If the excised, parturient rabbit uterus is repeatedly treated with a Krebs solution free from Ca, its tension in a tetanus drops gradually, and in 15 to 30 minutes becomes zero. We call such a uterus "Ca-deficient." The uterus in this condition has a high threshold, it is non-propagating, "inexcitable," fails to respond to oxytocics in a characteristic fashion, but retains maximum contractility. As Ca is gradually restored to the Krebs, these lost qualities return in a graded fashion and tension of the tetanized uterus becomes a log function of the [Ca]. If the [Ca] is kept low, i.e. 1/10 to 1/20 of the normal, tetanic tension is small but steady, and the preparation offers a full scale of tension increment for the measurement of oxytocic potency. Keeping the stimulus and the [Ca] constant, excitability (measured by tension increment) is a log function of the drug concentration. The recovery of excitability by restoring Ca to the Ca-deficient uterus is strongly temperature-dependent. The Ca-deficient uterus is a useful preparation for the study of the mechanism of regulation. When its excitability is partially recovered by Ca, the electrically stimulated uterus becomes an excellent tool for the quantitative measurement of oxytocic potency.

Animals↗

PubMed1959

The effect of the ovarian steroids on the membrane potential of uterine muscle.

Measurements with flexible intracellular microelectrodes show that the rabbit myometrium when insufficiently stimulated by estrogen has a low membrane potential of about 30 to 35 mv. Estrogen (endogenous or exogenous) increases this value up to 43 to 48 mv., whereas progesterone increases it still further up to 55 to 60 mv. Between the 20th and 29th days of rabbit pregnancy the membrane potential at placental implantation sites is about 10 mv. higher than between these sites. Progesterone treatment increases the value at the interplacental sites to the level of the placental sites. The onset of labor is preceded by a drop in membrane potential at the placental sites to the level of the interplacental sites. This change results in a more or less uniform potential of 45-50 mv. along the length of the uterine horn at term. The membrane potential of the uterus is a log function of the [K](0) at values greater than 12 mM/liter, as is the case in other excitable tissues. In Ca-free Krebs the membrane potential of the estrogen-dominated uterus drops to a low level; the progesterone-dominated myometrium exhibits only a transient and lesser decrease. The membrane potential of the myometrial cell is influenced by endocrine regulation. The correlation between membrane potential and threshold only holds for the estrogen-dominated uterus, however, in other endocrine conditions the relationship breaks down.

Animals↗

PubMed1959

[On molecular physiology and regulation of the uterus].

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Female↗

PubMed1958

The effectiveness of the longitudinal field, coupled with depolarization in activating frog twitch muscles.

EFFECTIVE EXCITATION, PRECEDING THE MECHANICAL RESPONSE IN FROG TWITCH MUSCLES, INVOLVES TWO DISTINCT EVENTS: depolarization of the excitable membrane and the flow of internal currents. To distinguish between the effects of these two potential factors in activation of the contractile machinery, experiments ought to be conducted in which one or the other is excluded. Our experiments are designed to distinguish between these effects by indirect methods. Depolarization in a longitudinal electric field can be expected to be greatest at the ends where current leaves the muscle fibers (space constant at [K] = 16 mM/liter is >1 mm.), whereas the internal longitudinal current is known to be greatest in the middle portion. Depolarization, therefore, should affect the ends more strongly and internal current the middle portion. Our experiments show that non-propagating frog twitch muscles shorten, during isotonic work, along their whole length and not only at their ends, when effectively stimulated in a longitudinal A.C. field. At a field strength about twice the new threshold value (at [K](o) = 16 mM) shortening is distinctly greater in the middle portion of the muscle than at the ends. The muscles, although temporarily non-propagating, remain intact throughout the experiment, as demonstrated by complete recovery after repolarization. These findings may be taken as an indication that internal currents are more directly linked to activation than is depolarization, but the latter is an essential priming step, which must precede or coincide with effective current flow.

Animals↗

PubMed1956