Calcium influx in resting conditions in a preparation of peptidergic nerve terminals isolated from the rat neurohypophysis.
1. Calcium accumulation in a preparation of nerve terminals isolated from the rat neurohypophysis was measured both in rapid (10-60 s) and long-term (up to 60 min) uptake experiments, by use of 45Ca2+ as radiotracer and ion-exchange chromatography as separation method. Unless otherwise stated all experiments have been performed in the absence from the incubation media of secretagogues or depolarizing agents. 2. The uptake of 45Ca2+ in nerve terminals was linear up to 30-45 s, with an apparent initial rate of uptake of 0.98 nmol Ca2+ (mg protein)-1 min-1. 3. The level of 45Ca2+ accumulation was sensitive to manipulations of electrochemical gradient for Na+ across the plasma membrane. Alterations of extracellular concentrations of Na+ affected secretory activity to a larger extent than manipulations of internal Na+. These effects were not qualitatively dependent on the nature of the replacement for Na+. 4. Removal of extracellular Na+ induced a significant increase of both the level of 45Ca2+ accumulation and of the apparent initial rate of uptake. The concentration for half-maximal stimulatory effect was 40 mM-Na+. 5. The analysis of the stimulatory effect of high extracellular K+ on the 45Ca2+ accumulation reveals at least two components: a depolarization and an intrinsic K+ effect. 6. Sodium channel inhibitors (TTX, 1.25 microM) decreased significantly the level of 45Ca2+ accumulation, an effect which was evident from the first minute of exposure to the drug. 7. A specific L-type Ca2+ channel blocker (nicardipine) inhibited 45Ca2+ uptake, in a dose-dependent manner. Simultaneous addition of both TTX and nicardipine (20 microM) decreases the 45Ca2+ uptake up to 50%. 8. In conclusion, the uptake of Ca2+ in isolated peptidergic nerve terminals, incubated in resting conditions, is mediated by at least three pathways: a TTX-sensitive and a nicardipine (dihydropyrine)-sensitive pathway and through a Na(+)-Ca2+ exchange-dependent mechanism. The principal route of Ca2+ entry appears to be through TTX-sensitive channels.
Toescu
E C
EC
Department of Human Anatomy, Oxford University.
D000494:Allosteric Regulation; D000818:Animals; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000187:drug effects / Q000378:metabolism; D002462:Cell Membrane / Q000378:metabolism; D066298:In Vitro Techniques; D017136:Ion Transport / Q000187:drug effects; D007700:Kinetics; D008564:Membrane Potentials; D009411:Nerve Endings / Q000187:drug effects / Q000378:metabolism*; D009529:Nicardipine / Q000494:pharmacology; D010904:Pituitary Gland, Posterior / Q000187:drug effects / Q000378:metabolism*; D011188:Potassium / Q000494:pharmacology; D051381:Rats; D012964:Sodium / Q000378:metabolism; D015222:Sodium Channels / Q000187:drug effects / Q000378:metabolism; D013779:Tetrodotoxin / Q000494:pharmacology
D015220:Calcium Channels; D015222:Sodium Channels; D013779:Tetrodotoxin; D012964:Sodium; D009529:Nicardipine; D011188:Potassium; D002118:Calcium