Potential Fields in Vascular Smooth Muscle Generated by Transmitter Release from Sympathetic Varicosities

A quantitative model is provided of how current flow occurs in the media of blood vessels upon the release of transmitter from autonomic varicosities onto ionotropic receptors located on smooth muscle cells at the adventitial surface of the vessel. In particular, the extent to which potential generated in cells at the adventitial surface (AS) conducts through to cells at the intimal surface (IS) is investigated. Experimental tests of the model have been made for the case of the rat tail artery. The model of the media is an extension of the discrete bidomain syncytium to the case where the smooth muscle syncytium is bounded on two sides by a volume conductor, as is the case with the media of blood vessels. The amplitudes and temporal characteristics of excitatory junction potentials (EJPs), recorded throughout this syncytium following the release of ATP from varicosities located on one side of the syncytium, are predicted by the theory. Current injection into a single cell at the AS will not give rise to a detectable membrane potential at the IS; however, simultaneous injection of current into all the cells at the AS can give rise to a membrane potential at the IS that has an amplitude of about 50% of that at the AS, in agreement with experimental findings. In addition, the effects of perturbing the electrical couplings between cells in the syncytium on the EJPs recorded at different sites in the syncytium are also predicted. This work shows that the discrete bidomain model of the syncytium gives a quantitative description of the current and potential fields that occur throughout the smooth muscle of the media of blood vessels following the release of transmitter from varicosities at the adventitial surface of the vessels. The theory can be applied to the media of blood vessels of any size to determine the relative effectiveness of sympathetic nerves in controlling the excitability of smooth muscle cells through the media.