THE EFFECT OF EXTRACELLULAR CA2-STIMULATED RAT VENTRICULAR CELLS( CONCENTRATION ON THE NEGATIVE STAIRCASE OF CA2+ TRANSIENT IN FIELD)

We performed experiments using the Ca2+ indicator dye, fura-2 to inves tigate the effect of extracellular Ca2+ concentration ([Ca2+](0)) on s arcoplasmic reticulum (SR) Ca2+ release and loading in single rat vent ricular cells. In normal Tyrode solution (1.8 mM [Ca2+](0)) repetitive stimulation (0.5 Hz) resulted in a gradual decrease in calcium transi ents (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 +](0). However, the peak of the first calcium transient was relatively invariant over a wide range of [Ca2+](0) (0.5-5 mM). The size of the calcium transient elicited by field stimulation was proportional to th at induced by 10 mM caffeine, applied following the field stimulation. These results suggest that the size of calcium transients depends mai nly on the Ca2+ content of the SR. The quiescent period favoured the r eplenishment of the SR and this effect was promoted further by increas ing the driving force for Ca2+ entry across the sarcolemma during this period. We conclude that in low [Ca2+](0), short stimulation interval may limit Ca2+ influx across the sarcolemma during the quiescent peri od to cause a gradual reduction in calcium content of the SR and thus the calcium transient.