Lowering temperature increases cardiac muscarinic receptor site affinity towards agonists and antagonists

Effect of temperature reduction on responsiveness to cholinomimetics in the taenia caeci of the guinea-pig

1. The effect of temperature reduction on the interaction of carbachol (CCh) and McN-A-343 (McN) with muscarinic receptors in the guinea-pig taenia caeci was investigated. 2. McN, a partial agonist, acted on the smooth muscle to produce contraction. The response was unaffected by tetrodotoxin and the pKB for inhibition by pirenzepine was 6.8, indicating that ganglionic M1 receptors were not involved in the response. 3. Reduction in temperature from 37 degrees C to 18 degrees C for 3 h led to a marked reduction in the contractile response to McN (2-200 microM) but no reduction in the response to CCh (0.1-3 microM). 4. The reduction in temperature was not accompanied by any change in the affinity of McN or CCh for muscarine receptors in binding experiments with [3H]-QNB. 5. The KA value for CCh determined after irreversible receptor inactivation with propylbenzilylcholine mustard followed by ca 60-min wash-out was 7.6 microM, a value similar to that obtained in binding experiments. 6. The EC50 for McN in producing contraction at 37 degrees C (2.1 microM) was similar to the KA value for the partial agonist obtained in experiments with the irreversible antagonist phenoxybenzamine (2.5 microM). It was also similar to the KB value determined at 18 degrees C (3.4 microM) when McN could be used as an antagonist of contractions to CCh. 7. At 18 degrees C, phosphatidylinositol (PI) hydrolysis by CCh was reduced to 23% of that at 37 degrees C. 8. It is concluded that reduction of muscarinic receptor activation of the PI pathway by cholinomimetics with lowering of the temperature could account for the findings with McN on contractility.

MgCl2-sensitive and Gpp(NH)p-sensitive antagonist binding states of rat heart muscarinic receptors: preferential detection at ambient temperature assay and location in two subcellular fractions

Some novel observations dealing with antagonist binding to cardiac particulate muscarinic receptors are described. Gpp(NH)p increased (2-3 fold) the specific binding of [3H]-QNB or [3H]-NMS, both potent muscarinic antagonists, to washed particles (WP), but not microsomes (MIC), when the binding was conducted at 30 degrees C. Magnesium, on the other hand, increased (2-3 fold) the binding of these antagonists to MIC, but not to WP, under the same condition. The treatment of subcellular fractions with 0.2 mM N-ethylmaleimide (NEM), a sulfhydryl reagent, failed to significantly modify the respective stimulatory actions of either Gpp(NH)p on WP binding or of magnesium on MIC binding of these antagonists; treatment with dithiothreitol (1 mM) was also ineffective in this regard. Gpp(NH)p decreased Kd (WP) while magnesium increased Kd (MIC) for [3H]-QNB. Repeated freezing/thawing of isolated subcellular fractions abolished the stimulatory effect of magnesium on antagonist binding to MIC but not of Gpp(NH)p on WP antagonist binding; the freeze/thaw procedure per se increased MIC binding but not WP binding of these antagonists. When the binding was conducted at 4 degrees C (24 hr), the stimulatory effect of Gpp(NH)p on [3H]-QNB binding was enhanced (6-fold) in the case of WP and was detectable (80%) in the case of MIC. Under this condition, the stimulatory effect of magnesium on [3H]-QNB binding was also enhanced (5-fold) in the case of MIC and became evident (200%) in the case of WP. The results of this work support the following views: (a) antagonist-occupied cardiac muscarinic receptors are capable of interaction with guanine nucleotide binding proteins (G protein like Gi, Go) and such interaction influences antagonist binding properties (e.g. increased affinity) of the cardiac membrane-associated muscarinic receptors (b) magnesium influences (decreased affinity) antagonist binding properties by interacting with multiple sites of which some are likely associated with components other than G proteins of the particulate fractions (c) a pool of NEM-sensitive sulfhydryls involved in the regulation of Gpp(NH)p-sensitive agonist binding to cardiac muscarinic receptors is not involved in the regulation by either Gpp(NH)p or magnesium of antagonist binding in these subcellular fractions and (d) membrane fluidity and microenvironment surrounding the receptor and G proteins contribute to the actions of Gpp(NH)p and magnesium on antagonist binding.

The mechanism of hypothermia-induced supersensitivity of guinea pig left atria to carbachol

Hypothermia has previously been demonstrated to induce supersensitivity (defined as a decrease in the ED50) of guinea pig left atria to the negative inotropic effect of carbachol. In the present investigation, the dissociation constant (pKA or -log KA) for carbachol, determined using benzilylcholine mustard, was found to be significantly increased at 25 degrees C compared to 37 degrees C. However, the increase in pD2 (-log ED50) for carbachol at 25 degrees C was much less than would be predicted from the increase in pKA. Increasing the extracellular Ca2+ concentration or the frequency of stimulation, both of which, like hypothermia, are believed to increase Ca2+ influx into cardiac cells, resulted in a decrease in sensitivity to carbachol. Carbachol had no significant effect on cAMP or cGMP levels at either 37 degrees C or at 25 degrees C. These results suggest that the hypothermia-induced increase in sensitivity of left atria to carbachol can be explained by an increase in the affinity of the muscarinic receptor for this agonist. However, the expression of this increased affinity appears to be limited. This may be due to a concurrent decrease in the efficacy of the carbachol muscarinic receptor complex.

