AJP - Cell Physiology, Vol 248, Issue 1 165-C169, Copyright © 1985 by American Physiological Society
"On" and "off" responses of guinea pig ureter
K. W. Hong, P. Biancani and R. M. Weiss
The in vitro guinea pig ureter responds to 5-s trains of electrical stimuli
with two contractions: the first, an "on" response, occurs within 0.1-0.3 s
after the onset of the stimulus train; the second, an "off" response,
occurs 0.2-1.0 s after the termination of the stimulus train. Force
decreases between the two responses during a time when the stimulus is
still being delivered. Longer duration and/or higher frequencies of stimuli
within the train are required to elicit the off response than the on
response. Neither the on nor the off response appears to be neurally
mediated, since both responses are unchanged by tetrodotoxin, phentolamine,
atropine, and pyrilamine. Decreasing temperature from 37 to 22 degrees C
decreases the amplitude of the on response and increases the amplitude of
the off response. Calcium-free solution, 2 mM ethylenediaminetetraacetic
acid (EDTA), 1 mM Mn2+, and 1 microM verapamil abolish the on response at a
time at which the off response continues to persist. Conversely, 0.5 mM
caffeine and 0.1 mM theophylline abolish the off response, whereas they
only slightly reduce the on response. These data suggest that the on
response depends on extracellular free calcium, whereas the off response is
more dependent on bound or stored calcium.
