Obligatory role of a Tris/choline allosteric site in guanine nucleotide regulation of [3H]-L-QNB binding to muscarinic acetylcholine receptors

Divalent cation-sensitive antagonist binding to heart muscarinic receptors

1. The binding of (-) [3H]quinuclidinylbenzilate (QNB), a potent muscarinic antagonist, to cardiac muscarinic receptors was examined in washed particles and microsomes isolated from rat heart atria. 2. Addition of divalent cations (Mg2+, Mn2+ and Ca2+) to the binding assays increased (6 to 8-fold) the antagonist binding to microsomes but exerted little or only weak (up to 35%) stimulation of the antagonist binding to washed particles. 3. Mg2+ ions present only in the dilution and washing buffer prior to rapid filtration of the binding reaction were capable of markedly increasing the antagonist binding to microsomes but not to washed particles. 4. Such differential action of Mg2+ on antagonist binding to subcellular fractions was seen when fractions were either isolated and or incubated in media containing either 10 or 50 mM imidazole-Cl, Tris-phosphate or Tris-Cl buffer, each at pH 7.5. While the modulatory effect of Mg2+ was not seen in Na-K-phosphate buffer, pH 7.5, the antagonist binding per se was much higher in Na-K-phosphate buffer compared to other buffers.

Effects of temperature and buffer composition on calcium sequestration by sarcoplasmic reticulum and plasma membrane of rabbit renal artery

45Ca electron microscopic autoradiography was used to examine the effects of buffer composition and temperature on the distribution of calcium in rabbit renal artery smooth muscle cells. The results show that the relative distribution of calcium is dependent on both the buffer used (Tris or Krebs) and the temperature of the bathing solution (25 degrees C or 34 degrees C). Krebs buffer at 34 degrees C gave the highest relative activity in the plasma membrane, sarcoplasmic reticulum, and mitochondria. Buffer and temperature had little effect on the relative activity of the nucleus or cytoplasm. Next, we identified the cellular sites of calcium accumulation after 5, 15, 30, or 60 min exposure to 45Ca in Krebs buffer at 34 degrees C. The results show that sarcoplasmic reticulum and plasma membrane are the primary sites of calcium accumulation during influx into these cells. Although the amount of 45Ca in the cell continues to increase with longer exposure, the relative distribution of calcium is essentially the same after 5 or 60 min. The data also indicate that the relative activity of plasma membrane + sarcoplasmic reticulum (a combination site that includes sarcoplasmic reticulum within a mean distance of 275 nm of the plasma membrane) is similar to the membrane alone and is lower than the sarcoplasmic reticulum alone.

Guanine nucleotide-induced positive cooperativity in muscarinic-cholinergic antagonist binding

The effect of guanine nucleotides on the binding of [3H] Quinuclidinyl benzilate to heart muscarinic receptors was studied. It was observed that GTP and Gpp (NH)p increased antagonist binding. Scatchard transformation of the data resulted in curvilinear plots (downward concavity) consistent with positive cooperativity. Graphic analysis for cooperativity indicated that guanine nucleotides: a) increased slightly but consistently the affinity for the antagonist (independent of receptor occupancy) and b) induce positive cooperativity in the binding of [3H]QNB (dependent of receptor occupancy).

Reduction of muscarinic acetylcholine receptor number and affinity by an endogenous substance

We examined the soluble fraction from homogenates of 12-day embryonic chick heart for the presence of an endogenous modulator of muscarinic acetylcholine receptors (mAChR). Homogenates were separated into 100,000 g soluble and crude membrane fractions by differential centrifugation. Aliquots of membranes were incubated in the presence or absence of the soluble fraction and the muscarinic antagonist, [3H]quinuclidinyl benzilate ( [3H]QNB), and the data subjected to Scatchard analysis. In the presence of the soluble fraction, mAChR number decreased up to 70% and the affinity for [3H]QNB decreased six- to eightfold. These results suggested that an endogenous soluble factor (ESF) affected cholinergic ligand binding to the receptor. The amount of ESF extracted from less than 10 mg of brain was sufficient to reduce by 50% [3H]QNB binding to 50 fmol mAChR. ESF activity was partially purified by heat and acid treatment. The loss of receptors was dependent upon the amount of ESF added and was time dependent. QNB protected some receptors from loss due to ESF. The change in mAChR affinity for [3H]QNB was observed only if ESF was present continuously during the [3H]QNB binding assay. Ultrafiltration and gel filtration showed that ESF was less than 10,000 daltons and probably less than 700 daltons. ESF activity was blocked by EDTA. However, ESF was not a divalent cation since it was base labile, and removal of divalent cations with Chelex-100 did not inhibit ESF activity. ESF activity was also blocked by catechol, catecholamines, ascorbate, and dithiothreitol. ESF was present in embryonic but not in adult heart.

Large scale preparation and characterization of membrane-bound and detergent-solubilized muscarinic acetylcholine receptor from pig atria

The muscarinic acetylcholine receptor (mAcChR) has been prepared from pig atrial membranes by new large scale procedures which result in 30-40 fold enrichment of the receptor in the membrane-bound state and a further three fold enrichment during solubilization. The membrane-bound receptor was prepared by differential and sucrose density gradient centrifugation in 25 mM imidazole, 1 mM EDTA, pH 7.4. A double extraction procedure using a mixed digitonin/cholate detergent was used to solubilize the receptor at a 60-70% yield. The membrane and solubilized preparations had specific activities of 3.5-5 and 8-12 pmol [3H]L-quinuclidinyl benzilate (QNB) binding sites per mg of protein, respectively. The presence of imidazole, which behaved as a weak muscarinic ligand, stabilized the receptor during solubilization and storage. Both the membrane-bound and detergent-solubilized mAcChR bound antagonists at a single class of sites and agonists at two subclasses of QNB sites. The proportion of high affinity agonist sites in the solubilized receptor was about 1/3 that in the membrane receptor. [3H]Propylbenzilylcholine mustard covalently labeled a single prominent atropine-sensitive component with an apparent molecular weight of 70-74,000 on SDS-polyacrylamide gels for both the membrane and solubilized receptor.