Run-down of the cardiac Ca2+ channel: characterization and restoration of channel activity by cytoplasmic factors

Possible mechanisms for run-down in the Ca2+ channel, such as proteolysis or dephosphorylation of the channel, were examined in guinea-pig ventricular myocytes. The Ca2+ channel current, recorded in inside-out patches using a pipette solution containing 50 mM Ba2+ and 3 μM Bay K 8644, ran down with a mean survival time of 2.35 min. The survival time was not significantly affected by adenosine triphosphate (ATP) (3 mM), 1,2-bis(2-aminophenoxy)ethane-N,N,N′,N′-tetraacetic acid (BAPTA) (2 mM), isoprenaline (l–5 μM), phosphate (l20 mM) and leupeptin (l0 μM). Stimulation of guanosine triphosphate (GTP)-binding proteins was also ineffective. The catalytic subunit of adenosine 3′,5′-cyclic monophosphate (cAMP)-dependent protein kinase (PKA, 0.5–2 μM) slightly and transiently increased channel activity, but had minimal effects on the channel when applied after complete run-down. On the other hand, cytoplasm from the heart, skeletal muscle, brain and liver, but not kidney, induced channel activity. There was a positive correlation between NPo (the product of the number of channels N and the open probability Po) value before run-down and that after the application of cytoplasm, suggesting that the activity of once-active channels was restored ba the exogenous cytoplasm. The potency of cytoplasm in tissues in inducing channel activity was not related to PKA activity nor to the number of dihydropyridine binding sites. These results suggest that the run-down of the cardiac Ca2+ channel is not mediated by dephosphorylation or proteolysis of the channel, but involves other factor(s), possibly interaction of the channel protein with a cytoplasmic regulatory protein.