Excitatory pathways in smooth muscle investigated by phase-plot analysis of isometric force development.
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.
Van Koeveringe
G A
GA
Department of Urology, Erasmus University, Rotterdam, The Netherlands.
Van Mastrigt
R
R
D000818:Animals; D004558:Electric Stimulation; D007537:Isometric Contraction; D008959:Models, Neurological; D009130:Muscle, Smooth / Q000294:innervation*; D013268:Stimulation, Chemical; D013552:Swine; D001743:Urinary Bladder