Vol. 284, Issue 1, 66-74, 1998

Carbachol Increases Contractions and Intracellular Ca⁺⁺ Transients in Guinea Pig Ventricular Myocytes¹

Lev Protas², Jian-Bing Shen and Achilles J. Pappano

Department of Pharmacology, University of Connecticut Health Center, Farmington, Connecticut

We tested hypotheses concerning the muscarinic receptor subtype and the involvement of L-type Ca current (I_Ca) in the stimulation of contractions by carbachol (CCh) in single guinea pig ventricular myocytes. When superfused with Tyrode's solution (36°C, 5.4 mM [Ca⁺⁺]_o) and stimulated at 0.2 Hz, CCh (EC₅₀ 18 µM) 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 I_Ca 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 µM) 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 I_Ca, the initial increase of intracellular Na⁺ by CCh may eventually act through Na-Ca exchange to enhance excitation-contraction coupling.