Factors affecting in vitro evaluation of angiotensin II sensitivity

Vascular smooth muscle (VSM) sensitivity to Angiotensin II (AII) is altered by the endothelium (endo) in some models. Assessment of this response to All may be adversely affected by the development of tachyphylaxis (TP). We sought a model that would eliminate TP, thus allowing assessment of endothelial effects on canine femoral artery VSM.VSM strips were studied at 37° C in oxygenated baths. They were preloaded at 10g then subjected to maximal KCl activation. 4 groups of 5 strips each were used to define the time required for full return of the contactile response to a second dose of 7×10^-9M AII. The response was 57% at 15 min, 84% at 30 min, 92% at 60 min and 99.9% by 120 min. Thus, 120 min is necessary for full recovery. The effect of endo was assessed by comparing the contractile response of intact (n=7) and denuded (n=8) canine vascular strips. Strips were subjected to 30 mM KCl one hour after the initial KCl activation. The presence of an intact endo was confirmed by a relaxation response to carbachol (1X10^-6M) in the intact group (53%) vs the denuded group of (0.64%). Strips were subjected to either a single dose (1×10^-7M) A- II before or after a 120 min, peripd then a dose response curve (1×10^-10 to 1×10^-7M). There was no significant difference in the absolute force at the single dose (1×10^-7M) in the intact, 0.75 +/- 0.09 kg/cm² vs the denuded, 0.69 +/- 0.25 kg/cm² (p=0.64). the EC₅₀ (nM) of the intact, 1.48 +/- 0.22 and the denuded, 1.76 +/- 0.25 were not significantly different (p=0.40). We conclude that the development of TP in the canine femoral VSM is a time dependent phenonenom requiring 120 min interval to abate.Also, while the endo produces a significant relaxation response it does not appear to contribute significantly to the contractile response to All in the canine femoral artery.