SOME FACTORS AFFECTING THE CONTRACTILE RESPONSES OF PREGNANT MOUSE MYOMETRIUM

Abstract

Effects of [K] ₀ and [Na] ₀ on isometric contraction of mouse myometrium were investigated in combination with the use of low temperature and Mn. Membrane activity was recorded by the sucrosegap method or with microelectrodes, and contraction by strain gauge transducer.
1. A sudden increase of [K] ₀ from the 5.6 mm contained in normal Locke solution to more than 40 mm caused a quick rise of tension to a maximal level (phasic response), which was followed in about 5 min by incomplete relaxation (tonic response). The magnitude of tonic response changed depending on [Ca] ₀.
2. Contraction was elicited by applying Ca to the solutions containing various [Na] ₀ and [K] ₀, after the muscle had been relaxed in Ca-free isotonic K solution. In the Na-containing solution, where [Na] ₀ was decreased by replacement with the same amount of [K] ₀, the contraction was maximal in about 30 mm [K] ₀. In the Na-free solution containing various [K] ₀, where the osmolarity was kept constant by the substitution of Tris for Na, the contraction decreased with increasing [K] ₀. Lowering temperature from 32° to 16°C caused the contraction in Na-containing solution to increase but had little affect on that in Na-free solution.
3. Treatment with 2 mM Mn caused a potentiation of tonic response of K-contracture, whereas the phasic response was depressed. As the Mn was washed out with normal Locke solution after the pretreatment with 2 mM Mn for 30-60 min, tonic response became more potentiated for several hours. During this period, the effect of [Ca] ₀ on the K-contracture was investigated.
4. The change in membrane potential in response to a 10-fold change of [K] ₀ was 35 mV for Na-containing and 10 mV for Na-free solution. Lowering the temperature from 32° to 18°C caused a depolarization of 15 mV in normal Locke solution.
Several smooth muscles produce contracture when immersed in external media with modified ionic compositions, such as those with excess K or those without Na, which is replaced by Li, choline, or sucrose (for review, see BOHR 1964; EVANS et al., 1958; JUDAH and WILLOUGHBY, 1964; IMAI and TAKED 1967; REUTER and BLAUSTEIN, 1972). Previous studies indicate that, apart from effects on the membrane potential, these external media modify the Ca transport across the membrane and change intracellular Ca concentration, which triggers the contractile mechanism of the tissue (REUTER and SEITZ, 1968; BLAUSTEIN and HODGKIN, 1969; KATASE and TOMITA, 1972).
The mouse myometrium was shown to exhibit contracture in low Na solution and a much smaller contracture in isotonic K solution (OSA, 1971, 1973a, b). In the present experiments, the contractile effects of [Na] ₀ and [K] ₀ were investigated in combination with the use of low temperature and Mn. Also studied was the possibility that a reduction of [Ca] ₁ in mouse myometrium depends on the Nadependent Ca exchange on one hand, and on Ca-sequestering to some inner membrane site on the other, the mechanism of which appears to be depressed by the treatment with Mn. By contrast, it was argued that external K is largely responsible for modifying the influx of Ca.