Properties of voltage-activated [Ca2+]i transients in single smooth muscle cells isolated from pregnant rat uterus.
1. The intracellular calcium concentration ([Ca2+]i) was measured at 35 degrees C using the fluorescent indicator indo-1 in patch-clamped, single uterine myocytes from pregnant rats to investigate the relationship between depolarization, Ca2+ current (ICa) and [Ca2+]i. 2. Membrane depolarization activated ICa and produced a [Ca2+]i transient. The rapid increase in [Ca2+]i occurred at the same time as the inward ICa. Both ICa and the increase in [Ca2+]i were abolished by nifedipine (10 microM). 3. When the membrane potential was held at -80 mV the threshold depolarization for an increase in [Ca2+]i was about -55 to -50 mV. As the magnitude of the depolarization was increased to about 0 mV there was an increase in the size of both ICa and the increase in [Ca2+]i. As the magnitude of the depolarization was further increased both ICa and the [Ca2+]i increase declined. 4. When the depolarizing pulses were applied at 3 Hz to mimic normal action potentials then the individual [Ca2+]i transients did not fully relax and a tetanic rise of [Ca2+]i was observed. Under these conditions, there was not a simple relationship between the magnitude of the Ca2+ response and Ca2+ entry. When pairs of depolarizing pulses were applied, the increase in [Ca2+]i produced by the second pulse was larger (in relation to the magnitude of the L-type Ca2+ current) than that produced by the first pulse. This facilitation was abolished by both ryanodine and cyclopiazonic acid suggesting a role for release from intracellular stores. 5. We conclude that the L-type Ca2+ current is the major source of Ca2+ ions entering the cell to produce the [Ca2+]i transient on depolarization. The magnitude of the increase in [Ca2+]i may, however, be amplified by Ca2+-induced Ca2+ release.
Department of Physiology, University of Liverpool, Liverpool L69 3BX, UK.
A V
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Shmigol
D A
DA
Eisner
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S
Wray
D000818:Animals; D002118:Calcium / Q000378:metabolism; D015220:Calcium Channels / Q000502:physiology*; D004558:Electric Stimulation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D005260:Female; D007211:Indoles / Q000494:pharmacology; D015640:Ion Channel Gating / Q000502:physiology*; D008564:Membrane Potentials / Q000187:drug effects / Q000502:physiology; D009130:Muscle, Smooth / Q000737:chemistry* / Q000166:cytology / Q000502:physiology*; D018408:Patch-Clamp Techniques; D011247:Pregnancy; D051381:Rats; D017208:Rats, Wistar; D012433:Ryanodine / Q000494:pharmacology; D014599:Uterus / Q000166:cytology*
D015220:Calcium Channels; D004791:Enzyme Inhibitors; D007211:Indoles; D012433:Ryanodine; D002118:Calcium; C000543:cyclopiazonic acid