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 exhibited a complete rundown of ionic currents when cell-attached patches were excised. The alpha(1C) channel was expressed alone or was coexpressed with the accessory beta(2a) or beta(1b) subunit. The catalytic subunit of protein kinase A (PKAc) and MgATP were capable of delaying the run-down of single-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 and were not subject to any run-down, even after the complete run-down of ionic currents. The results presented here reveal that PKAc is capable of delaying the run-down of currents in a subset of patches. The patch-cramming results suggest that a cytoplasmic factor, in addition to phosphorylation of the channel (by PKAc), may be involved in the maintenance of channel activity.