Potassium-induced contractions in pig ureter strips. Consequences for helical muscle-fiber model of ureter.

Contractions were chemically induced in vitro in pig uterus segments by a high potassium concentration in a metabolic solution. Slow contractions were found mainly in the transverse direction. The near absence of contractions in the longitudinal direction is ascribed to a nearly circular orientation of the muscle fibers. (In terms of the helical muscle-fiber model of the ureter, our findings would correspond to a pitch of 3 to 5 degrees for the helix.) It is further postulated that the coupling structure of fibers in the transverse direction, determining an annular contraction mode, differs from that in the longitudinal direction, determining the longitudinal propagation of the annular contraction mode. The over-all conclusions on the basis of our results is that the helical muscle-fiber model now generally assumed for the ureter must be greatly modified to correlate with the experimental facts that its continued use is misleading and undesirable.