Loss-of-activity-mutation in the cardiac chloride-bicarbonate exchanger AE3 causes short QT syndrome

Mutations in potassium and calcium channel genes have been associated with cardiac arrhythmias. Here, Jensen et al. show that an anion transporter chloride-bicarbonate exchanger AE3 is also responsible for the genetically-induced mechanism of cardiac arrhythmia, suggesting new therapeutic targets for this diseas

Role of cyclic nucleotide phosphodiesterase isoforms in cAMP compartmentation following β2-adrenergic stimulation of ICa,L in frog ventricular myocytes

The role of cyclic nucleotide phosphodiesterase (PDE) isoforms in the β2-adrenergic stimulation of the L-type Ca2+ current (ICa,L) was investigated in frog ventricular myocytes using double patch-clamp and double-barrelled microperfusion techniques. Isoprenaline (ISO, 1 nM to 10 μM) was applied on one half of the cell, either alone or in the presence of PDE inhibitors, and the local and distant responses of ICa,L were used to determine the gradient of local vs. distant cAMP concentration (α). IBMX (100 μM), a non-selective PDE inhibitor, reduced α from 40 to 4.4 indicating a 9-fold reduction in intracellular cAMP compartmentation when all PDE activity was blocked. While PDE1 and PDE2 inhibition had no effect, PDE3 inhibition by milrinone (3 μM) or PDE4 inhibition by Ro 20-1724 (3 μM) reduced α by 6- and 4-fold, respectively. A simultaneous application of milrinone and Ro 20-1724 produced a similar effect to IBMX, showing that PDE3 and PDE4 were the major PDEs accounting for cAMP compartmentation. Okadaic acid (3 μM), a non-selective phosphatase inhibitor, or H89 (1 μM), an inhibitor of cAMP-dependent protein kinase (PKA), had no effect on the distant response of ICa,L to ISO indicating that PDE activation by PKA played a minor role in cAMP compartmentation. Our results demonstrate that PDE activity determines the degree of cAMP compartmentation in frog ventricular cells upon β2-adrenergic stimulation. PDE3 and PDE4 subtypes play a major role in this process, and contribute equally to ensure a functional coupling of β2-adrenergic receptors with nearby Ca2+ channels via local elevations of cAMP

Pharmacological characterization of the receptors involved in the β-adrenoceptor-mediated stimulation of the L-type Ca(2+) current in frog ventricular myocytes

1. The whole-cell patch-clamp was used for studying the effects of various β(1)- and β(2)-adrenoceptor agonists and antagonists on the L-type Ca current (I(Ca)) in frog ventricular myocytes. 2. Dose-response curves for the effects of isoprenaline (non selective β-agonist), salbutamol (β(2)-agonist), dobutamine (β(1)-agonist) on I(Ca) were obtained in the absence and presence of various concentrations of ICI 118551 (β(2)-antagonist), metoprolol (β(1)-antagonist) and xamoterol (partial β(1)-agonist) to derive EC(50) (i.e. the concentration of β-agonist at which the response was 50% of the maximum) and E(max) (the maximal response) values by use of a Michaelis equation. Schild regression analysis was performed to examine whether the antagonists were competitive and to determine the equilibrium dissociation constant (K(B)) for the antagonist-receptor complex. 3. Isoprenaline increased I(Ca) with an EC(50) of 20.0 nM and an E(max) of 597%. ICI 118551 and metoprolol competitively antagonized the effect of isoprenaline with a K(B) of 3.80 nM and 207 nM, respectively. 4. Salbutamol increased I(Ca) with an EC(50) of 290 nM and an E(max) of 512%. ICI 118551 and metoprolol competitively antagonized the effect of salbutamol with a K(B) of 1.77 nM and 456 nM, respectively. 5. Dobutamine increased I(Ca) with an EC(50) of 2.40 μM and an E(max) of 265%. ICI 118551 and metoprolol competitively antagonized the effect of dobutamine with a K(B) of 2.84 nM and 609 nM, respectively. 6. Xamoterol had no stimulating effect on I(Ca). However, xamoterol competitively antagonized the stimulating effects of isoprenaline, salbutamol and dobutamine on I(Ca) with a K(B) of 58–64 nM. 7. We conclude that a single population of receptors is involved in the β-adrenoceptor-mediated regulation of I(Ca) in frog ventricular myocytes. The pharmacological pattern of the response of I(Ca) to the different β-adrenoceptor agonists and antagonists tested suggests that these receptors are of the β(2)-subtype

β3-adrenergic receptor activation increases human atrial tissue contractility and stimulates the L-type Ca2+ current

β3-adrenergic receptor (β3-AR) activation produces a negative inotropic effect in human ventricles. Here we explored the role of β3-AR in the human atrium. Unexpectedly, β3-AR activation increased human atrial tissue contractility and stimulated the L-type Ca2+ channel current (ICa,L) in isolated human atrial myocytes (HAMs). Right atrial tissue specimens were obtained from 57 patients undergoing heart surgery for congenital defects, coronary artery diseases, valve replacement, or heart transplantation. The ICa,L and isometric contraction were recorded using a whole-cell patch-clamp technique and a mechanoelectrical force transducer. Two selective β3-AR agonists, SR58611 and BRL37344, and a β3-AR partial agonist, CGP12177, stimulated ICa,L in HAMs with nanomolar potency and a 60%–90% efficacy compared with isoprenaline. The β3-AR agonists also increased contractility but with a much lower efficacy (~10%) than isoprenaline. The β3-AR antagonist L-748,337, β1-/β2-AR antagonist nadolol, and β1-/β2-/β3-AR antagonist bupranolol were used to confirm the involvement of β3-ARs (and not β1-/β2-ARs) in these effects. The β3-AR effects involved the cAMP/PKA pathway, since the PKA inhibitor H89 blocked ICa,L stimulation and the phosphodiesterase inhibitor 3-isobutyl-1-methylxanthine (IBMX) strongly increased the positive inotropic effect. Therefore, unlike in ventricular tissue, β3-ARs are positively coupled to L-type Ca2+ channels and contractility in human atrial tissues through a cAMP-dependent pathway