Behaviour of the smooth muscle of the cardiovascular system
Steedman, Wilma M.
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This investigation was undertaken in an attempt to study the behaviour of vascular smooth muscle at the cellular level, and in conditions as near physiological as possible. Microelectrodes were inserted into smooth muscle cells in the walls of small arteries and arterioles in the rat mesenteric circulation in the intact anaesthetized rat, and the electrical activity recorded at rest and under the influence of various physiological stimuli.Intracellular recording gave membrane potentials that were low and variable, showing slow fluctuations at frequencies of some 9 to 12/min, and occasional action potentials which appeared to arise from the slow waves. Both these types of electrical activity were capable of being influenced by the sympathetic nervous system, as shown by the effects of stimulating the splanchnic nerves and denervating by chemical means and by cutting the nerves. However, as it is not known whether any residual nervous activity remained after denervation was attempted, it remains uncertain whether this electrical activity was generated by nervous activity or by some mechanism inherent in the smooth muscle cell itself.Propagated action potentials could not be elicited by direct electrical stimulation of the blood vessels, and the application of a stimulating pulse across the cell membrane into the interior of the cell did not evoke an action potential, but this may have been due to technical limitations.It was found impossible to record for long periods with an electrode inside a cell due to the small size and continuous movement of the cells, and the effects of various stimulating and inhibiting agents were therefore studied using extracellular recording methods. In general, stimulating agents such as asphyxia, electrical stimulation of the sympathetic nerves, and local application of adrenaline, noradrenaline, and vasopressin, increased the electrical activity of the muscle, and inhibiting agents such as acetylcholine and removal of the nerve supply depressed it. High concentrations of adrenaline and noradrenaline appeared to be capable of causing the muscle cells to contract without firing action potentials, but whether this type of contracture is a Use other side if necessary. physiological phenomenon remains uncertain. None of these manoeuvres had any significant effect on the frequency of the slow waves, but under the influence of stimulating agents the percentage of waves with action potentials increased, and spikes appeared in twos and threes.It was concluded that the technical difficulties due to the small size of the smooth muscle cells relative to that of the penetrating electrode tip will limit further investigation using refined techniques until the development of methods for producing finer microelectrodes.