The skeletal muscle is one of the important target tissues for the actions of estrogen via both nuclear and extranuclear (non-genomic) pathways. However, there is a paucity of information about the receptor (ER) involved. The aim of this study was thus to explore the ER expression in skeletal muscle, and the influence of estrogen on it, by using C2C12 myoblasts derived from mouse skeletal muscle. Significant expression of a approximately 66-kD protein immunoreactive to ER type alpha (ERalpha) monoclonal antibody, which was comparable to that in ovary, was detected in the whole-cell (total) and nucleus-free (nonnuclear) fractions of C2C12 myoblasts. The expression level of these ER proteins increased in several hours with treatment with 17beta-estradiol (E2), which was preceded by the elevation of the ER mRNA level. This increase appeared to reflect the acceleration of de novo synthesis of ER protein, as proved by the (35)S-methionine immunoprecipitation method. A similar extent of fast increase in ER expression was also induced by a membrane-impermeable, BSA-conjugated estradiol (E2-BSA). Unexpectedly, the E2-induced increases in total and nonnuclear ER were further enhanced by the classic ER antagonists tamoxifen and ICI182,780 in a wide concentration range, implying some structural difference of the involved ER from the classical one. Treatment with the ERK1/2 inhibitor, PD98059 (10 microM), or the p38 MAPK-specific inhibitor, SB203580 (10 microM), greatly inhibited the E2-induced ER increase, while the protein kinase C (PKC) activator TPA (1 microM) enhanced it. These results collectively suggest that C2C12 skeletal myoblasts express a high level of ER, a considerable part of which is extranuclear. Further, the expression of ER in these cells may be significantly upregulated by estrogen itself via increased biosynthesis linked to membrane-bound ER and downstream MAPK-mediated signaling pathways.

The effects of lanthanum on the resting membrane potential, action potential, membrane resistance, twitch tension, and potassium contracture were investigated and the localization of the drug was studied electron microscopically in isolated frog ventricular muscle. Lanthanum in concentrations of 0.2 to 5 mm decreased the resting potential by about 5-8 mV, which was accompanied by an increase in the membrane resistance of about 43 % for the depolarizing and 40 % for the hyperpolarizing direction. Lanthanum caused a decrease in height and a prominent shortening of the action potential, and also, a depression of the plateau level. In addition, it increased the threshold for action potential generation depending on its concentration. The slow response action potential was inhibited by lanthanum in parallel with twitch inhibition. This finding suggests that the twitch inhibition resulted from the suppression of the slow inward calcium current. In contrast, potassium contracture was not inhibited by lanthanum. When the muscle preparation was treated with neuraminidase, the twitch inhibition caused by lanthanum was strongly depressed. Electron microscopic observation revealed that the precipitates of lanthanum were localized on the external lamina of myocytes as well as in the extracellular spaces but could never be found within the cytoplasm. No such precipitates could be detected in the neuraminidase-treated muscle. From these results it is suggested that lanthanum takes the place of calcium at the membrane surface : it modifies permeabilities to sodium, potassium and calcium ions and the excitation-contraction coupling of the ventricular muscle by replacing calcium bound to the membrane surface.

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Estrogens, like other steroid hormones, initiate their action via specific intracellular receptors [1]. The presence of estrogen receptors in the skeletal muscle cell was demonstrated in bovine [2], rabbit [3,4], and rat [5]. Skeletal myoblasts also expressed functional estrogen receptors [6]. The effects of estrogen on skeletal muscle contractility have been diversely reported. A long-term treatment of the developing rat with estrogen reduced the tetanus tension of skeletal muscle [7]. In women, the supplement with a supraphysiological level of estrogen did not affect the contractile force and fatigue process of the first dorsal interosseus muscle [8]. Neither did the administration of estrogen significantly affect the fatigue of the estradiol-injected extensor digitorum longus muscle in rat [9]. In frog skeletal muscle, however, Rana temporaria, the presence of diethyl-stilboesterol, a nonsteroidal estrogen, markedly potentiated the twitch tension, but apparently not the tetanus tension [10], and it also potentiated the tension response of the fatigued Key words: skeletal muscle, contraction, fatigue, 17␤-estradiol. Abstract:The effects of 17␤-estradiol (10an active estrogen, on the tension and fatigue responses of single fiber or fiber bundle prepared from frog skeletal muscle were investigated. The administration of 17␤-estradiol caused a transient potentiation of tetanus tension by field stimulation at every minute. This potentiation was not affected by the presence of nicardipine, suggesting that the action of 17␤-estradiol would place the excitation-contraction (E-C) coupling beyond T-tubule depolarization. Fatigue was produced by repeated tetanic stimulation every second until tension declined to approximately 40% of the initial level. Fibers were then allowed to recover by having tetani given to them every minute. In the normal Ringer solution, the time to 50% of the initial tetanus tension was 41.7 s.With the presence of 17␤-estradiol, the time to 50% tension was faster than that of control. The presence of 17␣-estradiol, a stereoisomer, caused no potentiation of tetanic tension to be stimulated every minute, and the rate of decline of fatigued response was almost the same as that of control, suggesting the existence of specific estrogen receptors in the frog muscle. In fatigued muscle with or without estrogen, the tension to field stimulation was transient and not sustained. When the fatigued muscle was again treated with field stimulation at every minute after the more-frequent stimulation, the recovery rate was increased in 17␤-estradiol. A prompt reduction in temperature to 5°C, from 20°C, in the presence of caffeine elicited the tension response, a rapid cooling contracture (RCC). The presence of 17␤-estradiol inhibited peak tension and maximum rate of rise of the RCC only after the repetitive electrical stimuli. These results suggest that the potentiation of contraction upon the electrical stimulation by 17␤-estradiol was induced by the increase of myoplasmic-free Ca 2ϩ concentration via an activ...