Involvement of the renin-angiotensin system in endogenous central histamine-induced reversal of critical haemorrhagic hypotension in rats

The study was undertaken to examine the involvement of the renin-angiotensin system in the reversal by endogenous central histamine of critical haemorrhagic hypotension in anaesthetised Wistar rats. Histamine N-methyltransferase inhibitor metoprine (20 µg) administered intracerebroventricularly at 5 min of critical hypotension 20-25 mmHg produced increases in histamine concentrations as measured 20 min after treatment in the hypothalamus (581.33 ± 63.23 vs. 488.26 ± 56.34 ng/g of wet tissue; P < 0.01) and medulla oblongata (53.42 ± 14.65 vs. 34.68 ± 13.52 ng/g of wet tissue; P < 0.05). That was accompanied by 34.7% higher plasma angiotensin II concentration in comparison to the control group. Metoprine produced dose-dependent (5-20 µg) rises in mean arterial pressure (MAP) and heart rate, which were significantly higher than those in normotensive animals. The resuscitating action of metoprine (20 µg) was associated with rises in renal, mesenteric and hindquarters blood flows, and a 100% survival at 2 h after treatment, while in the saline-treated group, all the animals died within 30 min. Angiotensin type 1 (AT1) receptor antagonist ZD 7155 (0.5 mg/kg; iv) decreased regional vascular resistance and inhibited metoprine-induced increase in MAP, whereas AT2 receptor blocker PD 123319 (10 mg/kg; iv) had no effect. Angiotensin-converting enzyme inhibitor captopril (30 mg/kg; iv) reduced the increase in plasma angiotensin II level and the haemodynamic effects of metoprine. Neither capropril, nor angiotensin receptor antagonists influence the survival at 2 h after treatment. In conclusion, the renin-angiotensin system is involved in central histamine-induced resuscitating action in rats.