Mobilization of calcium from intracellular stores, potentiation of neurotransmitter-induced calcium transients, and capacitative calcium entry by 4-aminopyridine.
In this study we analyzed the effect of 4-aminopyridine (4-AP) on free cytosolic calcium concentration ([Ca(2+)](i)) in basal conditions, after stimulation with neurotransmitters, and during capacitative calcium entry. Using fura-2 ratiometric calcium imaging, we found that 4-AP increased [Ca(2+)](i) in type I astrocytes, neurons, and in skeletal muscle cells. The [Ca(2+)](i) elevation induced by 4-AP was concentration-dependent and consisted of two phases: the first was dependent on intracellular calcium mobilization, and the second was dependent on extracellular calcium influx. 4-AP also increased the second messenger inositol trisphosphate in both neurons and astrocytes. In astrocytes, 4-AP treatment potentiated the sustained phase of the [Ca(2+)](i) elevation induced by ATP and bradykinin. In addition, capacitative calcium entry was potentiated severalfold by 4-AP, in astrocytes and muscle cells but not in neurons. These effects of 4-AP were completely and promptly reversible. 4-AP blocked voltage-sensitive K(+) currents in astrocytes. However, voltage-sensitive K(+) channel blockers inhibiting these currents did not affect agonist-induced calcium transients or capacitative calcium entry, indicating that 4-AP effects on [Ca(2+)](i) were not caused by the blockade of voltage-gated K(+) channels. We conclude that 4-AP is able to affect calcium homeostasis at multiple levels, from increasing basal [Ca(2+)](i) to potentiating capacitative calcium entry. The potentiation of capacitative calcium entry in astrocytes or muscle cells may explain some of the therapeutic activities of 4-AP as a neurotransmission enhancer.
Grimaldi
M
M
Laboratory of Adaptive Systems, National Institute for Neurological Disorders and Stroke and National Institute of Deafness and other Communicative Disorders, National Institutes of Health, Bethesda, Maryland 20892, USA. mgrimaldi@usuhs.mil
Atzori
M
M
Ray
P
P
Alkon
D L
DL
D015761:4-Aminopyridine / Q000494:pharmacology*; D000255:Adenosine Triphosphate / Q000494:pharmacology; D000818:Animals; D001253:Astrocytes / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D001920:Bradykinin / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D000252:Calcium-Transporting ATPases / Q000037:antagonists & inhibitors; D002478:Cells, Cultured; D003600:Cytosol / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005456:Fluorescent Dyes; D016257:Fura-2; D007295:Inositol Phosphates / Q000378:metabolism; D007424:Intracellular Fluid / Q000378:metabolism*; D017136:Ion Transport / Q000502:physiology*; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D018482:Muscle, Skeletal / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D009474:Neurons / Q000166:cytology / Q000187:drug effects / Q000378:metabolism; D018377:Neurotransmitter Agents / Q000378:metabolism* / Q000494:pharmacology; D018408:Patch-Clamp Techniques; D026902:Potassium Channel Blockers; D051381:Rats; D017208:Rats, Wistar; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases
D004791:Enzyme Inhibitors; D005456:Fluorescent Dyes; D007295:Inositol Phosphates; D018377:Neurotransmitter Agents; D026902:Potassium Channel Blockers; D000255:Adenosine Triphosphate; D015761:4-Aminopyridine; D053498:Sarcoplasmic Reticulum Calcium-Transporting ATPases; D000252:Calcium-Transporting ATPases; D001920:Bradykinin; D002118:Calcium; D016257:Fura-2