Specific and non-specific desensitization of guinea-pig ileal smooth muscle

The effects of cis-2-methyl-4-dimethylaminomethyl-1-3-dioxolane methiodide (CD), a muscarinic agonist, histamine, substance P and K+-stimulation on the mechanical responses, Ca2+-dependence and desensitization in guinea-pig ileal longitudinal smooth muscle have been studied. The mechanical responses to all four stimulants are highly dependent upon extracellular Ca2+(Ca2+EXT) and are blocked by the Ca2+ channel antagonist nicardipine. The tonic (slow) components of response are more dependent on Ca2+EXT and are more sensitive to nicardipine (IC50 values 5.0 X 10(-8) - 2.5 X 10(-9)M) than are the phasic (fast) components of response. Tissue exposure to CD (5 X 10(-7)M, 10 min) or histamine (3 X 10(-6)M and 3 X 10(-4)M, 10 min) produces short term nonspecific desensitization but substance P (5 X 10(-8)M, 10 min) produces only specific desensitization. K+-induced responses neither desensitize nor are desensitized. Desensitization is concentration- and time-dependent for both specific and nonspecific processes. Nonspecific desensitization is protected by elevation of K+ concentration (5.36mM) in the incubating medium, by dithiothreitol and by inhibitors (mepacrine,p-bromophenacyl bromide and phenylgloxal) of phospholipase A2 and is potentiated by mellitin, an activator of phospholipase A2. Desensitization produced by the muscarinic agonist CS is protected by Gpp(NH)p (10(-4)M), but histamine-induced desensitization is unaffected. There is no loss of muscarinic receptors, measured by [3H]QNB binding following tissue exposure to low concentrations of CD (5.0 X 10(-7)M) for up to 72 h. However, an apparent loss of receptors (20-30%) is measured following 10-90 min exposure of tissue to 10(-3)M CD. It is suggested that contractions of guinea-pig ileal longitudinal smooth muscle elicited by CD, histamine, substance P or K+ mobilize a common pool of Ca2+ through a Ca2+ channel antagonist (nicardipine) sensitive pathway. However, the existence of short term nonspecific desensitization (CD and histamine), specific desensitization (substance P) or no desensitization (K+ stimulation) indicates that significant differences exist in the pathways linking initial stimulus to mechanical response. The ability of elevated K+ to protect against nonspecific desensitization suggest that post stimulus membrane hyperpolarization may represent one contributing component to nonspecific desensitization. Products of phospholipid degradation may also contribute to desensitization since inhibitors or activators of phospholipase A2 prevented or potentiated respectively, nonspecific desensitization.