Interaction Between S2-Serotonergic and α1-Adrenergic Receptor Activities at Vascular Sites

Cirrhosis of the liver and receptor-mediated function in vascular smooth muscle

Altered regulation of receptors on the vascular smooth muscle has been proposed as one of the mechanisms that may account for the vascular abnormalities in patients with cirrhosis of the liver. Impaired contractility and down-regulation of contractile receptors have been demonstrated in cirrhotic patients and animal models, although interpretation of the literature is hampered by methodological variation and conflicting results. There is little evidence, however, that receptor down-regulation is the cause of contractile dysfunction in either patients or animal models. Receptor desensitisation may contribute to impaired contraction in human arteries, but further investigation is required to confirm this possibility.

Cardiovascular effects of ketanserin on normotensive rats in vivo and in vitro

In this work, we report for first time that: (1) low doses of ketanserin (0.2 mg/kg) produce a transient hypotensive response in anaesthetized rats, which is basically due to the blockade of 5-hydroxytryptamine (2A) (5-HT)2A receptors, whereas high doses (1 mg/kg) of ketanserin cause a sustained hypotension also mediated by the blockage of alpha1-adrenergic receptors; (2) the in vitro vasorelaxant action of high concentrations of ketanserin (>10 microM) involves Ca2+ antagonism, which may also be responsible, at least in part, for the inhibition of high-K+-induced 45Ca2+ uptake, the inhibition of Ca2+-induced contractions in initially Ca2+-free high-K+ medium, and the negative chronotropic effects on isolated atria. This Ca2+ antagonistic activity does not seem to contribute to the in vivo cardiovascular effects of ketanserin at therapeutic doses.

The synergistic effect of 5-hydroxytryptamine and epinephrine on the human platelet is related to the activation of phospholipase C

Simultaneous addition of 5-hydroxytryptamine (5-HT) and epinephrine synergistically elevated cytosolic free Ca2+ concentration ([Ca2+]cyt) and activated phospholipase C. These phenomena occurred in an agonist concentration-dependent manner and reflected an increase in maximal response rather than a, shift in affinity for either of the stimuli. Ketanserin, a 5HT2 receptor antagonist completely blocked the response of 5-HT plus epinephrine and this in the nanomolar concentration range in which the drug selectively antagonizes 5-HT2 receptor-mediated responses. The synergistic effect was not a consequence of the activation by the platelet-release products ADP and 5-HT nor of influx of extracellular Ca2+. It is concluded that an increased activation of phospholipase C plays a major role in provoking the synergistic effects between 5-HT and epinephrine at the level of signal transduction.

Augmentation of acute random pattern skin flap viability in the pig

Three experiments were conducted to study the effect of ketanserin and LY53857, S2-serotonergic receptor antagonists, on skin blood flow and viability in acute random pattern skin flaps (4 x 10 cm) in the pig. In experiment 1, the dose-response effect of intravenous ketanserin (0, 0.15, 0.25, 0.35, and 0.50 mg/kg) on skin flap capillary blood flow was studied 6 hr after skin flap surgery, using the radioactive microsphere (15 microns) technique and under pentobarbital anesthesia. Significant (P less than 0.05) increase in skin flap blood flow was seen at the dosages of 0.25 and 0.35 mg/kg compared with the saline-treated control. In experiment 2, the effect of five-day intramuscular ketanserin and LY53857 treatment (0.30 mg/kg/day; in divided doses) on skin flap viability was studied. The drug treatments were started two days preoperatively. It was observed that the length of skin flap viability in ketanserin (6.6 +/- 0.2 cm; n = 40 flaps) and LY53857 (6.8 +/- 0.3 cm; n = 40 flaps) treated flaps were significantly (P less than 0.05) higher than the saline-treated control (5.5 +/- 0.1 cm; n = 48 flaps). Ketanserin treatment started 30 min after flap surgery also significantly (P less than 0.05) increased the length of skin flap viability (6.1 +/- 0.1 cm) compared with the control. There was no significant difference in skin viability between ketanserin and LY53857 treated skin flaps. The preceding study on the effect of ketanserin treatment on random pattern skin flap viability was repeated in experiment 3. Again, it was observed that intramuscular ketanserin treatment significantly (P less than 0.05) increased the skin flap viability. It was concluded that ketanserin and LY53857 treatment resulted in significant augmentation of porcine acute random pattern skin flap viability. This is the first experimental evidence to indicate that S2-serotonergic receptors participate in the pathogenesis of skin flap ischemia.

An amplifying effect of exogenous and neurally stored 5-hydroxytryptamine on the neurogenic contraction in rat tail artery

1. The interactions between sympathetic neuroeffector transmission and 5-hydroxytryptamine (5-HT) were investigated in segments of rat isolated tail artery. 2. Contractile responses to field stimulation of the artery segments were abolished by tetrodotoxin (3 x 10(-7) M). A subthreshold concentration of acutely applied exogenous 5-HT (10(-8) M) markedly enhanced the contractions induced by sympathetic nerve stimulation, through an action on postjunctional 5-HT2-receptors. 3. The amplifying effect of 5-HT involved an enhanced influx of extracellular calcium into the smooth muscle cells. In contrast, the neurogenic contractions in vessels not exposed to 5-HT were not dependent on extracellular calcium. 4. The adrenergic component of the amplified response involved postjunctional alpha 1- but not alpha 2- adrenoceptor activation. 5. Exposure of the vessels to 5-HT (5 x 10(-7) M) for 30 min resulted in uptake of the amine into the perivascular sympathetic nerves, as demonstrated by immunohistochemistry. After chemical sympathectomy with 6-hydroxydopamine in vitro or in vivo, or surgical sympathectomy, there was little or no uptake. 6. Exposure to 5-HT followed by repeated washing resulted in an enhancement of the neurogenic contraction, which was still fully tetrodotoxin-sensitive. The enhanced response was blocked by ketanserin (10(-8) M) and prevented by the presence of the 5-HT uptake blocker, paroxetine (3 x 10(-8) M), during the period of exposure to 5-HT.

Serum and serotonin induce retraction of calf aortic smooth muscle (CASM) cells in vitro: inhibition by ketanserin, a 5-HT2 receptor antagonist

Calf aortic smooth muscle (CASM) cells cultured in vitro at high cell density (4 x 10(4) cells/cm2) on bacteriological petri dishes in the presence of serum pile up in clusters and create open spaces in the monolayer. This phenomenon is clearly visible 6 days after plating and is markedly enhanced by the addition of fetal calf serum. Serotonin is essential for the serum-induced retraction since (1) dialyzed serum has no effect, (2) of all the vasoactive agents we tested, only serotonin induced a similar degree of retraction, and (3) the serum-induced retraction was completely blocked by preincubating the cells with serotonin 5-HT2 receptor blockers such as ketanserin and ritanserin but not by preincubation with adrenergic-alpha 1 blockers or histamine antagonists. Serotonin caused CASM cell retraction in a dose-dependent way, with a maximum effect at 10(-6) M. The serotonin-induced retraction was reversible in time and was effectively blocked by ketanserin (IC50 = 1.2 x 10(-9) M). It is therefore concluded that serotonin induces retraction of CASM cells, mediated by the serotonin 5-HT2 receptor.