The suppression of Ca(2+)- and voltage-dependent outward K+ current during mAChR activation in rat adrenal chromaffin cells.
1. The mechanism by which muscarine, ionomycin or caffeine results in suppression of Ca(2+)- and voltage-dependent outward current in rat adrenal chromaffin cells was evaluated using both whole-cell voltage clamp and single channel recording. 2. The whole-cell current activated following the elevation of the cytosolic calcium concentration ([Ca2+]i) by muscarine inactivates with a time course comparable to that of single Ca(2+)- and voltage-dependent potassium (BK) channels. 3. The whole-cell inactivating current is pharmacologically similar to BK current. 4. The voltage dependence of inactivation and rate of recovery from inactivation are qualitatively similar for both whole-cell current and ensemble averages of single BK channels. Furthermore, changes in the rate of whole-cell current inactivation track expected changes in submembrane [Ca2+]. 5. The suppression of outward current can be accounted for solely by inactivation of BK channels and does not depend on the means by which [Ca2+]i is elevated. 6. Muscarinic acetylcholine receptor (mAChR) activation, changes in holding potential (-50 to -20 mV), and step depolarizations of different amplitude and duration were tested for their ability to elevate [Ca2+]i and thereby regulate the availability of BK current for activation. 7. Following muscarine-induced elevation of [Ca2+]i at holding potentials positive to -40 mV, the availability of BK current for activation was typically reduced by more than 50%. 8. Holding potentials in the range of -50 to -20 mV produced only slight alterations in the availability of BK current for activation. 9. Step depolarizations that cause maximal rates of Ca2+ influx (0 to +10 mV) must exceed 200 ms to reduce the availability of BK current by approximately 50%. 10. The results show that the muscarine-induced elevation of [Ca2+]i produces a profound reduction in the availability of BK channels for activation at membrane potentials likely to be physiologically meaningful. Although depolarization- induced Ca2+ influx can inactivate BK current, we propose that short duration depolarizations that occur during normal electrical activity will not significantly alter BK channel availability.
Herrington
J
J
Department of Anesthesiology, Washington University School of Medicine, St Louis, MO 63110, USA.
Solaro
C R
CR
Neely
A
A
Lingle
C J
CJ
D000200:Action Potentials / Q000187:drug effects; D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism / Q000502:physiology*; D002838:Chromaffin System / Q000166:cytology / Q000378:metabolism*; D004594:Electrophysiology; D066298:In Vitro Techniques; D015759:Ionomycin / Q000494:pharmacology; D008564:Membrane Potentials / Q000187:drug effects; D009116:Muscarine / Q000494:pharmacology; D018721:Muscarinic Agonists; D018408:Patch-Clamp Techniques; D015221:Potassium Channels / Q000187:drug effects / Q000378:metabolism*; D051381:Rats; D011976:Receptors, Muscarinic / Q000378:metabolism
D018721:Muscarinic Agonists; D015221:Potassium Channels; D011976:Receptors, Muscarinic; D002110:Caffeine; D015759:Ionomycin; D009116:Muscarine; D002118:Calcium