Figure 4. (A) Sustained vasoconstriction of a rat mesenteric resistance vessel after KCl-induced depolarization (KCl, [Potassium sup +]bathincreased to 40 mM). (B) Sequential additions of halothane (HAL) over the concentration range of 0.11–2.64 mM (0.2510 vol% at 37 degrees Celsius) produced transient vasoconstriction followed by complete relaxation of the KCl-induced constriction. The arrows (1) indicate the time halothane concentration changes were made. (C) Treatment of the KCl-constricted vessel with ryanodine (RYO, 10 micro Meter) blocks halothane-induced constrictions but has little effect on halothane-induced relaxation. (D) Ryanodine alone has little effect on the vasoconstricting response to KCl. The four experiments shown were carried out sequentially (A → D) in the same vessel. ID = inside diameter of the vessel; OD = outside diameter of the vessel.