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Contribution of sarcoplasmic reticular calcium to smooth muscle contractile activation: gestational dependence in isolated rat uterus
Author(s) -
Taggart Michael J.,
Wray Susan
Publication year - 1998
Publication title -
the journal of physiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.802
H-Index - 240
eISSN - 1469-7793
pISSN - 0022-3751
DOI - 10.1111/j.1469-7793.1998.133bi.x
Subject(s) - ryanodine receptor , myometrium , cyclopiazonic acid , medicine , endocrinology , endoplasmic reticulum , calcium , chemistry , contractility , contraction (grammar) , agonist , muscle contraction , biophysics , myocyte , uterus , receptor , biology , biochemistry
1 The contribution of Ca 2+ released from the sarcoplasmic reticulum (SR) to smooth muscle contractile activation remains poorly understood. By simultaneously monitoring cytosolic [Ca 2+ ] ([Ca 2+ ] i ) and force in isolated rat uterine smooth muscle, we report the influence of SR Ca 2+ release on contractility during conditions (a) of altered SR Ca 2+ homeostasis and (b) where the only source of activating Ca 2+ was derived from the SR. 2 In myometria of non‐pregnant rats, ryanodine (1‐50 μM), a modulator of calcium‐induced calcium release (CICR), had no effect on the spontaneous [Ca 2+ ] i or force transients. However, depletion of SR Ca 2+ by inhibiting the SR Ca 2+ ‐ATPase (with cyclopiazonic acid (CPA), 20 μM) resulted in an enhancement of spontaneous [Ca 2+ ] i and force transients. 3 In myometria of pregnant rats, although ryanodine had no effect in 40 % of tissues studied it produced a small but significant enhancement of the integrated spontaneous [Ca 2+ ] i and force transient in 60 % of cases. The potentiating effects of CPA were enhanced in myometria of pregnant rats compared with non‐pregnant rats, often resulting in maintained [Ca 2+ ] i increases and contraction. 4 In zero external Ca 2+ , agonist‐induced SR Ca 2+ release resulted in transient increases in [Ca 2+ ] i and force. The magnitude of these agonist‐induced [Ca 2+ ] i and force changes were significantly enhanced in myometria of pregnant rats. No evidence for agonist‐induced Ca 2+ ‐independent force production was observed. 5 These results indicate that CICR plays little role in SR Ca 2+ release from the myometrium, and that there are gestational‐dependent alterations in the ability of SR Ca 2+ mobilization to contribute to contractile activation. The implications of these findings for the co‐ordination of myometrial [Ca 2+ ] i signalling and contractility are discussed.