Ca++-cycling in hypertrophied cardiac myocytes

,J Mr,I

300

210

Cell

Car&l

24

(Supplement

V)

(1992)

October 2,199Z

Oral presentation I (Minisymposium

Regulation of Cardiac Hypertrophy

: A Target for Intervention

V): 830 - 10.10

?

MOLECULAR AND CELLULAR BIOLOGY OF HEART CONDUCTION SYSTEM. Luisa Gorza, Pompeo Volpe and Stefano Schiaffino. Department of Biomedical Sciences, University of Padova. and C.N.R. Unit for Muscle Biology and Physiopathology. Padova. ITALY. Cell biology of conduction tissue myocytes is poorly understood due to the paucity of these cells and the difficulty to isolate them from ordinary myocytes. Immunohistochemistry and in situ hybridization analysis show that specialized myocytes differ from ordinary myocytes in the composition of myofihrillar and cytoskeletal proteins. A unique cardiac myosin type, immunologically related to the embryonic skeletal myosin, is present in nodal tissue of bovine and rat hearts. Slow skeletal troponin I mRNA, that is expressed in ordinary myocytes only during development. accumulates in conduction tissue myocytes of the adult rat heart. Neurofilament subunits M and L are expressed in addition to dcsmm in rabbit conduction tissue, since early developmental stages. Thus, differcnccs in structural components may reflect the diversity of conduction versus ordinary myocytes either in ontogencrls or in function. Furthermore, conduction system myocytes are very heterogeneous, Large amounts of inositolphosphate (InsP) receptors are detected in a subset of conduction myocytes. Vescicles isolxed from bundles of bovine heart Purkinje fibers bind higher amounts 3H-InsP, as compared to VCSCIC~CS from ventricular myocardium. Strong reactivity for the InsP receptor in heart Purkinjc fibers of several mammalian species is detected at the protein level by Western blot analysis and immunohistochemistry and at the mRNA level by in situ hybridization. Higher density of InsP rcccptors in heart Purkinje fibers may be involved with changes in automaticity described in these myocytes following al-adrcnergic stimulation. IN HYPERTROPHIED CARDIAC MYOCYTES ca++- CYCLING Ruby U Nnqvi, Philip A Poole-Wilson & Kenneth T Macleod. Department of Cardiac Medicine, National Heart & Lung Institute, University of London, Dovehouse Street, London, SW3 6LY, U.K. Single cardiac myocytes isolated from hearts in failure show depressed contractility, slow Ca++transients with long relaxation times and prolonged action potential durations. The trigger for Cs++-release from the sarcoplasmic reticulum (SR), the dihydropyridine-sensitive Ca++-current. is still present so we have investigated whether SR Ca++-release and uptake and Ca*-efflux from the cell is altered. The experimental preparations used were ventricular myocytes isolated from normal guinea-pig hearts, hypertrophied cells, isolated from hearts of guinea-pigs not yet in failure and cells isolated from humans with heart failure. Cells were superfused with HEPES buffer (22°C) field-stimulated or voltage-clamped, contraction measured by an edge-detection system and intracellular Ca++ measured by fluorescent dyes. We have looked at the characteristic rest decay of contraction (and hence the rate of Ca++ loss from the SR), the size of the rapid cooling contracture (hence the amount of Cn+’ in the SR) and the speed of relaxation when the cells are re-warmed after the rapid cooling contracture (hence the SR Ca-uptake and sarcolemmal efflux mechanisms). The results will be discussed.

s.59