Abstract
The effect of histamine on electrical (recorded intracellularly) and mechanical (isometric contraction) activity of the auricular myocardial fibers of the guinea pig atrium was investigated after preliminary blocking of the fast sodium channels by prolonged depolarization with K+ ions (20 mM) or with tetrodotoxin (3.2×10−7−2.0×10−6 g/ml). Slowly increasing gradual responses to short stimuli were found to appear in the depolarized fibers. Histamine (10 μg/ml) increased the amplitude and duration of these responses, and at the same time the amplitude of the contractions of the preparation also increased. Compound D-600 (1 mg/liter), which specifically blocks calcium channels, inhibited the slow responses and mechanical activity of the preparation. After administration of tetrodotoxin alone orin combination with KCl, histamine also had a potentiating action of its own. It is concluded that histamine activates the slow sodium-calcium channels of the myocardial excitable membrane.
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