Prostacyclin infusion does not prevent ACTH-induced hypertension in sheep

Arachidonic acid metabolism in glucocorticoid-induced hypertension

1. Products of metabolism of arachidonic acid, such as 20-hydroxyeicosatetraenoic acid (20-HETE), thromboxane A(2) (TXA(2)) and prostaglandin I(2) (PGI(2)), regulate vascular tone. Among them, 20-HETE is a potent constrictor in small arteries that also has natriuretic properties. The present study investigated changes in urinary concentrations of 20-HETE and metabolites of TXA(2) and PGI(2) in glucocorticoid-hypertension in rats, a sodium-independent model. 2. Male Sprague-Dawley rats were treated with saline, adrenocorticotrophic hormone (ACTH; 0.2 mg/kg) or dexamethasone (20 microg/kg) by daily s.c. injection for 12 days. Systolic blood pressure (SBP) was measured using the tail-cuff method. Metabolic cages were used for 24 h urine collection. Thymus weight and urinary concentrations of 20-HETE, TXA(2) and PGI(2) were determined. 3. In the present study, SBP was increased by both ACTH (from 102 +/- 2 to 134 +/- 7 mmHg; n = 10; P < 0.01) and dexamethasone (from 106 +/- 5 to 122 +/- 4 mmHg; n = 10; P < 0.01). Thymus weight, a marker for glucocorticoid activity, was significantly decreased by both ACTH and dexamethasone (56 +/- 9 and 76 +/- 5 mg/100 g bodyweight, respectively; n = 10; P' < 0.01) compared with the saline control (151 +/- 5 mg/100 g bodyweight; n = 20). Urinary 20-HETE excretion was increased by ACTH (501 +/- 115 pmol/g creatinine; n = 10; P' < 0.05) but not by dexamethasone (126 +/- 13 pmol/g creatinine; n = 10) compared with the saline control (219 +/- 54 pmol/g creatinine; n = 20). Neither ACTH nor dexamethasone affected urinary excretion of TXB(2) or PGI(2) compared with the saline control. 4. In conclusion, ACTH but not dexamethasone increased urinary 20-HETE excretion in male Sprague-Dawley rats. Urinary concentrations of the metabolites TXB(2) and PGI(2) were unchanged in both models of glucocorticoid-hypertension. The vasoconstrictor 20-HETE may play a role in the genesis of ACTH-induced hypertension.

Modification of the haemodynamic effects of ACTH by angiotensin II in sheep

The study was performed to examine the hypothesis that adrenocorticotrophic hormone (ACTH) induced hypertension in sheep would be enhanced if the blood level of angiotensin II (ANG II), normally suppressed during ACTH administration, was kept at control levels by intravenous infusion of ANG II. Administration of ACTH at 1.0 microgram/kg/day for 6 days produced a half maximum rise in mean arterial pressure, delta 14 mmHg, associated with hypokalaemia and initial urinary sodium retention. A rate of ANG II infusion, (0.9 microgram/kg/day) for 6 days, was contrived to produce a small increase in peripheral resistance and mean arterial pressure, delta 10 mmHg. Pressor responses to concomitant infusion of ACTH and ANG II were additive, delta 25 mmHg. There was no potentiation of ACTH hypertension by ANG II in the sheep.