AJP - Regulatory, Integrative and Comparative Physiology, Vol 261, Issue 1 138-R144, Copyright © 1991 by American Physiological Society

ARTICLES

Excitatory pathways in smooth muscle investigated by phase-plot analysis of isometric force development

G. A. Van Koeveringe and R. Van Mastrigt
Department of Urology, Erasmus University, Rotterdam, The Netherlands.

Excitatory pathways in the smooth muscle of the pig urinary bladder were investigated using phase-plot analysis of isometric contractions. The phase plots, plots of the rate of change of the force as a function of the force itself, were dominated by a straight line described by the horizontal intercept (Fiso) and the vertical intercept (U). The quotient Fiso/U is a time constant that characterizes the rate-limiting step in isometric force development in the muscle. Bladder strips of 1 mm diameter were activated by electrical field stimuli, acetylcholine, potassium, and ATP in combination with selective pathway inhibitors such as verapamil, atropine, or a calcium-free solution containing ethylene glycol-bis(beta-aminoethyl ether)-N,N,N',N'-tetraacetic acid. When pathways that depended significantly on depolarization or intracellular calcium release were selected, the time constant was significantly smaller, indicating a faster process. The results indicated that the rate-limiting step in force development was determined by the influx of extracellular calcium.

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