Relationship between calcium current and cytosolic calcium in canine gastric smooth muscle cells.
We measured free intracellular Ca2+ concentration ([Ca2+]i) and Ca2+ current (ICa) simultaneously in voltage-clamped, indo-1-loaded smooth muscle cells isolated from the circular layer of the canine antrum. Resting [Ca2+]i averaged 144 +/- 20 nM in cells held at -70 mV. Depolarization positive to -50 mV elicited ICa and increased [Ca2+]i. Peak [Ca2+]i occurred between 0 and +10 mV and averaged 372 +/- 48 nM. On repolarization, [Ca2+]i decreased slowly (time constant 2-3 s) and the rate depended on the magnitude of [Ca2+]i. Cells were also voltage clamped with protocols that mimicked the upstroke and plateau phases of slow waves. With simulated plateau potentials of -55 to -45 mV, [Ca2+]i increased transiently as a result of the small transient ICa elicited by the upstroke depolarization. Sustained ICa was of sufficient magnitude with plateau depolarizations positive to -40 mV to cause a secondary rise in [Ca2+]i throughout the plateau phase. These data suggest that at the plateau potential of slow waves in situ, ICa is sufficient to cause a sustained increase in [Ca2+]i. The resulting accumulation of Ca2+ may couple the slow wave plateau to contraction and may increase the open probability of Ca(2+)-activated K channels. The latter may provide the outward current necessary to initiate repolarization.
Vogalis
F
F
Department of Physiology, University of Nevada School of Medicine, Reno 89511.
Publicover
N G
NG
Hume
J R
JR
Sanders
K M
KM
D000818:Animals; D002118:Calcium / Q000378:metabolism* / Q000494:pharmacology; D015220:Calcium Channels / Q000187:drug effects / Q000502:physiology*; D003600:Cytosol / Q000378:metabolism; D004285:Dogs; D005260:Female; D005456:Fluorescent Dyes; D007211:Indoles; D008297:Male; D008564:Membrane Potentials / Q000187:drug effects; D009130:Muscle, Smooth / Q000187:drug effects / Q000502:physiology*; D009543:Nifedipine / Q000494:pharmacology; D011706:Pyloric Antrum; D013270:Stomach / Q000502:physiology*
D015220:Calcium Channels; D005456:Fluorescent Dyes; D007211:Indoles; D009543:Nifedipine; C048960:indo-1; D002118:Calcium