Spontaneous rhythmic contractile behaviour of aortic ring segments isolated from pressure loaded regions of the vasculature.

Study Objective: The aim was to study contractile responses of segments of rat arteries taken from pressure loaded and pressure protected regions and to examine the role of endothelial derived factors on the spontaneous activity of pressure loaded ring segments.

Design: Rats were subjected to complete aortic coarctation between the origins of the renal arteries and allowed to recover for 8 d. After 8 d arterial pressures were measured in awake animals from the pressure loaded and pressure protected regions simultaneously. Ring segments (2-3 mm) were taken from the two regions and mounted in a tissue bath for isometric force measurements. Similar studies were conducted in sham animals and in animals in which the kidney distal to the coarctation had been removed.

Experimental Material: Female Sprague-Dawley rats, weight 200-250 g, were used.

Measurements And Main Results: Eight days after coarctation mean aortic pressure proximal to the occlusion was 168(SEM 1.29) mm Hg (n = 104) while distally it was 38(2.42) (n = 40). Of the rings tested 96% showed spontaneous rhythmic activity, having a mean frequency of 3.94(0.17) cycles.min-1. Spontaneous activity was not present in the pressure protected segments taken from the same animals. Rats with the distal kidney removed (n = 25) failed to become hypertensive and similarly prepared ring segments failed to show spontaneous rhythmic activity. Prior removal of the endothelial layer had no effect on the spontaneous contractile responses in pressure loaded segments. Histological examination showed that the media to lumen ratio was increased in coarcted rats in both pressure loaded and pressure protected regions compared to similar regions in sham operated animals.

Conclusions: Pressure loaded arterial segments show spontaneous contractile activity when compared to sham segments by mechanisms not dependent on endothelial derived factors. The increase in pressure proximal to the occlusion is dependent on the renin-angiotensin system, since pressure was not increased when the distal kidney was removed. We hypothesise that the chronic pressure load induces fundamental changes in membrane permeability which leads to spontaneous contractile activity.