Effects of Hyperosmolar Solution on the Twitch Force, Membrane Potential, and Intracellular Sodium Activity in Purkinje Fibers and Ventricular Muscles

Kim, Jin Sang; Gho, Jea Ki; Joo, Chan Uhng; Chae, Soo Wan

Abstract

BACKGROUND
Hypertonic solutions are using in emergency patients including refractory shock. The effects of the hyperosmotic solutions for the cardiac contractile effect has remained unclear. To study the mechanism of increase in twitch force by hypertonic solution, memberane potential, intracellular sodium activities(aNia), and twitch force were measured simultaneously in 1 Hz-driven canine Purkinje fibers and guinea pig papillary muscles.
METHODS
To increase osmolarity, 20, 40, and 80 mOsm glucose, NaCl or mannitol was added to normal Tyrode solution. We used the conventional and Na(+)-selective microelectrodes, to study the membrane potential and intracellular sodium activity. Changes in twitch force were evaluated also by tension tranducer.
RESULTS
1) Hyperosmolar glucose or NaCl added to normal Tyrode solution produced membrane pontential hyperpolarization, increase in aNia, and increase in twitch force in dog Purkinje fibers. Increase in twitch force was related to decrease in the ratio of aNia to extracellular sodium activity(aNoa). NaCl-inducedd aNia increase was not blocked by 10(-5)M tetrodotoxin, a fast sodium channel blocker. 2) Hyperosmolar glucose or mannitol added to normal Tyrode solution produced membrane potential hyperpolarization, increase in aNia, and increase in twitch force in guinea pig papillary muscles. However, the addition of hyperosmolar NaCl did not affect on membrane potential, but produced increase in aNia, and decrease in twitch force. 3) Prolonging effect of hyperosmolar glucose on duration of action potential was smaller than that of NaCl or mannitol in Purkinje fibers and papillary muscles. 4) Increase in twich force produced by ECF Na+reduction or by hyperosmotic solution was reated to decrase in the aNia ratio. 5) Relationship curve between increase in twitch force and aNoa/aNia ratio in hyperosmolr solution was less steeper than that in ECF Na(+)-reduced solution.
CONCLUSION
The above results suggested that hyperosmolar solution-induced twitch force change was related to aNoa/aNia ratio change which influenced intracellular calcium activity via Na(+)-Ca(2+)exchange.