Force development at elevated [Mg2+]o and [K+]o in myocardium from the freshwater turtle (Trachemys scripta) and influence of factors associated with hibernation

The effects of high [Mg2+]o on force development were examined for heart muscle of freshwater turtle. Plasma [Mg2+] during hibernation may increase drastically and like plasma [K+] approach values as high as 10 mM. Each experiment performed at either 20 or 5 °C involved four ventricular preparations of which one pair was exposed to 10, and one to 1 mMMg2+. One preparation of each pair was furthermore exposed to 10 mM K+, whereas the other was maintained at 2.5 mM K+. During oxygenation, high relative to low [Mg2+]o displayed a weak tendency to reduce twitch force; a tendency that was not reduced by elevations of [Ca2+]o. Severe hypoxia accentuated the negative effect of high [Mg2+]o. This effect disappeared, however, when hypoxia was combined with acidosis obtained by 24 mM lactic acid. In comparison to [Mg2+]o, high [K+]o strongly depressed force development under both oxygenation and hypoxia, but no consistent interplay between the two ions was revealed. The negative inotropic effects of both high [Mg2+]o and high [K+]o were reduced or eliminated by 10 μM adrenaline. In conclusion the cardiac effects of elevations in [Mg2+]o appear to be small during hibernation, in particular when considering the concomitant adrenergic stimulation and acidosis.