The effect of extracellular Ca2+ concentration on the negative staircase of Ca2+ transient in field-stimulated rat ventricular cells.

We performed experiments using the Ca2+ indicator dye, fura-2 to investigate the effect of extracellular Ca2+ concentration ([Ca2+]o) on sarcoplasmic reticulum (SR) Ca2+ release and loading in single rat ventricular cells. In normal Tyrode solution (1.8 mM [Ca2+]o) repetitive stimulation (0.5 Hz) resulted in a gradual decrease in calcium transients (the negative staircase phenomenon) without being accompanied by a gradual decrease in diastolic intracellular Ca2+ concentration. The rate of the slow decline in calcium transient was faster in lower [Ca2+]o. However, the peak of the first calcium transient was relatively invariant over a wide range of [Ca2+]o (0.5-5 mM). The size of the calcium transient elicited by field stimulation was proportional to that induced by 10 mM caffeine, applied following the field stimulation. These results suggest that the size of calcium transients depends mainly on the Ca2+ content of the SR. The quiescent period favoured the replenishment of the SR and this effect was promoted further by increasing the driving force for Ca2+ entry across the sarcolemma during this period. We conclude that in low [Ca2+]o, short stimulation interval may limit Ca2+ influx across the sarcolemma during the quiescent period to cause a gradual reduction in calcium content of the SR and thus the calcium transient.