Novel regulation of muscarinic receptors and their coupling with G proteins in smooth muscle: transient resensitization during desensitizing process

The affinity of histamine for Gq protein-coupled histamine H(1)-receptors is predominantly regulated by their internalization in human astrocytoma cells

We examined the regulatory mechanisms of the affinity of Gq protein-coupled histamine H(1)-receptors for histamine after histamine pretreatment in intact human U373 MG astrocytoma cells. In control cells, the displacement curves for histamine against the binding of 5 nM [(3)H]mepyramine, a radioligand for H(1)-receptors, showed the presence of two binding sites for histamine, that is, high and low affinity sites. Pretreatment with 0.1 mM histamine for 30 min at 37°C induced a significant reduction in the percentage of high affinity sites for histamine and a concomitant increase in the percentage of low affinity sites with no change in their pIC(50) values. These histamine-induced changes were insensitive to 30 µM KN-62, an inhibitor of Ca(2+)/calmodulin-dependent protein kinase II, but they were completely inhibited either by 0.4 mM ZnCl(2), an inhibitor of G protein-coupled receptor kinases (GRKs), or under hypertonic conditions, where clathrin-mediated endocytosis is known to be inhibited. These results suggest that histamine-induced conversion of high to low affinity sites for histamine is predominantly regulated by GRK/clathrin-mediated internalization of H(1)-receptors in human astrocytoma cells.

DESENSITIZATION OF GUINEA-PIG TAENIA CAECI SMOOTH MUSCLE INDUCED BY A LOW CONCENTRATION OF CARBACHOL

1. In guinea-pig taenia caeci smooth muscle we have found that 10(-4) mol/L carbachol-induced desensitization to muscarinic agonists develops within 15-30 s, followed by transient resensitization at 1 min, whereas the desensitization to depolarizing high K(+) develops with maximal desensitization at 1 min followed by sustained resensitization up to 30 min. In both cases, Ca(2+)-dependent processes play a crucial role in determining the development of desensitization. 2. To elucidate whether these peculiar processes of desensitization/resensitization may be induced by a lower concentration of carbachol, we examined the development of desensitization induced by 10(-6) mol/L carbachol, because at this concentration carbachol is known to induce biphasic changes in intracellular Ca(2+) concentrations, with a smaller transient increase followed by a larger sustained increase than seen with 10(-4) mol/L carbachol. 3. Contractile responses to muscarinic agonists (carbachol or AHR-602) and high K(+) were desensitized by pretreatment with 10(-6) mol/L carbachol for 30 min in a manner dependent on the presence of extracellular Ca(2+). 4. The development of 10(-6) mol/L carbachol-induced desensitization to these muscarinic agonists in the presence of extracellular Ca(2+) showed three successive phases: fast desensitization within 30 s, followed by transient resensitization at 1 min and the subsequent development of desensitization up to 30 min. In contrast, desensitization to high K(+) did not develop up to 10 min and significant desensitization occurred at 30 min, with no apparent resensitization phase. 5. These results suggest that the characteristics of the Ca(2+)-dependent development of desensitization to muscarinic agonists, but not to high K(+), are well maintained in desensitization induced by a lower concentration of carbachol.

DIFFERENTIAL DEVELOPMENT OF CARBACHOL-INDUCED DESENSITIZATION IN RECEPTOR-MEDIATED Ca2+INFLUX AND Ca2+RELEASE PATHWAYS IN SMOOTH MUSCLE OF GUINEA-PIG TAENIA CAECI

1. We have found that development of carbachol (CCh)-induced desensitization to receptor agonists, but not to receptor by-passed stimulation, is transiently interrupted by a Ca2+-dependent resensitization during the early stage in the smooth muscle of guinea-pig taenia caeci. To further characterize the receptor-mediated signal transduction pathways involved in this peculiar desensitization process, we examined the desensitization processes during Ca2+ influx- and Ca2+ release-mediated contractions in response to activation of muscarinic receptors or histamine H1 receptors. 2. Desensitization treatment with 10(-4) mol/L CCh for 30 min in the presence of extracellular Ca2+ resulted in desensitization to the muscarinic agonists McN-A-343 or AHR-602, which are known to induce contraction only in the presence of extracellular Ca2+ in taenia caeci. The development of desensitization to these agonists was interrupted by a transient resensitization at 1 min. In contrast, the transient resensitization phase was lost following removal of extracellular Ca2+ during the desensitization treatment with CCh; under these conditions, the desensitization developed gradually without an apparent resensitization phase. 3. Contractions to 10(-4) mol/L CCh and 10(-4) mol/L histamine in the absence of extracellular Ca2+ were gradually desensitized without a resensitization phase following the CCh desensitization treatment, irrespective of the presence or absence of extracellular Ca2+ during CCh treatment, although the onset of the desensitization was delayed under Ca2+-free conditions. 4. These results suggest that the receptor-mediated Ca2+ influx and Ca2+ release pathways are differentially desensitized to CCh and that the transient resensitization appears to regulate the desensitization process in response to Ca2+ influx-mediated contraction. Such differential processes of desensitization in receptor-mediated bifurcated signalling pathways may determine cellular responsiveness to certain types of stimuli, depending on the different Ca2+ sources required for contraction.

