Characterization of intestinal smooth muscle responses and binding sites for endothelin.
The contractile activity of and binding sites for endothelin-1 (ET-1) were investigated in isolated guinea-pig ileal longitudinal smooth muscle (GPILM). ET-1 produced concentration-dependent contractions of GPILM that either slowly subsided in the continued presence of ET-1 or rapidly subsided following washing of the tissue. The ED50 value for ET-1 contractions was 4.2 +/- 1.3 x 10(-9) M. The removal of extracellular calcium or pretreatment with nifedipine produced a complete inhibition of the contractions to ET-1. The IC50 value of nifedipine for inhibition of ET-1 mediated contractions was 3.0 +/- 0.8 x 10(-8) M. ET-1 produced a marked prolonged homologous desensitization of its contractile response but did not affect the responses mediated by carbachol, histamine, serotonin, substance P, and PLA2. High-affinity binding sites for 125I-labelled ET-1 were identified on microsomal membranes prepared from GPILM with Kd and Bmax values obtained by Scatchard analysis of 3.5 +/- 0.6 x 10(-10) M and 2138 +/- 159 fmol/mg protein, respectively. The binding of 125I-labelled ET-1 to GPILM microsomes was characterized by a rapid association (kob value of 0.077 min-1 at a radioligand concentration of 0.45 nM and an extremely slow dissociation (k1 value of 0.011 min-1; t1/2 value of 793 min). The binding was unaffected by the calcium channel antagonists nifedipine, verapamil, and diltiazem (10(-6) M); the receptor antagonists phenoxybenzamine, atropine, and naloxone (10(-6) M) and propranolol; and the peripheral benzodiazepine receptor antagonists Ro 5-4864 and PK 11195 and psychotomimetic drug phencyclidine (10(-5) M).(ABSTRACT TRUNCATED AT 250 WORDS)
Bolger
G T
GT
Department of Pharmacology, Bio-Mega Inc., Laval, Que., Canada.
Liard
F
F
Garneau
M
M
Jaramillo
J
J
D000818:Animals; D015220:Calcium Channels / Q000187:drug effects; D016232:Endothelins / Q000378:metabolism* / Q000494:pharmacology; D006168:Guinea Pigs; D066298:In Vitro Techniques; D007413:Intestinal Mucosa / Q000378:metabolism*; D007457:Iodine Radioisotopes; D008297:Male; D009119:Muscle Contraction / Q000187:drug effects; D009130:Muscle, Smooth / Q000187:drug effects / Q000378:metabolism*; D011956:Receptors, Cell Surface / Q000187:drug effects / Q000378:metabolism*; D017466:Receptors, Endothelin
D015220:Calcium Channels; D016232:Endothelins; D007457:Iodine Radioisotopes; D011956:Receptors, Cell Surface; D017466:Receptors, Endothelin