Effects of high K on relaxation produced by drugs in the guinea-pig tracheal muscle.

In the guinea-pig tracheal smooth muscle, effects of various relaxants were compared in normal (5.9 mM) and excess (40 mM) K media. The relaxing effect of calcium-channel blockers, nifedipine and verapamil (group I) was potentiated by increasing the external K concentration. The effect of the drugs which are supposed to increase intracellular cyclic AMP, such as isoprenaline, forskolin, isobutylmethylxanthine, theophylline, dibutyryl cyclic AMP (group II) was moderately reduced by excess K. Nitroprusside, 8-bromo-cyclic GMP and sodium nitrite (group III) are generally considered to increase intracellular cyclic GMP and their effect was markedly reduced by excess K. When the tension development was made the same at 5.9 mM K and 40 mM K by adjusting the Ca concentration, the relaxing effect was similar and independent of the K concentration both for group II and group III drugs. It seems that the group II drugs can better overcome a large influx of Ca than group III drugs.