A quantitative relationship between cellular Na accumulation and relaxation produced by ouabain in the depolarized smooth muscle of guinea-pig taenia coli
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An investigation was made of the effect of oubain on the membrane potential, tension development,45Ca uptake and intracellular Na and K contents of smooth muscle of the guinea-pig taenia coli depolarized by high-K solution. The results were compared with the effects of 2,4-dinitrophenol (DNP) and verapamil.
All drugs produced concentration-dependent relaxations of high-K induced contractures without affecting the depolarization of the membrane. After removal of Na from the medium, the contracture was inhibited by DNP or verapamil but not by ouabain. The inhibitory effects of ouabain and verapamil were antagonized by raising the concentration of external Ca ([Ca]0), but the relaxation produced by DNP was independent of [Ca]0.
Estimates of cellular Na were made by bathing the tissues in cold Li-solution. After an initial rapid loss of Na due to an exchange of extracellular Na with Li, the tissue Na reached a steady state. The residual after 1 h was regarded as cellular Na.
During relaxation induced by ouabain, the depolarized muscle gained cellular Na. The increase in cellular Na was dependent upon the concentration of ouabain. A smaller increase was noted with DNP, while verapamil had no effect on cellular Na.
The relaxation produced by ouabain was related to the intracellular Na content of the tissues. Also, the magnitude of the contraction produced by adding 1 mM Ca to Ca-free K-depolarized tissues was inversely correlated with the logarithm of the intracellular Na content, and an increase in the Na from 7.3–26.6 mmole/kg wet wt. halved the size of the Ca contracture.
45Ca uptake, which was measured by a modified “La-method”, was increased in high-K solution. This increase was inhibited by pretreatments with ouabain, DNP and verapamil.
It is suggested that the action of ouabain is closely related to an accumulation of cellular Na, which may cause the relaxation mainly by inhibiting Ca influx in response to depolarization with high-K. The data also suggest that the mechanism of relaxation induced by DNP or verapamil is different from that induced by ouabain.
Key wordsOuabain Taenia coli Cellular Na Relaxation Depolarization
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