Ca2+-dependent differential development of carbachol-induced desensitization to receptor agonists and high K+ in guinea-pig taenia caeci

1. Carbachol (CCh)-induced desensitization to CCh was interrupted by a transient resensitization during its early stage, with concomitant changes at the muscarinic receptor/G-protein level in smooth muscle of guinea-pig taenia caeci. To assess whether such a peculiar desensitizing process may heterologously regulate smooth muscle contraction, we examined the developmental processes of CCh-induced desensitization to histamine and high K(+) and compared it with that to CCh. 2. Under Ca(2+)-containing physiological conditions, treatment with 10(-4) mol/L CCh for 30 min induced heterologous desensitization to histamine and high K(+). The development of desensitization to histamine was interrupted by a transient resensitization at 1 min in a manner similar to that to CCh. In contrast, CCh-induced desensitization to high K(+) reached a peak at 1 min and was followed by a gradual resensitization up to a partial restoration at 30 min. 3. Under Ca(2+)-free conditions containing 0.2 mmol/L EGTA, treatment with 10(-4) mol/L CCh for 30 min failed to induce heterologous desensitization to either histamine or high K(+), whereas the CCh treatment developed homologous desensitization to CCh in a simple time-dependent manner without a resensitization phase. 4. These results suggest that cellular responsiveness to receptor agonists and non-receptor-mediated depolarizing stimulation is differentially regulated by Ca(2+)-dependent heterologous desensitization in smooth muscle.

Ca(2+)/calmodulin-mediated regulation of the desensitizing process in G(q) protein-coupled histamine H(1) receptor-mediated Ca(2+) responses in human U373 MG astrocytoma cells

We investigated Ca(2+)/calmodulin (CaM)-mediated regulation of the desensitizing process of the histamine H(1) receptor-mediated increase in intracellular Ca(2+) concentration in human U373 MG astrocytoma cells. The desensitizing process was evaluated by measuring the histamine-induced Ca(2+) responses in cells pretreated with histamine for 15 s-30 min under various conditions. Under normal physiological conditions, desensitization developed with three successive phases : a fast desensitization within 15 s, a transient resensitization at 45 s, and a prompt and sustained redesensitization from 1 to 30 min. Similar processes of desensitization/resensitization occurred even under hypertonic conditions, where histamine-mediated internalization of the histamine H(1) receptor is inhibited. The transient resensitization phase was selectively prevented by deprivation of extracellular Ca(2+) and, even more strikingly, by the presence of W-7 (a CaM antagonist). FK506 and cyclosporin A, Ca(2+)/CaM-dependent protein phosphatase (PP2B) inhibitors, mimicked such effects. In the presence of KN-62, a Ca(2+)/CaM-dependent protein kinase II (CaM kinase II) inhibitor, the early development of desensitization disappeared, allowing a slow and simple development of desensitization. The early processes of desensitization and resensitization were unaffected by W-5, okadaic acid, and KN-04 (less potent inhibitors against CaM, PP2B, and CaM kinase II, respectively) or by GF109203X and chelerythrine (protein kinase C inhibitors). The high-affinity site for histamine was converted to a lower-affinity site by histamine treatment, which also showed a transient restoration phase at 45 s in a manner sensitive to KN-62 and FK506. These results provide the first evidence that Ca(2+)/CaM plays a crucial role in determining the early phase of the desensitizing process via activation of CaM kinase II and PP2B, by regulating agonist affinity for histamine H(1) receptors.

