteric blood flow (P < 0.05), and renal vascular resistance (P < 0.05), and decreased renal blood flow (P < 0.05), but did not change calculated total peripheral resistance, hindquarter blood flow, mesenteric or hindquarter vascular resistance. L-arginine prevented corticotropin-induced rises in blood pressure (P < 0.001) and renal vascular resistance (P < 0.05), and a fall in renal blood flow (P < 0.05), but did not affect other hemodynamic variables. Conclusion The hemodynamic profile of corticotropin-induced hypertension in the rat is characterized by a rise in cardiac output and renal vascular resistance, a fall in renal blood flow, but no change in total peripheral resistance, hindquarter blood flow, mesenteric vascular resistance, or hindquarter vascular resistance. L-arginine prevented corticotropin-induced rises both in blood pressure and in renal vascular resistance in the rat. These data suggest that the increase in renal vascular resistance might play a role in corticotropin-induced hypertension in the rat.

VASOPRESSIN V1A RECEPTOR ANTAGONISM DOES NOT REVERSE ADRENOCORTICOTROPHIN-INDUCED HYPERTENSION IN THE RAT

1. The role of arginine vasopressin (AVP) was examined in adrenocorticotrophin (ACTH)‐induced hypertension in Sprague‐Dawley rats using the non‐peptide AVP V1a receptor antagonist OPC 21268.

Circadian blood pressure variability in adrenocorticotrophin-induced hypertension in the rat

Objectives Secondary hypertension is often characterized by loss of diurnal blood pressure variability. This study examined circadian (24 h) blood pressure variability in adrenocorticotrophin (ACTH)-induced hypertension in the Sprague–Dawley rat. Methods Male Sprague–Dawley rats were randomly allocated to sham (0.9% saline, s.c.), n = (9), ACTH (0.5 μg/kg per day, s.c., n = 8) or ACTH (100 μg/kg per day, s.c., n = 7) in a room with a 12 h light/dark cycle (0600 h to 1800 h). A radio telemetry transducer was used to measure blood pressure in unrestrained animals over 3 control days (C1–C3) and 10 treatment days (T1–T10). Heart rate, systolic (SBP), mean arterial (MAP) and diastolic (DBP) blood pressure were continuously recorded. Body weight was measured daily and serum corticosterone concentration ([B]) prior to death. Results Sham treatment had no effect on any parameters. ACTH 100 μg/kg per day increased SBP from 124 ± 2 pooled control (PC) to 134 ± 2 mmHg (T10) , MAP from 105 ± 2 to 115 ± 2 mmHg and DBP from 87 ± 1 to 99 ± 2 mmHg and decreased heart rate from 305 ± 6 to 249 ± 5 beats/min and body weight from 299 ± 6 (C3) to 280 ± 8 g (T10) (all P′ < 0.0036). Serum [B] was higher in ACTH- (881 ± 44 ng/ml) than sham-treated rats (384 ± 17 ng/ml, P < 0.001). There were no differences between sham treatment and ACTH 0.5 μg/kg per day. SBP, MAP, DBP and heart rate were consistently higher for ACTH 100 μg/kg per day and sham-treated animals during the dark cycle (1800 h to 0600 h) than the light cycle (0600 h to 1800 h). Conclusions ACTH 100 μg/kg per day raises blood pressure in conscious unrestrained Sprague–Dawley rats without any change in normal diurnal rhythm.