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

Abstract

SUMMARY1. 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. f-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 AMPdependent protein kinase (PKAc) resulted in recovery of Ca21 channel activity. Under these conditions channel activity could be even greater than under control cell-attached conditions, resembling channel activity after exposure to isoprenaline.

Mechanisms of L-Type Ca 2+ Current Downregulation in Rat Atrial Myocytes During Heart Failure

“…26 In addition, it has been suggested that Ca 2ϩ channels in cardiac myocytes are phosphorylated in the absence of any neurohormonal stimulation of the cell and that this basal phosphorylation is necessary to maintain normal channel function. 25,27 This is illustrated by the observation that the rundown of Ca 2ϩ channels following membrane patch excision is reversed by application of MgATP and PKA at the inside face of the membrane. 27 The present finding that interventions known to stimulate cAMP-dependent phosphorylation directly (␤-adrenergic cell exposure, intracellular cAMP application) or indirectly (phosphatase inhibition) restore I Ca in HF myocytes indicates that reduced basal cAMP-dependent regulation of I Ca could be an important mechanism underlying its decrease during HF.…”

“…25,27 This is illustrated by the observation that the rundown of Ca 2ϩ channels following membrane patch excision is reversed by application of MgATP and PKA at the inside face of the membrane. 27 The present finding that interventions known to stimulate cAMP-dependent phosphorylation directly (␤-adrenergic cell exposure, intracellular cAMP application) or indirectly (phosphatase inhibition) restore I Ca in HF myocytes indicates that reduced basal cAMP-dependent regulation of I Ca could be an important mechanism underlying its decrease during HF. The high value of the EC 50 of the ISO effect on I Ca compared with published data might reflect tissue specificity of, for instance, the density of the ␤-adrenergic receptors as reported in neonatal dog heart.…”

The Distinct Roles of Calmodulin and Calmodulin Kinase II in the Reversal of Run-Down of L-Type Ca2+ Channels in Guinea-Pig Ventricular Myocytes

“…This phenomenon is referred to as rundown and can be considered as a failure of the mechanism by which the basal activity of the channel is maintained (1,2). So far, several mechanisms have been proposed for the run-down of Ca 2+ channels, including dephosphorylation of protein kinase A (PKA)-mediated phosphorylation (3) and proteolysis induced by calpain, a Ca…”

Calmodulin Kinase II Activation Is Required for the Maintenance of Basal Activity of L-Type Ca2+ Channels in Guinea-Pig Ventricular Myocytes

“…This phenomenon is referred to as run-down (2) and can be considered as a failure of the mechanism by which basal activity of the channel is maintained (3). So far, several mechanisms have been suggested for run-down, including proteolysis of the channel and dephosphorylation of the phosphorylation mediated by cAMP-dependent protein kinase (PKA) (2,4). Recent work also shows that phosphatidylinositol-4,5-bisphosphate (PIP 2 ) can stabilize the activity of P / Q-type Ca 2+ channels (5,6).…”