Effects of short-term toluene exposure on ligand binding to muscarinic acetylcholine receptors in the rat frontal cortex and hippocampus

Changes in the binding affinity of the muscarinic acetylcholine receptor agonist carbamylcholine were determined in membranes isolated from the brains of rats exposed to toluene at concentrations of 500-2,000 ppm for 6 h. Membrane fractions of the frontal cortex and hippocampus were prepared and agonist-binding affinities were determined by measuring the displacement of [3H]N-methyl scopolamine-binding activity by carbamylcholine. In the frontal cortex, the affinity of high-affinity carbamylcholine binding was reduced following exposure to toluene at a concentration of 1000 ppm or higher. However, in the hippocampus, the affinity of high-affinity binding of carbamylcholine was increased following exposure to toluene. These observations suggest that toluene exposure affects binding affinity of carbamylcholine, and the effect differs by brain region.

Dependence of Ca(2+) sensitivity of arterial contractions on history of receptor activation

Stimulation of receptors causing arterial contraction may also cause attenuation of cell responsiveness to stimuli. This study tested the hypothesis that attenuation of receptor-induced contractions involves Ca(2+) desensitization. Renal artery rings were pretreated with 10 microM phenylephrine (PE), relaxed with PE washout (plus phentolamine), and then activated by histamine (HA). Pretreatment for 30 min resulted in a rightward shift in the concentration-contraction curve to HA by approximately 1/2 log without a reduction in the slope or maximum response. For example, control and PE-pretreated tissues responded to 0.56 microM HA with strong (0.95 F/F(o)) and weak (0.16 F/F(o)) contractions, respectively, where F/F(o) represents contractile force. This reduced reactivity was completely reversed within 90 min. In fura-loaded tissues, PE pretreatment caused less of a rightward shift in the HA concentration-intracellular free Ca(2+) concentration ([Ca(2+)](i)) curve than in the HA concentration-contraction curve. A dissociation between force and [Ca(2+)](i) was also produced when KCl was used instead of HA. These data suggest that the reduced reactivity produced by PE pretreatment involved, in part, a reduction in the ability of HA to increase the Ca(2+) sensitivity of contractions. These data support the hypothesis that the degree of stimulus-induced Ca(2+) sensitization of contractions is dependent on the history of receptor activation.

Evidence for homeostatic adjustments of rat somatosensory cortical neurons to changes in extracellular acetylcholine concentrations produced by iontophoretic administration of acetylcholine and by systemic diisopropylfluorophosphate treatment

We describe the responses of single units in the awake (24 cells) or urethane-anesthetized (37 cells) rat somatosensory cortex during repeated iontophoretic pulses (1.0 s, 85 nA) of acetylcholine, both before and after systemic treatment with the irreversible acetylcholinesterase inhibitor diisopropylfluorophosphate (i.p., 0.3-0.5 LD50). The time-course of the response to acetylcholine pulses differed among cortical neurons but was characteristic for a given cell. Different time-courses included monophasic excitatory or inhibitory responses, biphasic (excitatory-inhibitory, inhibitory-excitatory, excitatory-excitatory, and inhibitory-inhibitory), and triphasic (excitatory-excitatory-inhibitory, inhibitory-inhibitory-excitatory, and inhibitory-excitatory-inhibitory) responses. Although the sign and time-course of the individual responses remained consistent, their magnitude fluctuated across time; most cells exhibited either an initial increase or decrease in response magnitude followed by oscillations in magnitude that diminished with time, gradually approaching the original size. The time-course of the characteristic response to an acetylcholine pulse appeared to determine direction and rate of change in response magnitude with successive pulses of acetylcholine. Diisopropylfluorophosphate treatment, given 1 h after beginning repeated acetylcholine pulses, often resulted in a gradual increase in spontaneous activity to a slightly higher but stable level. Superimposed on this change in background activity, the oscillations in the response amplitude reappeared and then subsided in a pattern similar to the decay seen prior to diisopropylfluorophosphate treatment. Our results suggest that dynamic, homeostatic mechanisms control neuronal excitability by adjusting the balance between excitatory and inhibitory influences within the cortical circuitry and that these mechanisms are engaged by prolonged increases in extracellular acetylcholine levels caused by repeated pulses of acetylcholine and by acetylcholinesterase inhibition. However, this ability of neurons in the cortical neuronal network to rapidly adjust to changes in extracellular levels of acetylcholine questions the potential efficacy of therapeutic treatments designed to increase ambient levels of acetylcholine as a treatment for Alzheimer's disease or to enhance mechanisms of learning and memory.

Bethanechol activates a post-receptor negative feedback mechanism in rabbit urinary bladder smooth muscle

PURPOSE

Recent studies using vascular and gut smooth muscles indicate that contractile receptor agonists may activate post-receptor down-regulatory mechanisms causing a temporary reduction in the strength of subsequent contractions. Our data indicate a similar mechanism exists in detrusor smooth muscle of the urinary bladder.

