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Alpha-adrenoceptor mediated vascular effects of antipsychotic drugs

Zahra Nourian

Summary

The therapeutic action of most antipsychotic drugs in the treatment of schizophrenia is attributed to their central dopamine antagonist effect, but they also have effects on α1-adrenoceptors (AR) in blood vessels. Orthostatic hypotension, as the most common cardiovascular adverse effects during treatment with antipsychotics, probably cause due to their antagonist action on resistance vessel α1A-ARs. In this project, this hypothesis has been tested indirectly (via animal models) and directly (via transfected cell line stably expressing human α1A1-ARs). 

The animal experiments were performed to determine in rats how the orthostatic hypotensive effect of several antipsychotic drugs compares with their affinity for adrenoceptors in male Wistar rat mesenteric small arteries (MSA with mainly α1A-AR) and rat aorta (mainly α1D-AR). Using a tilt setup in in vivo experiments, orthostatic hypotension was measured in anaesthetized rats for prazosin and the antipsychotics haloperidol, sertindole, risperidone, clozapine, ziprasidone, domperidone, olanzapine and aripiprazole. For in vitro studies, segments of MSA and aorta were mounted on a wire myograph for isometric tension recording. Cumulative concentration-response curves were constructed to phenylephrine (PE) in the absence and presence of all above mentioned drugs. Apparent affinity (pA2 values) were calculated by Schild analysis. 

Animal studies results: Prazosin antagonized tilt-induced and PE responses in both studies (threshold 4 ng/ml, pA2 9.52 MSA, 10.1 aorta). The rank order of the potency of the antipsychotics in the tilt experiments correlated (r2=0.69, P=0.01) with the pA2-values in MSA. Risperidone and sertindole had the highest potency in the tilt test (threshold 159 and 97 ng/ml), and the highest apparent affinity in MSA (pA2 8.92 and 8.78), in contrast with aripiprazole and domperidone which had the lowest in each case (threshold 4.1 and 3.0 μg/ml, pA2 7.17 and 6.99). In aorta, the pA2-values did not correlate with the in vivo potencies; in particular sertindole had no functional affinity in aorta. Endothelial removal did not affect either pA2-values or the Schild slopes in the presence of sertindole or risperidone in MSA which could confirm the action of the tested compounds is being mediated through effects on the vascular smooth muscle. 

Molecular biology results: To investigate the direct effect of antipsychotics on human α1A-ARs, a transfected stable cell line with a relevant human α1A-AR splice variant was used. To determine a splice variant which was relevant, I used quantitative real-time Polymerase Chain Reaction (qPCR) to determine the prevalence in human subcutaneous small arteries of three of the five splice variants ADRA1A_v1-5, which encode functional protein: α1A1-, α1A3-, α1A4-ARs. The statistical analysis showed higher transcriptional levels of α1A1- than of α1A3- and α1A4-ARs (1.6 and 5.8 times, respectively). Therefore, a Flp-In-293 cell line stably expressing human α1A1-AR was developed. Functional activity of recombinant protein was demonstrated using a Fura-2 assay by a rise in intracellular calcium concentration ([Ca2+]i) when challenged with PE (EC50 = 1.61◊10-8 M). Then, the direct antagonist potency (pKB values) of prazosin, sertindole, risperidone and haloperidol were determined (8.99, 9.36, 8.52 and 6.59, respectively) using cumulative concentration-response curves for PE. In conclusion, the orthostatic hypotensive effect in rats of the antipsychotic drugs investigated is mediated through α1A-ARs. There is lower adrenoceptor affinity of haloperidol than of risperidone and sertindole on human α1A1-adrenoceptor which could explain the higher rate of risperidone and sertindole to cause orthostatic hypotension compared to haloperidol in humans.