Sub-plasmalemmal [Ca2+]i upstroke in myocytes of the guinea-pig small intestine evoked by muscarinic stimulation: IP3R-mediated Ca2+ release induced by voltage-gated Ca2+ entry

D.V Gordienko,M.I Harhun,M.V Kustov,V Pucovský,T.B Bolton

doi:10.1016/j.ceca.2007.04.012

D.V Gordienko, M.I Harhun + Show 3 more

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https://doi.org/10.1016/j.ceca.2007.04.012

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Abstract

Membrane depolarization triggers Ca2+ release from the sarcoplasmic reticulum (SR) in skeletal muscles via direct interaction between the voltage-gated L-type Ca2+ channels (the dihydropyridine receptors; VGCCs) and ryanodine receptors (RyRs), while in cardiac muscles Ca2+ entry through VGCCs triggers RyR-mediated Ca2+ release via a Ca2+-induced Ca2+ release (CICR) mechanism. Here we demonstrate that in phasic smooth muscle of the guinea-pig small intestine, excitation evoked by muscarinic receptor activation triggers an abrupt Ca2+ release from sub-plasmalemmal (sub-PM) SR elements enriched with inositol 1,4,5-trisphosphate receptors (IP3Rs) and poor in RyRs. This was followed by a lesser rise, or oscillations in [Ca2+]i. The initial abrupt sub-PM [Ca2+]i upstroke was all but abolished by block of VGCCs (by 5μM nicardipine), depletion of intracellular Ca2+ stores (with 10μM cyclopiazonic acid) or inhibition of IP3Rs (by 2μM xestospongin C or 30μM 2-APB), but was not affected by block of RyRs (by 50–100μM tetracaine or 100μM ryanodine). Inhibition of either IP3Rs or RyRs attenuated phasic muscarinic contraction by 73%. Thus, in contrast to cardiac muscles, excitation–contraction coupling in this phasic visceral smooth muscle occurs by Ca2+ entry through VGCCs which evokes an initial IP3R-mediated Ca2+ release activated via a CICR mechanism.

Highlights

Smooth muscle cells (SMCs) are stimulated to contract either by depolarization of the cell membrane, often in the form of an action potential (AP), or by activation of a variety of receptors usually coupled to G-proteins, or by a combination of these mechanisms [1]
This revealed that the discharge of each action potential (AP) following muscarinic receptor activation was associated with a [Ca2+]i transient which was initiated by an abrupt increase of [Ca2+]i at multiple sub-plasmalemmal regions
Even though after each action potential the cell membrane was repolarized to a resting level, each AP was associated with a stepwise increase of the global [Ca2+]i

Summary

Introduction

Smooth muscle cells (SMCs) are stimulated to contract either by depolarization of the cell membrane (electromechanical coupling), often in the form of an action potential (AP), or by activation of a variety of receptors (pharmacomechanical coupling) usually coupled to G-proteins, or by a combination of these mechanisms [1]. The transient BK channel currents evoked have been observed to hyperpolarize the membrane so reducing Ca2+ entry through VGCCs, reducing the rate of Ca2+ loading of the SR stores, and so reducing tension [11,12]. It seems that Ca2+ sparks are a negative feedback mechanism, triggered by store overload, which regulate vascular myocyte tension as they barely increase global [Ca2+]i, while the high local sub-PM [Ca2+]i they create trigger a membrane-potential dependent relaxation of tension through the opening of BK channels [13]

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