No abstract

Blockade of L-type Ca2+ channels in spontaneously contracting cultured neonatal rat ventricular myocytes causes contractile arrest, myofibrillar disassembly, and accelerated myofibrillar protein turnover. To determine whether myofibrillar protein turnover. To determine whether myofibrillar atrophy results indirectly from loss of mechanical signals or directly from alterations in intracellular Ca2+ concentration ([Ca2+]i), contractile activity was inhibited with verapamil (10 microM) or 2,3-butanedione monoxime (BDM), and their effects on cell shortening, [Ca2+]i, and myosin heavy chain (MHC) turnover were assessed. Control cells demonstrated spontaneous [Ca2+]i transients (peak amplitude 232 +/- 15 nM, 1-2 Hz) and vigorous contractile activity. Verapamil inhibited shortening by eliminating spontaneous [Ca2+]i transients. Low concentrations of BDM (5.0-7.5 mM) had no effect on basal or peak [Ca2+]i transient amplitude but reduced cell shortening, whereas 10 mM BDM reduced both [Ca2+]i transient amplitude and shortening. Both agents inhibited MHC synthesis, but only verapamil accelerated MHC degradation. Thus MHC half-life does not change in parallel with contractile activity but rather more closely follows changes in [Ca2+]i. [Ca2+]i transients appear critical in maintaining myofibrillar assembly and preventing accelerated MHC proteolysis.

Capacitative Ca(2+) entry is a recently discovered Ca(2+) entry pathway that is activated on depletion of intracellular Ca(2+) stores, providing an avenue for store refilling. Despite recent progress in elucidating the capacitative Ca(2+) entry pathway, the mysteries of its molecular identity, its biophysical properties, and the store depletion signal remain.

No abstract

Ca¥ release was initiated at the site of stimulation and frequently propagated some tens of micrometres into non-stimulated regions. Local Ca¥ release caused activation of capacitative Ca¥ entry (CCE). CCE was initiallycolocalized with Ca¥ release. Following repetitive focal stimulation, however, CCE became detectable at remote sites where no Ca¥ release had been observed. In addition, the rate of Ca¥ store depletion with repetitive local activation of release in Ca¥-free solution was markedly slower than that elicited by ATP stimulation of the entire cell. 4. From these experiments it is concluded that both intracellular IP×-dependent Ca¥ release and activation of CCE are controlled locally at the subcellular level. Moreover, redistribution of intracellular Ca¥ stored within the endoplasmic reticulum efficiently counteracts local store depletion and accounts for the spatial spread of CCE activation.