Acetylcholine, the main neurotransmitter of the parasympathetic nervous system, depolarizes various smooth muscles and initiates their contraction via activating muscarinic cholinergic receptors. In most visceral smooth muscle tissues, such as the gastrointestinal tract, airways, and urinary system, muscarinic receptors are comprised of the predominant M2 (about 80 %) and minor M3 (about 20 %) subtypes. Cholinergic excitation is generally mediated by the opening of ion channels selective for monovalent cations (under physiological conditions, <$E roman Na sup+ > and <$E roman K sup +>); among them the cationic channel of an about 60 pS unitary conductance has been recently identified as the main target for the acetylcholine action. The signal transduction leading to channel opening is very complex and involves activation of <$E roman Gsub {alpha o}> protein (an M2 effect), activation of phospholipase С (an M3 effect), and <$E [ roman Ca sup 2+ ] sub i> and voltage dependence of channel opening. These multiple signalling pathways were difficult to reconcile with the channel gating mechanisms since only a simplified two-state channel mechanism (e.g., one open and one shut state) was until recently available. However, our recent studies of channel gating in isolated outside-out membrane patches revealed a greater complexity. Thus, this cationic channel shows transitions between at least eight states, four open and four shut, with strong connections between adjacent shut and open states. Therefore, four pairs of connected states have been identified, which showed voltage-dependent transitions in each pair

of shut/open states. Since the membrane potential did not affect relative proportions between the pairs, we have assumed that these effects are controlled by ligands, which bind to the channel and, thus, stabilize its various open conformations. In this work, direct tests of the above hypothesis have been performed, and their results showed that spontaneous brief channel gating exists in the absence of the receptor or G-protein activation, which is strongly voltage-dependent (increasing at depolarized potentials). Furthermore, this activity was potentiated at a low agonist concentration, while channel openings generally remained brief. An increasing receptor occupancy by the agonist produced long channel openings indicating a shift of gating towards a long open/brief shut pair of the channel states. These findings are interpreted in the context of the context of the established signal transduction pathways; certain predictions for the whole-cell current are also examined.

• Кустов Максим Володимирович (національна безпека)

  Якщо, ви не знайшли інформацію про автора(ів) публікації, маєте бажання виправити або відобразити більш докладну інформацію про науковців України запрошуємо заповнити "Анкету науковця"