The actions of intravenously administered
5-hydroxytryptamine (5-HT) have been analysed in conscious
DOCA-
salt hypertensive rats using selective
5-HT receptor agonists and antagonists to determine the receptor mechanisms involved and to compare them with those in conscious normotensive rats. In both normotensive and hypertensive rats
5-HT, 3 and 10 micrograms i.v., produced a complex triphasic effect on blood pressure consisting of an initial short lasting depressor response, which was followed by a pressor response and then, finally, a hypotensive phase. Marked decreases in heart rate were observed immediately after dosing, which were followed by small increases in rate. The selective 5-HT3-receptor agonist, 2-methyl
5-HT, 3-30 micrograms i.v., produced immediate and marked dose-related decreases in blood pressure and heart rate in both normotensive and
DOCA-
salt hypertensive rats. The 5-HT3-receptor antagonist,
MDL 72222, 0.03 and 0.1 mg/kg i.v., antagonised these effects in both normotensive and
DOCA-
salt hypertensive rats. Treatment with
MDL 72222, 0.3 mg/kg i.v., abolished the initial depressor response and
bradycardia produced by
5-HT. The 5-HT2 receptor agonist, alpha-methyl
5-HT, 3-30 micrograms i.v., produced dose-related increases in blood pressure which were significantly greater in magnitude in
DOCA-
salt hypertensive than normotensive rats.
Bradycardia was observed consistently at 30 micrograms only. The
5-HT 2 receptor antagonist,
ketanserin, 0.03-0.3 mg/kg i.v., caused a dose-dependent antagonism of the pressor responses produced by alpha-methyl
5-HT, but had no effect on the increases in blood pressure produced by
angiotensin.(ABSTRACT TRUNCATED AT 250 WORDS)