Pressor effects of the α2‐adrenoceptor agonist B‐HT 933 in anaesthetized and haemorrhagic rats: comparison with the haemodynamic effects of amidephrine
Authors: MacLean Margaret R. , Thomson Marina , Hiley C. Robin 
Published: July 2012

Blood pressure responses to single and multiple bolus doses of the α2‐adrenoceptor agonist B‐HT 933 were analysed in intact anaesthetized rats which were either normotensive or hypotensive as a result of haemorrhage. Single bolus doses of B‐HT 933 in normotensive rats induced a fall in blood pressure, whilst further doses induced dose‐dependent pressor responses which were inhibited by the α2‐adrenoceptor antagonist yohimbine and unaffected by the α1‐adrenoceptor agonist prazosin. In the haemorrhagic, hypotensive animals, single bolus doses of B‐HT 933 induced immediate dose‐dependent pressor responses; the maximum pressor responses to the bolus of B‐HT 933 and its ED50 values were the same in both the normotensive and hypotensive, haemorrhagic animals.
Cardiac output, its distribution and tissue blood flows were determined with tracer microspheres in intact anaesthetized normotensive and haemorrhagic, hypotensive rats during depressor (normotensive) and pressor (normotensive and hypotensive) responses to B‐HT 933. Haemodynamics were also determined during pressor responses to the α1‐adrenoceptor agonist amidephrine.
In control normotensive rats, a single dose of B‐HT 933 (1 mg kg−1) reduced blood pressure by reducing cardiac output (through a decrease in heart rate). It increased the fractional distribution of cardiac output to the spleen and stomach, reduced it to the heart and liver and reduced cardiac and hepatic blood flow. A further dose of B‐HT 933 (1 mg kg−1 bolus followed by 100 μg min−1 infusion) increased blood pressure by increasing total peripheral resistance, which was accompanied by decreased proportions of cardiac output passing to the heart, liver and testes. There was also increased fractional distribution of cardiac output to the lungs, spleen, kidneys and stomach but blood flows through the liver and testes were reduced. Amidephrine (6 μg kg−1 bolus followed by 0.5 μg min−1 infusion) increased blood pressure by increasing cardiac output through an increased stroke volume. It increased cardiac output distribution to the kidneys and brain, increasing blood flow through the heart, lungs, brain, testes, epididimides, skin and large intestine.
Haemorrhage caused a fall in blood pressure which resulted from decreased total peripheral resistance and cardiac output (the latter due to decreases in both heart rate and stroke volume). It reduced the proportion of cardiac output distributed to the lungs, spleen, kidneys, testes and pancreas/mesentery and decreased blood flow through these organs as well as through the heart, liver, brain, epididimides, skin and the gastrointestinal tract. Haemorrhage also caused mild acidosis, hypocapnia and hyperoxia. B‐HT 933 administration to the haemorrhagic, hypotensive rats partially restored blood pressure, by increasing stroke volume, and increased the fractional distribution of cardiac output to the spleen, kidneys and stomach whilst decreasing it to the heart. B‐HT 933 restored blood flow through the spleen, kidneys, brain, testes, epididimides, skin, stomach and large intestine to control values. Amidephrine restored blood pressure in the haemorrhagic, hypotensive rats by increasing stroke volume; it also increased the proportion of cardiac output sent to the liver and restored blood flow through the heart, liver, kidneys, brain, testes, skin, stomach and large intestine. Haemorrhage potentiated the pressor responses to infusions of amidephrine whilst inhibiting those to infusions of B‐HT 933.
It is concluded that single doses of B‐HT 933 induce a fall in blood pressure, probably due to its known central action on α2‐adrenoceptors, whilst further doses induce pressor responses through activation of peripheral vascular α2‐adrenoceptors. In haemorrhagic, hypotensive rats, B‐HT 933 only induces pressor responses and both B‐HT 933 and amidephrine restore deficits in the circulation through vital organs. Selective α‐adrenoceptor agonists may, therefore, be of therapeutic value for the treatment of haemorrhagic shock.