A change in inhibitory mechanism of Na+ deficiency on high K(+)-induced contraction in rat uterus with the progress of pregnancy.

In the present paper, effects of high K+/Na+ deficient solution on the rat uterus with the progress of the pregnancy were examined on the mechanical response and wet weight. The progress of pregnancy was divided in four stages, early stage (Day 4 and 5 of pregnancy), middle stage (Day 8-10), late stage (Day 14 and 15) and end stage (Day 21). In the early stage muscle, an isosmotically substituted high K+/Na+ deficient (iso-154K+) solution induced a large contraction followed by a decrease in tension level, though a hyperosmotic KCI addition induced a large sustained contraction. Similar results were shown in muscles of the middle and late stages. However, a maximum tension level and a subsequent inhibition by the iso-154K+ solution in the end stage muscle were smaller than those of other stages. On the other hand, iso-154K+ solution remarkably increased the relative cellular water content in the early or middle stage muscle, but moderately in the late or end stage one. In the muscle of all stages, a hyperosmotic addition of sucrose reversed the inhibition of muscle tension and the cell swelling by the iso-154K+ solution. Moreover, a substitution of more impermeable anion (C2H5COO-) for Cl- in the iso-154K+ solution decreased the inhibition of the contraction in the early stage muscle, but increased it in the end stage one. The substitution for Cl- with more permeable anion (NO3- or I-) produced a greater inhibition of contraction in all stage muscles.(ABSTRACT TRUNCATED AT 250 WORDS)