High carbachol increases the electrically induced [Ca 2+] 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 μM on the electrically induced [Ca 2+] i transient of rat isolated ventricular myocytes. Carbachol at this dose range increased the amplitude and duration of the electrically induced [Ca 2+] 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 μM the increase in [Ca 2+] i transient was followed by a cluster of Ca 2+ oscillations in 50–83% of the cells studied. The effects were blocked by atropine, but not pertussis toxin. Depletion of Ca 2+ from sarcoplasmic reticulum by ryanodine, which itself reduced the amplitude of the [Ca 2+] i transient and increased resting fluorescence, abolished the effect of carbachol on the [Ca 2+] i transient without affecting its effect on resting fluorescence ratio. The caffeine-induced [Ca 2+] i transient was unaffected by prior addition of carbachol in a Ca 2+ free and low Na + solution. Inhibition of Ca 2+ influx by the L-type Ca 2+ channel blocker, verapamil, which itself reduced the amplitude of the [Ca 2+] i transient without affecting the resting fluorescence ratio, attenuated the augmentation of the amplitude of the [Ca 2+] i transient elicited by carbachol. Ni 2+, a non-specific Ca 2+ channel blocker and an inhibitor of Na +Ca 2+ exchange, abolished the effects of carbachol on both [Ca 2+] i transient and resting fluorescence ratio. Low external Na +, which increased the resting fluorescence ratio due to its inhibitory effect on Na +Ca 2+ 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 [Ca 2+] i transient in ventricular myocytes via a process which is not pertussis toxin sensitive. The increase in the electrically induced [Ca 2+] i transient may result from increases in Na +Ca 2+ exchange and influx of Ca 2+ via voltage-gated Ca 2+ channels, and mobilization of Ca 2+ from the intracellular store. The mobilization of Ca 2+ from the intracellular store is a secondary event. The study has provided evidence for the first time that muscarinic acetylcholine receptor stimulation by high concentrations of carbachol increases Ca 2+ influx via the Ca 2+ channel and mobilization of Ca 2+ from its intracellular store. The study has also demonstrated for the first time the occurrence of Ca 2+ oscillations induced by high concentrations of carbachol.