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A cytoplasmic factor, calpastatin and ATP together reverse run-down of Ca²⁺ channel activity in guinea-pig heart

Li-Ying Hao, Asako Kameyama and Masaki Kameyama

The cytoplasmic extract of bovine heart was separated into four fractions by gel filtration: H (molecular mass > 300 kDa), P (250-300 kDa), L₁ (180-250 kDa) and L₂ (< 180 kDa). The effects of these fractions on the run-down of L-type Ca²⁺ channel activity were investigated in guinea-pig ventricular myocytes.

After run-down induced by inside-out patch formation, Ca²⁺ channel activity was restored by P or H (+ 3 mM ATP) to 7·5 and 5·8 % of that in the cell-attached mode, respectively, but to as high as 86 % by P + H + ATP.

The reversal of run-down brought about by the P fraction was mimicked by calpastatin.

The restorative effect of calpastatin + ATP showed a biphasic time course: 38 % in the early transient phase and 11 % in the late phase. However, calpastatin + H + ATP showed a sustained effect: 66 % in the early transient phase, and 87 % in the late phase.

The effective component of the H fraction showed a protein-like nature: heat and trypsin sensitivity.

The activities of cAMP-dependent protein kinase, casein kinase I, casein kinase II, protein tyrosine kinase, protein serine/threonine or tyrosine phosphatases were measured. However, these kinases and phosphatases were not confirmed as the effective component of cytoplasm or the H fraction.

Run-down was not prevented by 2 µM phalloidin or 2 µM paclitexal, suggesting that neither actin filaments nor microtubules are directly involved in the run-down.

Our results support the view that the basal activity of the Ca²⁺ channel is maintained by at least three factors: a protein-like factor in the H fraction, calpastatin, and ATP.

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