AJP - Cell Physiology, Vol 263, Issue 3 C642-C652, Copyright © 1992 by American Physiological Society
Contractile arrest accelerates myosin heavy chain degradation in neonatal rat heart cells
A. M. Samarel, M. L. Spragia, V. Maloney, S. A. Kamal and G. L. Engelmann
Department of Medicine, Loyola University Stritch School of Medicine, Maywood, Illinois 60153.
Mechanical forces influence the growth and metabolism of a variety of
cells, including cultured neonatal rat ventricular myocytes. To determine
whether mechanical activity affected the synthesis and turnover of myosin
heavy chain (MHC) in these striated muscle cells, MHC fractional
degradative rates were measured in spontaneously beating cells and in
arrested myocytes in which contractile activity was prevented by L-channel
blockade (with verapamil, nifedipine, nisoldipine, and diltiazem) or K+
depolarization. MHC degradative rates were measured as the difference
between rates of MHC synthesis and accumulation and in pulse-chase
biosynthetic labeling experiments. Both methods indicated that contractile
arrest markedly increased MHC degradation. Contractile arrest produced by
L-channel blockade accelerated MHC degradation to a greater extent than K+
depolarization. The signal transduction pathway linking contractile
activity to alterations in MHC degradation did not involve protein kinase C
(PKC), because MHC degradation was unaffected by activating PKC in arrested
cells or inhibiting PKC in spontaneously beating cells. Chloroquine and
E-64 did not suppress the accelerated MHC degradation, suggesting that the
rate-limiting step in MHC turnover occurred before degradative processing
by cellular proteinases. Using a computer simulation, we hypothesize that
the rate-limiting step in MHC turnover preceded (or was coincident with)
MHC release from thick filaments. Thus mechanical forces may influence MHC
half-life by regulating the rate of myosin disassembly.
