Introduction: Atrial fibrillation (AF) is the most common tachyarrhythmia in humans and causes thromboembolic stroke, which is the most serious complication of AF. Canine atrial cardiomyocytes have a constitutively active acetylcholine-regulated K+ current (IKH) that is enhanced by atrial tachycardia (AT). It has showed that IKH contributes to action potential (AP) duration (APD) shortening in AT remodeled atrial cardiomyocytes; APD regulates cardiomyocyte contractility by controlling systolic Ca2+ level. This study investigated the role of IKH in the contractile dysfunction of single atrial cardiomyocyte. Methods: Dogs were divided into 2 groups: (1) unpaced control (CTL); (2) 1 week AT (atrial tachypacing (ATP): 400 bpm x 7 dats). Tertiapin Q (TQ), a highly-selective IKH blocker, was used to define IKH contributions in contractility. Single cell left atrial intracellular Ca2+ transient and cell-shortening were measured with microfluorimetry. Results: Ca2+ transient and cell-shortening were significantly decreased in ATP compare to CTL (at 1Hz 292짹35 vs. 147짹15, 7.8짹1.3 vs. 1.5짹0.3; both p < 0.001). Ca2+ transient amplitude and cell-shortening were significantly increased after TQ administration compare with baseline in AT cells (at 1Hz 65짹6 vs. 111짹16, 1.5짹0.3 vs. 2.6짹0.5; p < 0.05, p < 0.001 respectively, all data in arbitrary unit). TQ decreased basal calcium transient level in control and AT cells. Conclusions: Upregulation of IKH is associated with atrial contractile dysfunction and systolic Ca2+ entry abnormalities in atrial tachycardia remodeled atrial cardiomyocytes. Highly selective IKH blocker, TQ reverses AT-induced atrial contractile dysfunction and Ca2+ transient abnormality. Thus, IKH may be a novel target for the prevention of AF-related atrial contractile dysfunction.