Phosphorylation restores activity of L‐type calcium channels after rundown in inside‐out patches from rabbit cardiac cells.

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Abstract
1. Rundown of L-type calcium channels was studied in inside-out patches made from single isolated rabbit ventricular myocytes, using barium as the charge carrier. 2. In the cell-attached patches single-channel activity was stable for more than 15 min after the patch pipette sealed. beta-Receptor stimulation by isoprenaline caused a characteristic increase in opening probability and the appearance of prolonged openings. When the patch was excised to the inside-out configuration and exposed to a simple ionic solution, channel activity disappeared within 1-2 min and never reappeared spontaneously. 3. After rundown of L-type channel activity in the excised patch, exposure of the inside face of the patch to MgATP and the catalytic subunit of the cyclic AMP-dependent protein kinase (PKAc) resulted in recovery of Ca2+ channel activity. Under these conditions channel activity could be even greater than under control cell-attached conditions, resembling channel activity after exposure to isoprenaline. This recovery of activity persisted many minutes, usually until the patch was lost. Addition of MgATP alone caused a small transient increase in channel activity in some patches. 4. Recovery of activity by MgATP and PKAc could be prevented by prior exposure of the excised patch to protein kinase inhibitor (PKI), or it could be abruptly terminated by exposure to PKI after recovery of activity. Addition to the pipette solution of okadaic acid, a protein phosphatase inhibitor, greatly slowed rundown. These findings support the proposal that dephosphorylation is an important component of rundown, and that phosphorylation is needed for channel opening activity. 5. Single-channel conductance was not altered by patch excision, but it was reduced after exposure of the excised patch to MgATP and PKAc. Mg2+ was responsible for this effect, probably by direct channel block from the inside, and Mg2+ also caused a negative shift in the channel activation, as expected from shielding of inside fixed negative charges.