Ca2+ and Sr2+ entry induced Ca2+ release from the intracellular Ca2+ store in smooth muscle cells of rat portal vein.
1. Changes in cytosolic free Ca2+ concentration ([Ca2+]i) induced by membrane depolarizations were investigated using indo-1 microspectrofluorimetry in single patch-clamped smooth muscle cells of rat portal vein at room temperature (20-21 degrees C) and in the presence of 2 mM Ca2+. 2. During a 1 s depolarization from -50 to -30 mV [Ca2+]i rose, but, although the Ca2+ current was terminated by repolarization to -50 mV, [Ca2+]i continued to increase in a regenerative manner. The delay between the end of the voltage step and the peak of the [Ca2+]i rise was reduced by increasing the depolarization. 3. When a second identical depolarization was rapidly applied (8-13s) after the first one, it induced an identical Ca2+ current but a smaller increase in [Ca2+]i which started to decay upon repolarization. 4. A low concentration of caffeine (0.05 mM), applied to cells showing a small depolarization-induced [Ca2+]i transient which reached a peak at the end of the voltage step, produced an increase in amplitude and in duration of the [Ca2+]i rise without changing the amplitude of the depolarization-induced Ca2+ current. 5. The depolarization-induced [Ca2+]i rise was shortened and reduced in amplitude after noradrenaline- (NA 10 microM) or caffeine- (5 mM) induced release of Ca2+ store and when the patch pipette solution contained ryanodine (100 microM). Under these conditions, the depolarization-induced [Ca2+]i transient was maximal at the end of the voltage step and declined immediately when the membrane was repolarized at -50 mV. 6. Experiments were done by replacing extracellular Ca2+ by Sr2+. Depolarization-induced Sr2+ entry through voltage-dependent Ca2+ channels could evoke an increase in indo-1 fluorescence which occurred after the termination of the voltage step. This delayed component of fluorescence increase displayed properties similar to those of the regenerative [Ca2+]i rise recorded in the Ca(2+)-containing solution. 7. The inefficiency of the second of two successive depolarizations to produce the delayed component of [Ca2+]i rise was not due to the emptiness of the intracellular Ca2+ store since, under these conditions, caffeine was still able to induce a Ca2+ release. 8. It is concluded that depolarization-evoked Ca2+ or Sr2+ entry through voltage-dependent Ca2+ channels induced the release of Ca2+ from an intracellular store, which could occur in a regenerative manner, independent of the termination of the triggering current.
Grégoire
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Laboratoire de Physiologie cellulaire et Pharmacologie moléculaire, URA CNRS 1489, Bordeaux, France.
Loirand
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Pacaud
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D000818:Animals; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D015220:Calcium Channels / Q000378:metabolism; D066298:In Vitro Techniques; D007424:Intracellular Fluid / Q000378:metabolism; D017136:Ion Transport / Q000187:drug effects; D008564:Membrane Potentials; D009131:Muscle, Smooth, Vascular / Q000166:cytology / Q000187:drug effects / Q000378:metabolism*; D009638:Norepinephrine / Q000494:pharmacology; D011169:Portal Vein / Q000166:cytology / Q000378:metabolism; D051381:Rats; D017208:Rats, Wistar; D012433:Ryanodine / Q000494:pharmacology; D013324:Strontium / Q000378:metabolism*
D015220:Calcium Channels; D012433:Ryanodine; D002110:Caffeine; D002118:Calcium; D009638:Norepinephrine; D013324:Strontium