Effect of tetraethylammonium, 4-aminopyridine, cesium and magnesium on slow responses in the guinea pig papillary muscle

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Abstract
Having inactivated the sodium conductance by K-rich media, propagated slow responses were triggered by local electrical stimulation after addition of catecholamines. We determined the smallest noradrenaline concentration eliciting slow responses (noradrenaline threshold) and it was found to be 1.04 × 10−6mol·litre−1 ± 0.25. The rate of rise and the amplitude of cardiac slow responses increase with the external concentration in calcium, however their development is also influenced by altering potassium conductances. Substances inhibiting gK: tetraetylammonium, 4-aminopyridine, and cesium increased the maximal rate of rise and the duration of slow responses and except for the latter compound, decreased the noradrenaline threshold. On the other hand Mg ions exerted an inhibitory effect on slow responses since they markedly decreased the maximal rate of depolarisation and increased the noradrenaline threshold in spite of a very small increase in the duration of slow responses. The effect of Mg ions can be explained by an inhibition of the slow inward current.

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