Vasoconstriction reactions in tail artery in the rats with regional arterial hypotension

Abstract

Reactivity of the tall artery to norepinephrine was studied in rats with regional arterial hypotension. Constant-flow perfusion of the artery elicited less pronounced constriction in hypotensive rats in comparison with the controls, while the constant-pressure perfusion evoked stronger vasoconstriction. This is explained by possible involvement of myogenic component in the contractile response. When myogenic reaction was inhibited by enhanced potassium concentration in the perfusion solution, vasoconstriction in hypotensive rats was less pronounced under both perfusion modes. Key Words: regional hypotension; myogenic reaction; rat tail arteryThe vasomotor reactions in hypertensive rats (HTR) and in normotensive rats (NTR) differ considerably. Under conditions of constant-flow perfusion, the hypertensive vessels are contracted by norepinephrine (NE) stronger than the normal vessels [4][5][6]. This phenomenon is related to increased thickness of the smooth muscle wall and decreased vascular lumen in HTR [2].By contrast, the vasoconstrictor reactions were more pronounced in NTR, as compared to HTR both in the hind limb vascular system [11] and in isolated tail artery pert'used under constant pressure [9]. Different reactivity under various perfusion regimens is supposed to be determined by different distension of smooth muscle and, consequently, by various degrees of activation of the myogenic reaction [8].Our aim was to test this hypothesis by comparing vasoconstriction of isolated tail artery before and after smooth muscle depolarization by potassium ions, which prevent the responses of the blood vessels to distension by inhibiting the myogenic reaction [7,10]. After depolarization vascular reactivity was higher in HR at both perfusion regimens [9]. To further study the effect of the vascular wall thickness on the reactivity of blood vessels we have chosen vessels with a thinner muscle wall. The blood vessel wall becomes thinner under conditions of prolong decrease in arterial pressure [1,3]. Therefore, we studied vasoconstriction of isolated tail artery in rats subjected to regional arterial hypotension. MATERIALS AND METHODSExperiments were pertormed on 12-14-week-old Wistar rats. The abdominal aorta was dipped under Nembutal anesthesia (40 mg/kg intraperitoneally) distal to the branching of renal arteries. The internal diameter of the clip was 0.25 mm. Controls were sham-operated rats. Experiments were carried out 6-7 weeks after the operation. Control rats (CR) and rats with regional hypotension (low-tension rats, LTR) weighed 372+5 g (n=8) and 365__6 g (n=8), respectively.For morphological measurements the tail artery segments were fixed in 10% formalin and processed according to the standard procedure. Sections (8 ~tm) were stained by picrofuchsin-fuchselin. The wall crosssection area of smooth muscle layer was estimated from photographs.After decapitation the ventral tail artery was isolated and placed into oxygenated Krebs-Henseleit