Direct control of contraction force of single frog atrial cells by extracellular ions

We describe a method by which the ionic surround of an isolated frog heart cell can be changed within a small fraction of a contraction cycle while continuously measuring contraction force. With this method, we have investigated the effect on force development of changing the extracellular concentrations of Ca [( Ca]o) and Na [( Na]o) in the period between electrically driven contractions and during the rising phase of a contraction. Raising or lowering either [Ca]o or [Na]o more than 300 ms prior to a stimulus caused peak force of the next contraction to be changed 100% of the way to the steady-state value characteristic of the new ionic concentrations. Similar maneuvers at later times relative to the stimulus caused progressively smaller changes. Lowering [Ca]o from 2 to 1 mM or raising [Na]o from 78 to 110 mM 100 ms after stimulation brought twitch force 35 and 67% of the way to the new steady states, respectively. We conclude that extracellular Ca is the source of activator Ca in these cells and that extracellular Na plays a role in regulation of the intracellular Ca concentration early in the contraction cycle.