SK Current, beta-3 adrenoceptor activation and Sex Differences in Ventricular Arrhythmogenesis

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Project Summary The broad and long-term goal of this research project is to test the hypothesis that (1) the response of apamin- sensitive small conductance Ca2+-activated K+ (SK) current (IKAS) to sympathetic stimulation plays an important role in the mechanisms of sex differences in cardiac arrhythmogenesis and that (2) beta-3 adrenoceptor (AR) activation is antiarrhythmic and eliminates the sex difference of IKAS activation during sympathetic stimulation. In the current funding period, we discovered a significant sex difference of both the SK protein and IKAS at baseline and that the magnitudes of sex differences are amplified during isoproterenol or acetylcholine infusion. Apamin reduces the phase singularities (PSs) in ventricular fibrillation (VF) more effectively in the female than in the male ventricles. It is unclear which AR is responsible for the isoproterenol-induced IKAS activation. Isoproterenol activates the beta-1 and beta-2 ARs at lower concentrations than the beta-3 AR. Contrary to the effects of beta-1 and beta-2 AR activation, beta-3 AR activation exerts negative inotropic effects in the myocardium. Beta-3 ARs likely served as a buffer or rescuer of the excess catecholamines. We showed in a canine model of ventricular tachycardia that BRL-37344 (a beta-3 agonist) is antiarrhythmic. We hypothesize that beta-3 AR activation is antiarrhythmic and eliminates the sex difference of IKAS activation during sympathetic stimulation in the rabbit model. Specific Aim 1: SK current, beta-3 adrenoceptor activation, and Sex Differences in Ventricular Arrhythmogenesis. We will perform optical mapping and patch clamp studies with specific beta-1 and beta-2 antagonists in the presence of isoproterenol. We also will perform optical mapping studies to determine the effects of mirabegron on phase singularities (PSs) during VF with and without isoproterenol. In additional experiments, we will use BRL-37344 instead of mirabegron to test the hypothesis that this antiarrhythmic mechanism is a class effect shared by other beta-3 agonists. The antiarrhythmic action of mirabegron will then be tested in a rabbit model of myocardial infarction. These findings will be used to test the hypothesis that beta-3 AR activation rescues the proarrhythmic effects of beta- 1 and beta-2 stimulation, indirectly reduce IKAS, and eliminates the sex differences of PSs in VF. Specific Aim 2: Na+-K+ ATPase (NKA) activation during beta3 stimulation contributes to the sex differences of antiarrhythmic action. Beta-3 AR activates NKA, which may reduce Ca2+ overload-induced cardiac arrhythmias. We hypothesize that NKA activation plays a vital role in the antiarrhythmic effects of beta-3 AR activation. We will first perform studies to determine if there is a sex difference in beta-3 AR, NKA protein, and mRNA in rabbit ventricles. Optical mapping studies will be done in the presence of ouabain, a specific NKA blocker, to determine if the company of NKA is required for the antiarrhythmic effects of beta-3 activation. Rabbit ventricular action potential models will be used for computer simulation studies to assess the importance of NKA activation in the antiarrhythmic mechanisms of beta-3 AR activation.