Pflügers Archiv

, Volume 420, Issue 5–6, pp 428–433 | Cite as

Paradoxical decrease in cytosolic calcium with increasing depolarization by potassium in guinea-pig mesotubarium smooth muscle

  • M. -L. Lydrup
  • B. Himpens
  • G. Droogmans
  • P. Hellstrand
  • A. P. Somlyo
Excitable Tissues and Central Nervous Physiology


The free intracellular Ca2+ concentration ([Ca2+]i) was measured simultaneously with isometric force in strips of guinea-pig mesotubarium using the Fura-2 technique. [Ca2+]i and force were maximal at a relatively low (30 mM) concentration of extracellular K+ ([K+]o), and declined at 90 and 140 mM K+. Plateau values of both [Ca2+]i and force were higher in the presence of 5 · 10−6 M ryanodine, indicating that the sarcoplasmic reticulum (SR) contributes to the decline with depolarization. Force and [Ca2+]i at 90 mM K+ were both lower then the high-K+ solution was applied after a period in 30 mM K+ than after a period in normal solution (5.9 mM K+), consistent with inactivation of Ca2+ channels during prolonged depolarization. Addition of carbachol to the depolarized muscle caused a maintained increase in force without maintained increase in [Ca2+]i. We conclude that the decrease in force at increased [K+]o (the “calcium-potassium paradox”) is due to a membrane-potential-mediated decrease in [Ca2+]i and, to a lesser extent, to desensitization of the contractile-regulatory apparatus to Ca2+.

Key words

Calcium Mesotubarium Smooth muscle Fura-2 K+ depolarization 


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Copyright information

© Springer-Verlag 1992

Authors and Affiliations

  • M. -L. Lydrup
    • 1
  • B. Himpens
    • 2
  • G. Droogmans
    • 2
  • P. Hellstrand
    • 1
  • A. P. Somlyo
    • 3
  1. 1.Department of Physiology and BiophysicsUniversity of LundLundSweden
  2. 2.Physiological Laboratory of the K.U. LeuvenLeuvenBelgium
  3. 3.Department of PhysiologyUniversity of Virginia, School of MedicineCharlottesvilleUSA

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