title: Role of the transverse-axial tubule system in generating calcium sparks and calcium transients in rat atrial myocytes.
abstract: Cardiac atrial cells lack a regular system of transverse tubules like that in cardiac ventricular cells. Nevertheless, many atrial cells do possess an irregular internal transverse-axial tubular system (TATS). To investigate the possible role of the TATS in excitation-contraction coupling in atrial myocytes, we visualized the TATS (labelled with the fluorescent indicator, Di-8-ANEPPS) simultaneously with Ca2+ transients and/or Ca2+ sparks (fluo-4). In confocal transverse linescan images of field-stimulated cells, whole-cell Ca2+ transients had two morphologies: 'U-shaped' transients and irregular or 'W-shaped' transients with a varying number of points of origin of the Ca2+ transient. About half (54 %, n =289 cells, 13 animals) of the cells had a TATS. Cells with TATS had a larger mean diameter (13.2 +/- 2.8 microm) than cells without TATS (11.7 +/- 2.0 microm) and were more common in the left atrium (n = 206 cells; left atrium: 76 with TATS, 30 without TATS; right atrium: 42 with TATS, 58 without TATS). Simultaneous measurement of Ca2+ sparks and sarcolemmal structures showed that cells without TATS had U-shaped transients that started at the cell periphery, and cells with TATS had W-shaped transients that began simultaneously at the cell periphery and the TATS. Most (82 out of 102 from 31 cells) 'spontaneous' (non-depolarized) Ca2+ sparks occurred within 1 microm of a sarcolemmal structure (cell periphery or TATS), and 33 % occurred within 1 pixel (0.125 microm). We conclude that the presence of a sarcolemmal membrane either at the cell periphery or in the TATS in close apposition to the sarcoplasmic reticulum is required for the initiation of an evoked Ca2+ transient and for spontaneous Ca2+ sparks.
authors.affiliation: Department of Medicine, University of Maryland School of Medicine, Baltimore, MD 21201-1541, USA.
authors.forename: Malcolm M
authors.initials: MM
authors.lastname: Kirk
authors.forename: Leighton T
authors.initials: LT
authors.lastname: Izu
authors.forename: Ye
authors.initials: Y
authors.lastname: Chen-Izu
authors.forename: Stacey L
authors.initials: SL
authors.lastname: McCulle
authors.forename: W Gil
authors.initials: WG
authors.lastname: Wier
authors.forename: C William
authors.initials: CW
authors.lastname: Balke
authors.forename: Stephen R
authors.initials: SR
authors.lastname: Shorofsky
mesh_terms: D000818:Animals; D002118:Calcium / Q000378:metabolism*; D048429:Cell Size; D005456:Fluorescent Dyes; D006325:Heart Atria; D032383:Myocytes, Cardiac / Q000166:cytology / Q000378:metabolism* / Q000648:ultrastructure; D011726:Pyridinium Compounds; D051381:Rats; D017207:Rats, Sprague-Dawley; D012508:Sarcolemma / Q000502:physiology / Q000648:ultrastructure; D012519:Sarcoplasmic Reticulum / Q000378:metabolism* / Q000648:ultrastructure
chemical_list: C089669:1-(3-sulfonatopropyl)-4-(beta-(2-(di-n-octylamino)-6-naphthyl)vinyl)pyridinium betaine; D005456:Fluorescent Dyes; D011726:Pyridinium Compounds; D002118:Calcium