Abstract
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°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 ICa was not increased when CCh increased contractions. Carbachol increased intracellular Ca++transients and contractions reversibly, which indicated an effectvia 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 M2receptors 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.
Footnotes
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Send reprint requests to: Dr. Achilles J. Pappano, Department of Pharmacology, University of Connecticut Health Center, Farmington, CT 06030.
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↵1 This work was supported by USPHS Grant HL-13339.
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↵2 Present address: Department of Pharmacology, College of Physicians and Surgeons, Columbia University, New York, NY 10032.
- Abbreviations:
- ACh
- acetylcholine
- PTX
- pertussis toxin
- CCh
- carbachol
- Gi
- inhibitory guanine nucleotide binding protein
- AF-DX 116
- (11[[2-[diethylamino)methyl]-1- piperidinyl]acetyl]-5,11-dihydro-6H-pyrido[2,3–6][1,4]benzodiazepine-6-)one
- mAChR
- muscarinic receptor
- SR
- sarcoplasmic reticulum
- InsP3
- inositol 1,4,5- trisphosphate
- Ica
- L-type calcium current
- INa-Ca
- sodium-calcium exchange current
- HEPES
- N-(2-hydroxyethyl) piperazine-N-(2- ethanesulfonic acid)
- cAMP
- cyclic adenosine monophosphate
- cGMP
- cyclic guanosine monophosphate
- IK(ACh)
- acetylcholine-induced potassium current
- fura-2AM
- fura-2-acetoxymethyl ester
- EC50
- effective concentration for 50% maximum effect
- APD50
- action potential duration at 50% amplitude
- PKC
- protein kinase C
- TTX
- tetrodotoxin
- Received April 8, 1997.
- Accepted September 2, 1997.
- The American Society for Pharmacology and Experimental Therapeutics
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