Muscarinic cholinergic binding sites on rat lymphocytes

Receptors for neurotransmitters in blood cells could serve as useful markers for the same receptors in solid tissues. Muscarinic receptors have been identified in human, rat and mouse lymphocytes by binding of [3H]quinuclidinyl benzilate (3H-QNB); however, the biochemical and pharmacological characterization of such binding sites has not been complete. Spleen lymphocytes were isolated on a histopaque gradient and incubated in Hank's buffer with 3H-QNB. Binding of 3H-QNB was linear with increasing protein concentrations and was saturable. Binding constants were Bmax = 111 +/- 10.5 fmol/10(6) cells, and Kd = 29.7 +/- 3.9 nM (n = 7). An extensive pharmacological analysis of these binding sites indicated that several cholinergic muscarinic drugs were capable of inhibiting 3H-QNB binding. Muscarinic antagonists were more potent than agonists, and lipophilic drugs were more potent than hydrophilic drugs. Several non-cholinergic drugs were also capable of inhibiting 3H-QNB binding; however, they did so also in brain membranes, while a third group of non-cholinergic drugs and neurotransmitters were inactive. Similar results were also obtained in circulating lymphocytes and in lymphocyte membranes. These results suggest that lymphocytes possess muscarinic receptors which share several, although not all, characteristics of the same receptors in brain and other tissues. Measurement of these binding sites could be useful to monitor the status of muscarinic receptors in solid tissues.

Temperature sensitivity of cardiac muscarinic receptors in bat atria and ventricle

In contrast to other mammals, muscarinic receptors in the bat ventricle can mediate significant decrease in basal contractile force (greater than 50%), not only at 37 degrees C but also at hibernation temperature (12 degrees C). At frequencies of contraction that approximate in vivo values for 37-12 degrees C, no significant shift in receptor affinity or maximum response to applied acetylcholine was found for either ventricular or atrial muscle. Low temperature does not appear to compromise receptor function in hibernators. The atypical cholinergic innervation of the ventricle may maintain a regulative role during hibernation.

Temperature effect on subclasses of muscarinic receptors in rat colon, heart and cerebral cortex

The high-affinity muscarinic antagonist /3H/-Quinuclidinyl benzilate (/3H/-QNB) has been used to label muscarinic receptors in a crude membrane fraction of rat cerebral cortex, colon and heart. The inhibition of /3H/-QNB binding by Atropine, Oxotremorine and Pirenzepine was investigated at three temperatures: 37 degrees C, 22 degrees C and 10 degrees C. The IC50 values and the proportion of high (Rt1) and low (Rt2) affinity binding sites were determined for the three compounds. When the temperature were lowered from 37 degrees C to 10 degrees C, in the agonist and antagonist dissociation constants decreased in all tissues. Changes in temperature did not modify Rt1 or Rt2 values for Oxotremorine and Pirenzepine. The results show marked temperature-dependent modifications of IC50 values for muscarinic receptors of high- and low-affinity sites in rat cerebral cortex, colon or heart.

Membrane fluidization increases low-affinity muscarinic receptor binding in brain: changes with aging

Specific cholinergic muscarinic receptor binding was determined with L-[3H]quinuclidinyl benzilate ([3H]QNB) in homogenates from crude synaptosomal pellets prepared from mouse whole-brain homogenates. Specific total (high- and low-affinity) binding was determined in the absence of the agonist carbachol and low-affinity binding in its presence. These membrane preparations were fluidized by adding in vitro aliphatic alcohols ranging from ethanol to hexanol and by increasing the incubation temperatures. At 23 degrees C hexanol (14.7 mM) nearly doubled the low-affinity binding in the presence of carbachol (0.32 mM) and decreased high-affinity binding by the same amount. This suggested a change of muscarinic receptors from high- to low-affinity conformation. Increase of incubation temperature from 24 degrees C to 37 degrees C nearly tripled low-affinity binding. Brain homogenates from female C57BL/6J mice, ages 6, 12, 18, and 30 months, showed a progressively lower stimulation by hexanol of low-affinity [3H]QNB binding in the presence of carbachol. We postulate that this diminished change with age of [3H]QNB-receptor binding in response to alcohols may be a result of increasing membrane rigidity with advancing age. Rigidity of membranes may link aging at the membrane level, synaptic receptors, and impaired learning behavior.