AJP - Endocrinology and Metabolism, Vol 267, Issue 1 E77-E87, Copyright © 1994 by American Physiological Society

ARTICLES

[Ca2+]i signaling in pregnant human myometrium

S. E. Szal, J. T. Repke, E. W. Seely, S. W. Graves, C. A. Parker and K. G. Morgan
Department of Medicine, Brighman and Women's Hospital, Boston 02115.

The purpose of the present study was to determine the changes in intracellular ionized calcium concentration ([Ca2+]i) or [Ca2+]i sensitivity accompanying spontaneous and agonist-induced contraction of human myometrium at term pregnancy, as well as to quantify the response to three prototypical agonists: 1) oxytocin, 2) vasopressin, and 3) phenylephrine. Uterine biopsies were obtained at the time of cesarean section from patients who delivered at or near full-term gestation. These preparations were used to measure isometric force development and [Ca2+]i levels with the luminescent calcium indicator aequorin. Concentration-response relationships were determined with respect to isometric force development in the presence of the agonist. [Ca2+]i-force relationships were determined with respect to spontaneous phasic contractions, as well as agonist-induced phasic and tonic contractions. The results provide evidence that the phasic nature of term human myometrium is due to 1) the resting [Ca2+]i level being less than the calcium threshold for contractions and 2) the inability of the tissue to maintain high [Ca2+]i levels for prolonged periods of time. In addition, calcium-independent mechanisms of regulation were suggested by the relatively minor calcium sensitizing action of oxytocin and the observation that relaxation of tonic contractions preceded the fall in [Ca2+]i levels.

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