PDE type-4 inhibition increases L-type Ca(2+) currents, action potential firing, and quantal size of exocytosis in mouse chromaffin cells.
We studied the effects of the cAMP-hydrolyzing enzyme phosphodiesterase type-4 (PDE4) on the L-type Ca(2+) channels (LTCCs) and Ca(2+)-dependent secretion in mouse chromaffin cells (MCCs). The selective PDE4 inhibitor rolipram (3 microM) had a specific potentiating action on Ca(2+) currents of MCCs (40% increase within 3 min). A similar effect was produced by the selective beta(1)-AR agonist denopamine (1 microM) and by the unselective PDEs inhibitor IBMX (100 microM). Rolipram and denopamine actions were selective for LTCCs, and the Ca(2+) current increase remained unchanged if the two compounds were applied simultaneously. This suggests that at rest, LTCCs in MCCs are down-regulated by the low levels of cAMP determined by PDE4 activity and that LTCCs can be up-regulated by either inhibiting PDE4 or activating beta(1)-AR. No other PDEs are likely involved in this specific action. PDE4 inhibition had also a marked effect on the spontaneous firing of resting MCCs and catecholamine secretion. Rolipram up-regulated the LTCCs contributing to the "pace-maker" current underlying action potential (AP) discharges and accelerated the firing rate, with no significant effects on AP waveform. Acceleration of AP firing was also induced by the LTCC-agonist Bay K (1 microM), while nifedipine (3 microM) reduced the firing frequency, suggesting that LTCCs and intracellular cAMP play a key role in setting the pace-maker current regulating MCCs excitability. Rolipram increased also the size of the ready-releasable pool and the quantal content of secretory vesicles without affecting their probability of release. Thus, rolipram acts on MCCs by up-regulating both exocytosis and AP firings. These two processes are effectively down-regulated by PDE4 at rest and can dramatically increase the quantity of released catecholamines when PDE4 is inhibited and/or cAMP is raised.
Marcantoni
A
A
Department of Neuroscience, NIS Center, Torino, Italy.
Carabelli
V
V
Vandael
D H
DH
Comunanza
V
V
Carbone
E
E
D015056:1-Methyl-3-isobutylxanthine / Q000494:pharmacology; D000200:Action Potentials / Q000187:drug effects*; D000818:Animals; D020746:Calcium Channels, L-Type / Q000187:drug effects / Q000502:physiology*; D019439:Chromaffin Cells / Q000187:drug effects / Q000502:physiology*; D054703:Cyclic Nucleotide Phosphodiesterases, Type 4 / Q000502:physiology; D004983:Ethanolamines / Q000494:pharmacology; D005089:Exocytosis / Q000187:drug effects*; D051379:Mice; D008810:Mice, Inbred C57BL; D018408:Patch-Clamp Techniques; D058988:Phosphodiesterase 4 Inhibitors; D010726:Phosphodiesterase Inhibitors / Q000494:pharmacology*; D020889:Rolipram / Q000494:pharmacology; D022142:Secretory Vesicles / Q000187:drug effects; D015854:Up-Regulation
C485995:Cacna1d protein, mouse; D020746:Calcium Channels, L-Type; D004983:Ethanolamines; D058988:Phosphodiesterase 4 Inhibitors; D010726:Phosphodiesterase Inhibitors; D054703:Cyclic Nucleotide Phosphodiesterases, Type 4; D020889:Rolipram; D015056:1-Methyl-3-isobutylxanthine; C037293:denopamine
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