Denervated aortic smooth muscle tension and cellular membrane current in variations of calcium ion concentration in iso-osmotic solution

Abstract

Objective To study the influence of high calcium ion environment on the membrane current of denervated aortic smooth muscles in iso-osmotic condition, as well as study the myogenic spontaneous vasomotion of the aortic smooth muscles under different pre-load state and intervention of calcium-channel blockers. Methods Smooth muscle samples were taken from the aortic smooth muscle layer of Kunming mice. The samples were fixed at two-ends in relaxed state and infiltrated in Ringer's solution. After stabilization, a glass microelectrode was adsorbed on the smooth muscle membrane to form gigaohm-seal. The current state of the membrane was observed under the physiological osmotic pressure. Then, the calcium ions concentration in the Ringer's solution was increased from 0.9 mol/L to 1.2 mol/L, and the immediate change of membrane current was observed after changing of osmotic pressure. Finally, the membrane potential change in iso-osmotic condition was observed when the pre-load was increased to 1 g, and the spontaneous vasomotion curves of the samples were recorded. Results In the relaxation state, the membrane current of the smooth muscle was increased significantly, i.e. (10.25±1.34) pA vs. (24.91±3.27) pA (P<0.05), when the calcium ion concentration was increased from 0.9 mol/L to 1.2 mol/L in iso-osmotic condition. When the pre-load was increased, the variation amplitude of the membrane current was increased in iso-osmotic condition, i.e. (10.25±1.34) pA to (15.33±4.33) pA (P<0.05) for the lower calcium ion concentration, and (24.91±3.27) pA to (33.31±7.25) pA (P<0.05) for the higher calcium ion concentration. If only increasing the pre-load, the myogenic spontaneous vasomotion could be increased by 175% for the condition of lower calcium ion concentration (0.9 mol/L). When the calcium ion concentration was increased to 1.2 mol/L, the spontaneous vasomotion frequency of the specimen was increased, and the range of the vasomotion could be further increased by 40%. After pretreatment with the calcium-channel blocker (0.5 g/L nitrendipine), the membrane current and myogenic spontaneous vasomotion were significantly decreased in the higher calcium ion environment, which indicated that the membrane potential and spontaneous vasomotion was dominated by calcium ions. Conclusions The increase of extracellular calcium ion concentration not only can improve the excitability of the aorta smooth muscle cells, but also can cause significant spontaneous vasomotion, and can improve the compliance of the smooth muscle tissue in pre-load variation. The calcium-channel blockers can inhibit calcium-dominated transmembrane currents, reduce myogenic spontaneous vasomotion, and enhance the stiffness of smooth muscle, which may have negative effects on windkessel vessel function. Key words: Denervation; Quick stretching; Overloading; Spontaneous vasomotion; Membrane current