Complete reversal of run down in rabbit cardiac Ca2+ channels by patch-cramming in Xenopus oocytes; partial reversal by protein kinase A

The rabbit cardiac Ca2+ channel (alpha(1C)) expressed in Xenopus oocytes ex hibited a complete rundown of ionic currents when cell-attached patches wer e excised. The alpha(1C) channel was expressed alone or was coexpressed wit h the accessory beta(2a) or beta(1b) subunit. The catalytic subunit of prot ein kinase A (PKAc) and MgATP were capable of delaying the run-down of sing le-channel currents. In 33% of the alpha(1C) patches, and 26% of the alpha( 1C) + beta(2a) patches, inclusion of PKAc in the bath solution delayed the run-down for a maximum of 20 min. In experiments where PKAc in the bath was not sufficient to delay the run-down of channel activity, insertion of the patch back into the oocyte (patch-cramming) could restore channel activity . Gating currents were also measured in the alpha(1C) + beta(1b) channel an d were not subject to any run-down, even after the complete run-down of ion ic currents. The results presented here reveal that PKAc is capable of dela ying the run-down of currents in a subset of patches. The patch-cramming re sults suggest that a cytoplasmic factor, in addition to phosphorylation of the channel (by PKAc), may be involved in the maintenance of channel activi ty.