[Effect of lysophasphatidylcholine on sensitivity of the heart to acetylcholine and binding of quinuclidinyl benzilate to myocardial membranes]. / Vliianie lizofosfatidilkholina na chuvstvitel'nost' serdtsa k atsetilkholinu i parametry sviazyvaniia khinuklidinilbenzilata s membranami miokarda.

Chemical nature of the blood serum factor exerting cholinolytic effect on the animal heart, was determined. The effect is due to the serum lipid component: lysophosphatidylcholine. The latter in concentration 1-25 microM suppresses the sensitivity of the frog perfused heart ventricle to acetylcholine. Phosphatidylcholine was found to protect the heart from the lysophosphatidylcholine effect. Lysophosphatidylcholine also affected the binding of the acetylcholine high affinity antagonist [3H]-quinuclidinylbenzilate to the rabbit atria cell membranes. The mechanism of this effect is related to an increased ability of muscarinic receptors to form oligomeric complexes in presence of lysophosphatidylcholine, the latter manifesting a positive cooperativity on binding to ligand.

Effect of lysophosphatidylcholine on the structure and function of low density lipoproteins.

The treatment of LDL with bee-venom phospholipase A2 resulted in the formation of lipid-protein particles (phl-LDL) with an increased content of lysophosphatidylcholine (LPC). At the same time, the composition of other lipids and the protein structure remained unaffected. phl-LDL, as well as LPC, abolished the hormone-induced [Ca2+] increase in platelets and platelet aggregation induced by PAF, AMP and thrombin, whereas LDL produced no effect on the hormone-stimulated increase in the intracellular [Ca2+]. The effect persisted in a Ca(2+)-free medium, indicating that phl-LDL and LPC did not abolish the mobilization of intracellular stores with the above-mentioned inducers. Neither LPC no phl-LDL affected the [Ca2+]i level in platelets and suppressed the platelet aggregation evoked by tapsigargine, a specific inhibitor of endoplasmic reticulum Ca(2+)-ATPase, or by phorbol myristate acetate. The inhibitory effect depended on the LPC concentration and the time of platelet incubation with phl-LDL or LPC. The half-maximum efficient LPC concentrations were identical for LPC and phl-LDL (2-4 microM). The inhibitory effect was dependent on the LPC structure: lysophosphatidylethanolamine and phosphatidylcholine displayed no inhibitory effect. The results suggest that when added to washed platelets, free LPC and phl-LDL inhibit only the receptor-dependent increase of [Ca2+]i.

[Subtypes of the functionally active muscarinic receptors in the heart of the frog Rana temporaria]. / Issledovanie subtipov funktsional'no aktivnykh muskarinovykh retseptorov serdtsa liagushki Rana temporaria.

Radioligand analysis of ventricular membranes from the frog Rana temporaria with muscarinic antagonist 3H-QNB (qunuclidinul benzilate) revealed homogeneous population of highly specific muscarinic receptors with high affinity (Kd = 0.502 +/- 0.020 nM and Bmax = 789 +/- 18 fmol/mg protein). Competitive binding of M1-antagonist pirenzepine revealed two binding sites on the membrane with Kd values 5.6 +/- 1.7 and 237 +/- 45 nM, and total capacity of 20.6 +/- 10.2 and 742 +/- 58 fmol/mg protein respectively, which may be attributed to M1- and M2-subtypes in accordance with Receptor Nomenclature. The inhibitory effect of pirenzepine on negative inotropic action of Ach has been studied in the intact electrically stimulated ventricle. Analysis of the data by Schild procedure showed that KB for pirenzepine is equal to 13.9 +/- 2.4 nM. The results obtained suggest that the frog ventricle contains muscarinic cholinoreceptors of M2-subtype (possibly, M4-subtype) which account for negative inotropic effect of Ach. Small population of M1-like receptors was also revealed, their functional role still remaining obscure.

[Effect of macroergic nucleotides and lysolecithin on the cholinergic reception of the heart muscle in the frog Rana temporaria]. / Vliianie makroérgicheskikh nukleotidov i lizoletsitina na kholinoretseptsiiu serdechnoi myshtsy liagushki Rana temporaria.

In experiments on isolated frog ventricle, it has been demonstrated that the dose-response curve for negative inotropic reaction of the myocardium to acetylcholine exhibits a sigmoid form, Hill's coefficient (nH) of this reaction being more than 1. The value of nH depends on the interval from isolation of the ventricle and on the duration of perfusion of the latter with Ringer's solution. It was shown that ATP, UTP, UDP and GTP in physiological concentrations induce both the increase in nH and the increase of K50 (acetylcholine concentration evoking the effect which is equal to half of the maximal one) of the investigated physiological reaction. Similar effects are produced by lysolecithin. Possible causes of "physiological cooperativity" of negative inotropic reaction of the myocardium to acetylcholine and the role of energy-rich nucleotides in this process are discussed.