INHIBITORY MECHANISMS OF ISOPRENALINE IN THE GUINEA-PIG TAENIA COLI

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The beta-receptor stimulating effect of isoprenaline on the spontaneous activities of the guinea-pig taenia coli was investigated in the presence of an alpha-receptor blocking agent, phentolamine (2 × 10-6 M). Substitution of the external Na ion with Li, choline, tris-hydroxymethyl aminomethane or K ion greatly reduced the inhibitory effect of isoprenaline. A treatment with ouabain (4 × 10-6 M) also had effects similar to those of Na removal. The spontaneous activity was transiently suppressed when 5.9 mM K ion was re-admitted into K-free solution, and this suppressing effect was blocked by ouabain and also by reducing Na concentration to 5.9 mM. However, isoprenaline retained its effects in K-free solution containing normal Na concentration or 5.9 mM Na. The relaxation by isoprenaline in these solutions was hardly reduced by ouabain (4 × 10-6 M). These observations suggest that the beta-receptor stimulating effects of isoprenaline are partly mediated through an activation of the Na-pump and partly through some other mechanism which is inactivated when the preparation is deprived of Na or when the membrane is depolarized.
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