Deletion of amino-acid residues 1505-1507 (KPQ) in the cardiac SCN5A Na(+) channel causes autosomal dominant prolongation of the electrocardiographic QT interval (long-QT syndrome type 3 or LQT3). Excessive prolongation of the action potential at low heart rates predisposes individuals with LQT3 to fatal arrhythmias, typically at rest or during sleep. Here we report that mice heterozygous for a knock-in KPQ-deletion (SCN5A(Delta/+)) show the essential LQT3 features and spontaneously develop life-threatening polymorphous ventricular arrhythmias. Unexpectedly, sudden accelerations in heart rate or premature beats caused lengthening of the action potential with early afterdepolarization and triggered arrhythmias in Scn5a(Delta/+) mice. Adrenergic agonists normalized the response to rate acceleration in vitro and suppressed arrhythmias upon premature stimulation in vivo. These results show the possible risk of sudden heart-rate accelerations. The Scn5a(Delta/+) mouse with its predisposition for pacing-induced arrhythmia might be useful for the development of new treatments for the LQT3 syndrome.

Aqueous solutions of a number of metal complexes have been found to dissolve cellulose. Recently, a number of new metal complexes have been developed that completely dissolve cellulose by deprotonating and coordinative binding the hydroxyl groups in the C2 and C3 position of the anhydro glucose. A detailed comparative light scattering study is given for cellulose in Schweizer's reagent (cuoxam), Ni-tren, and Cd-tren. Cuoxam is the well-known solution of cupric hydroxide in aqueous ammonia, and the abbreviation tren stands for tris(2-aminoethyl)amine. Cuoxam and Ni-tren are deep blue solvents. The light scattering measurements were carried out with the blue line of an argon ion laser at wavelength λ 0) 457.9 nm, and the data from these solvents required an absorption correction according to the Lambert-Beer law. Cd-tren is almost colorless, and the data could be used without correction. Because of traces of colloid particles, possibly originating from the metal hydroxides, a special treatment for optical clarification became necessary. A large number of samples, cotton linters, various pulp celluloses, and bacterial celluloses, were studied. All three solvents exhibited good solution properties, but only Cd-tren was capable of dissolving also the highest degrees of polymerization of cotton linters and bacterial cellulose (DP w) 9700). The limits for the two other solvents were DPw < 6300 for Ni-tren and DPw < 5300 for cuoxam. A fairly high chain stiffness was found with Kuhn segment lengths of lK) 15.8 (1.4 nm for Cd-tren, lK) 10.2 (0.8 nm for Ni-tren, and lK) 13.1 (1.2 nm for cuoxam, corresponding to characteristic ratios of C∝) 24.6, 15.4, and 19.4, respectively. The problem of preferential adsorption is discussed.

Recent evidence suggests that biosynthesis of the human heart Na+ channel (hH1) protein is rapidly modulated by sympathetic interventions. However, data regarding the intracellular processing of hH1 in vivo are lacking. In this study we sought to establish a model that would allow us to study the subcellular localization of hH1 protein. Such a model could eventually help us to better understand the trafficking of hH1 in vivo and its potential role in cardiac conduction. We labeled the C-terminus of hH1 with the green fluorescent protein (GFP) and compared the expression of this construct (hH1-GFP) and hH1 in transfected HEK293 cells. Fusion of GFP to hH1 did not alter its electrophysiological properties. Confocal microscopy revealed that hH1-GFP was highly expressed in intracellular membrane structures. Immuno-electronmicrographs showed that transfection of hH1-GFP and hH1 induced proliferation of three types of endoplasmic reticulum (ER) membranes to accommodate the heterologously expressed proteins. Labeling with specific markers for the ER and the Golgi apparatus indicated that the intracellular channels are almost exclusively retained within the ER. Immunocytochemical labeling of the Na+ channel in dog cardiomyocytes showed strong fluorescence in the perinuclear region of the cells, a result consistent with our findings in HEK293 cells. We propose that the ER may serve as a reservoir for the cardiac Na+ channels and that the transport from the ER to the Golgi apparatus is among the rate-limiting steps for sarcolemmal expression of Na+ channels.

To identify the mechanisms underlying the faster activation kinetics in Kv1.2 channels compared to Kv2.1 channels, ionic and gating currents were studied in rat Kv1.2 and human Kv2.1 channels heterologously expressed in mammalian cells. At all voltages the time course of the ionic currents could be described by an initial sigmoidal and a subsequent exponential component and both components were faster in Kv1.2 than in Kv2.1 channels. In Kv1.2 channels, the activation time course was more sigmoid at more depolarized potentials, whereas in Kv2.1 channels it was somewhat less sigmoid at more depolarized potentials. In contrast to the ionic currents, the ON gating currents were similarly fast for both channels. The main portion of the measured ON gating charge moved before the ionic currents were activated. The equivalent gating charge of Kv1.2 ionic currents was twice that of Kv2.1 ionic currents, whereas that of Kv1.2 ON gating currents was smaller than that of Kv2.1 ON gating currents. In conclusion, the different activation kinetics of Kv1.2 and Kv2.1 channels are caused by rate-limiting reactions that follow the charge movement recorded from the gating currents. In Kv1.2 channels, the reaction coupling the voltage-sensor movement to the pore opening contributes to rate limitation in a voltage-dependent fashion, whereas in Kv2.1 channels, activation is additionally rate-limited by a slow reaction in the subunit gating.