A study of the mechanisms involved in the responses of cerebral arteries to 5-hydroxtryptamine

Clark, Angela Helen (1990) A study of the mechanisms involved in the responses of cerebral arteries to 5-hydroxtryptamine. University of Southampton, Doctoral Thesis.

Abstract

5-HT-induced contractions in the peripheral vasculature are largely due to activation of 5-HT₂ receptors, which are coupled to the production of inositol 1,4,5 triphosphate (Ins 1,4,5P₃) and diacylglycerol (DAG). The mechanism of contraction to 5-HT in cerebrtal arteries is less undertood than in peripheral arteries, and the 5-HT receptor mediating the contractile response has not been fully characterized. In this study, the mechanism by which 5-HT produces contraction, the nature of the 5-HT receptor, and the role of the endothelium in 5-HT-induced responses have been investigated in the rabbit basilar artery. 5-HT produced smooth muscle contraction and membrane depolarization. The onset of contraction generally preceded the onset of membrane depolarization, suggesting that 5-HT-induced contraction, at least in the early stages was largely a result of voltage-independent mechanisms. 5-HT (10^-8M-10^-4M) produced a concentration-dependent increase in membrane concentration of DAG, but did not stimulate phosphoinositide hydrolysis. DAG formation and 5-HT-induced contraction were inhibited by the phospholipase C inhibitor, 2-nitro-4-carboxyphenyl N,N-diphenylcarbamate (NCDC). Activation of protein kinase C with phorbol 12,13-dibutyrate (PDBu) (10^-10M-5x10^-5M) produced concentration-dependent increases in tension. Inhibition of protein kinase C (PKC) with 1-(5-isoquinolinesulfonyl 1)-methyl piperazine (H-7), potentiated DAG release, and inhibited PDBu- and 5-HT-induced contractions. Both NCDC and H-7 had no effect on membrane depolarization induced with 5-HT. These results suggest that DAG, derived from a non-phosphatidylinositol source, activates PKC which plays an important role in the contraction produced in response to 5-HT. Both 5-HT- and PDBu-induced contractions were rapidly abolished in the absence of extracellular calcium. 5-HT-induced contraction, but not membrane depolarization, was inhibited by the calcium antagonists, nifedipine (10^-8M) and verapamil (10^-5M), suggesting that 5-HT-induced contraction is largely dependent on an influx of calcium from outside the cell. The receptor(s) mediating the 5-HT-induced responses did not fit any of the current classified 5-HT receptor subtypes. Removal of the endothelium potentiated contractile responses to 5-HT, but 5-HT did not cause endothelium-dependent relaxation. In preconstricted arteries, both 5-HT and the 5-HT₁ agonist, 5-carboxyamidotryptamine (5-CT), but not the 5-HT_1A agonist, 8-hydroxy-dipropylaminotetralin (8-OHDPAT), caused further contractile responses, but only in tissues with intact endothelium, indicating the possible release of an EDCF. In conclusion, these data suggest that the mechanism of contraction in the rabbit basilar artery is different to that inthe peripheral vasculature. Unlike peripheral arteries, the DAG produced in response to 5-HT, appears to be generated froma non-phosphoinositide source. The subsequent PKC activation plays a major role in the 5-HT-induced contraction, but not membrane depolarization. The receptor subtype present on the smooth muscle cells remains uncharacterized, but endothelium-dependent contractions are a result of activation of a 5-HT₁-like (but not 5-HT_1A) receptor.

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