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Hypotensive effect of clonidine is not mediated by enhanced baroreflex gain in rats
1Department of Pharmacology, University of North Carolina, Chapel Hill 27599-7455.
Insights
Clonidine causes lower blood pressure in rats, but not by strengthening the depressor baroreflex. This study found clonidine
Area of Science:
- Pharmacology
- Cardiovascular Physiology
- Neuroscience
Background:
- Clonidine is a medication known for its hypotensive effects.
- The role of baroreflexes in mediating clonidine's actions is not fully understood.
- Baroreflexes are crucial for regulating blood pressure and heart rate.
Purpose of the Study:
- To investigate if enhanced depressor baroreflexes contribute to clonidine's hypotensive effect in rats.
- To compare clonidine's effects on blood pressure and baroreflex gain in conscious versus anesthetized rats.
- To elucidate the mechanisms underlying clonidine's influence on baroreflex control of heart rate.
Main Methods:
- Comparison of clonidine's effects in conscious rats with intact baroreflexes and pentobarbital-anesthetized (PA) rats with obtunded baroreflexes.
- Assessment of baroreflex gain (change in heart rate/change in blood pressure).
- Pharmacologic blockade of sympathetic (atenolol) and parasympathetic (methyl-atropine) pathways to assess their role.
Main Results:
- Clonidine induced greater hypotension and bradycardia in PA rats compared to conscious rats.
- This enhanced hypotensive effect in PA rats was not linked to an improvement in baroreflex gain.
- Clonidine decreased baroreflex gain in PA rats, primarily through inhibition of sympathetic control of heart rate.
- Hypotension in conscious rats was not associated with changes in baroreflex gain.
Conclusions:
- Clonidine's hypotensive effect in rats is not mediated by an enhancement of reflex bradycardic responses.
- The study suggests that clonidine may exert its blood pressure-lowering effects through mechanisms independent of baroreflex potentiation.
- Sympathetic nervous system inhibition plays a role in clonidine's effect on baroreflex gain in anesthetized rats.
Abstract:
We wished to examine whether enhanced depressor baroreflexes contribute to the hypotensive effect of clonidine in rats. Clonidine's hypotensive effect and clonidine's effect on baroreflex gain mean heart rate/mean blood pressure, (delta HR/delta BP) were compared under two conditions: in conscious rats with intact baroreflex control of HR, and in pentobarbital-anesthetized (PA) rats in which baroreflex control of HR is obtunded. Clonidine produced greater hypotension and bradycardia in PA rats than in conscious rats, but this greater hypotension was not associated with enhancement of baroreflex gain. Instead, clonidine-induced hypotension in PA rats was associated with a further decrease in baroreflex gain, whereas hypotension in conscious rats was not associated with any change in baroreflex gain. Selective pharmacologic blockade of sympathetic (i.e., with atenolol treatment) or parasympathetic (i.e., with methyl-atropine treatment) control of HR showed that clonidine decreased baroreflex gain in PA rats through inhibition of sympathetic control of HR. This study suggests that clonidine's hypotensive effect is not associated with enhancement of reflex bradycardic responses in rats.