Protas L, Shen J B, Pappano A J
Department of Pharmacology, University of Connecticut Health Center, Farmington, USA.
J Pharmacol Exp Ther. 1998 Jan;284(1):66-74.
We tested hypotheses concerning the muscarinic receptor subtype and the involvement of L-type Ca current (ICa) in the stimulation of contractions by carbachol (CCh) in single guinea pig ventricular myocytes. When superfused with Tyrode's solution (36 degrees C, 5.4 mM [Ca++]o) and stimulated at 0.2 Hz, CCh (EC50 approximately 18 microM) increased the early component of isotonic contractions by acting at muscarinic receptors indistinguishable from the M2 subtype because AF-DX 116 (M2-selective) was more potent than pirenzepine (M1-selective) as an antagonist of the CCh effect. Action potential duration decreased slightly and ICa was not increased when CCh increased contractions. Carbachol increased intracellular Ca++ transients and contractions reversibly, which indicated an effect via sarcoplasmic reticulum (SR) Ca stores. Ryanodine (1-10 microM) blocked the early contraction component increased by CCh, another indication that CCh action depends on SR Ca stores. We previously found that CCh increased a background Na+ current by occupancy of M2 receptors. We now report that the increased contractions by CCh can also originate at M2 receptors and that SR Ca stores are involved in the CCh effect. Because CCh did not significantly increase ICa, the initial increase of intracellular Na+ by CCh may eventually act through Na-Ca exchange to enhance excitation-contraction coupling.
我们测试了关于毒蕈碱受体亚型以及L型钙电流（ICa）在豚鼠单个心室肌细胞中卡巴胆碱（CCh）刺激收缩过程中所起作用的假说。当用台氏液（36℃，细胞外钙离子浓度[Ca++]o为5.4 mM）灌注并以0.2 Hz频率刺激时，CCh（半数有效浓度EC50约为18 microM）通过作用于与M2亚型难以区分的毒蕈碱受体增加了等张收缩的早期成分，因为作为CCh效应拮抗剂，AF-DX 116（M2选择性）比哌仑西平（M1选择性）更有效。当CCh增加收缩时，动作电位时程略有缩短，而ICa并未增加。卡巴胆碱可逆地增加细胞内钙离子瞬变和收缩，这表明其作用是通过肌浆网（SR）钙库实现的。ryanodine（1 - 10 microM）阻断了CCh增加的早期收缩成分，这是CCh作用依赖于SR钙库的另一个证据。我们之前发现CCh通过占据M2受体增加了背景钠电流。我们现在报告，CCh增加的收缩也可起源于M2受体，且SR钙库参与了CCh效应。由于CCh并未显著增加ICa，CCh引起的细胞内钠离子最初增加最终可能通过钠钙交换作用来增强兴奋 - 收缩偶联。
