Fabien Brette

T-tubule function in mammalian cardiac myocytes

Abstract —The transverse tubules (t‐tubules) of mammalian cardiac ventricular myocytes are invaginations of the surface membrane. Recent studies have suggested that the structure and function of the t‐tubules are more complex than previously believed; in particular, many of the proteins involved in cellular Ca2+ cycling appear to be concentrated at the t‐tubule. Thus, the t‐tubules are an important determinant of cardiac cell function, especially as the main site of excitation‐contraction coupling, ensuring spatially and temporally synchronous Ca2+ release throughout the cell. Changes in t‐tubule structure and protein expression occur during development and in heart failure, so that changes in the t‐tubules may contribute to the functional changes observed in these conditions. The purpose of this review is to provide an overview of recent studies of t‐tubule structure and function in cardiac myocytes. (Circ Res. 2003;92:1182–1192.)

Na+-Ca2+ Exchange Activity Is Localized in the T-Tubules of Rat Ventricular Myocytes

Abstract— Detubulation of rat ventricular myocytes has been used to investigate the role of the t-tubules in Ca2+ cycling during excitation-contraction coupling in rat ventricular myocytes. Ca2+ was monitored using fluo-3 and confocal microscopy. In control myocytes, electrical stimulation caused a spatially uniform increase in intracellular [Ca2+] across the cell width. After detubulation, [Ca2+] rose initially at the cell periphery and then propagated into the center of the cell. Application of caffeine to control myocytes resulted in a rapid and uniform increase of intracellular [Ca2+]; the distribution and amplitude of this increase was the same in detubulated myocytes, although its decline was slower. On application of caffeine to control cells, there was a large, rapid, and transient rise in extracellular [Ca2+] as Ca2+ was extruded from the cell; this rise was significantly smaller in detubulated cells, and the remaining increase was blocked by the sarcolemmal Ca2+ ATPase inhibitor carboxyeosin. The treatment used to produce detubulation had no significant effect on Ca2+ efflux in atrial cells, which lack t-tubules. Detubulation of ventricular myocytes also resulted in loss of Na+-Ca2+ exchange current, although the density of the fast Na+ current was unaltered. It is concluded that Na+-Ca2+ exchange function, and hence Ca2+ efflux by this mechanism, is concentrated in the t-tubules, and that the concentration of Ca2+ flux pathways in the t-tubules is important in producing a uniform increase in intracellular Ca2+ on stimulation.

K+ current distribution in rat sub-epicardial ventricular myocytes

Abstract. In the rat ventricle, the transient outward K+ current (ITO) is carried by heteromeric channels composed of Kv4.2 and Kv4.3. However its distribution in the cell membrane is unclear: immunohistochemical studies of Kv4.2 distribution in the cardiac ventricular cell membrane have given equivocal results, and there are no corresponding studies of Kv4.3. We therefore used detubulated cardiac cells to investigate the functional distribution of ITO between the t-tubules and surface membrane. ITO, the delayed rectifier (IK), the inward rectifier (IK1) and steady-state (ISS) K+ currents were monitored using the patch-clamp technique in control and formamide-treated (detubulated) cells from rat left ventricular sub-epicardium. Formamide treatment decreased cell capacitance by 20%, did not significantly change the density of ITO, IK or IK1 but decreased the density of ISS and L-type Ca current (ICa). These data suggest that ITO, IK, and IK1 are uniformly distributed between the surface and t-tubule membranes, but that ISS and ICa are concentrated in the t-tubules.