A study of the influence of the endothelium on the responses of the smooth muscle cells in rabbit cerebral arteries

Rand, Victoria (1991) A study of the influence of the endothelium on the responses of the smooth muscle cells in rabbit cerebral arteries. University of Southampton, Doctoral Thesis.

Abstract

Application of acetylcholine (ACh), and other muscarinic agonists, to arterial segments cause endothelium dependent smooth muscle relaxation and membrane hyperpolarization. The relaxation is resistant to indomethacin and has been suggested to result from the release of a factor called endothelium-derived relaxing factor (EDRF). The chemical identity of EDRF has been suggested to be nitric oxide or a nitroso-containing compound. Whether the hyperpolarization seen to cholinomimetics is due to nitric oxide or a separate hyperpolarizing factor (EDHF) is a matter under investi vation.

Simultaneous changes in membrane potential and tension were recorded from the rabbit basilar artery in response to application of ACh (10~7-10~4M). ACh caused an endothelium dependent smooth muscle relaxation and membrane hyperpolarization. Maximum relaxation of noradrenaline reconstricted artery segments was observed at 10~4M ACh. The hyperpolarization at this concentration had a maximum amplitude of 7.9±0.8mV and was relatively transient compared to the sustained relaxation. The hypcrpolarization preceded the relaxation by 3.0±0.7 seconds and desensitized easily upon repeated application of ACh.

Inhibitors of nitric oxide synthase, such as NG-monomethyl L-arginine (L-NMMA) (10-*M) and NG-nitro L-arginine (L-NAME) (10-*M), showed varying degrees of block of the ACh induced hyperpolarization and relaxation. In some cases, there was only a partial block of the hy-perpolarization and relaxation, the hypcrpolarization being split into two phases. In other situations, incubation of the artery segment with the inhibitors produced a more substantial block of both the smooth muscle relaxation and membrane hyperpolarization.

In noradrenaline preconstricted and depolarized artery segments, application of sodium nitroprusside (NaNP) and nitric oxide solutions in concentrations up to lOOjiM produced smooth muscle relaxation but failed to induce membrane hyperpolarization. Application of nitric oxide had no effect on the resting membrane potential. In the presence of higher concentrations of NaNP, a slow developing, reversible hyperpolarization ranging from 2.7±0.3mV at 100/iM to 19.3±2.4mV at lOmV was observed.

Application of carbachol (10~7-10~4M) induced an endothelium dependent smooth muscle relaxation. 10~* carbachol induced a transient hyperpolarization that desensitized easily and preceded the relaxation. Other muscarinic agonists such as oxotremorine and, less potently, bethanechol induced relaxations but at concentrations of 10~4M produced no change in membrane potential.

Atropine (10~TM) substantially inhibited the relaxation induced by ACh and inhibited the hyperpolarization. Pirenzipine (10~8M) did not significantly affect relaxations to ACh (10~7-10~*M) but blocked the hyperpolarization.

It seems likely that, in the rabbit basilar artery, nitric oxide may contribute to both the relaxation and the hyperpolarization induced by ACh. The exact role of the hyperpolarization in the smooth muscle relaxation is unclear, but it appears to be minor as dcscnsitization of the membrane events caused no change in the relaxation. In addition, the hyperpolarization and relaxation may be mediated by separate muscarinic receptors located on the arterial endothelial cells.

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