Abstract 1508: Phosphoinositide-3-Kinase γ Regulates Heart Rate in vivo and Modulates Membrane Currents in Isolated Mouse Sinoatrial Node Cells
Regulation of intracellular cAMP concentration is critical in determining the chronotropic and inotropic status of the heart. Furthermore, altered cAMP metabolism in disease settings is a major contributing factor to the progression of heart failure. Ablation of the enzyme PI3Kγ (PI3Kγ− /−) in mice causes elevated intracellular cAMP levels and enhances contractility in ventricular myocytes by increasing sarcoplasmic reticulum Ca2+ load and release without altering L-type Ca2+ current (ICa,L). The regulation of heart rate by PI3Kγ has not been investigated in detail; therefore, the purpose of this study was to determine the role of PI3Kγ in heart rate regulation in vivo and in isolated sinoatrial node (SAN) myocytes, which determine heart rate under normal conditions. Although heart rate is similar in anesthetized PI3Kγ −/ − and PI3Kγ+/+ mice, significant differences became apparent when the sympathetic or parasympathetic branches of the nervous system were inhibited. Specifically, intraperitoneal injection of atropine (1 mg/kg) increased heart rate from approximately 540 ± 20 to 580 ± 20 beats/min in PI3Kγ+/+ mice (n =6) and from 530 ± 18 to 630 ± 14 beats/min in PI3Kγ −/ −mice (n =7). Propranolol (1 mg/kg) reduced heart rate from 520 ± 10 to 340 ± 15 beats/min in PI3Kγ+/+ mice (n =6), but only from 520 ± 12 to 420 ± 10 beats/min in PI3Kγ −/ − mice (n =6). These results establish that the intrinsic regulation of the SA node differs between PI3Kγ+/+ and PI3Kγ −/ − mice. To examine this further, firing rates were measured in isolated SAN myocytes. Spontaneous action potential rate was 25% greater in knockout mice (150 ± 6 beats/min; n =18 myocytes) compared to wildtype (119 ± 7 beats/min; n =13 myocytes). Voltage clamp experiments revealed these differences in rate are due to enhanced hyperpolarization-activated current (If), a rightward shift in the If activation curve, and enhanced ICa,L. These current changes are cAMP dependent because application of a cAMP inhibitor, Rp-cAMP (100 μM), reduced If and ICa,L in PI3Kγ −/ − mice to levels similar to wildtype mice. Therefore, PI3Kγ plays a significant role in the regulation of heart rate by modulating sinoatrial node function. Most intriguingly, the mechanism of this effect in the SA node is unique from that in the working myocardium.