Cytosolic myocardial calcium modulation by ATP-dependent potassium channel openers and NO donors

The goal of this study was to evaluate, in rat cardiomyocytes, the effects on cytosolic calcium of a pure K-adenosine triphosphate (ATP) channel opener, aprikalim, and those of nicorandil, a dual-acting agent that increases cyclic guanosine monophosphate (cGMP) levels and opens K-ATP channels. These effects were compared with those of a pure NO donor, 3-morpholino-sydnonimine (Sin-1). Ventricular myocytes were isolated from the hearts of adult rats. Changes in cytosolic calcium concentration ([Ca2+]i) were measured by using a Ca2+ indicator, indo-1/AM. Alterations in indo-1 fluorescence were recorded during regular electrical stimulation. After 10 min of pacing, end-diastolic [Ca2+]i was significantly increased as compared with control without significant changes in calcium transient. For doses of 10(-7) to 10(-4) M, aprikalim and nicorandil did not affect significantly the calcium transient. Sin-1 produced a significant decrease in calcium transient (by approximately 20%), which was already maximal at 10(-7) M. When given with the potassium channel antagonist glibenclamide (10(-5) M), nicorandil induced the same effects as those observed with Sin-1. We conclude that potassium channel openers aprikalim and nicorandil do not not decrease calcium transient. Thus the NO-donor properties of nicorandil are not apparent when given alone but appear when ATP-dependent potassium channels are blocked.