Characterization of action potential-triggered [Ca2+]i transients in single smooth muscle cells of guinea-pig ileum

Authors

  • M. Kohda,

    1. Laboratory of Pharmacology, Department of Veterinary Science, Faculty of Agriculture, Gifu University, Yanagido 1–1, Gifu 501-11, Japan
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  • S. Komori,

    1. Laboratory of Pharmacology, Department of Veterinary Science, Faculty of Agriculture, Gifu University, Yanagido 1–1, Gifu 501-11, Japan
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  • T. Unno,

    1. Laboratory of Pharmacology, Department of Veterinary Science, Faculty of Agriculture, Gifu University, Yanagido 1–1, Gifu 501-11, Japan
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  • H. Ohashi

    Corresponding author
    1. Laboratory of Pharmacology, Department of Veterinary Science, Faculty of Agriculture, Gifu University, Yanagido 1–1, Gifu 501-11, Japan
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Laboratory of Pharmacology, Department of Veterinary Science, Faculty of Agriculture, Gifu University, Yanagido 1–1, Gifu 501-11, Japan.

Abstract

  • 1To characterize increases in cytosolic free Ca2+ concentration ([Ca2+]i) associated with discharge of action potentials, membrane potential and [Ca2+]i were simultaneously recorded from single smooth muscle cells of guinea-pig ileum by use of a combination of nystatin-perforated patch clamp and fura-2 fluorimetry techniques.
  • 2A single action potential in response to a depolarizing current pulse elicited a transient rise in [Ca2+]i. When the duration of the current pulse was prolonged, action potentials were repeatedly discharged during the early period of the pulse duration with a progressive decrease in overshoot potential, upstroke rate and repolarization rate. However, such action potentials could each trigger [Ca2+]i transients with an almost constant amplitude.
  • 3Nicardipine (1 μM) and La3+ (10 μM), blockers of voltage-dependent Ca2+ channels (VDCCs), abolished both the action potential discharge and the [Ca2+]i transient.
  • 4Charybdotoxin (ChTX, 300 nM) and tetraethylammonium (TEA, 2 mM), blockers of large conductance Ca2+-activated K+ channels, decreased the rate of repolarization of action potentials but increased the amplitude of [Ca2+]i transients.
  • 5Thapsigargin (1 μM), an inhibitor of SR Ca2+-ATPase, slowed the falling phase and somewhat increased the amplitude, of action potential-triggered [Ca2+]i transients without affecting action potentials. In addition, in voltage-clamped cells, the drug had little effect on the voltage step-evoked Ca2+ current but exerted a similar effect on its concomitant rise in [Ca2+]i to that on the action potential-triggered [Ca2+]i transient.
  • 6Similar action potential-triggered [Ca2+]i transients were induced by brief exposures to high-K+ solution. They were not decreased, but rather increased, after depletion of intracellular Ca2+ stores by a combination of ryanodine (30 μM) and caffeine (10 mM) through an open-lock of Ca2+-induced Ca2+ release (CICR)-related channels.
  • 7The results show that action potentials, discharged repeatedly during the early period of a long membrane depolarization, undergo a progressive change in configuration but can each trigger a constant rise in [Ca2+]i. Intracellular Ca2+ stores have a role, especially in accelerating the falling phase of the action potential-triggered [Ca2+]i transients by replenishing cytosolic Ca2+. No evidence was provided for the involvement of CICR in the action potential-triggered [Ca2+]i transient.

British Journal of Pharmacology (1997) 122, 477–486; doi:10.1038/sj.bjp.0701407

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