Effects of noradrenaline, isoproterenol and acetylcholine on ureteral resistance

Resistance to fluid flow in the canine ureter can be divided into two categories. The higher resistance is recorded at flow rates less than or equal to 2.16 ml./min. At these rates the ureter is able to completely coapt its walls so that urine is transported in individual boluses. The lower resistance is recorded at flow rates greater than or equal to 5.40 ml./min. At these rates the ureteral walls remained open and urine is transported as a column of fluid. Noradrenaline causes a marked increase in ureteral resistance at low flow rates and a small but statistically significant increase in ureteral resistance at high flow rates. Acetylcholine increases resistance only at the low flow rates. Isoproterenol significantly decreases resistance at both low and high flow rates. These findings are consistent with ureteral resistance to fluid flow being composed of two components. One is the ureteral peristaltic contraction which plays a principal role in urinary bolus transport at low flows; the other is ureteral wall tonus, which plays an important role in the transport of columns of urine by the ureter, which does not coapt its walls, at the higher flow rates.