Chronic endurance exercise increased AF susceptibility in rats, with autonomic changes, atrial dilation, and fibrosis identified as potential mechanistic contributors. Vagal promotion is particularly important and occurs via augmented baroreflex responsiveness and increased cardiomyocyte sensitivity to cholinergic stimulation, possibly due to RGS protein downregulation.

Activation of the K ATP channels results in faster fatigue rates as the channels depress action potential amplitude, whereas abolishing the channel activity has no effect in whole extensor digitorum longus (EDL) and soleus muscles. In this study, we examined the effects of abolished K ATP channel activity during fatigue at 37• C on free intracellular Ca 2+ (Ca 2+ i ) and tetanic force using single muscle fibres and small muscle bundles from the flexor digitorum brevis (FDB). K ATP channel deficient muscle fibres were obtained (i) pharmacologically by exposing wild-type fibres to glibenclamide, and (ii) genetically using null mice for the Kir6.2 gene (Kir6.2 −/− mice). Fatigue was elicited using 200 ms tetanic contractions every second for 3 min. This study demonstrated for the first time that abolishing K ATP channel activity at 37• C resulted in faster fatigue rates, where decreases in peak Ca 2+ i and tetanic force were faster in K ATP channel deficient fibres than in control wild-type fibres. Furthermore, several contractile dysfunctions were also observed in K ATP channel deficient muscle fibre. They included partially or completely supercontracted single muscle fibres, greater increases in unstimulated Ca 2+ i and unstimulated force, and lower force recovery. We propose that the observed faster rate of fatigue in K ATP channel deficient fibres is because the decreases in peak Ca 2+ i and force caused by contractile dysfunctions prevail over the expected slower decreases when the channels do not depress action potential amplitude.

Abstract-Heart rate is controlled by the opposing activities of sympathetic and parasympathetic inputs to pacemaker myocytes in the sinoatrial node (SAN). Parasympathetic activity on nodal myocytes is mediated by acetylcholinedependent stimulation of M 2 muscarinic receptors and activation of G␣ i/o signaling. Although regulators of G protein signaling (RGS) proteins are potent inhibitors of G␣ i/o signaling in many tissues, the RGS protein(s) that regulate parasympathetic tone in the SAN are unknown. Our results demonstrate that RGS4 mRNA levels are higher in the SAN compared to right atrium. Conscious freely moving RGS4-null mice showed increased bradycardic responses to parasympathetic agonists compared to wild-type animals. Moreover, anesthetized RGS4-null mice had lower baseline heart rates and greater heart rate increases following atropine administration. Retrograde-perfused hearts from RGS4-null mice showed enhanced negative chronotropic responses to carbachol, whereas SAN myocytes showed greater sensitivity to carbachol-mediated reduction in the action potential firing rate. Finally, RGS4-null SAN cells showed decreased levels of G protein-coupled inward rectifying potassium (GIRK) channel desensitization and altered modulation of acetylcholine-sensitive potassium current (I KACh ) kinetics following carbachol stimulation. Taken together, our studies establish that RGS4 plays an important role in regulating sinus rhythm by inhibiting parasympathetic signaling and I KACh activity. (Circ Res. 2008;103:527-535.)Key Words: RGS proteins Ⅲ sinoatrial node Ⅲ parasympathetic Ⅲ GIRK channels H eart rate (HR) regulation by the autonomic nervous system is integrated by specialized autorhythmic (pacemaker) cells located within the sinoatrial node (SAN). Sympathetic neurotransmitters work via G s -coupled ␤-adrenergic receptors to increase adenylyl cyclase activity, intracellular cAMP concentration and protein kinase A activity. As a result, cAMP-regulated effectors such as hyperpolarizationactivated cyclic nucleotide-gated cation (HCN) channels, delayed rectifier, and voltage-gated Ca 2ϩ channels are enlisted by sympathetic activity to increase pacemaker cell firing rate. 1,2 By contrast, vagal parasympathetic activity decreases HR via G␣ i/o -coupled cholinergic M 2 muscarinic receptors (M 2 Rs). Several effects, mediated by both G␣ i/o and G␤␥ subunits, may contribute to this reduction in HR. G␤␥ heterodimers directly activate G protein-coupled inward rectifying potassium (GIRK) channels, resulting in membrane hyperpolarization. By contrast, G␣ i/o can both modulate phosphodiesterase activity and inhibit adenylyl cyclase activity, reduce both intracellular cAMP levels and protein kinase A activity, leading to decreased depolarizing currents carried by HCN and L-type calcium channels. [2][3][4][5] Because dysregulation of parasympathetic activity occurs in heart failure, sick sinus syndrome, and selected cardiac arrhythmias, 6 it is of clinical interest to identify key molecular regulators of parasympathetic signa...