The vascular ring was allowed to equilibrate for at least 30 min, and
the preparation was refreshed with Krebs−Henseleit solution every 15
min. Passive tension was adjusted to maintain 1.5 g throughout the
equilibration and experimental periods. At the basal tension of 1.5 g,
the rings were found to develop maximal active tension in response to
stimulation with a contraction-inducing agent. After equilibration for
30 min, the maximal contractile response to a high concentration of
potassium (80 mM) in Krebs solution (potassium exchanged with
sodium on a molar basis) was determined. Then, rings were washed
three times with fresh Krebs solution and allowed to equilibrate for 30
min before further evaluation of the relaxation response to either ACh
10−9 to 10−5 M (Endo+) or SNP 10−9 to 10−5 M (Endo−) in vessels
precontracted with L-phenylephrine (Phe 10−7 M). The vasodilator
effect of increasing concentrations of ACh and SNP was expressed as
percent decrease of the peak Phe concentration.