title: High carbachol increases the electrically induced [Ca2+]i transient in the single isolated ventricular myocyte of rats.
abstract: In order to investigate the mechanisms responsible for the inotropic effects of muscarinic acetylcholine receptor stimulation by high concentrations of muscarinic receptor agonists, we studied the effects of carbachol at 30-300 microM on the electrically induced [Ca2+]i transient of rat isolated ventricular myocytes. Carbachol at this dose range increased the amplitude and duration of the electrically induced [Ca2+]i transient time and dose dependently. It also increased the resting fluorescence ratio and time to 80% decline of amplitude from the peak. At 100-300 microM the increase in [Ca2+]i transient was followed by a cluster of Ca2+ oscillations in 50-83% of the cells studied. The effects were blocked by atropine, but not pertussis toxin. Depletion of Ca2+ from sarcoplasmic reticulum by ryanodine, which itself reduced the amplitude of the [Ca2+]i transient and increase resting fluorescence, abolished the effect of carbachol on the [Ca2+]i transient without affecting its effect on resting fluorescence ratio. The caffeine-induced [Ca2+]i transient was unaffected by prior addition of carbachol in a Ca2+ free and low Na+ solution. Inhibition of Ca2+ by the L-type Ca2+ channel blocker, verapamil, which itself reduced the amplitude of the [Ca2+]i transient without affecting the resting fluorescence ratio, attenuated the augmentation of the amplitude of the [Ca2+]i transient elicited by carbachol. Ni2+, a non-specific Ca2+ channel blocker and an inhibitor of Na(+)-Ca2+ exchange, abolished the effects of carbachol on both [Ca2+]i transient and resting fluorescence ratio. Low external Na+, which increased the resting fluorescence ratio due to its inhibitory effect on Na(+)-Ca2+ exchange, also abolished the effects of carbachol. The results indicate that the inotropic effect of muscarinic acetylcholine receptor stimulation by high concentrations of a muscarinic receptor agonist may be due to an increase in the electrically induced [Ca2+]i transient in ventricular myocytes via a process which is not pertussis toxin sensitive. The increase in the electrically induced [Ca2+]i transient may result from increases in Na2(+)-Ca2+ exchange and influx of Ca2+ via voltage-gated Ca2+ channels, and mobilization of Ca2+ from the intracellular store. The mobilization of Ca2+ from the intracellular store is a secondary event. The study has provided for the first time that muscarinic acetylcholine receptor stimulation by high concentrations of carbachol increases Ca2+ influx via the Ca2+ channel and mobilization of Ca2+ from its intracellular store. The study has also demonstrated for the first time the occurrence of Ca2+ oscillations induced by high concentrations of carbachol.
authors.affiliation: Department of Physiology, University of Hong Kong, Hong Kong.
authors.forename: H X
authors.initials: HX
authors.lastname: Wang
authors.forename: W M
authors.initials: WM
authors.lastname: Zhang
authors.forename: J Z
authors.initials: JZ
authors.lastname: Sheng
authors.forename: T M
authors.initials: TM
authors.lastname: Wong
mesh_terms: D000818:Animals; D001285:Atropine / Q000494:pharmacology; D002110:Caffeine / Q000494:pharmacology; D002118:Calcium / Q000378:metabolism*; D002217:Carbachol / Q000494:pharmacology*; D004558:Electric Stimulation; D006321:Heart / Q000187:drug effects*; D066298:In Vitro Techniques; D018721:Muscarinic Agonists / Q000494:pharmacology*; D009532:Nickel / Q000494:pharmacology; D037342:Pertussis Toxin; D051381:Rats; D017207:Rats, Sprague-Dawley; D012433:Ryanodine / Q000494:pharmacology; D012964:Sodium / Q000378:metabolism; D014700:Verapamil / Q000494:pharmacology; D010566:Virulence Factors, Bordetella / Q000494:pharmacology
chemical_list: D018721:Muscarinic Agonists; D010566:Virulence Factors, Bordetella; D012433:Ryanodine; D002110:Caffeine; D001285:Atropine; D009532:Nickel; D002217:Carbachol; D012964:Sodium; D014700:Verapamil; D037342:Pertussis Toxin; D002118:Calcium