Effects of noradrenaline and acetylcholine on electro-mechanical properties of the guinea-pig portal vein.

Effects of acetylcholine (ACh) and noradrenaline (NA) on mechanical properties of smooth muscle cells of the guinea-pig portal vein were investigated using intact and skinned muscle preparations. In some preparations, the electrical activity was also recorded. In addition to ACh and NA, the effects of caffeine, procaine, excess concentrations of [K]o and MnCl2 were investigated. NA enhanced the mechanical response due to increase in the spike generation, receptor activated depolarization and release of Ca stored in the cell. ACh also enhanced the mechanical response, but this agent had little effect on the Ca release from the storage sites. Caffeine and procaine (10 mM) depolarized the membrane and enhanced the electrical activity. Caffeine enhanced the mechanical activity due to an increase in the membrane activity and the release of Ca stored in the cell, while procaine inhibited the contraction. Procaine inhibited and caffeine accelerated the Ca-induced Ca-release mechanism in the cell. NA released the Ca from the storage sites to a greater extent than did caffeine or ACh. MnCl2 inhibited the spontaneous membrane activity and contraction; however, low concentrations of MnCl2 increased the caffeine- or NA-induced contraction in Ca-free solution. When the Ca-tension relationship was observed in saponin-treated skinned muscles, the minimum concentration of Ca required to produce the contraction was 10(-7)M, as observed in other vascular tissues. Caffeine released the stored Ca, Na and ACh had no effect on the Ca release and procaine inhibited the caffeine-induced Ca release in skinned muscles. It was concluded that the membrane properties of smooth muscle cells in the portal vein are much the same as observed in other spontaneously active visceral muscles. Differences observed in the actions of NA and ACh on mechanical properties seems to be mainly due to different receptor-operated Ca release mechanisms.