Force development at elevated [Mg2+]o and [K+]o in myocardium from the freshwater turtle (Trachemys scripta) and...

The effects of high [Mg(2+)](o) on force development were examined for heart muscle of freshwater turtle. Plasma [Mg(2+)] during hibernation may increase drastically and like plasma [K(+)] approach values as high as 10 mM. Each experiment performed at either 20 or 5 degrees C involved four ventricular preparations of which one pair was exposed to 10, and one to 1 mMMg(2+). 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 [Mg(2+)](o) displayed a weak tendency to reduce twitch force; a tendency that was not reduced by elevations of [Ca(2+)](o). Severe hypoxia accentuated the negative effect of high [Mg(2+)](o). This effect disappeared, however, when hypoxia was combined with acidosis obtained by 24 mM lactic acid. In comparison to [Mg(2+)](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 [Mg(2+)](o) and high [K(+)](o) were reduced or eliminated by 10 muM adrenaline. In conclusion the cardiac effects of elevations in [Mg(2+)](o) appear to be small during hibernation, in particular when considering the concomitant adrenergic stimulation and acidosis.