AJP - Gastrointestinal and Liver Physiology, Vol 253, Issue 5 666-G672, Copyright © 1987 by American Physiological Society
Receptors for substance P on isolated intestinal smooth muscle cells of the guinea pig
J. C. Souquet, K. N. Bitar, J. R. Grider and G. M. Makhlouf
Department of Medicine, Medical College of Virginia, Richmond 23298-0001.
Two radioligands, 125I-labeled substance P (125I-SP) and 125I-labeled
substance K (125I-SK), were used to characterize the kinetics and
stoichiometry of binding of mammalian tachykinins [substance P (SP),
substance K (SK), and neuromedin K (NK)] to smooth muscle cells isolated
from the longitudinal muscle layer of guinea pig intestine. Specific
binding of 125I-SP and 125I-SK was rapid, saturable, reversible, and
temperature dependent. Binding attained 63-70% of steady-state binding
within 1 min, coincidentally with the time of optimal contraction. The
order of potency with which mammalian tachykinins and the SP antagonist,
[D-Pro2, D-Trp7,9]SP, inhibited the binding of both radioligands was
identical: SP greater than SK greater than NK greater than [D-Pro2,
D-Trp7,9]SP, implying preferential interaction with a site that had highest
affinity for SP. SK was 2-3 times, NK 3-4 times, and [D-Pro2, D-Trp7,9]SP
7-23 times less potent than SP (IC50 0.36 nM). Except for NK, the order of
potency was similar to that for contraction of isolated muscle cells. The
existence of binding sites with even higher affinity was suggested by the
ability of muscle cells to contract in response to concentrations as low as
10(-13) M. These binding sites were not detectable at the concentration of
radioligands used. It was concluded that a SP receptor is the only
tachykinin receptor subtype present on intestinal muscle cells of the
guinea pig.