MATERIALS AND METHODS

Each isolated strip of female rabbit detrusor was placed in a tissue bath, secured to an isometric force transducer, and length-adjusted until depolarization with 110 mM KCl produced a maximum contraction (S0). Subsequent contractions were normalized to S0 (S/S0) or to a first stimulus with 30 mM KCl or caffeine (S/S1). Tissues were pretreated with the muscarinic receptor agonist, bethanechol (BE), then stimulated with KCl, caffeine, or Bay k 8644 to identify potential post-receptor down-regulation.

RESULTS

Contractions induced by 30 mM KCl had three phases labeled fast peak (FP), slow peak (SP) and steady-state (SS). In tissues exposed for 30 min. to a maximum BE concentration then washed for 5 min., the KCl-induced FP and SP, but not SS, responses were reduced by approximately 40%. Smaller reductions in peak KCl-induced contractions occurred in tissues pretreated for a shorter duration or with a 100-fold lower BE concentration. This down-regulation induced by bethanechol pretreatment was reversible, lasting approximately 1-2 h. Not only were KCl-induced contractions reduced by BE pretreatment, but also those produced by the intracellular Ca(2+)-mobilizer, caffeine, and the L-type Ca2+ channel agonist, Bay k 8644.

CONCLUSIONS

Pretreatment of isolated strips of rabbit detrusor with a muscarinic receptor agonist produced short-term down-regulation of KCl-induced peak contractions that may have involved inhibition of both influx of extracellular Ca2+ and release of intracellular Ca2+. Reductions in the degree of this novel modulatory response during disease conditions and aging could enhance contractile activity, possibly causing detrusor instability.

Contrasting effects of carbachol, McN-A-343 and AHR-602 on Ca(2+)-mobilization and Ca(2+)-influx pathways in taenia caeci

1. We compared the binding profiles and contractile mechanisms of putative muscarinic M1 agonists McN-A-343 and AHR-602 with those of carbachol in smooth muscle of guinea-pig taenia caeci. 2. McN-A-343 and AHR-602, as well as carbachol, completely displaced the atropine-sensitive binding of [3H]-quinuclidinyl benzilate to muscarinic receptors present in the membrane preparation. The potency order for the affinity of these agents for muscarinic receptors was carbachol > McN-A-343 >> AHR-602. 3. In the presence of 2.2 mM extracellular Ca2+, McN-A-343 and AHR-602 induced contraction corresponding to 79 and 85%, respectively, of the maximal contraction to 0.1 mM carbachol. Contractions induced by these agents were mediated via activation of the muscarinic receptor subtype that had a high affinity for 4-DAMP (M3 selective) but a low affinity for pirenzepine (M1 selective) and AF-DX 116 (M2 selective). These contractions were inhibited by an L-type Ca2+ channel blocker, verapamil. 4. In Ca(2+)-free solution containing 2 mM EGTA, carbachol elicited a transient contraction whereas no contraction was observed in response to McN-A-343 and AHR-602. Application of McN-A-343 or AHR-602 inhibited the carbachol-induced contraction in Ca(2+)-free solution, and this inhibition was surmounted by a higher concentration of carbachol. 5. The EC50 value for carbachol-induced contraction in the presence of extracellular Ca2+ was approximately 175 times lower than that in the absence of Ca2+. After treatment with propylbenzilylcholine mustard, carbachol induced contraction only in the presence of extracellular Ca2+. 6. The results suggest that in the taenia caeci there is a greater receptor reserve for muscarinic M3 receptor-mediated Ca2+ influx than for M3 mediated Ca2+ release. The compounds McN-A-343 and AHR-602 are agonists of the Ca2+ influx pathway, but do not appear to stimulate the Ca2+ release pathway.

Carbachol-induced desensitization of rat basophilic leukemia (RBL-2H3) cells transfected with human m3 muscarinic acetylcholine receptors

1. Carbachol-induced homologous desensitization of the secretory response was investigated by transfecting RBL-2H3 cells with cDNA encoding the human m3 muscarinic acetylcholine receptor (RBL-m3). 2. Exposure of RBL-m3 cells to 100 microM carbachol for 30 min in Ca2+-free medium inhibited the secretion induced by the subsequent addition of 10 microM carbachol plus Ca2+. 3. Desensitized cells bound [3H]quinuclidinyl benzilate with a similar Bmax and Kd to those of control cells. 4. The carbachol-induced transient increase in levels of inositol 1,4,5-trisphosphate was not changed by desensitization. 5. Homologous desensitization persisted when desensitized cells were permeabilized with Staphylococcal alpha-toxin.