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Changes in the mechanisms involved in uterine contractions during pregnancy in guinea‐pigs
Author(s) -
Coleman H. A.,
Hart J. D. E.,
Tonta Mary A.,
Parkington Helena C.
Publication year - 2000
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.1469-7793.2000.00785.x
Subject(s) - myometrium , depolarization , nifedipine , medicine , contraction (grammar) , endocrinology , pregnancy , chemistry , egta , uterine contraction , calcium , guinea pig , muscle contraction , prostaglandin f2alpha , biophysics , prostaglandin , uterus , biology , genetics
1 The mechanisms involved in contraction in guinea‐pig myometrium were compared at mid‐ and late pregnancy. Tension was recorded simultaneously with either membrane potential or cytoplasmic calcium ([Ca 2+ ] i ) in strips exposed briefly to prostaglandin F 2α (PGF). 2 PGF‐induced increases in tension were underpinned by action potentials followed by sustained depolarization and biphasic increases in [Ca 2+ ] i at mid‐ (peak, 879 ± 199 nM; sustained, 298 ± 35 nM, n = 11) and late pregnancy (peak, 989 ± 302 nM; sustained 178 ± 33 nM, n = 8). 3 At mid‐ and late pregnancy, nifedipine (10 −6 M) reduced (a) the PGF‐induced increase in tension to 84 and 35 %, (b) the level attained during the depolarization by 2 and 12 mV and (c) the peak rise in [Ca 2+ ] i to 42 and 17 %. The sustained rises in [Ca 2+ ] i were resistant to nifedipine. 4 In Ca 2+ ‐free solution (containing 1 mM EGTA), PGF elicited an increase in tension that was 26 % of that in 2·5 mM Ca 2+ and an increase in [Ca 2+ ] i (24 % of the sustained level) at mid‐pregnancy but no increase in tension or [Ca 2+ ] i at term. 5 At both stages of pregnancy, PGF decreased the level of [Ca 2+ ] i required to elicit increases in tension comparable to those evoked by high K + o . The slope of the tension‐[Ca 2+ ] i curves were steeper in mid‐ than in late pregnancy. 6 In conclusion, at mid‐pregnancy, the contractile response of the guinea‐pig myometrium to PGF involves Ca 2+ influx through L‐type voltage‐operated Ca 2+ channels (VOCCs) and by receptor‐operated mechanisms, release of Ca 2+ from intracellular stores, and an increase in the sensitivity of the contractile apparatus to Ca 2+ . At term the situation is different: a modest increase in the sensitivity of the contractile apparatus to Ca 2+ persists and there is a major reliance on Ca 2+ influx through VOCCs.