Acetylcholine (ACh) increases the amplitude of cardiac K⁺ current (I_K.ACh) through activation of the muscarinic K⁺ (K_ACh) channel, but the amplitude gradually decreases to a quasi-steady-state level within seconds. This phenomenon, known as short-term desensitization, is believed to play a role in cellular adaptation to external input. However, the precise mechanism and physiological role of short-term desensitization is still unclear. In the present review, we introduced our experimental and theoretical studies on the mechanisms underlying short-term desensitization of I_K.ACh. In atrial myocytes, short-term desensitization features the unique dose responses of the transient and quasi-steady state I_K.ACh, effects of ACh preperfusion and recovery from short-term desensitization. In simulation analysis, two conditions are required for the mathematical I_K.ACh model to reconstitute short-term desensitization. The first condition is distinct muscarinic receptors (M₂Rs) with different affinities for ACh, which conferred an I_K.ACh response over a wide range of ACh concentrations. The second condition is 2 distinct K_ACh channels with different affinities for the G-proteinβγsubunit, which contributes to reconstitution of the temporal behavior of I_K.ACh. Under these conditions, the model quantitatively reproduced the unique properties of short-term desensitization observed in experiments. Furthermore, the present model conferred vagal escape on the mathematical action potential models of sinus node cells. Therefore, 2 different populations of K_ACh channels and M₂Rs may be responsible for short-term desensitization and vagal escape at nodal cells.

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We report a case of atrial tachycardia due to pregnancy. A 37 years old woman was admitted to our hospital for the treatment of palpitation. She was aware of palpitation from the 15th week of pregnancy, but her condition was diagnosed as sinus tachycardia. The electrocardiogram on admission showed long RP'tachycardia (170/min), which was difficult to terminate with ATP, Pilsicainide and Verapamil. β blocker was effective with this patient, and she was able to give birth at 38weeks of pregnancy without any difficulties. Atrial tachycardia has not recurred. The exact mechanism of increased arrhythmia burden during pregnancy is unclear, but has been attributed to autonomic changes related to pregnancy. We concluded that a β blocker was effective in this case.

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